Note to Readers – Dr. Martin H. Goodman is a physician, scientist, environmentalist and a pro-nuclear advocate. He wrote this guest OP ED at the request of Neutron Bytes after he submitted this column as a long comment. It was felt that his views deserved a wider readership which is why it is published here.
by: Martin H. Goodman MD
Recently there has been talk about development by the USA of nuclear reactor powered rocket engines for future space missions. In the aftermath of the recent cruise missile explosion in Russia, the media buzzed with rumors of Russian development of “nuclear rockets.” In 2020, Roscosmos (the Russian Federal Space Agency) plans to launch a spacecraft utilizing nuclear-powered propulsion systems (developed at the Keldysh Research Center), which includes a small gas-cooled fission reactor with 1 MWe.
It may surprise the reader to learn that half a century ago the USA had a very active program to develop fission reactor-powered rocket engines, which involved many highly successful tests of nuclear rocket engines on the ground, between 1958 and 1973, right up to the point of using these to launch spacecraft. This included the Kiwi (ground tests of engines named for the flightless bird), Phoebus, and PeeWee programs.
NERVA solid core nuclear rocket. Image: Los Alamos National Laboratory
The US Kiwi program included, as its final test in January 1965, a deliberate test to destruction of a nuclear rocket engine, in which a prompt criticality was deliberately caused and an explosion estimated comparable to 300 lbs of black gunpowder. Radioactive material was scattered over a 1700 foot radius (that is, very much contained). This dramatically indicates how concerned with even very unlikely untoward events this program was. Note this test required major modifications of the KIWI engine, involving having controls that could be slammed into full on position using pneumatic cylinders.
Nuclear fission reactor-powered rocket engines, in which a fission reactor heats propellant that then is ejected in conventional reaction rocket fashion, offer at a minimum specific impulses of twice that of the theoretically very best chemical rocket engines, and may be able to be designed to be many fold more superior than that. Thus reducing travel times for space missions by those factors.
See also – Los Alamos National Laboratory – Nuclear Rockets to Mars and Beyond
The Soviet Union also had a nuclear rocket development program, active between 1965 and 1986 (ended after the Chernobyl disaster. Released records claim they ground-tested a “RD-0410” nuclear rocket engine.
Nothing New About Nuclear Fission Reactors in Orbit
NERVA specifications; Data via Los Alamos National Laboratory
While to date no nuclear fission reactor / reaction motor rocket engine has either been used to launch a spacecraft or power one once in space, nuclear fission reactors have been placed into earth orbit as much as half a century ago, when in 1965 the US orbited one of its “SNAP 10” fission reactors, using it to provide electricity to a satellite.
The Soviet Union powered around a dozen different satellites with their “Topaz” fission reactor. In both cases, heat from the reactor was directly converted into electricity by solid state (thermionic) means. Research is being actively pursued now to develop Stirling engines for converting such heat into electricity, thus gaining a 4 fold increase in efficiency over current thermo-couple-like methods with something light weight and reliable enough to be used in a deep space mission.
A much simpler means of using nuclear power in space than that of a fission reactor has been for almost a half century the source of operational electricity for some of the most inspiring achievements of our species: The spectacularly successful robotic space probes that explored our solar system, especially the more distant reaches of it.
This source of electricity is the Radioisotope Thermonuclear Generator (RTG), in which heat from a simple “lump” of highly radioactive material (typically plutonium 238 in RTGs for spacecraft) throws off physical heat (for many decades), and this heat is turned into electricity using a kind of thermocoule (Peltier device).
Radioisotope Thermonuclear Generator – Image: NASA
The RTG has no moving parts, so is ideal for space missions lasting 20 to 50 and more years. The impressive robotic space missions using RTGs as a source of electricity include the Voyager probes, Viking missions to Mars, the current Mars rover, the Galileo (Jupiter system), Cassini, (Saturn system), and New Horizons (Pluto and Kuiper belt) missions.
At least one highly successful spacecraft and space mission (the Dawn mission to Ceres and Vesta) has already been powered by ion drive engines, which allowed the robot spacecraft to put itself in orbit around one asteroid, study it, then leave orbit and fly to another asteroid, and enter orbit around it to study it. Dawn’s ion drive drive engine was powered by 1.4 kilowatts of electricity from solar panels, but that same amount of electrical power can also be obtained on a spacecraft from either a fission reactor or even a simple RTG.
References for further reading
- https://en.wikipedia.org/wiki/Nuclear_power_in_space
- https://en.wikipedia.org/wiki/Systems_for_Nuclear_Auxiliary_Power
- https://en.wikipedia.org/wiki/SNAP-10A
- https://en.wikipedia.org/wiki/Nuclear_thermal_rocket
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There’s more in this great book, Atomic Adventures: Secret Islands, Forgotten N-Rays, and Isotopic Murder: A Journey into the Wild World of Nuclear Science, by James Mahaffey.
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Cool technology, but alas, the nuclear thermal rocket is a solution looking for a problem. Back in the days when the all rockets were used only once then discard, a costly but high performing nuclear upper stage made sense because the big first stage was the most expensive part. Now that SpaceX (and soon Blue Origin) are re-using the first stage, the single-use upper stage is the expensive part.
Additionally nuclear is never a good choice for the first stage. The engine thrust-to-weight ratio is one problem: at least 30:1 is needed, but NERVA struggled to hit 1.2:1. Also, for a given rocket first stage tank volume, the rocket performance is proportional to the engine performance (Isp) and the propellant density, and the hydrogen propellant used for nukes is only about 7-15% as dense as normal chemical propellants. The second stage is still partially dependent on propellant density, but the third stage (which is ignited after the vehicle is in orbit, and mass is only a couple of percent that of lift-off) is pretty tolerant of low density propellant, so that’s where mission planners consider nuclear engines.
So far, SpaceX has not talked about using a third stage; they prefer to re-fuel the 2nd stage on-orbit. For Mars missions, NASA talked about using an outbound nuclear stage to depart Earth orbit, then another nuclear stage to return from Mars orbit. SpaceX instead plans to re-fuel on the Martian surface, which is better than bringing hydrogen to Mars orbit from Earth. Once on the Martian surface, again chemical engines are preferred over nuclear, since chemical propellants like hydrogen-oxygen or methane-oxygen are more effective than plain hydrogen, for a given amount of hydrogen produced (oxygen is always produced as a free byproduct of making hydrogen on Mars or the Moon).
So the nuclear thermal will have to wait for a future mission architecture/destination.
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