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A Fusion Runway to Deep Space?

Beamed propulsion concepts are usually conceived in terms of laser or microwave beams pushing a lightsail. But as we’ve seen over the years, there are other ways of thinking about these things. Clifford Singer went to work back in the 1970s on the concept of pellet streams fired by an accelerator, each pellet a few grams in size. The idea here is to vaporize the pellets when they reach the spacecraft, their energy being redirected as a plasma exhaust.

There are enough interesting variations on the idea that I’ll probably return to it soon. But over the weekend, an email from Jeff Greason reminded me of Jordin Kare’s unusual ‘fusion runway’ idea, to which he attached the moniker the ‘Bussard Buzz Bomb.’ Kare is an astrophysicist and space systems consultant with a background in laser technologies. He’s been involved in studies of laser launch methods, in which beamed energy is focused on an onboard heat exchanger that converts liquid propellant into a gas to produce thrust. Currently he serves as chief scientist for LaserMotive, a laser power transmission firm in Kent, WA.

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Image: Astrophysicist and space systems consultant Jordin Kare.

In the interstellar community, Kare is best known for SailBeam, where pellet propulsion is supplanted by tiny micro-sails that are pushed at huge velocities to the spacecraft, to be vaporized there much in the manner of Singer’s pellets. His sails turn out to require a smaller optical system than would be needed to push a large sail, and they can be driven to high accelerations while still close to the beam, reducing pointing and collimation challenges.

But I haven’t said much in these pages about Kare’s fusion runway, which he presented at a Workshop on Advanced Space Propulsion at the Jet Propulsion Laboratory back in the late 1990s. In 2003, when I interviewed him for my Centauri Dreams book, Kare gave me a breakdown of the concept. The idea harkens back to pellets, in this case fusion fuel pellets made up of deuterium and tritium that are slammed together to achieve ignition.

The pellets are laid down in an outbound track for the spacecraft that will eventually use them, deployed in advance by small spacecraft seeding the runway along the route of flight. Kare thinks in terms of a runway about half a light-day in length. The accelerating spacecraft would gobble up the fusion pellets one at a time, taking about ten days to exit the Solar System, moving along a runway track that stretched from near Earth to beyond the orbit of Pluto.

What kind of a craft would this be? Think in terms of a vehicle in the shape of a doughnut, or perhaps in more elongated form as a cylinder. The spacecraft would have its own supply of fusion fuel pellets. As the craft accelerates, it drops a pellet into the central ‘hole’ when one of the pellets of the fusion runway is about to be encountered. Nearing the end of the fusion runway, the spacecraft is being driven by fusion explosions at the rate of thirty per second.

The fusion runway relies on impact fusion, with the departing spacecraft first needing to reach speeds high enough (about 200 kilometers per second, by Kare’s reckoning) to ignite the reaction. Once ignition is achieved, the craft continues to accelerate along the runway track. The runway length would have to be adjusted depending on the mission, with robotic probes obviously capable of coping with far higher accelerations than humans. Add a human crew at 1 g of acceleration and a fusion runway might need to stretch out to a tenth of a light year.

String enough fuel pellets along the runway and the spacecraft gets up to ten percent of c. Although Kare built his workshop presentation around a one-ton interstellar probe, he sees the concept as scalable, telling me in that interview: “The fusion runway doesn’t care if you’re working with a ten or a hundred ton probe. You just need more pellets. You don’t need to build larger lasers. So it probably scales up better than most other schemes.”

Several other advantages emerge in the fusion runway concept. So-called ‘impact fusion’ doesn’t require the exquisitely symmetrical fuel pellets demanded by inertial confinement methods, nor does it demand that each pellet be fed energy simultaneously from every direction. Remember that Kare is assuming a spacecraft that is already moving — through some other energy source — at 200 kilometers per second to achieve ignition. From that point on, velocity depends upon the number of fuel pellets available in the runway ahead.

When I think about possible show-stoppers here, I wonder about accuracy. After all, each runway pellet has to hit the ship-borne pellet precisely, though Kare believes that this could be managed by laser pulses guiding the pellets internally. Perhaps the ship can be designed so as to channel runway pellets to the exact point of collision. Also challenging is the magnetic nozzle that will be necessary to contain the fusion explosions and direct their energy.

As far as getting up to speed, Geoff Landis told me some years back that a close pass by the surface of the Sun could be used to reach somewhere in the range of 500-600 kilometers per second. That could give you the velocity needed for ignition. Line the fuel pellets up so as to begin hitting them outbound and the method could work. In our recent email exchange, Landis does question Kare’s 200 kilometers per second as the sufficient velocity to ignite impact fusion — some figures in the literature point to 3500 km/s for a deuterium/tritium mixture.

Bundesarchiv_Bild_146-1975-117-26,_Marschflugkörper_V1_vor_Start

Image: Not exactly an interstellar prototype, but the German V1 gives its name to an advanced propulsion concept because of how it would sound (if you could hear it). Credit: Bundesarchiv, Bild 146-1975-117-26 / Lysiak / CC-BY-SA 3.0.

The ‘buzz bomb’ reference? That one is easy. The German V-1 was a pulse-jet rocket that gave off a characteristic staccato buzzing sound much like what a fusion runway spacecraft would sound like if you could hear it at all. The nod to Robert Bussard stems from the latter’s work on interstellar ramjet concepts, craft that pull in interstellar hydrogen to serve as fuel. Thus we have, as so often in interstellar studies, a hybrid design putting two distinct propulsion concepts together in ways that attempt to enhance the performance of each.

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{ 64 comments… add one }
  • Alexander Tolley June 16, 2017, 12:00

    Staying on the runway might not be trivial. The pellets, once released are going to follow a trajectory that is partially influenced by gravity. In contrast, the ship is also going to accelerate so I would expect its trajectory to deviate from that of the runway. How would this be overcome? isn’t this going to be an issue especially if a sundiver approach is used to accelerate the ship to a velocity needed for collision fusion.

    It is an interesting idea though, as it turns on its head the problem of fractional c impacts that need to be shielded into a mechanism that allows “simple” fusion propulsion.

    I wonder if the ship even needs to carry pellets to collide with. Could runway pellets be captured in pairs and reorientated so that they can collide with each other instead? Given the size of tanks needed for interstellar fusion rockets, reducing their size by 50% is useful, but not groundbreaking. Getting rid of them entirely, except for a small emergency supply, would make a big difference to ship design, making them far more like the classic Bussard ramscoop concept.

    Wandering off the interstellar runways and how to return to them might make a good hard SF story.

    • Joe H. June 16, 2017, 14:50

      Trying to make a round trip using this system would be even more complicated.

      • Alexander Tolley June 16, 2017, 15:37

        The same problem with any initial approach that needs an Earth base. However, once interstellar travel is more frequent, any system can use facilities at both ends of the trip. Whether beams, pellet streams or dilithium crystals and anti-matter.

        In practice, one can imagine a starship waiting several days for a pellet stream to have been laid down before departure under fusion acceleration can begin.

        It is unfortunate that we don’t have a good means to use natural phenomena to propel us to the stars in an analogous way to the age of sail. We can use sails in interplanetary space, but they are useless in interstellar space. The Bussard ramscoop was a possibility but that idea has been discarded as unworkable. So we seem to need to start at the age of steam, taking on fuel at ports, creating strategic bunkers and dock facilities within the volume of our interstellar expansion to power our starships.

        • Brett Bellmore June 18, 2017, 13:32

          I wouldn’t say that sails are useless for interstellar travel; The problem lies not in the stars, but ourselves. We just don’t live long enough, and consequently are rather impatient.

          • Alex Tolley June 18, 2017, 21:21

            We not only have short life spans, but we require a lot of energy to accelerate us up to high velocities. Embryos, or even better, information, would be a more energy efficient way to get humans to the stars.

          • ljk June 19, 2017, 10:42

            This assumes in large part that humans, especially like those of the early 21st Century (aka, mostly unenhanced biologically and technologically) will actually be aboard interstellar vessels. Or that those waiting at “home” will also be pretty much as we are now.

            Human nature in general may not change, but assuming our species is still around in some form, in many key areas our descendants will have likely modified themselves to improve upon life spans, mental agility, physical strength and appearance, etc. You do not have to search very hard to find advertisements and articles about medicines that improve performances, reduce aging, plastic surgery, and so on.

            These improvements may include waiting patiently for starships to reach their destinations.

            • ljk June 19, 2017, 12:02

              It is funny how futurist can image the most wild advanced technologies yet when it comes to the human condition, it might as well still be 1950.

              Here is a prime example from Walt Disney’s Magic Highway USA, made in 1958:

              http://2719hyperion.blogspot.com/2007/01/road-ahead.html

              The animated segment shows, with that wonderful vintage Disney style, fantastic concepts for future transportation and the homes and cities they will presumably shape along the way. However, when it comes to human family structures, dad is the one who goes to work while mom (and son!) go shopping during the day.

              Another example of why we need more than just one or two disciplines when it comes to building interstellar vessels carrying passengers or searching for ETI.

      • Andrei June 18, 2017, 8:56

        A full round trip might not be easy, but not impossible.
        If the ship carry enough pellets it could ‘seed’ its path ahead, then using the magnetic scope to decelerate further and finally use those pellets for deceleration but perhaps not stop destination.
        Those pellets would still be moving toward the target star, magnetic scope braking against stellar wind perhaps for final stop.

        Last leg returning to Earth is no problem, we would seed pellets ahead of the ship arrival.

        The problem is departing from the distant star, carrying the machinery to make a runway in that distant system might be the major problem with this approach.
        But if we have reason to build a two way interstellar transport, this might be a creative and interesting idea on how to do this.

    • David H June 16, 2017, 20:24

      The ignition happens because of the big difference in the velocities of the two pellets. That’s why one needs to be in the ship’s inertial frame and the other one not. But those things won’t be very heavy. What worried me more is that the seeding ship will itself have some sort of outbound velocity, so the pellets it poops out will almost match it. This means that the seeding ship needs to be very slow relative to the ship that consumes the pellets. Either that, or it shoots the pellets backwards with some kind of mass driver. Hey, that would also produce some bonus thrust for the seed ship! The ideal mission plan would be to launch before all the seeding is done, and time it so that the seeding ship gets passed just after it releases its last pellet. Any extra waiting allows the pellets to drift further from their intended positions.

      About the accuracy issue, I’m sure the mention of Bussard comes from the electromagnetic mechanism that scoops the pellets and orients them down the midline of the ship. It sounds tricky but not crazy.

      A crazy thing to do to make the ship fuel-less would be this: You have the seeding ship shooting pellets backwards, and you have some sort of a launcher in the solar system shooting pellets outwards at 200+ km/sec. You calibrate things so that the pellets are on a collision course, and that collision happens to be exactly in the belly of the ship just as it’s passing by. The ship just surfs the explosions. (Obviously, it would also deflect the pellets a bit to make sure they actually collide.)

      • Alexander Tolley June 16, 2017, 21:42

        I see a solution is to have both pellets arrive just slightly out of sync. The first pellet is captured and stopped with magnetic fields. The 2nd then collides with it. The ship loses velocity stopping the 1st pellet, but more than gains it back with the fusion blast as exhaust.

        By getting rid of the 1/2 of the pellets needed as stored mass, the propulsion delivers more acceleration as te ship is much less massive, especially in the initial period of the flight.

        • Michael June 18, 2017, 8:57

          Might be better to have the runway concept and fire fuel sails towards the craft to collide with runway fuel units, timing again and accuracy will be a must. In this configuration there is no fuel carrying penalty and the rocket equation is not applicable.

          • Alex Tolley June 18, 2017, 11:25

            That is increasing the complexity even further. The pellets ahead and behind must collide exactly as the ship intersects the 2 pellets. The smallest discrepancy will create “misfires”. This is asking even more from the pellet launcher. It must create 2 streams of pellets, which collide as the ship reaches each unique collision point. And I thought the Starshot laser aiming was going to be hard.

            • Michael June 19, 2017, 2:53

              A compromise may be better where the spacecraft uses lasers to slow and control the incoming fuel pellets, it acts as a damper. And as you say it will require very, very good accuracy but a laser on-board the spacecraft could allow timing to be fairly accurate.

        • Michael June 19, 2017, 6:45

          Perhaps if we used the sails as kinetic energy pushers to get the craft up to +200 km/s and then used a laser to slow alternate fuel units down so they impact each other. A laser would work well here because the light could be bounced back and forth allowing greater efficiency and there would be no need for fuel ‘tanks’.

      • J. Jason Wentworth June 17, 2017, 5:29

        Yes–the pellet-dispensing ship could utilize the pellets as *its* reaction mass, ejecting them rearward with an on-board mass driver (Gerard K. O’Neill’s proposed tugs and interplanetary spaceships used mass driver propulsion, with compacted lunar or asteroidal regolith pellets being the reaction masses), and:

        If we had William Escher’s lunatron electromagnetic launchers (see: http://www.google.com/#q=William+Escher+lunatron ) set up on the Moon, they could launch pellets aimed at the rear of departing fusion runway ships. The Fiesler Fi 103 (the V-1 “buzz bomb”–it would be something if that firm [or its descendant] supported fusion runway propulsion research) also brings to mind another possibility:

        If other civilizations use fusion runway technology, might such starships–when under thrust–sound like “buzz bombs” to our astronomical instruments, by means of the rapid “flashes” of radiation, light, and radio emissions produced by their propulsion systems? If we ever detect strange pulsars that periodically go silent, have high proper motions–and *don’t* gradually slow down in their pulse cycles, as pulsars do–maybe they won’t be natural phenomena…

  • Andrew Higgins June 16, 2017, 13:51

    A great reference I would recommend for anyone looking at impact fusion (how fast the pellets need to be going, how to launch them, etc.) is the Proceedings of the Impact Fusion Workshop that was held at Los Alamos in 1979, now available online:

    https://www.osti.gov/scitech/servlets/purl/5841385

    Alas, this was the *last* Impact Fusion Workshop, due to the conclusion that the accelerator to launch a macroscopic pellet to the necessary speeds would need to be too long.

  • Egad June 16, 2017, 15:25

    Along the line of pre-positioned propulsion units, why not do it with Orion-esque pulse units (aka nukes)? Some precision trajectory matching would be needed, of course, but an Orion unit could carry a beacon and have some smarts itself.

    • Brett Bellmore June 18, 2017, 10:26

      As always, Orion is the most technically feasible, and least politically feasible, approach to interstellar propulsion. But the former is the reason for the latter: Virtually any real propulsion technique is going to follow “The Kizinti Lesson”: “A reaction drive’s efficiency as a weapon is in direct proportion to its efficiency as a drive.” Reaction drives capable of interstellar application are thus only politically feasible so long as they are technologically infeasible; The moment they become feasible as a matter of technology, they get evaluated as weapons, and ruled out on that basis.

      I fully expect that real interstellar travel will not begin until we have significantly colonized the solar system, and it will be the colonies engaging in it, not Earth. And the colonies will be more pragmatic, colonies typically are, having had recent history where anything but pragmatism meant death. They’ll still understand the fundamental reality of reaction drives being weapons, but use them anyway.

      • Alex Tolley June 18, 2017, 11:50

        Orion fell prey to the treaty banning atmospheric nuclear weapons tests, as well as the ban on nuclear weapons in space. Assemble the ships nukes a long way from earth and it might be acceptable to build such a ship. I think it is an inelegant, brute-force, approach to building interplanetary ships, and it will suffer the same scaling problems (rocket equation) as fusion-powered ships for interstellar travel.

        The huge energies involved in moving mass across interstellar distances are best met by reducing the payload mass. Accelerating 100kg meat puppets at significant fractions of c doesn’t make a lot of sense to me. Worldships, yes, as they move slowly and need far less propulsion. Robotic probes, yes, as they can be miniaturized to reduce mass. But fast ships carrying human crews, that suffers from irreducible mass requirements which predicate massive energy requirements. Not impossible, but wasteful. It is rather like communicating by physically traveling rather than using electronic media. Send a seed autofac that can create humans in situ or build a body for a transmitted mind. Go with the technology flow that is advancing in this direction.

        • Brett Bellmore June 19, 2017, 6:39

          It may be inelegant, but as the saying goes, “If brute force doesn’t solve your problems, you’re not using enough.” I’m an engineer, I do appreciate elegance, but given a choice between elegance and something that actually works, I’ll go with works every time.

          Let me tell you something: I was near-sighted from an early age. I got my first pair of glasses in elementary school, went out that night, and saw the stars for the first time in my life. Contra Asimov, I didn’t go insane. But I did go interstellar crazy, formed my life’s ambition to be on a star ship. Why do you suppose I became an engineer? I didn’t want to be Kirk, I wanted to be Scotty.

          Well, life is full of disappointments, isn’t it? But it really sucks when the disappointments were dictated not by the laws of physics, or engineering constraints, but instead political stupidity. When you realize we had the keys to the solar system in our hands, and threw them away to go play with oversized fireworks.

          You think Orion inelegant, that we can’t use fission. I hear complaints about how sour those stupid grapes are. Well, guess what: They’re the only thing on the menu for now, so stop complaining, and jump higher.

          • J. Jason Wentworth June 19, 2017, 11:09

            Brett, I find it annoying and frustrating that even NEP (Nuclear Electric Propulsion–ion drives powered by Topaz-type reactors) spacecraft are politically impossible. In the past, I have jokingly suggested that we should contract with North Korea for NEP and Orion-type nuclear systems, but maybe it wouldn’t be a bad idea to actually do such a thing, and:

            I loathe the self-limiting prohibition on using nuclear power for space propulsion (even RTGs and radioisotope heaters for spacecraft are hard to get approved). If the human race can’t overcome its collective failure of nerve and summon the “will to wield” these propulsion technologies, they don’t deserve to learn about in detail–let alone benefit from in any way–the stars and planets beyond this solar system, because using great powers responsibly is a hallmark of societal maturity. Such folk (I’m thinking of our leaders here and abroad, not you or I) might as well content themselves with fictional interstellar adventures on television or in movies.

          • Alex Tolley June 19, 2017, 11:36

            Engineers are also prudent and cautious. In the Star Trek series, especially TNG, they have occasional “warp core breaches”. These always happen in space. In the first ST movie reboot, we see the Enterprise being built in a dockyard on earth. Now just suppose they built the complete ship and had a core breach in the dockyard. You really, really, don’t want that to happen.

            The original Orion was going to detonate nukes in rapid succession to launch the ship. I think we can say that this is not a good idea. Which means that to use the technology today, the Orion must be ship to orbit incrementally by chemical rockets and assembled there. The nuclear bomblets need to be shipped to orbit in a safe way so that any launch failure doesn’t cause contamination. That may mean final assembly in orbit.

            We are talking about ground based nuclear security ramped up for such a project. This doesn’t strike me as prudent.

            The brute force use of nuclear bombs was propsed to build canals. Didn’t happen. MacArthur wanted to use nukes to win the Korean War. Prudent thinking kiboshed that.

            We don’t just build things because we can. We also think about the nth order effects and balance negatives against the benefits. Nuclear power seems like a relatively good use of such technology. Yet with relatively low usage, we have had some bad accidents, one still ongoing. If nuclear had become teh main source of power on earth, we might be having blowups every few years or even more frequently. (As Heinlein suggested, “Blowups Happen”.) Maybe the consequences of GW will prove worse than such accidents, although I would prefer we try to transition to more benign techologies instead of resorting to nuclear as the main alternative.

            Orion may have been a wondrous piece of engineering, but the benefits seem rather small compared to teh costs. But find a way to make it safe and I will support such a use. It may be a good interplanetary technology, but the rocket equation makes it a real brute for interstellar trips. I would hope fissile material extraction can be from extra terrestrial sources, because I would’t want to see a large expansion of Uranium mining on Earth to feed its propellant tanks.

            • J. Jason Wentworth June 20, 2017, 1:42

              “Star Trek” is fiction, so it’s irrelevant. There were designs for Orion interplanetary ships that were sized to fit atop Saturn launch vehicles, and they would not have started thrusting until they were in orbit.

              I also wouldn’t extrapolate from the U.S. nuclear power industry, in which so many power plants’ reactors are one-off designs. The French nuclear power program, which has an excellent safety record (as does the U.S. Navy’s surface vessel and submarine reactor program) utilizes common reactor designs (with the generator units “sized ” to match the loads at the various power plants), which makes maintenance and upgrades much easier. The French and U.S. Navy examples are the ones to emulate regarding in-space reactors.

              • ljk June 20, 2017, 8:54

                You want details on Orion? I got your details on Orion right here:

                http://www.projectrho.com/public_html/rocket/realdesigns2.php#id–Project_Orion

                • Alex Tolley June 20, 2017, 12:00

                  Very cool. Almost TMI.

                • J. Jason Wentworth June 21, 2017, 5:34

                  THANK YOU for posting that link! Not only does that site cover Orion (I didn’t know there were so many different Orion designs and sizes), but it also covers the MEM (Mars Excursion Module, also called the Martian Taxi) and NASA’s Nuclear Shuttle, among numerous other interplanetary spacecraft! I agree with the site author that Orion is one torchship (having both high thrust *and* high exhaust velocity) that we could actually built today. Also:

                  If we built O’Neill-type space colonies which mined asteroids, they could build Orion ships, using asteroidal uranium (of which there is plenty) for the pulse units…all well away from the Earth. Fission pulse units for these ships could be quite simple. They could be made like the M-28/29 “Davy Crockett” warhead, (see: http://foxtrotalpha.jalopnik.com/this-is-what-it-looks-like-when-the-worlds-smallest-nuk-1684923814 and http://www.google.com/#q=davy+crockett+nuclear+warhead ), whose W54 warhead (see: http://en.wikipedia.org/wiki/W54 ) had a single ellipsoidal mass of fissile material. The detonating high explosives forced it into a spherical shape, whose cross-section was then high enough to capture enough neutrons to trigger the runaway fission reaction. We must embrace atomic power (even in NEP ion drive or electrothermal drive form) if we are to *really* go anywhere, in the solar system or beyond; chemical propellants will only carry us so far before the propellant mass and the cost become impractical.

        • David H June 19, 2017, 20:52

          I totally agree about the impracticality of transporting humans across interstellar distances. We might never actually do it. But since I still want interstellar colonies of humans, I think we should work on tools for building humans in faraway places. This weekend Craig Ventner mentioned machines that could one day “print a baby” on Mars. No thanks, I’d rather have Martian babies “printed” by human women. But finding a way to build such a baby printer from a tiny starter kit we launch to another star system… well, that sounds like a good idea.

          • ljk June 20, 2017, 8:51

            But what kind of humans do you want for these missions? Or are expecting? Do you think the humans that exist now will be the same in even a century from now – assuming we make it to another century, that is.

            • J. Jason Wentworth June 21, 2017, 5:50

              I have never liked the idea of trying to “breed better space travelers”, via any method. Even if it can be done (which I have doubts about), it’s a stretch to assume that they would share a burning desire to explore space. I am even more doubtful about the idea of “uploading” ourselves into electronic devices or android bodies, and about whether these “Humans Si/Ge” (silicon and/or germanium) would share the interest–or want to risk their longer lives–in exploring space. We can go as we are–even if generation or hibernation starships are the best we can manage for a long time, and the vast majority of such *true astronauts* (star voyagers) would also rather go as they are.

              • ljk June 21, 2017, 11:38

                I do not know if we will ever deliberately breed humans to travel in deep space. I figure if humanity ever comes to that, the first types they will breed are supersoldiers. Or superathletes, which may be one and the same. Whichever can be done the cheapest while returning the most revenue.

                What I was thinking of was that people are already using technology and the biological sciences to improve themselves without the impetus of star travel. So that by the time we are able to send people across the galaxy, our descendants may already be better suited mentally and physically for such journeys.

                • J. Jason Wentworth June 22, 2017, 1:15

                  Neither of those possibilities is appealing to me (with the exception of using science and technology to advance the diagnostic and healing arts of medicine). This is the only sense in which I am glad that I will not live to see interstellar travel (with crewed starships).

  • Robert June 16, 2017, 15:44

    These kinds of ideas need to be explored if only in order to eliminate them as realistic alternatives.

    I’ve been enjoying the proceedings and videos of the recent Estes Park Workshop with prof. Woodward and others.

    http://ssi.org/2016-breakthrough-propulsion-proceedings/

    My money’s on Woodward’s Mach effect or MEGA drives or possibly some form of EMdrive or Cannae drive. Those may be Mach effect devices too. At least I hope one of those technologies pans out. Propellent-less propulsion seems the only realistic alternative to get to the stars with both a reasonable time and energy budget.

    • Betto Davis June 19, 2017, 18:42

      In other words, magic.

      I’m not against such things, though. Empiricism and some tolerance for serendipitous/unexpected discoveries should make part of humanity’s scientific endeavors.

      Because sometimes you really need a proverbial quantum leap in technology and science for allowing some things to happen.

      And it seems that for the moment being and without such miraculous things, we are very much stuck at very slow speeds, at least too slow for hauling our meat to the stars, all while we live our lives in a tin can.

      Which means it probably won’t happen as it is currently foreseen (or plainly, ever).

      • ljk June 20, 2017, 9:08

        For a frighteningly large percentage of the global population these days, modern technology and science might as well be magic.

        This is why a disproportionate amount of people also focus on warp and hyper drives over more plausible modes of interstellar propulsion, and even with those methods the term plausible is relative. They also assume that an ambiguous person or persons of the future will automatically come up with the solution, or better yet aliens will come to Earth and grant us this technology, just because. It doesn’t require any real effort on their part, just turn on the television and watch a bunch of fictional characters go to Warp 9 in their sleek and sexy starships.

        • J. Jason Wentworth June 22, 2017, 1:58

          I’ve never had a problem with magic (either the subject or the name), because it is real–calling the Placebo Effect by that term doesn’t make it one whit less magical. Nor is an atomic bomb, especially a simple gun-type one such as “Little Boy”; just bringing together its two pieces of enriched uranium (totaling 141 pounds in weight) on a tabletop, or even in ones’ hands (without any of the other parts of the weapon) would have released the energy of 15 thousand tons of TNT (such power could also be used creatively, as in Orion spaceships). Magic–whether it is worked by means of one’s mind and will (the Placebo Effect is just one example of this) or by mind and will plus other materials and tools (nuclear bombs and reactors are examples of this)–is the art of harnessing the forces of nature, both seen and unseen, to make specific things happen, and:

          Regarding the desire of many people for warp drives, hyper drives, and other forms of fast (faster-than-light or even instantaneous) interstellar travel, it isn’t due to ignorance on their part; they simply prefer to see probes (and/or crews) reach the stars during their lifetimes. This is a wholly sensible preference, especially if they will be footing the bill (through national space agencies’ expenditure of their tax money on the research); had President Kennedy said “…before the ^next century^ is out” (instead of “…before this decade is out”) regarding the Moon landing, who would have been excited about paying for it (or even working on it) in 1961? Unless slow (more than a human lifetime transit time) interstellar probes make interesting observations while en route, neither the taxpayers, nor the politicians, nor even the scientific community, are going to be very interested in such projects.

          • ljk June 22, 2017, 9:20

            The problem is there are many who do not see “magic” as you do and cannot tell the difference. Then there are those who look at science as a belief system, whose tenants can be changed on a whim or by popular vote as the case seems to be these days.

            The Universe does not give a flying fig about human wants and needs. As anyone who is more than a casual reader in this blog can attest, interstellar travel is going to be difficult and time consuming no matter what method is found. The problem with all known forms of FTL propulsion is that their issues are so far beyond the issues with STL methods that they may not be resolved for centuries if ever and take away from the methods that do have promise, to say nothing of more than a toehold in reality.

            Yet this is what the public and science fiction focuses on, all in the name of wanting to get to some destination they know nothing about and will likely become bored with the moment they realize it is nothing like what they see on Star Trek (aka, no aliens).

            • Alex Tolley June 22, 2017, 12:11

              (…) will likely become bored with the moment they realize it is nothing like what they see on Star Trek (aka, no aliens).

              Especially as the aliens, if they exist, won’t likely look even remotely human and certainly will not be possible to enjoy sex with.

              James Tiptree jr. wrote a short story about humans wanting to be with aliens to have sex with them.

              As anyone who has traveled to “exotic” places knows, even the most beautiful landscape becomes like wallpaper after a few weeks and disappears into the inattention. I suspect early Mars colonists will soon find the excitement of the new world fade away and the irritations that accompany living there become more of concern. It will take a special person to stick with it. On an analogous note, it is said that Brits who went to Australia on the free passage plan from the government got up to 2 tries for free. Supposedly those that couldn’t stick it soon complained of not being able to get a good cup of tea (very important!) and returned home. Long duration star flight is going to make the great oceanic voyages of exploration and the Polynesian migrations seem like short walks in the park.

              • J. Jason Wentworth June 23, 2017, 4:48

                In his TIME LIFE series book “Man and Space,” Arthur C. Clarke expressed a concern that “So much has been written about the moon, and so many thousands of lunar touchdowns have occurred in films and fiction, that one sometimes wonders if the real M-Day may be an anticlimax. But that possibility is very remote.”

                I wonder, though, if the first manned Mars landing may be somewhat anticlimactic, because–even more so than with the Moon, before Apollo 11–so many robotic orbiters, landers, and rovers have examined (and will have examined) the Red Planet in detail, even in the more scientifically interesting (and thus often dangerous for human explorers) regions.

                • Alex Tolley June 23, 2017, 13:02

                  As we know, Moon exploration rapidly wilted from the public’s interest, so Clarke was almost right. The same has happened with the Mars rovers. I recently realized I didn’t even know where Curiosity was on Mar’s surface after the initial excitement of the landing. We are already focusing on the next rover mission that really, really, cross-my-heart, pinky finger swear, will look for life.

                  Humans love novelty and each novelty reduces attention to previous novelties. Novelty might have been rare on those East African plains, but today it comes at us like a firehose. Toffler worried that we would be future-shocked. I would propose that we are novelty-shocked.
                  A human crewed Mars mission will be interesting at launch, peak at landing, bump up at leaving Mars and be high-ish on a safe return. Most of the trip and surface stay will be of low interest to the general population.

                  • ljk June 23, 2017, 14:10

                    This is probably why the films Apollo 13 and The Martian were made: Just sending humans on pioneering missions to the Moon and Mars were not enough, there had to be some life-threating disaster in the mix.

                    Of course such space missions are not about the mere entertainment of the public, but that point is often lost or ignored despite what their PR campaigns say.

                    • Alex Tolley June 23, 2017, 16:15

                      It might be even worse than that. With increasing novelty, the level of excitement has to be constantly raised, rather like an addiction. Just look at the tempo of tv and movies today compared with half a century ago. A movie like “Transformers” would be unwatchably frenetic then, yet kids seem to enjoy that pace today. Multi-tasking may be another symptom of needing more excitement. I know from experience that many people cannot watch 2001L A Space Odyssey because the pace is far too slow for them and the excitement level is very low as the danger is much less dramatic. You would need a slasher crewman or rapacious alien these days.

                      When we have discussed which humans would colonize Mars or the planets, my sense is that you want farmers from the 3rd world. They might be able to withstand low sensory input, hardship, lack of comfort, and to use local resources. Wealthy western city dwellers might be the last type one would want on a colony ship. Too addicted to convenience, technology and sensory input to induce excitement.

                      Best of all are robots for the initial stages. They do not have such human needs, both physiological and cognitive. They can patiently explore, and toil away developing infrastructure and food sources for later human colonists.

                    • J. Jason Wentworth June 24, 2017, 1:58

                      That is a characteristic (the need for ever more novelty) that has always felt utterly alien to me–I have always found fascination, and often profundity, in the simplest, commonest, and most humble of things. For example, I have *never* found flying boring–while other passengers around me read or watched screens of one kind or another, I looked out the window and thought about how many people, in past times, dreamed of flying and of viewing the world from above. Also:

                      I prefer old astronomy and astronautics books (although I do have new ones), because the hand-painted artwork and diagrams of illustrators such as Helmut K. Wimmer and Chesley Bonestell inspire far more wonder in me than the sharpest photographs (which I do like, of course) and computer-generated diagrams in more recent books.

                  • J. Jason Wentworth June 24, 2017, 5:45

                    Mars is, in some ways, a “teaser.” Looking out the window of a spaceship or a building on Mars will eventually become frustrating–people will see scenery that might as well be in Arizona, with clouds in the sky, and at times they will wonder, in annoyance (even though they will know why intellectually), “Why *can’t* I just walk out there and enjoy a cool drink?!”

                    The Moon, by contrast, will not tease settlers. Looking out at the stark yet beautiful lunar vistas, with a space-black sky always overhead, they will never, even for a moment, feel (emotionally) that they *should* be able to stroll about out there sans spacesuits.

              • ljk June 23, 2017, 9:14

                Well, I wasn’t saying that people might become bored with interstellar travel because they couldn’t have sex with aliens, so I am not quite sure how we got there….

                • J. Jason Wentworth June 24, 2017, 6:10

                  Having once had an equine girlfriend (nothing untoward–in human terms–occurred physically; our relationship was like that between a maiden and a unicorn, but the other way around), it’s not so strange to contemplate aliens in that light, and:

                  It’s actually a positive thing, because it shows that not all human beings assume that aliens–who are the ultimate “others”–must be evil/ugly/grotesque/predatory, as they are so often depicted to be in science fiction. There may be aliens who, to human eyes, are achingly beautiful (even if their forms are not even vaguely humanoid) and who are personally charming.

  • Brett Bellmore June 16, 2017, 16:59

    If you can launch the pellets fast enough, you can start out with a stationary ship accelerated up to operating speed by its pellets being struck from behind, and then transition to impacting relatively stationary pellets.

    But, impact fission is a lot easier to manage….

  • Dr. Eric Davis June 16, 2017, 21:41

    Dr. Vince Teofilo, retired Lockheed-Martin Space & Missiles Co./Advanced Technology Center’s nuclear space propulsion and power expert, has told me numerous times during our several recent discussions on the technical problems with laser inertial-confinement fusion is that the cost to produce a single deuterium-tritium pellet is $100,000+. So Jordin’s hybrid propulsion concept is not financially feasible in addition to the other difficult technical problems to overcome.

    • Andrew Palfreyman June 17, 2017, 23:22

      If you mean Livermore’s NIF – the hohlraum tech – then the article makes plain that the constraints for Kare’s pellets are far looser.

  • Michael June 17, 2017, 3:04

    Moving the fuel ‘pellets’ outwards may be as simple as using a ‘Starshot’ laser and onboard guidance system to blow them out there. An onboard laser system could be used to guide them into the ignition chamber. I would prefer fission materials as although they have less energy per mass they are very dense by a factor of over 200 that of hydrogen.

  • ole burde June 17, 2017, 5:50

    The general idea of a ‘runway’ is the right direction , but for a scheme that only reduces the propellant weight 50% , this one creates some heavy problems …it would seem more logical to accelerate charged particles or Lacerligt or microwaves or anything else that does NOT demand impossibly long equipment , more so because before or later it will become clear that we need a whole series of accelerating stations , and not just a single earthbased one …but the term Runway is Great !

  • Greg Matloff June 17, 2017, 7:39

    Dear Paul

    I did my version of the ramjet runway for a 1979-vintage JBIS paper. In deriving Newtonian kinematics, I used techniques from calculus including integration by parts and trigonometric substitution. The effort took about 6 months and I was very proud of the results. About a decade later my pride was dashed when my ancient Mac-Plus with no math co-processor replicated the results using (I believe) MathCad in about 45 seconds!

    Regards, Greg

    • ljk June 19, 2017, 10:45

      Just think about all those folks in the pre-computer era who only had pencil, paper, and their brains to work with – or quill pens, or sticks digging into wet clay.

  • Jim Early June 17, 2017, 7:43

    The simplest form of a runway is a deceleration runway when you arrive at your destination. For this runway no fusion reaction is required. The pellets are simply ionized passing through a very thin foil. The resulting ions have their velocities relative to the spacecraft reversed by a magnetic field. This scheme only works for two way traffic since someone has to make the runway at the target system.

    If you have ever seen one of the rocket sled tests at Sandia, they provide a clear visual example. The sled is stopped by lowering a scoop into a trough of water.

    Impact fusion for an accelerating “ram jet” is much more complicated, and the pellet design depends on the velocity. At relativistic speeds the two small pellets pass through each other on time scales (femptoseconds) two fast for fusion. It may be possible to induce fusion and utilize the slower fusion energy release with larger scale magnetic fields generated by the spacecraft.

  • Michael June 17, 2017, 9:05

    To slow the craft down in the target system we could turn the craft around and then collect fuel sails fired a short time later from our system (timed). The last fuel sails will be coming in very fast towards the craft aiding impact fusion, accuracy will be a must.

    • Michael June 17, 2017, 11:39

      Actually this guidance of the sails if controlled from the spacecraft would be very easy as the fuel sails will all be in line that can be controlled by a laser on the spacecraft.

  • david lewis June 17, 2017, 10:16

    Fascinating. Imagine the possibilities not for interstellar travel but getting to the sun’s focal point. At 1000 m/s, a runway of only 500,000 km would suffice to get a probe to 1/300 the speed of light. It would take less than 3 years for a probe to get to the focal point. While 100 gravities is a bit extreme it would be, of course, unmanned.

    If it were possible to get the pellets up to 200 km/s then it would be possible to launch as many of those probes as we wanted, barring the cost of the pellets. I would assume however that once manufactoring began costs would drop.

    Also if the orginal speed of the pellets were 200 km/s we would have a method of getting probes and craft anywhere we want in the solar system without having to wait decades. Even to Pluto we would only need around 200 km/s to get there in a year.

    At what speed would two pellets of uranium cause a fission explosion? If lower then maybe start with fisson and then switch to fusion?

    • Michael June 22, 2017, 3:00

      ‘At what speed would two pellets of uranium cause a fission explosion? If lower then maybe start with fisson and then switch to fusion?’

      This is highly dependant on the density reached by the impacting pellets, at very high velocities very high densities should be obtained and substantial fission will take place. Perhaps a sandwich will be better, lithium deuteride with a coating of fissile material as a backing. I have often wondered if a coulomb explosion would take place in a fissile materials nucleus when all the electrons are removed in a violent collision which would occur at high fractions of c.

  • Al Jackson June 19, 2017, 8:19

    I have got a question. Had someone put forward the ‘fusion pellet runway’ before 1977? Dan and I never did any calculations , only talked about the concept. I don’t know if this qualifies us as the originators?*
    The idea is mentioned at the end of the paper:
    “Laser Powered Interstellar Ramjet,” Journal of the British Interplanetary Society Vol. 30 (1977), pp. 223-226.

    *After all, Columbus did not discover American first, he did discover it best.

    • Paul Gilster June 19, 2017, 10:45

      Interesting! I’ve got the laser ramjet paper around here and will dig it up. Surely Jordin would have seen it.

    • ljk June 19, 2017, 10:49

      No, Columbus was not the best at it, he just had better PR. :^)

      I have read that English fishermen knew about the excellent fishing in the Grand Banks long before Columbus went sailing to find India, but naturally they did not want to announce such a resource to the rest of the world, or at least Europe for obvious reasons.

      • Alex Tolley June 19, 2017, 14:05

        There are a lot of people who want it to return to that way of life. :(

  • Isaac Kuo June 20, 2017, 12:32

    I had independently come up with this idea in the 1990s, but I never figured out one huge problem with it – there’s no way to test the idea except at full scale. How the heck do you get a physical pellet up to hundreds of km/s, much less many thousands of km/s, to make sure it works at the required speeds?

    For this reason, I have modified the concept to use bomblet designs more similar to thermonuclear bombs. In these, the primary fission device is used as an extremely powerful x-ray flashbulb, with a plastic liner to smooth out the thermal x-rays to implode the lithium deuteride fusion fuel. In addition, the ablative liner and outer casing are made of cheap common uranium to absorb fast neutrons (which are useless for propulsion) and create more fission fragments (which are useful for propulsion). This lets you test the bomblet at rest, using fission primaries at various distances to simulate the collision flash. (To minimize testing costs, you have several bomblets at different distances from the fission primary.)

    Unlike just using normal bombs directly, this system allows scaling down the bomblets and it eliminates most of the cost of the expensive fission primaries. The starship contains an on board store of impactors, and uses a magloop to deflect charged particles from the bomblets for thrust.

    As for the problem of deceleration and return – these things can actually be provided from the home system. One thing that’s not obvious about these runway propulsion schemes is that the runway does not need to be at rest. The runway can be moving at relativistic speeds and it still works. This is the key idea, and for some reason hardly anyone understands this.

    So the sequence of events is:

    1) Lay a (slow) launch runway
    2) Accelerate the main starship with this runway up to .2c
    3) Lay another (slow) launch runway
    4) Launch an auxiliary starship, at .22c, so it would catch up with the main starship near Alpha Centauri.

    5) The auxiliary starship contains a bunch of bomblets and nothing else. It releases the bomblets, which spread along a line to create the braking runway. Note that this runway is moving at .22c
    6) The runway catches up with the main starship. The starship uses this runway to accelerate back toward Earth. Note that it starts off at .02c relative velocity with the bombtrack. It ends up with .22c relative velocity with the bombtrack – at rest with Alpha Centauri.

    7) Lay another (slow) runway to launch yet another (fast) runway. Actually, do this soon after you launched the auxiliary starship. The earlier you do it, the slower it needs to be to arrive at Alpha Centauri after the exploration mission is complete. Since the exploration mission might last a decade or two, this one might only need to be moving at .1c to .15c.

    8) The second aux starship releases its bomblets to spread over a linear formation for the return runway.
    9) When this runway arrives in Alpha Centauri, the main starship uses it to accelerate back towards Earth.
    10) Lay a final braking runway, but this one doesn’t need to be bomblets. It’s okay for it to just be inert pellets because the kinetic energy of impact will be good enough for braking.
    11) When the main starship arrives, it uses a minimal amount of on board sacrificial impactor mass to vaporize the inert pellets for braking.

    • Andrew Palfreyman June 21, 2017, 3:09

      I love the concept, Isaac. And of course there are umpteen knobs to twiddle to extract optimisations for various success metrics.

      I did a similar (and much less realistic) thing when I extended the black hole starship paradigm of Crane & Westmoreland to produce staggeringly more efficient propulsion.

    • Michael June 21, 2017, 3:18

      I looked at this years ago and found that the magnetic pellets scale badly with the rail launcher becoming huge. I found with the laser fuel sail concept the design was much more favourable provide the guidance system was accurate enough.

      I find that the laser fuel sail system allows us not only to send a spacecraft out, stop it but also allows a return via another stream of these sails and like wise stopping it.

      The concept of ‘Starshot’ is very wide ranging and should be a national/international priority.

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