I’ve read everything I’ve been able to find via nuclearspace.com about the “nuclear lightbulb” gas core nuclear fission rocket motor and the “Liberty Ship” heavy lifter to be built of 7 of them, and didn’t find much serious engineering detail or less than wildly optimistic speculation.
I’ll try to outline, in a somewhat rambling narrative, the basics of the nuclear lightbulb engine:
Like the simpler “open cycle” motor, coolant (hydrogen, which in an open cycle motor is also propellant) is flowed
inside the reactor vessel (which is transparent glass), in order to “swirl around” the hot fissile material containing gas to prevent it from contacting the vessel walls. Magnetic fields may also be used to keep the hot gas away from the vessel walls. This allows the gas to reach a very high temperature (most sources say something like 20000 – 25000 C, though my
Wein’s law calculations put it around 18000) without melting the vessel.
Unlike the open cycle motor, the heating of the hydrogen within the vessel is not the heating of the propellant. Where the usual open cycle motor is surrounded by neutron reflector and a gamma photon (30 MeV) absorbing shell (typically of beryllium oxide), which also absorbs the much lower energy ultra violet photons (10 eV). The ultraviolet photons, which were created by the hot gas inside the vessel, potentially carry many times more energy (as much as 5-6 times) that the neutrons and gamma photons, because they are produced by collision of the much more massive atoms (mostly uranium, its lighter fission products, such as krypton and barium, and “carrier” elements such as fluorine) that carry most of the energy of nuclear fission. (nuclear physics source: wikipedia article “nuclear fission”)
Hydrogen absorbs UV photons well, so the liquid and gas hydrogen inside the outer vessel and outside the inner should absorb nearly all of the reaction energy. The hydrogen inside the inner vessel absorbed UV photons, too, but because it is a much thinner layer than the outer, not as much – otherwise, the motor would just be an open cycle motor. What a NLE does with its heated core hydrogen, I didn’t notice any reference to – apparently either it’s stored, or refrigerated and recycled.
About this time, I was asking myself “so why not just have an open cycle motor?” There are a couple of obvious answers:
Even if the LH2 does a perfect job of keeping them from melting the vessel, fission fuel (uranium) and products will get mixed into the coolant. If this is also your propellant, as it is in an open cycle motor, your rocket exhaust is full or dangerous radioactive waste.
Because you’ve got to have a big exhaust hole in an OCM, it’s presumably harder to keep it swirling in the proper pattern and at the proper pressure to protect the vessel than with a closed inner vessel.
After learning this, I next wondered how difficult it is to make a gas core reaction of any kind. The answer is pretty daunting – though it’s been studied for more than 50 years, nobody has built even a small, lower-power gas core test reactor.
Now, pretty smart technologists as far back as the 1960s seemed to think that not only could GCNRs be built, but be very low mass, high power, and flown in spacecraft. At first glance, this seems like unreasonable optimism, but a factor supporting such optimism is that much of the difficulty of juggling a core of 20000 C fissioning gas with swirling cold H2 and magnetic fields is lessened if the vessel is not experiencing much acceleration, as an Earthbound research reactor (which is, of course, under a constant 1 g of acceleration) would be. The rocket motors they had in mind produces very small accelerations, on the order of 0.01 g, and would be fired only in space, where their radioactive exhausts wouldn’t be a problem. In other words, it’s easier to operate a GCNRM in space than on Earth.
All this leads me to what I think are a couple of big problems with the whole Liberty ship idea:
Because it will be subject to varying accelerations of 0 to 3 or more gs, stabilizing the fissioning gas in the reactor will be very hard. Because the glass inner vessel wall is thin (if you make it thick, it will be increasingly opaque to UV, defeating the main reason for it being glass), even a brief failure that allows the 20000 C gas to touch the 1650 C melting-point vessel will burn through it.
A very fast reacting feedback system might be able to meet this challenge, but it’s a scary, intrinsically unstable juggling act.
A nuclear lightbulb reactor emits a lot of gamma photons. One the same mass and size of a
NERVA 2 (which has actually been built and test run), will emit a gamma photon flux proportional to its power, about 10 times the NERVA 2. The 12000 kg NERVA 2, including a 240 kg shield, gave a total radiation dose of about 130 grays to its payload, 26 times the human fatal dose. Because they have minimal shielding, nuclear rocket motors also release an extraordinary number of neutrons, many in the low-velocity range prone to absorption by atomic nuclei, so they transmute their external materials into radioactive isotopes.
Extra shielding for sensitive or living payloads, and operating the rockets only in remote locations, such as ocean launch sites, as Anthony Tate’s
“Opening the Next Frontier” proposes, could address these issues, but describing these or other nuclear rocket motors as “clean” is, while in a ecological sense true, common sensically deceptive. In other words, I wouldn’t want one in my back yard. More seriously, I fear their high neutron fluxes limit the reusability of the proposed motors and entire vehicles – though, given their potential for tremendously economical payload launching, this is, IMHO, more of an aesthetic flaw than a practical one. A rocket that can orbit a million kg could be single use only, and still be a tremendous advance in spaceflight.
While I think “Opening the Next Frontier” is technically underweight, and understates the challenges of nuclear fission rocket motors, I strongly agree that it’s a shame there’s been so little study and actual prototyping of NFRMs since the early 1970s. While there’s not been a complete absence of research (eg: the
Pratt & Whitney TRITON solid core nuclear rocket motor), I agree Tate’s feeling that, had the level of engineering R&D effort of the 1950s-70s been sustained, nuclear rockets might currently exist, and have dramatically improved spaceflight technology.
