Research at the Marshall Space Flight Center has picked up on an old thread to advance rocket technology. Known as the Nuclear Thermal Rocket, this technology uses the heat generated by nuclear reactions to produce rocket thrust. As the space agency once again eyes destinations beyond the orbit of Earth, NASA must review sources of propulsion better suited for long-duration missions to distant destinations—and that means nuclear.
Normal chemical rockets use the heat generated by chemical reactions between fuels and oxidizers to create hot gas which is forced through a nozzle to produce thrust. Nuclear thermal rockets use the heat generated either by the radioactive decay of unstable isotopes or nuclear fission reactions to heat gas, force it through a nozzle, and create thrust. The key difference is that nuclear thermal rockets are more efficient at harvesting the energy of their heat source, and thus can produce higher thrust and greater exhaust gas velocities—two key measures of rocket performance. The Space Shuttle Main Engine’s combustion temperature is about 3500 K. While NASA-Marshall’s simulated NTR has a similar internal temperature, real NTRs can get much hotter, as hot as the materials that make them up can stand.
NASA-Marshall’s NTREES (Nuclear Thermal Rocket Element Environment Simulator) experiments do not involve actual radiological materials, but simulated radioactive substances. The engineers use materials that might be used to contain the reactor-fuel and heat it to the incredible temperatures an NTR rocket would endure. They also pass cryogenically cold hydrogen over the containment element to simulate the way the rocket would produce thrust. NTRs are not a new idea by any means. Science fiction writers speculated about nuclear engines during the 1940s and ’50s. NASA itself, in concert with several other federal agencies, produced a working version of an NTR under the NERVA (Nuclear Engine for Rocket Vehicle Application) project. The final product, the NRX/XE engine, was considered safe enough to use on an actual manned Mars mission, until Congress and then-President Richard Nixon canceled the program by slashing the budget for manned Mars exploration.
But the Nuclear Thermal Rocket is the key to the future. Consider now the expense of using expendable rockets over and over—not to mention the accumulation of space junk as spent upper stages hang around in useless orbits over the Earth, sometimes breaking up or even exploding spontaneously and creating more debris, which can go on to threaten more spacecraft and create more debris.
Now imagine a reusable upper stage powered by an NTR. The “space-tug,” as the concept has long been known, and an NTR-powered one would only need one propellant. Liquid hydrogen would be best (in general, the lower the atomic weight of a propellant, the better it is for use in rocketry), but methane could work as well.
In such a scenario, a small expendable carrier rocket would launch a payload into LEO, where the space-tug would rendezvous with it and carry it up to the correct orbit, then maneuver away to intercept its next payload.
Such a tug would need only the occasional refueling, as its engine might last over a decade.
It could also be of enormous use to the manned spaceflight program. NERVA meant for its engines to send men to Mars, and Marshall’s NTREES experiments have the same ultimate purpose, but such an engine could meet needs closer to home as well.
The problem with the Apollo program was that every time we needed to put a man on the Moon, we had to launch a massive Saturn V rocket. What if, instead, we could launch only a medium-lift rocket and send a crewed capsule to a space station at the Earth-Moon L1 point (the point in space where the Earth’s and the Moon’s gravity balance out, and an object placed there would tend to stay there)? The capsule could rendezvous with the station, which might or might not be permanently inhabited. Think of the science that could be done at such a station—Earth observation, Moon observation, stellar astronomy, solar astronomy, solar wind physics … the possibilities are endless—which would have one or more reusable lunar landers docked, each using these NTR engines. The space station would double as a fuel depot for them. From that station, any point on the Moon could be explored at any time with no need for a Saturn V/SLS class launch from the ground. In fact, teams could be maintained on the Moon as on the ISS, switching out via the reusable landers and ascenders and the capsules to carry them back to Earth, or an Earth-orbital hub station.
And, of course, there is the engine’s usefulness in a manned Mars mission. By allowing a near-constant acceleration (there would be need for only one propellant to be carried, instead of the greater weight, complexity, and expense of multiple tanks to maintain multiple propellants) and the greater thrust of the engine, the travel time to Mars could be slashed enormously.
And even unmanned exploration could benefit from the boon of an NTR. Right now, the greatest hurdle in the way of exploring the outer solar system is the time it takes probes to get to their destinations. In that time, the teams that built and operated the craft grow old, retire, and even die, and the spacecraft components themselves are exposed to degrading radiation and the harsh environment of space. By the time they arrive at their targets, they have already lost much of their useful lives.
Consider the New Horizons probe, launched in 2006 and not expected to arrive at Pluto until 2015—and this is the fastest object humans have ever launched into space. Furthermore, it won’t even be able to stop at Pluto and carry out a thorough characterization of the planetary system. It’s traveling too fast and has too little fuel to slow down, so the best we can hope for are some fleeting glimpses as the spacecraft tears by the dwarf planet at a relative velocity of 13.8 km/s.
And Uranus and Neptune have yet to have dedicated probes visit them, and, again, time is the greatest hurdle (not to dismiss the budgetary hurdles, however). But a Uranus or Neptune probe with an NTR engine could arrive in a reasonable amount of time. At orbit insertion, the NTR stage could be ditched into the target planet (like Galileo’s RTG into Jupiter) or kept and used to make the kind of drastic orbital maneuvers Galileo and Cassini could never make. An NTR-powered Uranus or Neptune probe could sweep close enough to the cloud-tops to get an actual sniff, while still having the power to explore the planets’ moons in detail.
The thrust available to an NTR stage would allow a heavier payload to be sent to the target. Perhaps that would enable smaller “carry-on” probes, like the Galileo Jupiter entry probe and the Huygens Titan landing probe. Such probes could enter the atmospheres of their targets or land on their moons. A sufficiently resilient probe might even be able to endure direct exploration of the outer planets’ ring systems. An NTR-powered spacecraft bus might enable a whole fleet of probes to explore a single planetary target.
But right now all of this remains in the future. The biggest problem is that Americans are inherently afraid of the words “radiation” and “nuclear.” A systematic educational campaign could ease baseless fears about NTR-driven space-tugs, manned-mission engines, and planetary-probe engines, and a write-in campaign to Congress urging them to support the future of humanity itself by supporting the future of space exploration could be vital.