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The Joy of Extreme Possibility

Nuclear rocket designs are hardly new. In fact, it was clear as early as the 1950s that conventional chemical rocketry was inefficient, and programs like Project Rover, set up to study the use of nuclear reactors to heat liquid hydrogen for propulsion, aimed at the kind of rockets that could get us beyond the Moon and on to Mars. The NERVA rocket technology (Nuclear Engine for Rocket Vehicle Application) that grew out of all this showed great promise but ran afoul of political and economic issues even as the last Apollo missions were canceled. Nor is the public wariness of nuclear methods likely to vanish soon, yet another hurdle for future ideas.

But making people aware of what has done and what could be done is good practice, as Kenneth Chang does by example in his recent piece on the 100 Year Starship Symposium, which bears the optimistic title Not Such a Stretch to Reach for the Stars. In interstellar terms, propulsion is the biggest problem of all. Chang’s article suggests a pathway through conventional rocketry and into nuclear-thermal designs, with reference along the way to using nuclear engines to generate the electrical fields that power up an ion engine. The goal on this pathway is fusion, though Chang admits no one has yet built an energy-producing fusion reactor.

The Daedalus concept was fusion-based, and the ongoing Icarus project that followed is now examining Daedalus to note the effect of thirty years of new technology. But Chang has also talked to James Benford, whose interest in laser and microwave beaming remains strong. Leave the propellant behind and you’ve maximized payload, in addition to working with known physics and apparently achievable engineering. And there continue to be startling new concepts like those of Joseph Breeden, who finds a more extreme way to create an engineless vehicle:

From his doctoral thesis, Dr. Breeden remembered that in a chaotic gravitational dance, stars are sometimes ejected at high speeds. The same effect, he believes, could propel starships.

First, find an asteroid in an elliptical orbit that passes close to the Sun. Second, put a starship in orbit around the asteroid. If the asteroid could be captured into a new orbit that clings close to the Sun, the starship would be flung on an interstellar trajectory, perhaps up to a tenth of the speed of light.

“The chaotic dynamics of those two allow all the energy of one to be transferred to the other,” said Dr. Breeden, who came toting copies of a paper describing the technique. “It’s a unique type of gravity assist.”

What I call the ‘joy of extreme possibility’ has animated interstellar studies since the days of Robert Forward. It works like this: We know the distances between the stars are so vast as to dwarf the imagination. Indeed, most people have no notion of them, seeing an interstellar mission as merely a next step once we have explored the outer system, a kind of juiced-up Voyager. The scientists and engineers who work on these matters, knowing better, realize how far beyond our current technologies these journeys really are. So they’re not afraid to speculate even at the absolute far end of the plausible (and often beyond that). Work your way through interstellar papers like these and you pick up an infectious, jazzy brainstorming. It’s the kind of mental riffing on an idea that a John Coltrane or a McCoy Tyner does with a musical theme.

And by the way, Chang is careful to get those distances across to readers. I’m always interested in homely comparisons because you can use them to boggle audience minds when speaking about interstellar flight. This is useful, because a boggled mind often becomes a curious one, and while you can never predict these things, occasionally interstellar studies gain a new adherent. Chang cites a Richard Obousy analogy: If the Earth were Orlando and Alpha Centauri were in Los Angeles, then the Voyager spacecraft would have traveled but a single mile.

Even after all these years, that one still boggles my own mind. Chang again:

Another way of looking at the challenge is that in 10,000 years, the speed of humans has jumped by a factor of about 10,000, from a stroll (2.6 m.p.h.) to the Apollo astronauts’ return from the Moon (26,000 m.p.h.). Reaching the nearest stars in reasonable time — decades, not centuries — would require a velocity jump of another factor of 10,000.

It’s good to see the 100 Year Starship Study steadily percolating in the news. Maybe one day these concepts will not seem as esoteric as they do today. I note as I write this, for example, that my word processor flags the word ‘starship’ as a spelling error. We need to set deeper roots into the culture than that. We can start by doing what conference organizer David Neyland told Chang he wants to do, to establish a bar high enough that people “will actually go start tackling some of these really hard problems.” Of course, the real bar is set by nature, and it’s the highest bar we as a species have faced in terms of travel times and distance. But the joy of extreme possibility only ignites the spirit when everything is on the table and the challenge is immense.

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  • Avatar2.0 October 27, 2011, 14:06

    @ljk:

    “That is right – all any Earthling needs to wipe out someone else on Earth is the right kind of planetoid, a sufficiently efficient rocket engine, and a guidance computer.”

    This Earthling will need another essential ingredient in order to succeed: that no one else on Earth has the same capability.
    If this is not the case, the planetoid could again be diverted long before it caused any damage – even a lax surveillance from a civilization capable of such feats will detect such threats long before they are actually a problem.

    Traditional WMDs (nuclear warheads) are FAR more efficient weapons of mass destruction – under any relevant criterion you’d care to analyse – than asteroids with altered trajectory.

  • Christopher Phoenix October 27, 2011, 18:06

    @ljk:

    Asteroids offer massive amounts of resources for building moon-bases, space stations, spacecraft, and starships. However, they leave plenty wanting as weapons. Altering an asteroid’s orbit will require a rocket ship, crew of trained astronauts, a mass driver or rocket to alter the rock’s trajectory, and plenty of patience. A missile tipped with a nuclear warhead is decades old technology. No new technology like space rockets that actually travel beyond Earth orbit, techniques to alter an asteroids orbit, etc. are required. A program to colonize and manipulate asteroids will be high profile, with absolutely no way to hide. Nuclear subs are stealthy, as are suitcases. Even if you do throw a space rock at an enemy country, it will take some time to get here- much too long to be useful in a nuclear war. Imagine if America launched their missiles, and then it took a month for one to show up at Russia!!

    The final nail in the coffin, as Avatar2.0 pointed out, is that the only way this plan can succeed is if no one else on Earth has the capability to deflect an asteroid. A fleet of Mark IX Hawks will blast the snot out of this nefarious corporation and then put the asteroid back onto a safe course. Moving an asteroid will require shuttle spacecraft and heavy mining equipment- which are not efficient war machines.

    Your plan to threaten other nations with space rocks is similar to the pronouncement that “He who controls space controls the world”. In a sense, this is true, since space is the next area to explore and thrive in. In the sense that this pronouncement was used, it is not really entirely true. An evil space station or orbiting warship cannot simply attack anyone with impunity. The orbiting weapons platform is totally exposed, open to attack from the entire hemisphere below it. It can be attacked by missiles launched from fighters, laser beams, or propaganda by anyone below it- and it has no cover, nowhere to hide. The people below can hide in jungles, underwater, or beneath the ground. They can attack from submarines, aircraft, or with lasers mounted on truck beds. Laser beams don’t suffer from bullet drop when being fired from ground to orbit, so the space station has no real advantage in a laser duel. It is easier to power and cool a laser on the ground than in space.

    Noted rocket engineer G. Harry Stine designed a vehicle called the “Space Scout” to deal with enemy space stations in the early 1950’s. He figured that manned space stations would be controlled by the nation that build them, making it possible for a crazed dictator to convert her scientific space stations to martial moons at the trumpet call. Armed with atomic missiles, these stations could strike at any point on the Earth’s surface with impunity!! The Space Scout is designed to deal with these evil space stations by blowing them up with atomic missiles before they can strike. It is launched by a chemical rocket, completes its mission, and then flies back with jet engines and lands on its tail. Without them the world stand unarmed and helpless before the threats of a technologically advanced dictator. At least, according to Mr. Stine. In reality it is probably much more cost effective to launch flight after flight of surface-to-orbit missiles at the space stations. In fact, a fighter plane can launch an anti-satellite missile, so the evil space stations have to worry about every single fighter aircraft!!

    Later, G. Harry Stine further developed the design into the Mars Snooper- a craft with a nuclear thermal rocket that can fly to Mars. I would guess a Mars Snooper could also blast the Planetoid Bombardment Corp. team of asteroid movers to smithereens with its payload of atomic missiles. You don’t even need a space base to launch a Mars Snooper from- it can take off from any Earth spaceport on its mission to save the planet, then return later and land with its air-fed nuclear ramjets. Just like any proper rocketship should.

    What about nuclear missiles on the Moon? They’d take a few days to reach Earth unless you had a very large rocket. Perhaps some sort of kinetic bombardment is practical- but in the end, conventional subs are more cost effective. A rebellious lunar colony might bomb the Earth with missiles, but for a ground based country a plain old missile works just fine. Why develop all these expensive space technologies when ICBM’s can easily end civilization many times over?

    The asteroids will become very important in the future. Millions of tons of iron, titanium, iridium, gold, platinum, etc. waits to be mined- all in much higher concentrations than is found on Earth. Volatiles like water, oxygen, etc. are also found in the asteroids. Space colonies will need these resources. Earth, however, is not going to run out of resources anytime soon- plenty of untapped ores remain on the ocean floor and elsewhere. Don’t take the Donner Party’s decision because of some “peak lithium” scare!!

    You are right that Mars probably does not offer as much for a developing space civilization as the asteroids. Going for Mars with a single minded obsession of colonizing it, i.c. Zubrin’s “Case for Mars”, while romantic, is a little narrow-minded. Zubrin argues that Mars has all the resources needed to support civilization, but he does not consider the other living options we have in space. We can live in space habitats free of any planet, built with materials from asteroids and containing any environment we want. This may be preferable- such habitats will have 1G spin gravity, easy access to solar energy, no gravity well to blast out of, and none of those dust storms that occasionally cover all of Mars and sometimes carry flying rocks!! Mars is a challenge, to the say the least.

  • ljk October 29, 2011, 18:24

    Most Fascinating NASA Man You’ve Never Heard Of

    August 2nd, 2011

    By Matthew Van Dusen

    If Congress found a half trillion dollars tomorrow for a down payment on a mission to Mars, how would we get there? For more than 50 years, a hard core of rocket scientists has promoted a propulsion system that holds, in roughly equal measure, promise and danger: nuclear.

    Finger in the early 1960s.

    No nuclear rocket aficionado has been as active as Harold B. Finger, the former head of the U.S. nuclear rocket program and a man whose involvement with the American space program predates NASA’s creation.

    This week, the 86-year-old Finger will advocate for nuclear propulsion at a space conference in Dallas. He has written, “the technology of nuclear rocket propulsion was fully demonstrated as ready for flight mission applications… Let’s do it!” Harold Finger is, far and away, the most fascinating rocket scientist you’ve never heard of.

    Full article here:

    http://www.txchnologist.com/2011/nuclear-space-rockets-and-the-most-fascinating-nasa-man-youve-never-heard-of

  • Rob Henry October 30, 2011, 19:25

    Christopher Phoenix, you’re right, ljk does get confused about the utility of a space programme towards military purpose, but you seem to have continued his mistake! The militarisation of space makes absolutely no economic sense as a means in itself, the real danger is that once commercial concerns have established space based operations as viable, the marginal extra cost of transferring weapons systems makes them so cost effective as to give any power with space supremacy total world domination if one of its future leaders so desires.

    Lets also add a little subtlety here – after all the human world attaches high status to professional diplomats. Imagine that China achieves such supremacy, and at a crucial time in a trade dispute with America announces that “you know that asteroid that we have been bringing to Earth orbit to mine its metals – well the mass driver on it is broken. We have calculated that it will probably land somewhere in the United States but our technicians are working night and day to fix the problem and the thoughts of our nation are with you in this dark hour.”

    Note that actually following through with such a threat is counterproductive, but the time lag can actually be used to the advantage of the aggressors. Also note that the advantages of playing such games of chicken over a ground based nuclear exchange are nullified by the accentuated risk created by the large first strike advantage. Those on the high ground really do rule!

  • Christopher Phoenix October 31, 2011, 21:09

    @Rob Henry

    I don’t think I continued ljk’s mistake. I simply pointed out that the effort and time lag involved in altering an asteroids orbit makes a plain old thermonuclear strike a lot simpler.

    What “large first strike advantage”? China says, “We’ll drop an asteroid on you that’ll show up two months later!!” How is that a first strike advantage over an ICBM launch or something more subtle, like a weaponized Ebola virus introduced to Chinese territory? Not to mention that if America has a space military, they can attack and destroy the asteroid moving team and nudge the asteroid toward China. Asteroids are a Rube Goldberg machine.

    Also note that I was debunking the notion that space based weapons will provide world supremacy. An asteroid is a Rube Goldberg machine that fascinates SF readers- but really is not very practical. A space station can be attacked by any fighter jet or ground based missile launcher. It can’t hide. A spaceship can be seen coming- especially if other countries are looking for possible threats. I would not want to give another country the run of space, mainly due to the vast resources available up there and the advances in technology these programs will provide, but space doesn’t offer any clear advantage for placing weapons systems.

    “the real danger is that once commercial concerns have established space based operations as viable”

    This is like saying that it would not cost much to mount machine gun turrets on an oil rig. It would not- but the machine gun turrets would not be very useful. Threatening to drop an asteroid on someone is a bit like parking an oil tanker in their waters and threatening to spill oil. An oil tanker spill can do a lot of damage to ecosystems- but it is not a very good weapons system.

    “Those on the high ground really do rule!”

    This sounds like those arguments scientists used to use to try to get Congress to fund their space programs. How do you rule? You are either in orbit, within range of anti-satellite weapons, or you are millions of miles away. If you are millions of miles away, your weapons won’t be of much use in a ground based conflict.

    I’m not saying space weapons platforms couldn’t exist someday to control or defend space operations. But these weapons will be there be to wage war on the new frontier- not to be used in some James Bond movie plot to dominate other countries by throwing space rocks at them.

    This seems to me to be counterproductive. I was hoping that the people of Earth could explore and share space peacefully. We are all human, after all.

    Who knows what aliens might make of a species that thinks of every new frontier as a place to wage war and dominate? Wouldn’t you consider such a race achieving star travel to be a security threat, as an advanced alien? Any race as primitive to try to dominate even their own kind has no right to be in space. Some of you might think that there is no such thing as a “right” to do something or not, but shows like Star Trek show that quite a few people believe the notion that continuously seeking ways to dominate and control are primitive and contemptible.

    On the star flight approach, how do we even hope to explore other stars if we spend all our time and energy waging tribal warfare? Maybe we need new leaders.

  • Christopher Phoenix October 31, 2011, 21:22

    @Rob Henry

    Even if you are right about countries playing “chicken” with asteroids in the future, China’s plot is as flawed as ljk’s. As Avater2.0 said, the asteroid threat only works if no other nations has the capacity to deflect asteroids.

    Using your example, America could respond by sending “assistance” to the ailing Chinese technicians in the form of a several Cerberus class space warships that just happen to be carrying heavy neutral particle beam cannons and missiles tipped with fusion warheads. This game can be played both ways- are you assuming that only China travels into space in the future?

  • Eniac November 1, 2011, 11:21

    Space is already the high ground. Satellite imagery, GPS, and communications satellites are all highly valuable strategic assets, based in space. Asteroids are indeed useless, but warheads and kinetic missiles may one day join the other assets in orbit. That would be dangerous, because they could be delivered even faster than ICBMs, and without the early warning and vulnerability provided by a rocket launch. Detected, you guessed it, by an IR sensing early warning satellite, from space. Fourth existing strategic space asset, right there. Space is bristling with military hardware today, any economic activity would be arriving way late at the party.

  • Christopher Phoenix November 1, 2011, 23:36

    @Eniac

    Quite right- space is bristling with military hardware today. Weapons platforms carrying kinetic impactors and nukes are possible. However, placing nuclear weapons in space is prohibited by the Outer Space Treaty, as is testing weapons, conducting military maneuvers, or establishing military bases, installations, or fortifications on any kind in other planets. Conventional weapons, like kinetic energy strikes or laser weapons are allowed.
    http://en.wikipedia.org/wiki/Outer_Space_Treaty

    Space is of tremendous strategic importance. This makes it a very unsafe place to be in the event of an all out war, as these satellites and space stations are not invulnerable. Ground to orbit missiles can be launched from aircraft or silos on the ground to take out satellites with their sheer kinetic impact. Laser weapons work too- and they don’t have to totally disintegrate the target satellite to damage it. A low power “dazzler” laser can blind sensors. A higher power laser can burn out sensors and damage antenna or solar panels. A really powerful zap can vaporize the satellite.

    If you are willing to really make a mess, there is another technique. How many nukes does it take to destroy every single satellite? Only one. If you detonate a nuclear weapon at a high altitude, it will create a manmade radiation belt. This radiation belt will disable satellites, zapping their electronics into so much space junk. The Starfish Prime high altitude nuclear test created an artificial aurora. It also created a radiation belt that crippled a number of satellites in Low Earth Orbit, including the first commercial relay communications telescope, Telstar. Imagine what an enterprising warlord with a few nukes could do to the electronic fairyland that orbits above our head.
    http://en.wikipedia.org/wiki/Starfish_Prime

    Don’t assume that your military satellites will keep functioning- planetary defense systems can strike at orbiting satellites or approaching spacecraft. Laser cannons on the ground can duel with orbiting spacecraft, and it is easier to energize and cool lasers on the ground. Swarms of antisatellite missiles will take down satellites.

    If, in the future, Earthlings have to deal with a fleet of armed spaceships coming to conquer the planet, the planet will have more defenses then you might at first realize.

    I prefer to focus on exploration and economic activity. Space is already a dangerous enough place to be without having bullets, lasers, and artificial radiation belts bombard and zap anyone in orbital space.

  • Eniac November 2, 2011, 10:34

    All military assets are vulnerable to attack, more so if they are nearer to the ground where they do not require a space launch to attack.

    Your “enterprising warlord with a few nukes” will have more vulnerable targets to chose from, such as large cities, or military bases. No matter his choice, though, you can bet that this particular enterprise will end badly for him.

  • Christopher Phoenix November 2, 2011, 17:14

    @Eniac

    My point was that satellites are pretty vulnerable to attack- especially radiation based attacks that can destroy electronics. I read an article on man-made radiation belts, which noted that you could cripple the fleet of vital military satellites that orbit overhead fairly easily with a modest rocket carrying a nuke. So if it satellites we are worried about, there are ways to take the out.

    The best option for the “enterprising warlord with a few nukes” is to fry all of America’s electronics with an EMP- just detonate a thermonuclear bomb hundreds of kilometers above Earth’s surface. ZAPP!!! There goes the power grid, the internet, communications systems, cars, aircraft, etc. After this attack, most of the U.S. population will be without light, water, or food- leading to a total panic. The military has some EMP proof equipment, and most of their vehicles are somewhat shielded, so they will still be functional. Time for my warlord to initiate the second part of his fiendish plot… antigravity airships with disintegrator rays!!

    Just kidding… this warlord has some orbiting weapons platforms to strike from once he has hit his enemies with the EMP. Naturally, this warlord has invested plenty of effort in his fledgling Yoyodyne corporation, which is trying to build a nuclear-powered interplanetary spacecraft with ion propulsion. These craft are quickly converted into orbiting weapons platforms to strike at his enemies after he has taken over a quarter of the world. His name is Kahn… Kahn Noonien Singh.

    http://en.wikipedia.org/wiki/Khan_Noonien_Singh

    And yes… his enterprise did turn out badly, although he escaped in a DY-100 Class interplanetary sleeper ship.

  • Rob Henry November 2, 2011, 18:49

    Sorry Christopher but you seem to have misconstrued the context in which I meant “large first strike advantage”. I was actually trying to use it to show that in a ground based nukes v ground based nukes situation the advantages that might be gained by the superior side by brinkmanship were more than offset by the disadvantages of the inferior side panicking and launching (I was alluding to the elevated possibility of this due to that first strike factor). Such a reasoned launch would not occur with a asteroid threat situation.

    As for that help that you suggest America hopes to send, I can just see China announce “we are delighted by the prospects of your help, but our calculations are that you don’t have a craft with sufficient delta V to reach it in time for a safe deflection”, and if they did their despot could not have used that threat in the first place!

  • Christopher Phoenix November 3, 2011, 23:21

    By the way- I don’t believe Richard Shawyer’s EmDrive works. So far, all I have seen is pictures of some metal device sitting on a stand, doing nothing. The EmDrive is no more convincing then the Dean Drive. I’ll need a lot more evidence than that to believe this thing works. Extraordinary claims require extraordinary evidence.

    All these mechanical antigravity devices don’t work. The Breakthrough Propulsion Physics Program responded to them in a paper titled “Response To Mechanical Antigravity” because would-be inventors kept sending their schemes in, all of which were variations of designs involving vibrating weights or gyroscopes.

    http://gltrs.grc.nasa.gov/reports/2006/TM-2006-214390.pdf

    Many of these submissions display paranoia and delusions of grandeur…

    Here is the story of the Dean Drive, yet another scheme for a reactionless thruster. Apparently, Mr. Dean was so afraid his device would be stolen that the wouldn’t let anyone see it.

    http://www.jerrypournelle.com/sciences/dean.html

    According to Jerry Pournelle, this is what you should do if you think you have invented a reactionless drive.

    If anyone does have a candidate device for producing reactionless acceleration — that is, linear acceleration without throwing mass overboard and without reacting with a medium such as air or water — the first test is to suspend it on two wires attached so that the plane of the two wires is normal to the direction of thrust– that is, make a swing and put your gadget on it facing in the normal direction of travel of the swing. Now turn it on. If it will hang non-vertically, get interested. Now cover it with a plastic garbage bag and see if it will still hang non-vertically. If it will still do so, turn it off, and if it settles to a vertical angle, and you can do this repeatedly, and it hasn’t lost any mass during the experiments, call your local physics professor. Or call me. I’ll take care of notifying the Swedish Academy. But until it will do that, I don’t need to look at it…

    If the would-be inventors would do this test first, then they would probably not be sending unsolicited claims to NASA. Claims and hypotheses don’t convince anyone. A good solid test showing a reactionless drive worked would eventually convince everyone.

    I don’t think a breakthrough lies in this direction- the key to a “space drive” is finding ways to push against all of space or alter the space around you so it will push back on you, if such things are possible. Mechanical devices or electromagnetic ones like the EmDrive don’t work, and until someone demonstrates one in controlled conditions and publishes a peer-reviewed paper, I won’t believe claims that someone invented one.

  • Eniac November 5, 2011, 2:47

    Again I feel the need to emphasize for the umpteenth time that “throwing mass overboard” is not the problem, the problem is to provide the energy for propulsion. Presumably, your reactionless drive still needs energy, and thus fuel, and therefore has no real advantage over a rocket which simply uses its spent fuel for reaction mass.

    When you have that free energy, sucked from the vacuum or wherever from, then you can talk about a reactionless drive to go with it. Before that, your quest is pointless. Of course, you’d be stinking rich from that first part, so you would not have to worry about funds for the second. You would first have to be willing, though, to joint the throng of crackpots that is currently working on free energy (newspeak for good old perpetual motion), not the most sweet-smelling of prospects.

    If you want real breakthrough propulsion that is not pure fantasy, the best I can think of is the “torch drive”. You somehow convert regular matter (the fuel) into an energetic beam of other matter (the spent fuel) or a pure photon beam (for the best efficiency) or a pure neutrino beam (for best efficiency and least murderous exhaust) without requiring antimatter or a black hole. Good luck with that!

  • Christopher Phoenix November 5, 2011, 19:20

    When you have that free energy, sucked from the vacuum or wherever from, then you can talk about a reactionless drive to go with it. Before that, your quest is pointless. Of course, you’d be stinking rich from that first part, so you would not have to worry about funds for the second. You would first have to be willing, though, to joint the throng of crackpots that is currently working on free energy (newspeak for good old perpetual motion), not the most sweet-smelling of prospects.

    Under the framework of current physics, drives of the “antigravity” kind, the kind that can propel a spacecraft using nothing more than a source of power, break the entire mathematical foundation of physics. However, they are desirable, in order to overcome the limitations of rocket performance- including the massive amounts of propellent rockets require. Most of the Saturn V rocket was composed of the propellent and fuel that was required to lift the later stages of the rocket that contained still more propellent and fuel.

    In order to build a “star drive”, physicists will need to find new propulsion physics, exactly the goal of the BPPP. The new technologies the BPPP researched include new force production technologies, breakthroughs in energy production physics, and FTL travel. All of these are very disruptive concepts, and very hard to research without attracting the lunatic fringe or inciting the “giggle factor”, but it is worthwhile research that attracts bright young minds to interstellar research- I noticed that the students associated with the Icarus Project all mentioned having great interest in speculative concepts like warp drives and wormholes.

    I’ll never join the ranks of “free energy” crackpots- what an odious idea!! Is that really what you think of me, Eniac?

    Well….If you use chemical engines like those that are on the Shuttle, well…, sorry, there isn’t enough mass in the universe to supply the rocket propellant you’d need.

    So let’s step up to next possibilities, nuclear rockets with a predicted performance that’s 10 to 20 times better!

    Well…it’s still not looking all that good. For a fission rocket you would need a BILLION SUPERTANKER size propellant tanks to get you there, and even with fusion rockets you would still need a THOUSAND SUPERTANKERS!

    Even if we look at the best conceivable performance that we could engineer based on today’s knowledge, say an Ion engine or an antimatter rocket whose performance was 100 times better that the shuttle engines, we would need about ten railway tanker sized propellant tanks.

    That doesn’t sound too bad, until you consider that we didn’t bring along any propellant to let us stop when we get to the other star system…or if we want to get there quicker than 9 centuries.

    Once you add the desire to actually stop at your destination, or if you want to get there sooner, you’re back at the incredible supertanker situation again, even for our best conceivable rockets.

    In conclusion, we’d really like to have a form of propulsion that doesn’t need any propellant! This implies the need to find some way to modify gravitational or inertial forces or to find some means to push against the very structure of spacetime itself.

    Yes, energy is a major issue, even if you have some non-rocket space drive that can transform energy directly into motion without propellent.

    Our third big challenge is energy. Even if we had a nonrocket space drive that could convert energy directly into motion without propellant, it would still require a lot of energy. Sending a Shuttle-sized vehicle on a 50 year one-way trip to visit our nearest neighboring star (subrelativistic speed) would take over 7 x 10^19 Joules of energy. This is roughly the same amount of energy that the Space Shuttle’s engines would use if they ran continuously for the same duration of 50 years. To overcome this difficulty, we need either a breakthrough where we can take advantage of the energy in the space vacuum, a breakthrough in energy production physics, or a breakthrough where the laws of kinetic energy don’t apply.

    Note that the laws of kinetic energy might not apply in some propulsion schemes. In that case, energy will not be such a big issue. For instance, the laws of kinetic energy don’t apply to warp drives- though making a warp bubble is probably even harder than accelerating to near light speed!

    http://www.nasa.gov/centers/glenn/technology/warp/scales.html

  • Christopher Phoenix November 5, 2011, 20:30

    If you want real breakthrough propulsion that is not pure fantasy, the best I can think of is the “torch drive”. You somehow convert regular matter (the fuel) into an energetic beam of other matter (the spent fuel) or a pure photon beam (for the best efficiency) or a pure neutrino beam (for best efficiency and least murderous exhaust) without requiring antimatter or a black hole. Good luck with that!

    The torch drive!! The most powerful rocket engine imaginable- one with both extremely high thrust and specific impulse. An engine that converts matter into energy and spew out a stream of relativistic particles, photons, or neutrinos. Torch drives are now though of as some form of fusion or antimatter annihilation drive, but the original concept from Father Heinlein was a total mass-to-energy conversion drive that can accelerate for months at 1 g to reach near light speed or reach the outer planets in days.

    The Torchship is one of my favorite SF spacecraft designs- both because of its evocative name and plausible implied physics. A fusion drive would be similar to a torch drive, though for a true, Heinlein-style torch drive, you need some means to convert matter completely to energy. The main problems with a torch drive is sustaining a suitable nuclear reaction and not being vaporized by the intense energy released from your drive.

    A fusion torch drive will likely be composed of a latticework of superconducting magnetic coils creating a magnetic nozzle, with plenty of gaps to shed heat. The whole drive might appear like a delicate lantern. A ship like this would probably not take off from Earth, since a vacuum is needed to sustain the fusion reaction, which space provides free of charge. However, a fusion drive is not a true Heinlein torch drive- fusion reactions convert only a small fraction of the mass of the fuel to heat and light. Fusion and antimatter annihilation drives would be good, but not as powerful as a true mass-to-energy conversion drive.

    To create a true “torch drive”, some means of converting protons and neutrons into heat and light must be found. Antimatter and Hawking radiation from black holes can convert matter into energy, but obtaining antimatter or small black holes is difficult. One possibility is using magnetic monopoles to catalyze proton decay, as theorized by some grand unified theories. Of course, obtaining magnetic monopoles in probably very difficult. In the Standard Model of particle physics, the number of protons and neutrons is almost always conserved. Still, Gerard t’ Hooft showed there is a process that could conceivable convert all of the mass of matter into neutrinos and usable energy, but it is normally extremely slow except in the extremely hot conditions found in the Big Bang.

    Perhaps there is some exotic nuclear decay that can convert all of the mass of matter into energy- and if this process is ever found, it would be a major breakthrough for energy production and propulsion technologies. A torchship that spewed out photons and relativistic particles couldn’t take off from the Earth because it would vaporize the entire launch site. It would have to serviced by more traditional nuclear thermal rockets in orbit. A pure neutrino beam, however, would be harmless- so the a neutrino torchship could take off from Earth-like planets without causing harm. If this neutrino torchship scoops up fuel with a ramfield, it can accelerate indefinitely and then land on Earth-like planets without melting continents into slag- such are the things dreams are made of…

    Maybe you are right- the key to advanced space travel may be finding new energy-production technologies instead of hyper-fixating on the reaction mass issue. It is a pity that the breakthrough propulsion physics program did not study mass-energy conversion torch drives. Torch drives are a breakthrough that works within the physics we already have but needs a breakthrough in energy production technology that can convert all the protons and neutrons in matter to light and heat- truly a physics problem. A rocket propelled starship would have to be a torchship to reach relativistic speeds.

  • Eniac November 5, 2011, 21:00

    Chris:

    Most of the Saturn V rocket was composed of the propellent and fuel

    Wrong. The Saturn V contained no propellant. Only fuel. The propellant was free: it was the spent fuel. Even with a “space drive”, you would still need the fuel, and it would be a similar amount, including the exponential penalty from having to carry it with you. Please read this three times, I will not say it again.

    A space drive would be of use for slow, solar powered craft, which could operate for long periods of time if they did not need propellant. Obviously, though, those do not really have interstellar applications. There is no energy out there, you have to take it with you. No escape from the rocket equation without free energy.

    I’ll never join the ranks of “free energy” crackpots- what an odious idea!! Is that really what you think of me, Eniac?

    I said you would have to, for what you are asking. I did not say you would. That’s obviously up to you…

  • Rob Henry November 6, 2011, 18:05

    Eniac, I see your point. If we converted the first stage of a Saturn V to an air breathing rocket, we would have difficulty defining what we meant by propellant, but none as to defining what constitutes the fuel.

    It is sad to think that if I discover an amazing propellantless drive, connect up its batteries, then hop in my home made spaceship to explore the galaxy, I will find myself no better off (and probably a lot worse off) than if I had used a conventional rocket. That’s not fair!!

  • Rob Henry November 6, 2011, 19:16

    Wait a minute Eniac… Couldn’t we come to some arrangement whereby I throw away those batteries once in orbit, and you send me the energy as a beam of microwaves. Assuming I could convert their energy to usable form, wouldn’t such a scheme be several orders of magnitude more efficient than a light sail?

  • Christopher Phoenix November 6, 2011, 20:36

    Wrong. The Saturn V contained no propellant. Only fuel. The propellant was free: it was the spent fuel. Even with a “space drive”, you would still need the fuel, and it would be a similar amount, including the exponential penalty from having to carry it with you. Please read this three times, I will not say it again.

    No, you are in error- the Saturn V rocket’s chemical fuel was both fuel and propellent. After being burned, the burned fuel and oxidizer was ejected as a jet of flaming gasses. However, in most rockets, the fuel and propellent are separate. Case and point- the nuclear thermal rocket’s fuel is the nuclear fuel in the reactor, and the propellent is the hydrogen that the nuclear engine super-heats. The chemical rocket has many people confused on the difference between fuel and propellent.

    I will say this only once, so read this carefully- fuel is not the same as reaction mass!! Rockets use Newton’s 3rd Law of Motion- the principle of action and reaction- in order to move. Newton’s 3rd Law says that every action has an equal and opposite reaction, and don’t tell me this is something you have never seen before. When you walk, your foot pushes on the ground which in turn pushes on you. When you drive a car, the wheels of the car push on the road, which in turn push on the car and move it. When an airplane flies, its jets or propellers push on the fluid medium surrounding the craft- the air- and the air pushes back on the craft and moves it. If you fire a pistol, the action of propelling the bullet down the barrel leads to an equal reaction that causes the phenomenon known as “recoil”.

    However, in space there are no roads, nor is there a fluid medium. A spacecraft has to bring reaction mass with it, which it expels from a rocket engine. The reaction accelerates the spacecraft, per Newton’s 3rd Law of Motion. A rocket pushes on its propellent, and resulting reaction pushes on the rocket. The total momentum of the propellent/spaceship system remains zero, satisfying Conservation of Momentum. Since Newton’s Laws of Motion apply to all situations, including spacecraft far away from Earth, this should be obvious.

    Rockets have to bring all their reaction mass with them. All rockets, including electric ion thrusters, need to carry a supply of propellent. In a rocket, there is a big difference between fuel and reaction mass.

    Fuel is the energy source of the rocket- whatever is generating the energy to throw the reaction mass away from the rocket. In a classic atomic rocket, the fuel is U-235 rods the generate heat from nuclear fission. The propellent is hydrogen, which is boiled and expelled at high velocity. In an ion drive, the fuel is whatever generates power for the electric thruster- a solar panel, RTG, or nuclear reactor- and the propellent is Cesium atoms or some other easily ionized gas. In a plasma rocket, the fuel is a nuclear reactor that generates electrical power, and the propellent is some easily ionized matter that is superheated and expelled through a magnetic nozzle.

    There are a few confusing cases where fuel and propellent are the same thing. This is the case with the chemical rockets used to propel the space shuttle, the Saturn V moon rockets, and all the launch vehicles used today, which is how the misconception got started in the first place. In a chemical rocket, energy is released by burning fuel and oxidizer- such as hydrogen and oxygen- and the reaction products are expelled through a nozzle in a sheet of flame. This is the action that leads to the reaction- the rocket is accelerated. Chemical rockets have a terrible specific impulse- the measure of how much change in velocity you get for a certain amount of propellent expelled- because chemical rockets have a low exhaust velocity. Thus, chemical rockets need a huge amount of fuel to reach orbit. We have to used staged rockets that throw off stages as they ascend to reach orbit. This is why we don’t have SSTO’s.

    If we ever have rockets that use the mighty power of the atom to lift cargoes to orbit, we will have SSTO’s- but all attempts at SSTO’s today are pathetic attempts to shave off mass using advanced materials or gain some extra velocity with ramjets or maglev sleds so the rocket will have a chance to each orbit. This a very high risk venture, since so much as a 20% gain in mass would be enough to prevent the would-be SSTO from being able to reach orbit.

    Airplanes, boats, and cars are sizable vehicles with relatively small fuel tanks. Not so with rockets- an incredibly powerful rocket might have half its mass composed of reaction mass and the other half structure, hull plates, crew members, and everything else. But it is more likely that 75% of its structure will be reaction mass. Or worse. Most rockets are huge propellent tanks with a rocket engine stuck on one end and a tiny crew habitat stuck on top.

    For trips around our Solar System, the amount of propellent needed is manageable. For interstellar flight, it is an entirely different situation. Just read the website Warp Drive When? at NASA- in particular the section “Why is Interstellar Travel So Tough?”.

    And before you mention it- photon rockets still have a reaction, although in this case it is just reaction momentum. The exhaust is not a stream of matter but a beam of electromagnetic radiation. The advantage is that the photon rocket has the maximum possible exhaust velocity and thus the maximum possible specific impulse. The disadvantage is the ludicrously high power requirements- one lousy newton of thrust takes 300 freaking megawatts!! The extra mass of shielding, support struts, and the body of the reactor lowers the specific impulse to boot. You could build a photon rocket in orbit with modern technology- but it would take forever to get anywhere.

    A powerful nuclear rocket is no use without propellent. The idea then occurs- what if we had some means to propel a spacecraft without propellent? We could just plug the “space drive” into a nuclear reactor and never have to worry about feeding the hungry rocket engines ever again. The idea is nice- but modern physics says a drive like this is impossible, unless it is part of an open system like a solar sail. Researchers like Mark Millis are looking into ideas like manipulating gravitational and inertial forces or pushing against the fabric of space-time. These ideas rely on new physics, thus the “Breakthrough Propulsion Physics Program”, not the “Incremental Advances in Rocketry Program”.

    Don’t get me wrong- I’m all for powerful nuclear thermal rockets and suitable rocket engines for interplanetary travel coming into use. At the same time, I recognize the incremental advances in the propulsion systems we have today are not enough for star travel, and that pioneers seek revolutions, not refinements to existing technologies. After all, if Michael Faraday had worked on better oil lamps instead of studying electromagnetic induction, we would not have light bulbs today. Michael Faraday was not interested in putting his research to practical use- but most researchers today will be quite happy to put their research to practical use if they find a breakthrough that will lead to new propulsion systems.

    http://www.nasa.gov/centers/glenn/technology/warp/warp.html

    Even if we had a space drive that could convert energy directly into motion, obtaining the energy required to reach the high speeds required to reach the stars in a reasonable period of time is a challenge, except for breakthroughs that don’t follow the laws of kinetic energy. However, a space drive will lower the amount of energy required. Mark Millis places the launch of the first interstellar probe around 200 years from now due to the high energy cost of such a mission. He projected our energy usage in the future based on patterns of energy growth and usage throughout history- although making predictions like this is always risky. Kevin Long does not agree with Mark Millis’s projections. He thinks that disruptive technologies and exponential grown patterns will lead to a much earlier launch date.

  • Eniac November 7, 2011, 11:10

    Rob,

    That would most certainly be a good idea. For slow, interplanetary craft beamed energy plus space drive (or even just an ion drive) are much better than rockets running on conventional fuel. Beamed energy is a way around the rocket equation, unfortunately it has some (near?) fatal flaws for interstellar missions. Even within the solar system most beams are too divergent to be of much use.

  • Eniac November 7, 2011, 11:34

    There are a few confusing cases where fuel and propellent are the same thing.

    Sigh…

    These happen to be the optimal cases. If you are taking extra propellant and throwing the spent fuel away (or worse, keep it on board as dead weight) you are wasting valuable mass. That is why nuclear thermal rockets are not usable for star flight, while fission fragment rockets could be. Same with nuclear/ion drives. Not at all efficient, unless your power source is external (=solar).

    The most efficient rocket is one that expels ALL its spent fuel as reaction mass, and nothing else. You can easily calculate the optimal specific impulse from that: The velocity to which the spent fuel can be accelerated given the energy that it has produced. Chemical rockets are very efficient, they just don’t use a very dense energy source. For a nuclear rocket to be efficient, you have to expel the fission fragments at the original velocity without allowing them to thermalize. The specific impulse comes out at around 0.05-0.1 c. Unfortunately that is difficult, but there are plenty of engineering ideas around.

  • Eniac November 7, 2011, 11:42

    It is sad to think that if I discover an amazing propellantless drive, connect up its batteries, then hop in my home made spaceship to explore the galaxy, I will find myself no better off (and probably a lot worse off) than if I had used a conventional rocket. That’s not fair!!

    Sad indeed. Note, also, that there is no battery in the world (nor will there ever be, barring nuclear ones) that stores enough energy to lift itself into orbit. Even if it is allowed to pull itself up on a string, as a poor man’s reactionless drive. This is why the space elevator requires power beaming.

  • Eniac November 7, 2011, 12:25

    Chemical rockets have a terrible specific impulse … because chemical rockets have a low exhaust velocity. Thus, chemical rockets need a huge amount of fuel to reach orbit

    Actually, chemical rockets have the ideal specific impulse, given the energy yield of the fuel. It is the lousy energy yield that forces you to take so much fuel. It would be the same with a space drive, if you were to power it by a gas turbine or fuel cell. You can use nuclear power, but then your space drive is up against the fission fragment rocket, where again it would only show a minor advantage, even if it were 100% efficient.

  • Eniac November 7, 2011, 12:37

    If Michael Faraday had worked on better oil lamps instead of studying electromagnetic induction, we would not have light bulbs today

    True, but tread carefully. This is a favorite argument of the aforementioned crackpots.

  • Christopher Phoenix November 7, 2011, 21:01

    @Eniac

    Let’s get this straight- you said that the Saturn V rocket carried no propellent at all, which is completely incorrect. The burnt fuel was used as reaction mass, while the energy came from rearranging the outer electron shells of the fuel into a lower energy state. Not much energy can be gained this way, which explains the chemical rocket’s low exhaust velocity. The fuel and oxidizer was used as reaction mass- if you meant to say that chemical rockets don’t carry a separate tank of propellent, then that was somewhat pedantic. You can’t argue that most of the Saturn V’s mass was chemical fuel and oxidizer that was ejected out the nozzle of the rocket as a sheet of flame once it was burned, fulfilling its role as reaction mass, which is what I meant.

    The most efficient rocket is one that expels ALL its spent fuel as reaction mass, and nothing else. You can easily calculate the optimal specific impulse from that: The velocity to which the spent fuel can be accelerated given the energy that it has produced. Chemical rockets are very efficient, they just don’t use a very dense energy source. For a nuclear rocket to be efficient, you have to expel the fission fragments at the original velocity without allowing them to thermalize. The specific impulse comes out at around 0.05-0.1 c. Unfortunately that is difficult, but there are plenty of engineering ideas around.

    I’ve been thinking about that. It seemed to me that the problem with nuclear thermal rockets is that they let the energy released to thermalize, which limits the rockets exhaust velocity. The higher the temperature, the higher the exhaust velocity- but at high temperatures, rocket engines tend to glow blue-white and vaporize.

    The key to truly impressive nuclear drives is not having a solid engine to vaporize- which is what Project Orion did. The Project Orion nuclear pulse drive let the explosions occur in free space behind the ship, so there was no limit on the energy that could be released from the bomb- thus the atomic bomb fragments from the blast had the maximum velocity possible given the energy released. Nuclear reactors are limited in the energy they can release (due to the fact you don’t want your reactor to melt or vaporize), while nuclear bombs are not.

    Once again, the nuclear pulse rocket beats all competitors that can be built using near future technology. Not to mention that Orion-style spaceships, with their massive pusher plates and shock absorbers, would only be the first generation. I can imagine several technological levels of nuclear pulse ships-

    Mark I: solid pusher plate and shock absorbers
    Mark II: Electromagnetic coupling incorporated into plates and shocks
    Mark III: pusher plate extensions such as canopy, segments, cables
    Mark IV: external pulse driver unit such as laser, antimatter etc.
    Mark V: magnetic fields used instead of solid pusher plate
    Mark VI: matter-antimatter pulse units (photon torpedoes), theoretically capable of attaining a speed of 50% to 80% light speed

    When building nuclear powered spaceships, direct the reaction products directly out the back without thermalizing them or even attempting containing them in a combustion chamber. Don’t let the reaction products thermalize, or you will be limiting your rocket’s performance to only a few times better then a chemical rocket!!

  • Christopher Phoenix November 7, 2011, 21:25

    Actually, chemical rockets have the ideal specific impulse, given the energy yield of the fuel. It is the lousy energy yield that forces you to take so much fuel. It would be the same with a space drive, if you were to power it by a gas turbine or fuel cell. You can use nuclear power, but then your space drive is up against the fission fragment rocket, where again it would only show a minor advantage, even if it were 100% efficient.

    Stop talking about “space drives” as though they are real, working technologies that can be analyzed with the physics we have today. A non-reaction drive does not make sense with the physics we have today. All rockets work by action-reaction. Something shoots out the back and the rocket accelerates forwards. Even the photon rocket has a reaction, even though it is just reaction momentum, and the drive efficiency is really terrible with a photon rocket- 300 megawatts for 1 lousy newton of thrust!! A reactionless drive, on the other hand, violates not only the conservation of momentum but the conservation of energy. This makes them nonsensical in the current mathematical framework of physics.

    Read this link, for a discussion why reactionless drives violate both Conservation of Momentum and Conservation of Energy (apparently, viewers in different reference frame disagree on how much energy the reactionless drive is using unless there is a reaction of some sort, as in a rocket, leading to violations of the conservation of energy).

    http://www.projectrho.com/rocket/reactionlessdrive.php

    I should just shut up about anything remotely resembling a reactionless drive. The best concept for a “space drive” is really the Alcubierre metric- even if the warp drive cannot be used for FTL travel, it might make a nifty way to reach high subluminal speeds without using rockets. The ship really isn’t moving- space moves around the ship, so this does not have any of the problems of a “reactionless drive”. Barring a complete change in the theories of physics, reactionless drives are not only impossible, but physically nonsensical.

    Mark Millis is trying to find a way to create propulsive forces anywhere in the cosmos using the fundamental properties of matter and space-time without needing to expel reaction mass, which is not the same as a classic “reactionless drive”. This is what NASA has to say about it:

    A “space drive” can be defined as an idealized form of propulsion where the fundamental properties of matter and spacetime are used to create propulsive forces anywhere in space without having to carry and expel a reaction mass. Such an achievement would revolutionize space travel as it would circumvent the need for propellant. A variety of hypothetical space drives were created and analyzed by Millis to identify the specific problems that have to be solved to make such schemes plausible. These hypothetical drives are just briefly introduced here. Please note that these concepts are purely hypothetical constructs aimed to illustrate the remaining challenges. Before any of these space drives can become reality, a method must be discovered where a vehicle can create and control an external asymmetric force on itself without expelling a reaction mass and the method must satisfy conservation laws in the process.

    Can we talk about something else please, maybe nuclear pulse propulsion? I’m getting royally sick of “space drives”, shame on me for even mentioning them. Our descendants may use bizarre space propulsion techniques base on exotic physics, but until the BPPP turns up something, it is kind of pointless to discuss it.

    Any long term star travel effort should try to reach a fraction of light speed with a drive based on physics we have, not physics we wish we had. A nuclear pulse drive is not based on new physics, and we could have built one in the 1960’s if we had the political will. There is no way for us to manipulate gravitational or inertial forces yet, nor can we push against space-time or build warp drives, at least not for the foreseeable future. I am interested in potential breakthroughs, but most ideas in that field of research probably have no more reality to them then the Blackett-Dirac equations that led to the spindizzy from Cities in Flight.

    http://en.wikipedia.org/wiki/Spindizzy

    At any rate, it would be nice to see someone speculating on how to power a spacecraft intended to reach near light speed without resorting to vacuum energy schemes.

    I’ll close this with a quote from a NASA paper:

    No prospects are now apparent for realization of propulsion schemes of the “antigravity” variety, because the negation or reversal of the gravitational attraction of matter would violate basic physical laws as presently understood. Pending discovery of a new class of physical phenomena, the notion of antigravity now stands in a state similar to that of the perpetual motion machine.

    The same goes for FTL travel and “space drives”. Until a new class of physical phenomena is discovered, these ideas are in a state similar to the perpetual motion machine.

    Here is the link to the paper the quote is from: http://history.nasa.gov/conghand/propulsn.htm

  • Paul Gilster November 8, 2011, 9:16

    Christopher Phoenix writes:

    Our descendants may use bizarre space propulsion techniques base on exotic physics, but until the BPPP turns up something, it is kind of pointless to discuss it.

    Be aware that BPP was defunded in 2002 and is no longer in existence. Marc Millis is now devoting full time to the Tau Zero Foundation and is no longer with NASA.

  • Christopher Phoenix November 8, 2011, 18:18

    @Paul Gilster

    Thank you, but I was already aware that the BPP is no longer in existence. I really meant that until a researchers discovers some new class of physical phenomena that might lead to new propulsion techniques, it really isn’t much good to compare “star drives” with advanced rocketry. Drives of the “antigravity” or “warp drive” just aren’t possible, not with the physics we have today. Some scientists speculate on such matters, but these ideas are just that- speculation.

  • Eniac November 9, 2011, 23:25

    Read this link, for a discussion why reactionless drives violate both Conservation of Momentum and Conservation of Energy

    That’s all I have been tying to say all this time. Useful reactionless drive equals perpetual motion. Glad to see I finally broke through… :-)

  • Eniac November 9, 2011, 23:46

    Let’s get this straight- you said that the Saturn V rocket carried no propellent at all, which is completely incorrect.

    I said it carried fuel, not propellant. You can split hairs and say “WRONG, the fuel is also propellant”, but that does not change my point: With a reactionless drive that is not also a perpetual motion machine you would save the propellant, but still require the fuel. All of it. No matter whether you call it fuel or propellant. Since the two are the same, you would gain nothing. Nothing at all. It is really not that hard to understand. Why do you refuse to?

  • Eniac November 10, 2011, 17:52

    Mark Millis is trying to find a way to create propulsive forces anywhere in the cosmos using the fundamental properties of matter and space-time without needing to expel reaction mass, which is not the same as a classic “reactionless drive”.

    It is not? How is it different?

  • Christopher Phoenix November 10, 2011, 22:21

    @Eniac

    That’s all I have been tying to say all this time. Useful reactionless drive equals perpetual motion. Glad to see I finally broke through… :-)

    Don’t be so pleased with yourself. I knew that all along. Also, useful reactionless drive does not equal perpetual motion, it simply breaks the entire mathematical foundation of physics- not quite the same thing.

    I said it carried fuel, not propellant. You can split hairs and say “WRONG, the fuel is also propellant”, but that does not change my point: With a reactionless drive that is not also a perpetual motion machine you would save the propellant, but still require the fuel. All of it. No matter whether you call it fuel or propellant. Since the two are the same, you would gain nothing. Nothing at all. It is really not that hard to understand. Why do you refuse to?

    Okay, Eniac, this is not so very difficult to understand. Does a car carry propellent? Does a boat carry propellent? Does an airplane carry propellent? Do you carry propellent to walk to your computer and respond to me? NO!!!! Of course not. Does a car need fuel? Does a boat need fuel? Does an airplane carry fuel? YES!!! Fuel and propellent are not the same thing, even if a very efficient rocket uses its spent fuel as propellent!! Fuel is the energy source for a rocket, propellent is the stuff the rocket carries to push against.

    If you suspend a car beneath a ballon and push down on the accelerator, does it move? No!! Do propellers work in space? No!! Are their roads or oceans in space for a wheel or propeller to push against? No, that is why we need to use rockets. Why do rockets work? They carry the stuff they need to push against, the reaction mass, with them. Is it really that hard to understand?

    Here- read this excerpt from the “Getting There” page of the Tau Zero website. Obviously you need a little reminding of the basic needs of spaceflight. http://www.tauzero.aero/site/html/getting_there.html

    The term “fuel” is used here for familiarity rather than because it is the accurate way to convey the needs of spaceflight. If you drive a car you should be very aware that you need fuel to go anywhere. That basic notion also pertains to spaceflight, but there are additional details. “Fuel,” in the strictest sense, is an energy source – a chemical energy source. When that expensive gasoline fuel reacts with free ($0) oxygen in the air, they burn and expand and push pistons which then turn a crankshaft which then turns the wheels that push against the road to move your car. So, in short, the fuel is the energy source for the car.

    But there is another important piece of that story that typically goes unnoticed – the road. The road plays an important part in moving your car. Imagine your car floating in the air, hung under a balloon. You press on the gas pedal, hoping to move forward, but your wheels just spin. The view might be great from up there under that balloon, but your car’s wheels aren’t connected to anything. You just drift. In order for you to move in one direction, another mass needs to be moved in the other. Newton’s laws are all over this situation, and if you’ve already learned them well, this will be obvious. Back on the ground, the road is your connection to that other mass – the entire Earth. When your wheels turn, they push the entire Earth one way (which actually moves ever so slightly) propelling you in the other direction. Some of you are wondering about this because you can’t fathom how you could move the Earth. Well, the amount of push is the same on your car and on the Earth – action and reaction – but the Earth is so much more massive than your car. The ratio of how much the Earth moves compared to how much your car moves is the same as the ratio of the mass of your car (tiny) to the mass of the Earth (gargantuan).

    In space there are no roads. There is no air either, so aircraft propellers are useless. Spacecraft have to bring all there reaction mass with them – that is the stuff they push against. The proper term for this onboard reaction mass is “propellant.” All rockets, even electric ion thrusters, need propellant. For chemical rockets, their propellant also serves double duty as their energy source. There is a fuel propellant and an oxidizer propellant, that react and dramatically expand out the rocket nozzle in one direction, thrusting the rocket in the other. For short journeys, the amount of propellant required is quite achievable, but for interstellar flight it is an entirely different situation.

    See, it isn’t that hard. Fuel is not propellent!!! Defining the difference between fuel and propellent is not “splitting hairs”. Being exact in the definitions you use is very important in science. The difference between fuel and propellent is one of the MAJOR misconceptions that the Atomic Rockets website debunks. http://www.projectrho.com/rocket/misconceptions.php

    If we had a spacecraft with a “space drive” that did not need reaction mass to create motion, it would lower the energy cost of spaceflight, a factor Mark Millis factored into his calculations of when the first interstellar probe will be launched. Said “space drive” would not follow the laws of rocketry since it is not a rocket. You can’t analyze the QE2 with the rocket equation, can you? No, because it is a boat, thus it does not need to carry reaction mass. It uses propellers. You can’t analyze the hypothetical “space drive” with the rocket equation either.

    Using the rocket equation to describe something that is not likely to involve a rocket is about as misleading as using the metrics of sailing ships to assess steam- ships.11 Although reduced sail area is indeed a consequence of steamships, the true benefit is that shipping can continue regardless of the wind conditions and with far more maneuvering control. Similarly, the benefits of breakthrough inertial or gravity control would likely surpass the operational conventions of rocketry.

    Above excerpt is from http://gltrs.grc.nasa.gov/reports/2005/TM-2005-213998.pdf Don’t take my word for it, read the introduction of the paper.

    Once again I ask you, if space drives have no advantages over rockets, why does Mark Millis discuss building one? Why does he think it is a desirable goal, if there is no real advantage in building one? Do you think he is an idiot?

    Mark Millis’s idea is that we might find some way to push against the entire universe or alter the properties of space-time to produce propulsive forces. These ideas rely on new physics being discovered, so we can’t build one today. This is not the same as a traditional reactionless drive like the “Dean Drive” which tries to produce propulsive forces by jiggling weights or swinging weighted flywheels around, without interacting with anything around it. If a Millis space drive worked, it would interacting with the space-time around it to produce propulsive forces. Not the same thing at all.

    Any breakthrough on the exotic propulsion physics front would completely surpass the operational constraints of rocket ships. A drive of the “antigravity” or “space drive” kind does not have to carry propellent, and while it still has to carry fuel, it does not need as much energy as a rocket drive does.

    I concede nothing to you in this argument. You are wrong about the limitations of space drives compared to rockets. You are completely wrong about fuel and propellent being the same. I have provided sources for my arguments, all of which are either NASA papers or explanations written at either NASA or the Tau Zero website. I have conceded that the energy cost for a space drive is considerable, and a breakthrough space drive that converts stored energy into motion still leaves the problem of finding means to power such devices- one of the goals of the now ended BPP project. I have explained the difference between fuel and propellent many times, but you still stubbornly refuse to see my point! :-((

  • Eniac November 12, 2011, 0:48

    It is getting very hard to find anything in your lengthy reiterations of trivial and undisputed facts that would address the issue we are talking about.

    I did seem to detect, though, that you think I said propellant is always the same as fuel. This appears to be a severe misconstruction of my actual position, which is: In an efficient rocket, the propellant is the spent fuel. A reactionless drive would eliminate the need for propellant, but not the need for fuel. Therefore, nothing would be eliminated. Are you going to dispute this?

    In another hidden place in between your ramblings and pastings you state that the reactionless drive requires less energy than a rocket, which would serve to make your point, if you could back it up. Why would this be so? I did not see any arguments or references for this, perhaps I missed it in the jumble.

    If the reactionless drive did somehow miraculously require much less energy, you could accelerate a small craft using the drive and then convert its kinetic energy into electric energy. Use some of that to power the drive, and enjoy the rest for free. Voila, the long sought perpetual motion machine, finally realized.

    If, on the other hand, we accept that the reactionless drive does require a similar amount of energy as a rocket, for the same thrust, then we still end out with the rocket equation. Yes, there is no rocket, but it is the same principle and the same equation. You need to carry a lot of fuel, and that fuel needs to be accelerated, for which you need more fuel, leading to the same exponential relationship we know and love (NOT!) as the rocket equation. You may re-christen it “powered flight equation”, if the name bothers you.

    Your mention of cars and boats is very droll. Trust me, if you wanted to accelerate your car or boat to 11 km/s, and you needed to take oxygen with you, your fuel tank would be just about the same size as the Saturn V. Call it the “powered motion equation”, then, to satisfy your sense of semantics.

    I expect none of the above will convince you, you will again miss the point and bombard us with another volley of information-free lectures on obvious but irrelevant facts. Therefore, I will let this be my final words in this by now rather pointless discussion.

  • Christopher Phoenix November 12, 2011, 17:20

    @Eniac

    I did seem to detect, though, that you think I said propellant is always the same as fuel. This appears to be a severe misconstruction of my actual position, which is: In an efficient rocket, the propellant is the spent fuel. A reactionless drive would eliminate the need for propellant, but not the need for fuel. Therefore, nothing would be eliminated. Are you going to dispute this?

    Yes, I am going to dispute that, and you did say at one point that fuel and propellent are the same. From one of your earlier comments:

    No matter whether you call it fuel or propellant. Since the two are the same, you would gain nothing. Nothing at all. It is really not that hard to understand. Why do you refuse to?

    If that is not saying propellent is the same as fuel, I don’t know what is. Now, however, that you have made your position somewhat clearer (in between your ramblings on fuel and accusing me of splitting hairs), I finally see what your argument really is. You claim that a space drive will have no advantage over a rocket since some rockets use their spent fuel as reaction mass (arguably the most efficient arrangement) and a space drive still requires a source of energy. You claim this source of energy, or “fuel”, will increase exponentially, so the space drive is still subject to the rocket equation.

    In another hidden place in between your ramblings and pastings you state that the reactionless drive requires less energy than a rocket, which would serve to make your point, if you could back it up. Why would this be so? I did not see any arguments or references for this, perhaps I missed it in the jumble.

    The paper titled “Energy Consideration of Hypothetical Space Drives” clearly states that for deep space travel with a space drive, the energy required is proportional to the square of delta-V, whereas rocket energy scales exponentially.

    The energy requirements of hypothetical, propellant-less space drives are compared to rockets. This serves to provide introductory estimates for potential benefits and to suggest analytical approaches for further study. A “space drive” is defined as an idealized form of propulsion that converts stored potential energy directly into kinetic energy using only the interactions between the spacecraft and its surrounding space. For Earth-to-orbit, the space drive uses 3.7 times less energy. For deep space travel, energy is proportional to the square of delta-v, whereas rocket energy scales exponentially. This has the effect of rendering a space drive 150-orders-of-magnitude better than a 17,000-s Specific Impulse rocket for sending a modest 5000 kg probe to traverse 5 ly in 50 years. Indefinite levitation, which is impossible for a rocket, could conceivably require 62 MJ/kg for a space drive. Assumption sensitivities and further analysis options are offered to guide further inquires.

    Here is the paper: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070031912_2007031208.pdf

    Please READ THE PAPER before you comment again and accuse me of having no references. Clearly, space drives would use much less energy than rockets. Do note that such space drives do not yet exist, and these numbers should be thought of as the goals of breakthrough propulsion researchers.

  • Christopher Phoenix November 12, 2011, 17:46

    If, on the other hand, we accept that the reactionless drive does require a similar amount of energy as a rocket, for the same thrust, then we still end out with the rocket equation.

    By the way- the phrase “energy of thrust” is about as meaningful as “mass of time”. Thrust is a force, not an energy. Force multiplied by distance gives an energy. A force of one pound applied over a distance of one foot equals one foot-pound of energy. The same force, applied over a greater or lesser distance, comes to proportionally more or less energy.

    To give an example: a force of 10 newtons acting along the path of 5 meters will do the work
    (10 newtons)(5 meters)=50 newton-meters= 50 joules

    While a force of 10 newtons applied along a distance of 2 meters will do the work
    (10 newtons)(2 meters)=20 newton-meters= 20 joules
    Same force, shorter distance, less energy!!

    But a force of 5 newtons acting along a distance of 10 meters will do the work
    (5 newtons)(10 meters)=50 newton-meters= 50 joules
    Smaller force, greater distance, same energy as first example.

    Please note that work, which is equivalent to energy, is a force times a distance! It is very hard to take someone seriously when they make silly errors like confusing energy with thrust.

  • Eniac November 13, 2011, 6:11

    If that is not saying propellent is the same as fuel, I don’t know what is.

    Read the context. I was talking about rockets, where you have said the same thing.

    This from the paper you cite (D.3):

    For these first-step exercises, the source of the stored potential energy need not be specified.

    Apparently, for their estimate of energy used with a space drive, they chose to assume a massless energy source and did not consider that fuel would have to be carried.

  • Eniac November 13, 2011, 6:19

    By the way- the phrase “energy of thrust” is about as meaningful as “mass of time”.

    I did not say “energy of thrust”, I said “similar amount of energy at the same thrust”. Big difference, and quite legitimate.

    Again, you misunderstand what was said and proceed to paste unattributed text (this one copied from Atomic Rockets, I believe) which is correct, but irrelevant.

  • Eniac November 13, 2011, 14:46

    In an efficient rocket, the propellant is the spent fuel. A reactionless drive would eliminate the need for propellant, but not the need for fuel. Therefore, nothing would be eliminated. Are you going to dispute this?

    Yes, I am going to dispute that

    Ok, let’s hear, please… which part of this is incorrect?

  • Rob Henry November 13, 2011, 16:39

    Eniac has pointed out quite clearly that the identical requirements for this costless and propellantless drive must give us energy for free. The effects if advanced intelligent life with exponential growth tendencies exist, even in other galaxies, would be so obvious that we should have noticed already. Perhaps quasars are examples, but I doubt it.

  • Christopher Phoenix November 13, 2011, 21:54

    @Eniac

    I think it is obvious why a “space drive” would consume less energy than a rocket to achieve the same delta-V.

    A rocket propels itself by spewing out a stream of exhaust gasses and moves due to the reaction. The rocket stores propellent and fuel on board to provide energy and reaction mass. However, some of the energy the rocket expends goes into accelerating the propellent rather than the spacecraft!! For all engines that carry their reaction mass onboard prior to use, some energy must go into accelerating the reaction mass. Even assuming 100% efficiency (which won’t happen, as all engines waste some energy), the engine will need energy amounting to (1/2)M*V^2 (where M is the mass of the propellent expended at V is the exhaust velocity), which is simply the energy required to accelerate the exhaust. Not all of the energy released from the fuel ends up in the rocket- most ends up as kinetic energy of the exhaust.

    A “space drive”, on the other hand, is defined as an idealized form of propulsion that converts stored potential energy directly into kinetic energy using only the interactions between the spacecraft and its surrounding space. This drive does not accelerate any propellent, so assuming 100% efficiency (again, not a real-life possibility, but we gave the rocket the same advantage), all the stored energy ends up as the kinetic energy of the spacecraft. In a real drive, there would be some energy losses, but the space drive does not lose energy as kinetic energy of the exhaust since there is no exhaust. That is why a space drive requires less energy than a rocket.

    If I’m wrong, I will be quite glad to hear why.

  • Christopher Phoenix November 13, 2011, 22:17

    @Eniac

    I did not say “energy of thrust”, I said “similar amount of energy at the same thrust”. Big difference, and quite legitimate.

    Again, you misunderstand what was said and proceed to paste unattributed text (this one copied from Atomic Rockets, I believe) which is correct, but irrelevant.

    Oh really- explain the difference. You seemed to be relating the energy use to the amount of thrust, rather than the change of speed of the rocket. You know, maybe when someone misunderstands what you intend to say, it is your fault instead of theirs.

    By the way, I did not copy that text from Atomic Rockets. I happen to be studying physics- and one of the first things you learn about energy is that work=force*distance. I don’t have to run all over the internet stealing snippets of other people’s discussions to argue with you, Eniac. I can think for myself, even if I am wrong sometimes..

    Read the context. I was talking about rockets, where you have said the same thing.

    No, I haven’t. I have stated, emphatically, multiple times that fuel is not the same as propellent. Even if some rockets expel the burnt fuel as propellent, the fuel is doing double-duty as both energy source and reaction mass. The point is that terrestrial vehicles move by interacting with their surroundings- for the most part- while a rocket applies acceleration to itself by expelling part of its mass at high speed. You keep saying fuel and propellent are the same- they aren’t!!!

    As for rechristening the Rocket Equation the “Powered Flight Equation”, that is just silly. Space drives don’t expel a part of their mass at high velocity- I don’t even know what numbers we’d plug in.

  • Eniac November 15, 2011, 0:30

    By the way, I did not copy that text from Atomic Rockets.

    You:

    By the way- the phrase “energy of thrust” is about as meaningful as “mass of time”. Thrust is a force, not an energy. Force multiplied by distance gives an energy. A force of one pound applied over a distance of one foot equals one foot-pound of energy. The same force, applied over a greater or lesser distance, comes to proportionally more or less energy.

    Atomic Rockets:

    There is the complication that “energy of the thrust” is as meaningless a phrase as, e.g., “mass of the time”.

    Thrust is a force, not an energy. Force, multiplied by distance, gives an energy. A force of one pound, applied as an object moves over a distance of one foot, equates to (unsurprisingly) one foot-pound of energy. The same force, over a greater or lesser distance, comes to proportionately more or less energy.

  • Eniac November 15, 2011, 0:34

    In an efficient rocket, the propellant is the spent fuel. A reactionless drive would eliminate the need for propellant, but not the need for fuel. Therefore, nothing would be eliminated. Are you going to dispute this?

    Yes, I am going to dispute that

    Still waiting… Which part of this is incorrect?

  • Eniac November 15, 2011, 0:49

    You are right that some of the energy of fuel is expended accelerating the fuel, but AFAIK just some, not most. This just means a rocket can never be 100% efficient, but this is unlikely for space drives as well. In any case, this does not change the fact that space drives, once you consider the need for fuel to power them, succumb to the same exponential relationship between delta-V and mass fraction as rockets. This is often overlooked, not least by Marc Millis in the paper you cite, although rarely so vehemently opposed against all reason as by you ….

  • Christopher Phoenix November 15, 2011, 16:58

    @Eniac

    Still waiting… Which part of this is incorrect?

    Your statement that “nothing would be eliminated” is not true. First of all, propellent would be eliminated. Only an energy source to power the space drive is needed. Your argument is that the space drive would succumb to the same exponential relationship between fuel and delta-V, but I dispute that as well.

    You are right about one thing. Inventing a drive that converts stored energy directly to motion does not give one a “miracle drive” that you can just hook up to a generator and soar into the sky in. If such a drive worked, you’d need a powerful nuclear reactor to power the space drive. It would, however, be more efficient than a rocket.

  • Eniac November 15, 2011, 21:31

    First of all, propellant would be eliminated

    Not true. The propellant doubles as fuel, and you will still need that. If you eliminate the propellant, you are left without fuel and your space drive will go nowhere.

    If you power it with a nuclear reactor, you have to be fair and compare it with an efficient nuclear rocket. Then, again, it will have little advantage. In all cases, the space drive is subject to the same exponential law, generally known as the rocket equation. The only way it could not be is if it had an external energy source, or mysteriously generated energy out of nothing.

  • Eniac November 15, 2011, 22:07

    Your argument is that the space drive would succumb to the same exponential relationship between fuel and delta-V, but I dispute that as well.

    You do? You say so, but you do not follow up with reasons. How exactly do you imagine the space drive can get around being weighed down by the fuel it needs to operate? The fuel to accelerate the payload, the fuel to accelerate that fuel, the fuel to accelerate that fuel, etc?

    Using chemical energy, you would need a fuel tank the size of the Saturn V, for the same payload. With nuclear fuel you could do a lot better. Much like rockets, really. Only rockets exist.

    Rockets also have an incredibly high power/mass ratio which no other engine known to man can match. Hooking a space drive to an electric generator would involve a large mass penalty. The generator alone is orders of magnitude heavier than a rocket engine, at equal power. It is hard to estimate the power/mass ratio of the space drive, given its physical impossibility, but presumably it will not be massless, either.

  • Christopher Phoenix November 15, 2011, 23:02

    @Eniac

    You are right that some of the energy of fuel is expended accelerating the fuel, but AFAIK just some, not most. This just means a rocket can never be 100% efficient, but this is unlikely for space drives as well.

    You’re kind of missing the point. Not all of the energy provided to a rocket ship ends up in the vehicle- usually most of it ends up as kinetic energy of the exhaust. Not just some, but most. This is referred to as propulsive efficiency. Your rocket loses quite a bit of its energy in the kinetic energy of the exhaust- which a space drive would not.

    It is true that no engine can be 100% efficient. Some of the energy will always be lost as waste heat. This doesn’t have much to do with propulsive efficiency, which shows how much energy ends up as kinetic energy of the exhaust even in the case of 100% efficient rocket. You are wrong, however, that no rocket can ever have all of the energy it expends show up in the rocket. There is a theoretical situation where the rocket has 100% propulsive efficiency.

    If the specific impulse is fixed, for a mission delta-v, there is a certain specific impulse that minimizes the energy used by the rocket. This comes out to an exhaust velocity of about 2/3 of the mission delta-v. Drives with a very high and fixed specific impulse have a much higher exhaust velocity than this ideal, and are thus limited by their power source and give very low thrust.

    If, on the other hand, the exhaust velocity of a rocket can be made to vary so that at each instant it is equal and opposite to the vehicle velocity, the rocket will use the absolute minimum energy possible. The exhaust will be seen to stop in space and have no kinetic energy, so the propulsive efficiency is 100% and all the energy ends up in the vehicle (at least in theory, in reality there would be thermal losses from the drive system and residual heat in the exhaust). In most cases this will use an impractical quantity of propellent, but it is a useful theoretical consideration.

    Yes, I know that some engines like VASIMR can vary their exhaust velocity to fit various missions. This does help reduce propellent usage and improve acceleration at various stages of a flight. However, the best energetic performance and acceleration is obtained when the exhaust velocity is close the vehicle speed, and ion or plasma drives usually have exhaust velocities enormously higher than this ideal (even when VASIMR is in low gear).

    Before you tell me that I reiterated undisputed facts, I want to make sure you know I told you that you were wrong about how much energy is used accelerating the propellent AND that a rocket can never have 100% propulsive efficiency, at least theoretically. It just isn’t practical because the rocket would have to use an impractically large quantity of propellent. Ironic, given the nature of our argument…

  • Christopher Phoenix November 16, 2011, 1:22

    Not true. The propellant doubles as fuel, and you will still need that. If you eliminate the propellant, you are left without fuel and your space drive will go nowhere.

    Sigh… not this again… FUEL IS NOT THE SAME AS PROPELLENT!!!!!! The fuel is the energy source for the rocket. The propellent is the mass the rocket ejects at high velocity to propel itself- the stuff it “pushes” on. Do you dispute this? Then go to http://en.wikipedia.org/wiki/Spacecraft_propulsion#Effectiveness and read the part about reaction mass.

    If I eliminate the need to carry propellent, I am left with only the need to carry a power source or have energy beamed in from elsewhere.

    You do? You say so, but you do not follow up with reasons. How exactly do you imagine the space drive can get around being weighed down by the fuel it needs to operate? The fuel to accelerate the payload, the fuel to accelerate that fuel, the fuel to accelerate that fuel, etc?

    Why do you keep insisting that the fuel a space drive will consume to operate will behave just like the reaction mass a rocket expels to propel itself, despite all reason? The fuel is onboard energy. That energy will be transformed into the motion of the craft by the space drive. In a rocket, on the other hand, you don’t just carry fuel. A fraction of your ship’s mass will be expelled at high speed- and the mass ratio tells you how many kilograms of propellent the ship has to carry for each kilogram of ship structure and mass. Why would fuel for a space drive behave anything like this? You aren’t throwing mass overboard and moving due to the reaction with a space drive- thus there is no mass ratio. Your argument is absurd, even though “inertialess thrusters” are physically nonsensical with the physics we have today.

  • Christopher Phoenix November 16, 2011, 2:14

    @Eniac

    If you power it with a nuclear reactor, you have to be fair and compare it with an efficient nuclear rocket. Then, again, it will have little advantage. In all cases, the space drive is subject to the same exponential law, generally known as the rocket equation. The only way it could not be is if it had an external energy source, or mysteriously generated energy out of nothing.

    Your comparison of a space drive to a nuclear rocket is rather droll. Once again, you simply repeat your mantra “the space drive is subject to the same exponential law, generally known as the rocket equation” without following up with a demonstration or the reasons. Not only that, but once again you have revealed your lack of knowledge of rocketry.

    Let’s look at your “efficient nuclear rocket”. Let’s assume it is a classic “atomic rocket” like the Project NERVA engines. Those are real and have been test-fired as prototypes, after all. Let’s take a look at the fuel- look, it is just a bunch of U-235 rods!!! Strange, there doesn’t seem to be several kilograms of U-235 rods for every kilogram of ships mass- I thought fuel followed the rocket equation- oops I meant the “powered flight equation”. Well, there is a huge tank of hydrogen propellent. This is the reaction mass, the stuff that is ejected at high velocity to propel the rocket. And, yes, there is a lot of propellent for every kilogram of ships mass and payload, as in most other rockets.

    Let’s launch our rocket!! Up we go, riding a blast of hot hydrogen. Our ship expels a good portion of its mass. Soon, we run out of propellent. The nuclear core is still capable of generating plenty of energy, but the Law of Conservation of Momentum won’t allow us to provide an impulse to our craft unless the momentum of something else changes as well. That was what the propellent was for, by golly!!! I’d have never guessed, and I’ll never go on a spaceflight with Eniac again, if I get out of this drifting rocket…

    Annoyed at these results, I design a more powerful drive that is more efficient with its reaction mass while waiting for a rescue ship. I design an engine with a much higher exhaust velocity. This rocket engine can achieve the same impulse as our original rocket with less reaction mass. However, I run against a new issue. The energy for the impulse is proportional to the exhaust velocity, so the more mass efficient engines require much more energy. A lot of energy is lost as the kinetic energy of the exhaust, and I am limited by my power source.

    I want a lot of thrust, but I need to expend a lot more energy per second to achieve a large amount of impulse per second with a more mass-efficient engine. I’d need to expend huge amounts of energy per second to achieve high thrust, so my high- ISP drive is also low thrust unless I have some extremely powerful energy source. My drive would lose a lot of energy as the kinetic energy of the exhaust, as well.

    Why is this true? Momentum is mass times velocity. I want to carry less mass, but to achieve the same impulse while using less fuel, I have to accelerate the exhaust to a higher speed. However, kinetic energy=mass*velocity^2 This exhaust will carry much more energy than the original rocket’s exhaust. I have to release more energy to accelerate this exhaust, so I need a more powerful energy source. This higher exhaust velocity engine also has a lower propulsive efficiency, as a lot of energy is lost as the kinetic energy of the exhaust. See, I explained myself. A higher exhaust velocity calls for more energy expenditure, all explained with basic physics.

    That is why it is amusing that you compare the energy use of a space drive to an “efficient nuclear rocket” – the mass efficient rocket is also less energy efficient. Remember what I said about propulsive efficiency? If you want a rocket with a lower mass ratio with high thrust, you need to expend huge amounts of energy per second, and a lot of that energy does not end up in your rocket ship at all- it ends up as the kinetic energy of the exhaust. Note also that the mass of the fuel that provided all the energy used by the rocket was insignificant compared to the mass of the propellent.

    Finally, I am picked up by a Solar Guard cruiser and transported back to Earth. Once there, I am chosen to pilot a new craft with a non-rocket space drive that converts stored energy directly into the motion of the craft. When we tour the craft, we note that the craft simply has a power plant that feeds energy to the “Horst-Morlay Impellers”. There is no huge tank of reaction mass to throw overboard. The energy for the craft is provided by a nuclear power plant similar to the one on the rocket- but it doesn’t need to carry any more fuel to propel the space drive craft. Why? The energy from the drive is directly converted into the motion of the craft. The drive uses even less energy than a rocket, since no energy is lost accelerating the propellent.

    Now, why don’t you explain to me why the ship with the space drive will have to carry as much U-235 fuel as the rocket ship had to carry propellent in order to achieve the same velocity? The rocket didn’t need to carry that much fuel- even though it carried a lot of propellent. My craft doesn’t need propellent. Why would it need more fuel? It might even need less, since it doesn’t lose energy accelerating the exhaust.

    Please, I really want to know. I’m tired of you just telling me to accept your statements without explaining why your statement is true. If you can give me a convincing argument of why the space drive ship needs to carry as much U-235 fuel as the nuclear rocket has to carry propellent, even though the rocket’s fuel is not nearly that massive as you say the space drive’s fuel has to be and the space drive is likely to be much more efficient than a rocket, I promise I will accept your view and repeat the mantra “the space drive is subject to the same exponential law, generally known as the rocket equation” to everyone else I converse with on Centauri Dreams.

    I expect you will just ignore my challenge and bombard me with your mantra again, all the while accusing me of reiterating undisputed but irrelevant facts.