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The Sciences

We Shouldn’t be Scared of Nuclear Rockets in Space

Vintage SpaceBy Amy Shira TeitelMarch 29, 2019 12:00 AM

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A few weeks ago, a spending bill passed by Congress included $100 million earmarked for NASA to develop nuclear thermal rocket engines. In spite of the ever-present backlash to nuclear material, it’s not uncommon when it comes to space exploration. The Curiosity rover is just one of many NASA missions powered by nuclear material, in this case, a multi-mission radioisotope thermoelectric generator (MMRTG) that converts heat from decaying plutonium-238 into electricity. But that’s robotic and doesn’t impact humans, you say? The Apollo lunar landing missions also had nuclear generators on board; the ALSEPs used radioisotope thermoelectric generators to power some of the surface experiments.

NERVA engine NASA/AEC
A schematic of the NERVA engine. NASA/AEC

Of course, a nuclear power plant for an instrument isn’t the same as a nuclear engine, but the challenges of working with the dangerous material remain. And yet, this won’t be the first time the space agency has tried to harness nuclear power for space travel. A small number of programs tried to develop nuclear power for manned space missions in the 1960s, one of which was called NERVA.

This primer on nuclear engines isn’t detailed but the visuals might help! 

A Brief Background on Nuclear Power

America began exploring the power of nuclear reactions during the Second World War with the Manhattan Project. Scientists working on this top-secret government project harnessed the power of a controlled nuclear chain reaction and put it into a bomb, two of which were dropped on the Japanese cities of Nagasaki and Hiroshima to end the War in the Pacific Theatre. In post-war America, scientists and engineers needed something new to do with their understanding of powerful nuclear reactions. 

One of the first ideas was the nuclear airplane. The idea was that if a plane was powered by a nuclear core rather than fuel, it could stay aloft much longer, easily carrying bombs to anywhere in the world. In light of the budding war with the Soviet Union, many scientists thought the first country to develop the nuclear airplane would own the skies. Another similar early application for nuclear power was the nuclear submarine, this time dominating the oceans by staying underwater longer. (The nuclear submarine, we know, came to be.)

And so the US Army Air Force began investigating nuclear-powered planes in 1945. In 1951, the National Advisory Committee for Aeronautics (the NACA) began exploring its own nuclear program with the same goal in mind. The NACA gave control over its nuclear program to the Lewis Flight Propulsion Laboratory in Cleveland, Ohio (now the Glenn Centre) largely because of its proximity to the Plum Brook Ordnance Works. This former ordnance site that saw one billion pounds of explosives produced during the War had some of the infrastructure needed to test and operate a nuclear reactor. 

The NACA brought the Atomic Energy Commissions (AEC) on board in 1956 to approve construction of the test reactor, still with an eye on building a nuclear airplane. Five years later in 1961, the site had been decontaminated from its previous life, unnecessary buildings were gone, and a reactor was ready to go critical meaning it was ready to sustain a nuclear reaction. But the dream of the nuclear-powered airplane was dead. Rumours that the Soviets were close to a nuclear plane proved untrue, and President Kennedy lost interest because “the possibility of achieving a militarily useful aircraft in the foreseeable future is still very remote.” But he saw other potential uses for this technology.

Nuclear Propulsion and the Space Race

Among President Kennedy’s most often-cited speeches might be his May 1961 address to Congress. On that day, he challenged the nation to “achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to the Earth.” Less often cited is what came immediately after: “Secondly, an additional 23 million dollars, together with 7 million dollars already available, will accelerate development of the Rover nuclear rocket. This gives promise of some day providing a means for even more exciting and ambitious exploration of space, perhaps beyond the moon, perhaps to the very end of the solar system itself.”

It’s hardly surprising that mention of nuclear rockets came right after the pledge of the Moon landing. The space race was an incarnation of the Cold War, which was itself spurred on by the Soviet Union developing nuclear weapons. (Yes, I know that’s a gross oversimplification but we’re not doing a deep dive into Cold War roots in this blog!) So why are nuclear rockets interesting for spaceflight?

The rockets of the early space age — in 1961 these were the Redstone and Atlas rockets NASA was working with for its Mercury missions — were chemical rockets. Burning highly refined kerosene and liquid oxygen yielded a powerful reaction that creates enough energy to lift a rocket off the Earth and into space. Later rockets like the Saturn V that took astronauts to the Moon were also chemical rockets with upper stages using a mix of liquid hydrogen and liquid oxygen with the same effect. But for all their power at the moment of launch, chemical rockets have their limitations. Though they’re necessary for getting a rocket off the ground, chemical engines don’t produce all that much power. So if you want to go to, say, Mars with a chemical rocket, you need to take advantage of optimal planetary alignment. A better so-called “launch window” gives you a bit of a gravitational boost to get there faster, but the window comes around about once every two years. Another problem with chemical rockets is they can only carry so much fuel. At some point, the fuel becomes too heavy to get off the ground in the first place. Thus chemical rockets rarely have a sustained burn; typically, the engine burns to get the spacecraft on its trajectory, then it coasts until the engine fires again to make some adjustment.

Nuclear propulsion solves a lot of these problems. It’s a lighter and therefore more efficient alternative. Rather than burning tons upon tons of propellant, a nuclear reactor creates energy by splitting atoms and releasing kinetic energy in the process, giving these engines a higher specific impulse! Ok, let’s break that down a bit…

Nuclear Engines vs. Chemical Engines

With a rocket engine, exhaust velocity determines propulsion efficiency: the lighter the exhaust gas the higher the exhaust velocity, and higher exhaust velocity means higher thrust. If a nuclear engine’s exhaust is hydrogen, the lightest element, it’s thrust will be very high compared to the exhaust from a chemical rocket at the same temperature. The chemical rocket exhaust contains heavier elements and therefore has less thrust. 

Thrust is measured in specific impulse. More simply, specific impulse means the time in seconds that one pound of propellant generates one pound of thrust. The higher the seconds of specific impulse the more economic and efficient the rocket. Chemical rockets in the 1960s had a specific impulse anywhere between 300 to 450 seconds. A nuclear rocket engine using hydrogen has a specific impulse in the range of 800 to 900 seconds. That means a nuclear rocket with lightweight exhaust is twice as efficient as a chemical rocket. This could shorten the time between planets, simultaneously shortening the time a crew is exposed to radiation in space and their time spent in microgravity with their muscles atrophying. The trade-off is the crew’s proximity to the radioactive components of the nuclear power plant. Therein lies the challenge.

Nevertheless, scientists explored this potential in the earliest days of spaceflight. At the Los Alamos Scientific Laboratory, the output was Project Rover, a program that was eventually folded into NASA and saw three development phases: Kiwi (from 1955 to 1964), Phoebus (from 1964 to 1969), and Pewee (from 1969 to 1972 when the project was ended). In 1961, NASA and the AEC created a joint office called the Space Nuclear Propulsion Office. SNPO oversaw another nuclear propulsion program that would build off the lessons learned in Los Alamos. With Westinghouse Electric Corporation and Aerojet-General Corporation as the main contractors, the Nuclear Engine for Rocket Vehicle Application, or NERVA, was born.

NERVA

Working primarily at the Plum Brook site in Ohio, NERVA scientists and engineers worked to develop practical rocket engines that could survive the shock and vibration of a space launch. Between 1964 to 1969, they built and tested various NERVA reactors and rocket engines, which stood 20 feet from the top flange to the base of the nozzle. Here’s how it worked. (For simplicity, we’re going to imagine that the engine is upright, so the fuel tank is on the top and the exhaust nozzle is on the bottom.) 

The heart of the engine is the reactor, a cylindrical core consisting of graphite elements impregnated with a Uranium-235 fuel. Inside the core, fission of uranium atoms produces heat. Keeping that heat in place is a reflector made of beryllium, completely surrounding the core. Inside the reflector are cooling rods also made of beryllium, but one side is coated with boron. Because the fissioning uranium constantly emits neutrons that are reflecting back to the core, the heat is sustained so has to be managed. Turning the rods with the boron side in absorbs the neutrons and lowers the temperature, eventually stopping the reaction. Turning the rods with the beryllium side in sustains the fission reaction. 

To carry more fuel, hydrogen is stored supercooled in a liquid state at -420 degrees Fahrenheit above the engine. That hydrogen is pumped down to the engine nozzle by an external line, though internal passaged back up to the nozzle, and into the reactor where it passes through small channels in the reflector. This serves the dual purpose of cooling the engine and heating the hydrogen such that by the time it enters the core it’s in a gaseous state. The hydrogen then moves through channels (coated with niobium carbide to resist corrosion) that run through the core from top to bottom. Passing through these channels, the hydrogen picks up heat from the fissioning uranium, heating to about 4,000 degrees. It passes through the exhaust nozzle and expands, producing thrust.

A bleed line carries some of the hydrogen back to the turbo pump to keep the engine running, making it an incredibly compact and powerful nuclear rocket engine for a deep space mission.

For all the benefits of the nuclear core, there are problems as well. Namely, the high operating temperature and extreme temperature variations make reactors a challenge to both build and operate. That and the crew’s living on a spacecraft propelled by a rocket with nuclear material on board presented its own problems owing to potential exposure. Nevertheless, the potential of nuclear propulsion in space was so promising there were already plans to mount the NERVA upper stage to the Saturn V as the base. Swapping out the S-IVB third stage for NERVA would have resulted in 40 to 75 percent more payload landed on the moon than with the chemical Apollo mission. And with that much power, it would have been a relative cinch to get to Mars.

NERVA Extinct

By 1968, engineers had built enough components to create a space-ready prototype version of NERVA that could have been mated with existing boosters to form a hybrid rocket. The Saturn C-5N as the NERVA variant was called could have increased the payload of the standard Saturn V to from 118,000 kg to 155,000 kg to low Earth orbit. It could have supported a manned mission to Mars by 1980, with a transit time of just 4 months instead of the 8 or 9 we get with the optimal launch window. Wernher von Braun envisioned an even larger mission: sending 12 men to Mars aboard two rockets, each propelled by three NERVA engines, launching in November 1981 and landing on Mars in August 1982.

This could have been the key to reducing both launch costs and time of manned missions long before the space shuttle tried and failed to meet thsoe same goals. But big dreams in space for the Apollo era ended in the early 1970s when the Nixon Administration cancelled the Saturn V program and ordered the end of nuclear activities at Plum Brook. The site was mothballed via a handwritten memo in 1973.

There is a lot to say about the history of nuclear propulsion and nuclear material in general in space, and I can hear you guys yelling “but what about Orion!?” I’ll certainly be coming back to nuclear topics in the future, and I have a whack of Orion docs sitting on my floor right now waiting to be read, so sit tight. I just need time to get through it all!

Sources, beyond those that are hyperlinked in the body of the blog: WiredLos Alamos LaboratoryJFK Library; NASA’s Nuclear Frontier: The Plum Brook Reactor Facility by by Mark D. Bowles and Robert S. Arrighi.

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