Nuclear Propulsion Essential For Interplanetary Space Travel

I believe Wernher von Braun was completely right when he suggested, in the 1960s, that a manned Mars landing would require nuclear propulsion. (Incidentally, Sergei Korolev, von Braun’s Soviet counterpart, reached the same conclusion). Had the United States Congress implemented von Braun’s suggestions, we would have astronauts on Mars by now — or at least we would have the technical ability for it. Unmanned Mars landers and spacecraft sent to explore the outer planets would also carry much larger scientific payloads.

Unfortunately, a confluence of several factors ended the nuclear space age before it even began. Although there were projects to develop small nuclear reactors, the main impetus didn’t come from NASA, but from the military. The US Navy wanted scaled down power reactors to drive its latest generation of submarines and aircraft carriers. For a while, the US Air Force also studied nuclear propulsion to power long range bombers. Not surprisingly, this idea was abandoned due to safety concerns and after it was realized that ICBMs, not long range bombers, would be the main thrust of nuclear deterrence.

A strong military involvement, and the secretive culture of the Atomic Energy Commission as well as the US nuclear weapons labs, meant that much of the research into small nuclear reactors was highly compartmentalized and classified. Especially during the Cold War, nearly everything containing the word “nuclear” also had a “national security” tag attached.

The secrecy also meant that many of America’s leading scientists and engineers had no access to the most important research papers in the field. This led to a lack of scientific discourse among space experts, few of whom would be pushing for a technology about which they knew no technical details. It also meant that very few political leaders fully understood the potential and necessity of nuclear propulsion in space. In addition, there was the question of which reactor technology to advance. As it happened, the US Navy prevailed in its pursuit of a reactor type suited for marine applications, but ill suited for space travel, where no external cooling medium is available.

Still, as director of Marshal Space Flight Center, von Braun’s word carried weight. Linking together NASA, the Atomic Energy Commission (AEC) and Los Alamos National Laboratory (LANL), the newly created Space Nuclear Propulsion Office took over the previously launched (and classified) AEC directed Project Rover.

“Project Rover could be divided into three phases: Kiwi, between 1955 and 1964, Phoebus, taking place between 1964 and 1969, and Pewee, taking place between 1969 and the project’s cancellation along with the cancellation of the NERVA rocket at the end of 1972,” writes James Dewar in “To The End Of The Solar System: The Story Of The Nuclear Rocket” (University Press of Kentucky, 2003; Apogee Books, 2008). “Kiwi and Phoebus were large reactors; Pewee 1 and Pewee 2 were much smaller and they conformed to the smaller budget available after 1968. Both Kiwi and Phoebus became part of the NERVA program.”

Many of the papers and background stories relating to all aspects of these programs remain classified and unpublished to this day. At least in part, this could be attributed to self-protective motives of the various entities involved. There is little question that many errors were made in the course of these programs, and that some of them led to serious incidents, mishaps, wasteful spending and questionable decisions.

For instance, in a highly classified test in 1965 at Jackass Flats (an area within the Nevada Test Site), a reactor core was allowed to overheat and explode. The event was recorded by high-speed cameras.

When the reactor blew itself to smithereens, bits and pieces of highly radioactive debris were ejected over 2,600 feet into the sky. Aircraft flew through the debris cloud to take samples. The cloud drifted east at first, then west toward Los Angeles. To this day, the full set of radiation measurement data remains secret.

One minor problem: the test was a violation of the 1963 Limited Test Ban Treaty with the Soviet Union. (But one could of course argue that it was merely an “accident”). What goes up, must come down, and radioactive debris rained back down over a wide area.

Five months later, in June of 1965, a nuclear rocket engine code-named “Phoebus” suddenly overheated and exploded. Large chunks of radioactive fuel were ejected, and what was left of the reactor fused into a highly radioactive, hot pile.

Of course, such incidents provided a welcome opportunity to conduct “decontamination exercises”. From whatever can be learned out of declassified documents, the “cleanup process” took 400 people and two months. Almost 50 years later, the entire area around Jackass Flats is strictly off limits to the public.

Were the radioactive fireworks shows necessary? Perhaps, given the circumstances, the knowledge base and needs of the time. But this certainly did nothing to advance the concept of nuclear propulsion for spacecraft. And it did not exactly alleviate the fears of taxpayers that such programs tend to be out of control, over budget, irresponsible, lacking oversight, and of questionable value to the public. As a result, politicians generally find it difficult to justify and back such programs before the public, which is why they often can only exist in secret. In turn, this creates a hopeless feedback loop.

Despite its problems, NERVA was considered widely successful in achieving its technical objectives. But in the end, it was killed when the funding spigot closed off the flow of cash.

In a first blow, the incoming Nixon administration slashed NASA’s budget dramatically. The remaining Apollo moon landings were canceled, even though the hardware for them had already been built and all personnel and infrastructure were in place. Worse: the new administration and others in NASA’s leadership maneuvered von Braun into a rather innocuous desk job. In plain terms, he was pushed aside by internal power struggles. Overruling his objections, NASA decided to cancel the entire Saturn rocket program — a decision I consider to be the worst blunder in the entire history of  human spaceflight.

Now lacking a large launch vehicle to lift large payloads to Earth orbit, there was no more way to the Moon or beyond, and there was no way to launch a nuclear-propelled spacecraft to Mars or elsewhere.

Meanwhile, the Soviet Union (and later, Russia) pursued a more steady course with its TOPAZ and ENISY reactors — although the objectives were different from NERVA. The most important objective behind these Soviet programs was the generation of electricity, not thrust. The US had, and probably still has, similar secret programs to develop small reactors for powering spacecraft.

What is publicly known is that the Soviets flew some of their systems in space on a large scale — with rather mixed results. There is less public information about US reactors flown in space. On April 3, 1965, SNAP-10A was launched from Vandenberg AFB in California into an unusual, retrograde orbit. After 43 days, a voltage regulator aboard the satellite failed, and the reactor was shut down. The satellite was left in a 1300 km high orbit. It is of some concern, because it has shed debris after some event — possibly a collision or meteorite hit — in 1979.

As part of Upravlyaemy Sputnik Aktivnyj (Russian: Управляемый Спутник Активный), or US-A, the Soviets launched 33 reactors aboard Earth-observing radar satellites in low Earth orbit. In addition, the Soviets are known to have launched at least two larger (6 kW) TOPAZ reactors aboard their Kosmos 1818 and Kosmos 1867 satellites.

Russian satellites were generally designed to either eject their radioactive components, or boost the entire satellite, into a “safe” storage orbit further away from Earth. To make a long story short: the reliability record of this technique has been abysmal. In quite a few cases, radioactive debris was either lost in orbit or re-entered the atmosphere. In some instances, radioactive debris fell back to Earth.

This brings me to a question often asked: isn’t it generally extremely dangerous to strap a nuclear reactor on a gigantic, possibly malfunctioning rocket? The answer is a little complex and could fill a book.

In simple terms, here is what I believe: when used for interplanetary travel (as opposed to use in near Earth orbit), the risks are quite manageable. As Wernher von Braun suggested, a reactor for spacecraft propulsion would be sent to orbit “cold”, on a conventional rocket. In this first stage of the trip, the reactor fuel would be tucked away in a strong safety container. Only when the spacecraft is at a safe distance from Earth would the reactor be made operational and start up for the first time. It is only then when the most dangerous, radioactive results of the fission reaction are produced.

After its initiation, the reactor core would remain in space and never return to Earth. However, by sending it on a return orbit around the Sun, it would even be possible to reuse it for multiple missions, for example to “cycle” spacecraft from Mars to Earth and back. (Buzz Aldrin has been fervently advocating one such concept, the “Aldrin Cycler”).

The arithmetics for a manned mission to Mars have not changed since von Braun’s time. As he pointed out, relying on chemical fuel for the interplanetary journey soon becomes a case of diminishing returns. The reason is that it takes a huge amount of rocket fuel to get the payload on course to Mars in the first place. The longer the journey takes, the heavier the load of supplies to be carried for the astronauts. And on the return trip, more fuel is needed, the weight of which has to be figured into the total mass of the outward bound mass.

By means of its very high power-to-weight ratio, a nuclear reactor can reduce the mass to be transported to and from Mars by a significant amount. And it might be possible to speed up the trip, so that fewer supplies and less shielding against cosmic radiation are necessary.

So where do we stand?

Electricity generating, small nuclear reactors like the Russian TOPAZ could be used as part of a propulsion system, possibly employing some type of ion engine. Perhaps these could even be scaled up to a certain degree. But since ion engines provide only low (yet steady) thrust, they are best suited for unmanned spacecraft in which travel time is not as important a factor.

Los Alamos National Laboratory has recently introduced a clever, innovative nuclear power generator for spacecraft:

An interesting idea, but again: even scaled up, this is still a low-power concept.

For serious human travel to Mars and beyond, a much more powerful reactor and propulsion system would be needed — something similar to NERVA. And this is just what von Braun already knew 50 years ago. Trying to make plans to land humans on Mars without an available nuclear propulsion system amounts to putting the cart before the horse.

If you have questions or information about this subject, or if you would like me to write about it for a commercial publication, please e-mail me.


AEC paper on NERVA:
Mars Cycler:
LANL Small Nuclear Reactor:

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