Nuclear Rockets

Once nuclear energy became commonplace in the 1950s, fairly straightforward engineering calculations could be made on the design of a nuclear rocket. These showed that designs using conventional materials might achieve high thrust levels along with high I_sp. Values in the range of 750-950 seconds were typical — roughly three times as high as the best I_sp from chemical rockets.

By the time NASA established its Space Nuclear Propulsion Office (SNPO) in 1961, several government laboratories had spent years on the concept and contractors were doing paper studies. SNPO was a joint effort with the Atomic Energy Commission or AEC 1 — a similar organization to the Navy-AEC effort that produced Nautilus and subsequent atomic submarines.

The SNPO program was known as "NERVA" — Nuclear Engines for Rocket Vehicle Applications. NERVA's first contract went to Aerojet General for engine systems, with subcontractor Westinghouse Astronuclear Division providing the nuclear reactor. Astronuclear's reactors were built in Large, Pennsylvania and tested at Jackass Flats, Nevada. As many as twenty-three 2 tests were conducted, achieving core power densities of 2kW per cm³ and coolant outlet temperatures of 2,500° C at pressures of 560 psia. Steady progress was made in engine efficiency and controllability, and in lowering the release of radioactivity. 3 The XE-prime, the last engine in the series, was tested under simulated space conditions. 4 It operated for a total of three hours 48 minutes (11 minutes at full power), starting and stopping 28 times.

XE-prime was tested in September 1969. (Here is a table of NERVA tests, unfortunately rather sketchy.) The NERVA program was cancelled in 1971, mostly because NASA had no missions that required nuclear rockets. Apollo might have used them, but chemical rockets were ready sooner; and as everyone knows Apollo was a dead end as far as boldly going where no one has been before is concerned. In fact, Apollo itself was winding down at that time, though the fact was not made public then.

Politics also played a part in the demise of NERVA. The 1963 Nuclear Test Ban Treaty — generally a good thing — was interpreted to apply to nuclear rockets as well as nuclear bombs. Thus, NERVA engines required exhaust scrubbers and other measures to prevent contamination of the environment, adding significant time and cost to the program.

For the three decades after that demise, all things nuclear struggled under the onus of distrust 5 in the minds of most American citizens. Many nuclear reactors were cancelled, despite the promise of the technology. In such a political climate, even theoretical studies of nuclear rockets were proscribed.

But things may be changing. NASA's new administrator, Sean O'Keefe, has talked up nuclear propulsion for about a year. He requested $125M to start the effort in NASA's 2003 budget. Now, Project Prometheus has been announced, and appears to have the blessing of the Bush White House. The objective of Project Prometheus is to speed up travel around the solar system. O'Keefe has compared today's space missions to travelling in covered wagons. He cites as a benchmark the one-way travel time of a Mars mission, which nuclear engines could cut from six months to two. Note that this does not imply a Mars mission is planned; but it is certainly true that such engines would drastically cut travel times. It is also true that — if operated beyond Earth orbit — they would pose no significant hazard to Earth. Prometheus is a new project and rumors about it are flying; we will just have to wait and see how things shake out.

Here, thanks to Christopher M. Jones, is a brief review of the various types of nuclear (fission) rockets.

Solid Core Nuclear Thermal Rocket

The solid-core NTR is the simplest type (simplest to build, compared to the others, but definitely not simple in any absolute sense.) The NERVA engines were of this type. Basically, it just replaces the chemical reaction which heats up the propellant with a regular fission reactor. Some working fluid (NERVA used hydrogen) is pumped through it. The fluid heats up to near the temperature of the core and then is expelled as rocket exhaust. The advantages of this are that it's simple and straightforward, produces a lot of thrust and has decently high I_sp. The disadvantage is that to maximize I_sp you need to maximize the temperature of solid materials, which becomes a very difficult problem very rapidly. Maximum I_sp for this design is about 850s without using really exotic designs or materials, up to maybe 1,000s for bleeding edge designs.

Nuclear-powered Electric Rockets

Electric rockets (such as the ion engine) also have high I_sp values. However, they also have low thrust — typically because to throw large quantities of reaction mass out of an electric rocket, you need immense amounts of electric power. As of today, we have just one compact, long-lasting source of such amounts of power, megawatts and above: the nuclear fission reactor. So the idea is to couple a fission reactor to an ion engine (or another type of electric rocket such as a Hall-effect thruster or a VASIMR). This gives you very high I_sp without putting a lot of stress (meaning high temperatures) on the reactor design. The disadvantage is that nearly all conceivable electric rockets are very low thrust designs, even with nuclear power. This means they cannot launch payloads from Earth; they are only suitable for use in space and they may not provide much benefit in terms of lower travel times for many conceivable space missions. Electric rockets have widely varying I_sp values,from 2,000 to 3,000 seconds for off-the-shelf systems up to maybe 10,000 seconds or arbitrarily higher for future designs.

Gas Core Nuclear Thermal Rocket

Farther from realization are gas core nuclear thermal rockets. In these, the core of the NTR (which must be the hottest component of any NTR) is gaseous and confined within the reactor vessel by clever means (e.g. magnetic fields). The heated propellant is expelled, and the plumbing can be actively cooled with these designs so you can achieve very high I_sp while still maintaining the very high thrust (and thrust to weight ratio) of solid core NTRs. The disadvantages of this design are that it is fairly complex and calls for technical advances. It also emits fairly large amounts of radioactivity, which makes it unsuitable for use near Earth. I_sp ranges are difficult to peg accurately for gas core NTRs but the best guess would be about 1,500 seconds as a lower limit up to about 3,000 seconds with probably a fairly substantial amount of headroom beyond that.

Nuclear Pulse Rocket

This type is popularly known as Orion. It uses nuclear bombs to provide thrust via a large pusher plate, or via a leading sail (Medusa). The advantage of the Orion design is that it provides truly enormous thrust at high I_sp. The disadvantages are that it doesn't scale down very well, requires the use of thousands of nuclear bombs, spews radioactive fallout as "exhaust" (making it unsuitable for use from Earth's surface), and involves quite a bit of tricky engineering. Designs vary, but achievable I_sp values for Orion drives range upward from about 10,000 seconds.

Much theoretical work on Orion was done during the 1950s and early 1960s by former Manhattan Project scientists including Freeman Dyson. (See the book Project Orion by his son George Dyson.) A small model was built and flown in a famous test using chemical explosives. Details are available on the Web. Enough is known about the engineering questions that an Orion-type vehicle appears feasible. But of course it would be "dirty", hence unsuitable as a launch vehicle — except in the hypothetical case of needing to reach a collision-course comet or asteroid in a big hurry.

Nuclear Salt Water Thermal Rocket

The nuclear salt water rocket creates a continuous fission chain reaction in a nuclear salt water solution within the rocket engine. The fission chain reaction is kept from spreading back into the fuel lines by the flow of the nuclear salt solution; the idea (due to Robert Zubrin) is to maintain the flow at a rate which pushes the fission zone just back of the nozzle, keeping the engine temperatures within tolerable limits. The advantage of this design is that it can produce truly astounding thrust levels as well as incredibly high I_sp values. The disadvantages are that it produces extraordinarily radioactive exhaust and would be very technologically challenging to make work. I_sp values for NSWRs (assuming they can be made to work) range from about 5,000 seconds at the low end up to >100,000 seconds to ~1,000,000 seconds at the ultra high (and very speculative) end.

Nuclear Fission-fragment Rocket

Finally there is the totally speculative fission fragment rocket design, which uses the fisson fragments from a highly fissionable isotope (such as Am-242) and directs them to create thrust (e.g. using magnetic fields). The advantage of this design is that it can produce very high I_sp values. The disadvantages are that it is very complex, requires enormously expensive6 fuels, and produces fairly low thrust. The I_sp achievable with such a rocket is on the order of 1,000,000 seconds.

1 The AEC grew too friendly with the industry it was supposed to regulate. It was reorganized to preclude the possibility of such cozy relationships, in token of which it was renamed the Nuclear Regulatory Commission.

2 Sources I found differ. One says 12 tests, one says 23.

3 In designing a solid-core nuclear thermal reactor, you avoid radioactive release at all costs — not because of concern for the environment (though this is certainly a valid concern), but because if it's happening, your engine is eroding away down the tail pipes and the mission using it might not get home.

4 It's not clear if this was done in a vacuum chamber.

5 Sad to say, this distrust was warranted. You only have to read (or remember) the nuclear power industry's record of poor designs, shoddy construction (sometimes concealed by faking the x-ray photos), and unsafe operating practices to see why.

6 Mr. Jones knows whereof he speaks. Am-242, Americium, is an artificial element. Making it in sufficient quantity for a space mission would take years, and probably require several times the U.S. GDP for funding. You might as well wish for antimatter.

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