The Nuclear Rocket Option

by Paul Gilster on January 14, 2013

Tim Folger and Les Johnson (NASA MSFC) stood last summer in front of a nuclear rocket at Marshall Space Flight Center in Huntsville, Alabama. Johnson’s work in advanced propulsion concepts is well known to Centauri Dreams readers, but what he was talking to Folger about in an article for National Geographic was an older technology. NERVA, once conceived as part of the propulsion package that would send astronauts to Mars, had in its day the mantle of the next logical step beyond chemical propulsion. A snip from the story:

nerva

Johnson looks wistfully at the 40,000-pound engine in front of us… “If we’re going to send people to Mars, this should be considered again,” Johnson says. “You would only need half the propellant of a conventional rocket.” NASA is now designing a conventional rocket to replace the Saturn V, which was retired in 1973, not long after the last manned moon landing. It hasn’t decided where the new rocket will go. The NERVA project ended in 1973 too, without a flight test. Since then, during the space shuttle era, humans haven’t ventured more than 400 miles from Earth.

I’m looking forward to getting back to Huntsville and seeing Les, as well as a number of other friends in the interstellar community, at the 2nd Tennessee Valley Interstellar Workshop, coming up this February, where it may be that NERVA will have a place in the discussion of how we go about building a system-spanning civilization. You’ll want to give Folger’s article a look for comments not only from Les but Freeman Dyson and Andreas Tziolas (from the Icarus team), as well as Elon Musk, the 100 Year Starship’s Mae Jemison, and NASA’s Mason Peck.

Image: NERVA nuclear rocket being tested. (Smithsonian Institution Photo No. 75-13750).

In fact, there are a number of issues presented here that I’ll want to get back to later, but I can’t cover the rest of the story today. I’m all but out the door for a brief but intense period of Tau Zero work that will leave me no time to keep up regular posts here or even to moderate comments. More about this later, and more about Folger’s essay as well, and please bear with me through the temporary slowdown. Things should get back to normal by mid-day Thursday.

Speaking of NERVA, though, I’ll leave you with an interesting petition Gregory Benford alerted me to with regard to the development of nuclear thermal rockets, one that calls for an effort to:

Harness the full intellectual and industrial strength of our universities, national laboratories and private enterprise to rapidly develop and deploy a nuclear thermal rocket (NTR) adaptable to both manned and un-manned space missions. A NTR (which would only operate in outer space) will jump-start our manned space exploration program by reducing inner solar system flight times from months to weeks. This is not new technology; NTRs were tested in the 1960s (President Kennedy was a guest at one test). The physics and engineering are sound. In addition to inspiring young Americans to careers in science, technology, engineering and mathematics, a working NTR will herald a speedy and economical expansion of the human presence in the cosmos.

Going significantly beyond the Moon demands advances in propulsion of the kind that nuclear thermal rockets can deliver. Getting NERVA concepts out of mothballs and updating them with modern materials are necessary steps as we push out into the Solar System.

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Eniac January 24, 2013 at 22:38

You might want to consider those thousands of nuclear weapons ready for use on this very planet.

Ready to incinerate innocent civilians. Not fly spaceships. Not even a little bit.

Eniac January 25, 2013 at 0:53

Freeman Dyson:

You’re using only less than one percent of the mass with any kind of nuclear reaction whether its fission or fusion. So it means the velocities you get with nuclear energy are limited to much less than a tenth of lightspeed.

I am not sure this adds up. According to Dyson himself, the theoretical maximum exhaust velocity achievable with fusion is 30,000 km/s [1]. With a reasonably high mass ratio you can achieve 3-5 times that as burnout velocity, or total delta-v. That makes 0.3-0.5 c, according to my calculation. Much more than “much less than a tenth”. What gives?

Of course, bombs are not going to get us there. Some type of fission fragment rocket may get us into the general vicinity, though, one might hope.

[1] http://galileo.phys.virginia.edu/classes/109.jvn.spring00/nuc_rocket/Dyson.pdf

Alex Tolley January 25, 2013 at 11:40

@Eniac – I think you have you decimal places wrong.

c = 300,000 km/s
fusion exhaust = 30,000 km/s, or 1/10 c.

GaryChurch January 25, 2013 at 14:38

“Of course, bombs are not going to get us there. Some type of fission fragment rocket may get us into the general vicinity,”

Freeman Dyson is not at liberty to discuss the classified aspects of his work and these articles are interesting but dated. A half century later supercomputers are now being used to study pulse propulsion.

Bombs might have to “get us there” but it would be an incredibly long trip and would require cryopreserving the crew for the centuries necessary to reach even nearby stars.

I do not think fission fragment has the power to do it either- I agree with
Dyson. I consider Beam Propulsion to be the way to get out of town at an acceptable percentage of the speed of light and bombs to slow down after centuries of travel. Megaton range pulses can slow down a big ship- fission fragment is not in this league of energy release.

10 percent is actually completely acceptable for star travel if you can freeze the crew. I have read that energy requirements go up in a steep curve after 30 percent and considering the energy required just to hit 10 percent it may be that we will going to the stars at the rate of 4 years for every light year traveled for a century or two or three.

What people should understand is that launching slow ships is a necessary step. Insurance against an extinction level event is the whole idea. Much faster ships, probably using black hole propulsion, can always intercept these slow boats centuries down the road and transfer the passengers and crew.

And thanks for the PDF- never seen this article before.

Eniac January 26, 2013 at 14:26

Alex Tolley:

@Eniac – I think you have you decimal places wrong.

c = 300,000 km/s
fusion exhaust = 30,000 km/s, or 1/10 c.

That would be 0.1c, and 3-5 times that would be 0.3-0.5c. That is not wrong, is it?

Note that the Saturn V, or the Space Shuttle, are able to accelerate payloads to that 3-5 times multiple of their exhaust velocities.

Eniac January 26, 2013 at 14:51

I agree with Dyson. I consider Beam Propulsion to be the way to get out of town at an acceptable percentage of the speed of light and bombs to slow down after centuries of travel.

Unfortunately, there are a great number of problems with beam propulsion that need to be solved, some of which look like very hard nuts to crack.

Perhaps the most serious is that any receiver of a beam must be very large and tenuous. A sail, essentially. Such a structure is impossible to protect from the oncoming interstellar medium(ISM). At high speeds, the ISM turns into radiation (compared to which cosmic rays are a joke), and is bound to be corrosive. There is a minimum material thickness below which a structure cannot remain intact at a given velocity for a given travel time. There is also a maximum material thickness dictated by the beam power density and desired acceleration. If the former is much larger than the latter, the whole thing is a no-go. I do not think there is much quantitative data on this, but from what I gather there is not much hope.

GaryChurch January 27, 2013 at 14:01

“Unfortunately, there are a great number of problems with beam propulsion that need to be solved, some of which look like very hard nuts to crack.”"-any receiver of a beam must be very large and tenuous. A sail, essentially.”
“I do not think there is much quantitative data on this, but from what I gather there is not much hope.”

We can generate and project several times the amount of energy the entire Earth uses by way of a Lunar Solar Power infrastructure. Not much hope? Puh-leez.

Alex Tolley January 27, 2013 at 15:45

@Eniac – my apologies. For some reason I read your post as 0.003-0.005 c – I must have been very tired or I need new specs.

You are correct about being able to multiple exhaust velocity a several fold. However I think your Dyson quote explains the issues. Firstly not all the mass can be converted to material at that speed. So your M1/M0 is not the same as with a chemical rocket. Secondly, unless you can fully channel all the exhaust away from you, much of it will be useless for propulsion. The Orion pusher plate design being a particular problem in this regard, although the Daedalus fusion engine design would be much more efficient in this regard.

Eniac January 27, 2013 at 19:07

Alex Tolley:

Firstly not all the mass can be converted to material at that speed. So your M1/M0 is not the same as with a chemical rocket.

I am afraid that does not explain it either. A chemical rocket converts even less mass to energy. How much mass is converted to energy determines the exhaust velocity, and has already been baked into the 30,000 km/s number. If all mass were being converted, the exhaust velocity would be c. This is known as the photon rocket and requires either antimatter or a black hole, neither of which are easy to come by.

The M1/M0 is simply a measure of how much fuel you are packing per payload, and determines how far above the exhaust velocity you can go, in the dreaded exponential relationship of the rocket equation. It works the same for chemical, nuclear, or even photon rockets. More than about 5 times is unrealistic.

Secondly, unless you can fully channel all the exhaust away from you, much of it will be useless for propulsion. The Orion pusher plate design being a particular problem in this regard, although the Daedalus fusion engine design would be much more efficient in this regard.

Right. The ideal fission/fusion rocket would use a magnetic field to guide the fission/fusion fragments (what’s left after each reaction) out straight back without slowing them down in any way. A pusher plate is very much less than ideal in this respect. It is not clear that we can do better, but magnetic fields can do this in principle, and it usually does not pay to bet against the ingenuity of engineers…

Of course, a nuclear fireball in itself slows down (i.e. “thermalizes”) the reaction products, even before they hit a pusher plate or magnetic nozzle. The ideal fission/fusion fragment rocket would have to employ a more controlled type of nuclear reaction. Not a bomb, that is.

GaryChurch January 28, 2013 at 15:03

“The ideal fission/fusion fragment rocket would have to employ a more controlled type of nuclear reaction. Not a bomb, that is.”

The “enginuity of engineers” has not made fusion reactors a reality for the half a century that has been going on; you cannot contain nulcear reactions which makes the bomb the ideal solution. No containment problems.

We have to think outside the box like Stan Ulam or we are going nowhere.

Peter January 29, 2013 at 16:10

The nearer-term cost-benefit ratio favors LEO applications over interplanetary…That’s where the TRL might be lower…Does anyone else see the potential for using NTRs for single stage to orbit?

Eniac January 30, 2013 at 23:53

… you cannot contain nulcear reactions …

50 years of nuclear power prove you wrong on this one. And fusion research still may do so on the other one. Some problems are harder than others.

GaryChurch January 31, 2013 at 14:10

“50 years of nuclear power prove you wrong on this one”

A half century of boiling water is not propelling a spaceship. A nuclear reaction system to efficiently propel a spaceship is just a little “harder” than that.

I do not think I am wrong.

ljk February 11, 2013 at 11:56

Solar System Exploration: The Missing Drive

by admin of Icarus Interstellar

This guest blog is a contribution from Glen Thornton. Glenn is recently retired from a career at Los Alamos National Laboratory. He worked on underground nuclear testing at the Nevada Test Site for four years and then joined the Lab’s satellite design group. He made important contributions to several satellite projects, some NASA funded projects among them. He designed the data acquisition systems for the Lab’s neutron spectrometers that flew on Lunar Prospector and a similar instrument for Mars Odyssey.

Lunar Prospector was the first orbiting lunar probe to use remote sensing (neutron spectrometer) to definitively identify water ice deposits in the permanently shadowed craters at the lunar poles. Mars Odyssey is still orbiting and returning data from Mars. It has produced detailed global maps of subsurface ice deposits on Mars. The neutron spectrometer can see about a meter deep and identify the percentage of water ice within a “footprint” that the instrument sees as it travels over a region. Glenn currently lives with his wife Pam and their mischievous cat, Tilly, in Santa Fe, New Mexico.

Ful article here:

http://www.icarusinterstellar.org/solar-system-exploration-the-missing-drive/

To quote:

I’m concerned about the present time, because our only path to the future is through the present. Each day, it seems, Kepler reveals a new, more Earth-like planet than the day before. As Kepler’s search compiles more data, the statistics favor ever greater numbers of possible new “Earths” and point to the probability that at least one will be in our neighborhood. Current predictions, fueled by data from Kepler, indicate that an Earth-like planet may well be found within a dozen light years of our home world. Still, a light year is long haul, much less a dozen of them.

Some form of nuclear power will be an absolute necessity to future interstellar travelers. They’ll need a formidable propulsion system and the power to maintain themselves through what we hope will be a “relatively” short trip, from their perspective. The path to an advanced nuclear propulsion system needs a beginning. I think the fission-based NTR is that beginning.

Like the chemical rocket that preceded it, the NTR is no panacea; it simply has twice the efficiency of its predecessor with very good T/W potential. Future development and use will undoubtedly widen that gap. We can stick with the chemical rocket and continue to explore LEO, while we wait for fusion power plants that fit in a briefcase, or we can pick up where Apollo left off and start exploring the solar system now. The NTR, is the next rung on the ladder. We should step up and use it.

GaryChurch February 12, 2013 at 14:33

“The NTR, is the next rung on the ladder. We should step up and use it.”

Many seem to be stuck on this idea that NTR is the easiest (cheapest) solution so we should pursue it as the only one.

There is no cheap. Pay now or pay more later. Bomb systems have an ISP in the tens and if large enough hundreds of thousands. The larger you make a bomb system the more efficient it gets. You make the pusher plate or megachute or whatever you are using bigger and you add a teaspoon more of hydrogen to the bomb and a few pounds more to the slug you are using as plasma stock- and you get more bang for your buck.

In addition, the bomb “pits” are the main engine; it explodes and that is it. A nuclear thermal engine has a far more complex operating system. KISS

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