Robert Bussard on Fusion Drives, NERVA & the Bussard Ramjet
By: Tim Ventura
Tags: Space, Physics, Futurism, Nuclear, Technology

Robert, let’s start out with your research into nuclear-thermal propulsion. Back in the 60's, you played a pivotal role in the NERVA program. Can you tell me about that experience and what lessons you learned from it?

Oh heavens, I invented the program, really. I wrote a paper in 1953 on nuclear rocket propulsion back when I was working at Oak Ridge National Laboratory.

I based some of it on work done earlier by Howard Seifert, Arthur Rurik, George Gamow, Nick Smith at Johns Hopkins, along with several people at North American in the late forties, as well as a very fine paper by Hsue-Shen Tsien, the Chinese physicist who later built the Chinese ICBM in a book on nuclear energy that came out of MIT.

Dr. Robert W. Bussard

So I put a good definitive paper together, which had a lot of nuclear calculations in it due to Carroll Mills work at Oak Ridge and could show by systems analysis that nuclear rockets would outperform chemical rockets for heavy payload missions over long ranges and interplanetary missions.

The paper I wrote attracted the attention of the Air Force and it’s science advisory board, which eventually led to the start of a national nuclear rocket program in spring of 1955, both at Livermore and Los Alamos — so I left Oak Ridge and joined Los Alamos.

We did in fact build reactors, I think there were 17 of them in total, and we ended up with a quarter million pound thrust engine that could be run 40 times on and off. We had a perfectly brilliant program that was technologically very successful.

We were running hydrogen through graphite cores at very high temperatures, all carefully controlled, which would give specific impulses 2.5 or three times higher than anything you could get from general chemical propulsion. The original idea was to use it to carry well, you know, the weapons on an ICBM, as the second stage rocket. That goal went away after a while because the bombs got smaller and the rockets got better.

NERVA XE Engine in ETS-1

NERVA ended up being a program that was controlled by NASA aimed at a space flight — and we even had a target. It was the 1978 opposition of Mars. They called it “Manned Mars in ’78”, but the program fell flat because of internecine political wars between the chemical rocket community, the nuclear rocket community, and the budget offices for Livermore & Los Alamos.

The long story about that is recounted in a book called To the End of the Solar System, by Jim Dewar. It’s the best book ever written on the history of that program. The program was very successful, but of course along came the Vietnam War and that basically killed the program. Nonetheless, over 50 years ago we had nuclear rockets that could take people to Mars.

Well, if NASA wanted to deploy NERVA now, how much of the ‘60s experience and technology would still be applicable?

I think it would all be applicable — all you’d have to do is resuscitate it. Most of the people who worked on it are either retired or dead, but the technology isn’t hidden. Everything is documented and could be easily brought back to life if you spent a little time and money on it. I don’t see that happening right now, however, because NASA’s budget is too tight as it is.

NERVA rocket engine cutaway (wikipedia)

A lot of progress has happened since the ’60s. Do you think modern technology would make such a space propulsion system even more practical than it was at the time?

I think so, but frankly you don’t need to go very far beyond what already existed, it’s just a matter of rebuilding the program we used to have. There’s a lot of new ideas around, but they haven’t been proven in reactor tests. Start by building what you already know how to build, then fly them, and improve it from there.

In any case, we already know the directions for improvement and I think we already know how far it can be improved with modern materials. For instance, we could increase the specific impulse from 860 seconds Isp up to 1,200, and performance would get vastly better.

We can open the solar system to human transport if we revive that program, but that takes time and money and NASA doesn’t have the time or money.

How much time and money do you think it would require for a program like that?

I think it would take about half the NASA budget for about 10 years.

So you’re talking about more than $8 billion a year?

Yeah, I think so. Then you’d have a real fleet of nuclear rocket powered spacecraft that could carry people anywhere you want in the solar system. Well, at least anywhere in the inner solar system —out to Mars anyway, and probably the moons of Jupiter.

Artists conception of a Bussard Ramjet for interstellar propulsion

In terms of another space subject, Jerry Pournelle and several other science fiction writers have adopted the Bussard Ramjet as a means of traveling between solar systems. I’m wondering if you could tell us about the origin of this concept and how Pournelle and the science fiction crowd picked it up.

Oh, well that’s a different story entirely. I don’t know, it occurred to me one night at dinner, actually back in 1960. The idea of traveling in between the stars, interstellar travel — people had looked at that for years, with chemical rockets, antimatter rockets, and all sorts of ideas.

If you’re going to try interstellar travel with rockets, though, you have a real problem because you have to carry all your reactive propellant onboard — which means extra space to carry it and energy to accelerate it with you.

The papers were written on it by very brilliant people. You know, the Eugen Sänger, Val Cleaver, several people in England, and so on.

So it occurred to me, sitting there at dinner one night — why bother to carry fuel when the fuel is sitting out there in the spaces between the stars? It’s hydrogen. What do the stars run on? The fusion of hydrogen.

So all you need to do in principle — this is only in principle — is collect the hydrogen that’s between the stars as you fly, put it somehow magically into a fusion engine, and then use the fusion engine to propel the ship so you don’t have to carry your fuel. So you fly along, scooping up hydrogen and using it to power an interstellar Ramjet.

I did the mathematics for the physics constraints, and it turns out to be very interesting. It tells you what engineering conditions you have to achieve mass per unit, area of the scoop and so forth. In principle, from a physics point of view, it looked like I’d solved the problem. Of course, the engineering is virtually impossible today, but give us a hundred years of advanced engineering and it may be possible.

There was a key feature missing in my solution, however, and a fellow named Daniel Whitmire at the University of Louisiana solved that. The problem is that the reaction rate for hydrogen-hydrogen fusion is very, very small. You have to do certain strange nuclear things to make it go.

Daniel found a way to enhance that reaction by the order of 10¹⁶ and wrote a paper on it, which then made the idea very practical. So now have a working concept for an Interstellar Ramjet, and I published a paper in 1960 on it in a journal, called Acta Astronautica.

Apparently that paper caught the attention of Jerry Pournelle, Larry Niven and other people in the science fiction writing community, and I had no idea that had happened.

Ten years later, I got an invitation from Jerry to come talk at a Nebula Award dinner. I asked him why, and he said, “well, we’re all using your Ramjet.” So I went and gave the talk.

The Bussard Polywell fusor, using magnetic fields instead of charged screens.

Amazing! What about your current IEC fusion technology, the Polywell reactor? How difficult would it be to deploy that for space travel?

Well, that topic was the burden of eight papers that we wrote between 1993 and 2005. You see, we were under an embargo by the Navy not to write papers on our physics for 11 years. We weren’t allowed to publish or go to conferences.

During that time, however, they didn’t care if we wrote about papers about spaceflight. So we did, and speculated that if we had an engine like Polywell, then how could we use it for spaceflight? That was OK — they just didn’t want us to talk about the source. So I wrote eight different papers on aspects of engine design for space propulsion and they were given at international conferences, AIAA Propulsion Conferences, and so on.

The bottom line is that with our approach, you build an engine system, that outperforms any other competitor by a factor of about a thousand — either a thousand times higher specific impulse at the same thrust to weight ratio, or vice-versa.

When you apply this engine system to missions in the solar system, everything changes. It gives you single-stage transit to Mars in three weeks with a single stage, 20% payload vehicle. You’ll also have transit around trips to the moon in 24 hours, as well as HTOL to Leo at a cost of $25 a kilogram, not $5,000 a kilogram. The whole paradigm becomes totally different.

If you can ever get these engines built, which would take another 10 or 15 years and another five to $7 billion, give it to Marshall space flight center and just let them run with it, you know?

What fuel would you use for space? There’s been a lot of talk about mining the moon for helium-3 and I’m wondering what your thoughts are on that concept.

I don’t think it’s either practical or necessary — and it’s also very difficult. There’s no question you’ll find helium-3 on the moon, and I know you can mine it at whatever large cost it may incur, but as I see it, it’s a completely unnecessary thing to do.

There’s no helium-3 on earth, but if you have a fusion reactor that can run on D-D (deuterium and deuterium), half of the reactions will produce helium three. Then you just take the helium-3 you have produced in the DD reactor, pipe it out of the vacuum system, feed it back into the machine and burn it — and you’re manufacturing your own fuel as you go. So why would go to the moon? If you have a D-D reactor, you have a helium-3 producing reactor.

Create it, pipe it back into the reactor, and burn it? That almost sounds like the turbocharger in automotive engine, using the exhaust products to boost your overall output.

Well, this is for a ground power plant really — you wouldn’t want to try and do this for a spacecraft. I mean why bother?

You’re going to do pB¹¹ for space flight — you don’t want any neutrons. But on a ground power plant, D-³He is fine. You can’t make a D-³He reactor neutron free , because as long as you have D you’re going to have D-D side reactions that are going to produce a neutron and the other half of the chain. You cannot avoid that.

So D-³He does not give you a pure, clean fusion system. It’ll reduce the level by factors of 10, 20 or even 40 but it will still generate enough neutrons that you‘ll need significant shielding.

In any case, the idea of using helium-3 to get more energy is perfectly fine, but you’re going to have a D-D plant that makes helium-3 anyway — so you don’t really need to go to the moon to do any of this, it can all start right here on Earth.