Hybrid NTR/HET Spacecraft
One Reactor, Two Propulsion Systems
Some time ago in Hybridization: the Takeaway Lesson I speculated on the possibilities for hybrid spacecraft, and emphasized that the more a system can be designed for dual use the more integrated the hybridization can be. In earlier posts I have emphasized the importance of hybrid propulsion systems, such as the “combined cycle” SABRE engine designed by Reaction Engines LTD for the Skylon spaceplane. The SABRE engine uses the same systems to operate as an air-breathing engine within the atmosphere and can then shift to liquid oxygen, becoming a rocket that can operate outside the atmosphere, or in the upper atmosphere where there is too little oxygen to run the engine. This innovative hybrid engine will allow for a single-stage to orbit (SSTO) spacecraft. While our attention is now focused on the successes of reusable spacecraft, if and when the Skylon spaceplane (or something like it) flies, the impression will be as dramatic as SpaceX simultaneously landing two booster stages for its Falcon Heavy.
Another form of hybrid propulsion that I would like to see explored in more engineering detail would be a single nuclear fission reactor core that could be used both as a rocket or for power generation for electric propulsion. By a “rocket” I mean nuclear thermal rocket (NTR) and by “electric propulsion” I mean some kind of ion engine, such as a Hall Effect Thruster (HET). I call this hybrid an NTR/HET spacecraft.
There has been a great deal of research on NTRs, and, back in the day, the NERVA system was tested, but was abandoned primarily due to proliferation concerns. NTR designs have proliferated (e.g., DUMBO is another interesting design), but the political imperative to restrict fissionables spelled the end of nuclear rockets despite their promise. It is difficult today to recapture the frame of mind when nuclear technology was flourishing, and we actually orbited a satellite (SNAP-10A) powered by a fission reactor. In my post Secrecy and the STEM Cycle I pointed out that nuclear rockets represent an entire industry that never happened because of proliferation concerns.
There has also been a great deal of research on electric propulsion, and many small spacecraft use electric propulsion today. An ion engine can attain very high exhaust velocities, and uses far less fuel than conventional chemical rockets. This makes ion engines ideal for space travel if the engineering problems can be solved. The engineering problems for scaling up electric propulsion are difficult, but not likely insuperable.
A few years ago I noted recent work by French researchers on a wall-less Hall-effect thruster, or HET (a kind of an ion engine), announced in their paper, Optimization of a wall-less Hall thruster, by Julien Vaudolon, Stéphane Mazouffre, Carole Hénaux, Dominique Harribey, and Alberto Rossi. The French researchers may have found a way to make a ion engine (i.e., a Hall thruster) last much longer, which potentially has applications for long distance human spaceflight in our planetary system.
This recent research points the way toward solving some of the engineering problems of HET powered spacecraft. The next problem is that, although ion engines need a lot less fuel, they need electricity to operate. Close to the sun solar power could supply electricity, but more electricity promises a more powerful engine. The obvious thing to do would be to use nuclear power to generate a lot of electricity to make a really powerful ion engine (fission is a proven technology), but if you’re going to go to the trouble of lifting a nuclear reactor off Earth, why not just make a nuclear rocket? At this point, spacecraft propulsion becomes a political problem.
If we can get past the political problem of nuclear fission, we can open up the solar system to human exploration at a reasonable cost, and one way to do this is to combine the complementary characteristics of NTR and HET technology. A nuclear rocket produces a lot of thrust, but also goes through a lot of fuel; ion electric engines use very little fuel, but do require electrical power, and the more electrical power you have available, the more thrust you can get. The complementary parameters and requirements of nuclear and ion rockets suggests the possibility of a hybrid propulsion system that employs a nuclear reactor that can be used either for short periods of time to power a nuclear rocket or for longer periods of time as a power generator for ion engines.
There have already been designs for hybrid nuclear reactors for dual use in an NTR. It was with great interest that I learned about the paper “Innovative concept for an ultra-small nuclear thermal rocket utilizing a new moderated reactor” by Seung Hyun Nam,Paolo Venneri, Yonghee Kim,Jeong Ik Lee, Soon Heung Chang, and Yong Hoon Jeong, as it details the design for a nuclear rocket (nuclear thermal rocket, or NTR) that also doubles as a nuclear reactor for electrical generation, called the Korea Advanced NUclear Thermal Engine Rocket (KANUTER).
Winchell Chung brought my attention to several other projects, such as the Bimodal Hybrid NTR NEP, A Crewed Mission to Apophis Using a Hybrid Bimodal Nuclear Thermal Electric Propulsion (BNTEP) System, and A One-year, Short-Stay Crewed Mars Mission Using Bimodal Nuclear Thermal Electric Propulsion (BNTEP) — A Preliminary Assessment. David S. F. Portree pointed out that a Bimodal Nuclear-Thermal Rocket (BNTR) had been designed in the 1990s for a mission to Mars (cf. NASA Glenn Research Center’s 2001 Plan to Land Humans on Mars Three Years Ago), so the idea of hybrid nuclear systems has been around for a few years, and there are a variety of engineering solutions to the design challenge of using a single nuclear reactor both as an NTR and for electrical energy generation. I have also heard of some designs for NTRs that could be transformed into a stationary reactor for electrical power generation upon arrival at Mars.
Given the range of design ideas suggested by KANUTER and BNTR, it would probably be possible to tailor a reactor design specifically for the hybrid propulsion system I have suggested. A hybrid NTR/HET spacecraft could be engineered into a package like the Skylon spaceplane, which could then be used either for automated cargo delivery or human missions, depending upon whatever spacecraft uses this propulsion package as its booster. Alternatively, an NTR/HET design could be a stand-alone spacecraft as an SSTO, by which I mean that the nuclear rocket might lift the craft into LEO and then the spacecraft, after further using its nuclear rocket to break free from Earth’s gravity, could shift to ion engine mode, using its nuclear reactor to power the ion engines. As the technology improved through use and experimentation, the range of the spacecraft could be pushed to the limits to solar system transportation and beyond.
The most advanced nuclear reactor currently used in US submarines, the S9G (which eventually will be replaced by the S1B, which will have superior specs) is designed to operate for 33 years without refueling, and is said to have a power output of 29.8 megawatts (all the detailed specs of naval reactors are classified, so these figures should only be understood as approximations). The use of such a reactor would provide robust and long-lasting power for a spacecraft.
Given contemporary or near-contemporary technology — a nuclear generator that could go 30 or 40 years without refueling, and an ion engine that could exploit 30–40 MW of power for thrust and could operate continuously — how far and how fast could such a spacecraft go? Obviously this would be enough to get us around the solar system, but would it be enough to get us to the stars? And if the answer is “no” to the latter question, imagine this: two or more nuclear reactors, each of which could operate for 30–40 years, where the second reactor takes over after the first reactor is exhausted (or multiple stages, each with its own nuclear reactor). That pushes the time frame for interstellar flight with contemporary or near-contemporary fission/HET technology out to 60–80 years. Is that long enough to reach another star? Ion engines take much less fuel than chemical or nuclear rockets, but they still take some fuel. How much fuel are we talking about? As much fuel as the Daedalus project was to carry? More? Less?
