I understand what the DARPA contracts are working toward - nuclear heated propellant rocket engines. That is basically the Project Kiwi and NERVA technology of the 1960s, hopefully updated to use current knowledge and materials. But, I do wonder about the advantages of using those in cis-lunar space, especially if we are talking about manned vehicles. The shielding requirements for nuclear reactors make the resulting rocket engines very heavy. If only "shadow shields" are to be used, then the maneuvering of such vehicles in the vicinity of other manned vehicles becomes a real concern, so that any radiation sensitive things, such as people, always remain in the shadows of the shields for all reactors in the vicinity. Loss of attitude control, as has happened on the International Space Station more than once, could have lethal consequences without involving actual collisions.
On the other hand, the article's discussions of the two "advanced" systems seems much more "way out" to me.
One is "a chargeable, nuclear radioisotope battery" that is "able to scale to 10 times higher power levels, compared to plutonium systems". All I could find is this:
https://usnc.com/embercore/ . It is very non-specific - but sounds like it uses various specifically selected radioactive isotopes to produce heat in a ceramic material. So, producing more heat than our current plutonium-238 isotopic systems is not surprising. But, being able to select isotopes for high heat generation
and low external radiation exposures seems like the trick for making something that is actually better in real use. The linked site talks about "individual “embers” [ceramic pellets with specific radioactive isotopes blended into each of them] made from a family of commercially available, inert isotopes charged with neutrons in a nuclear reactor." So, it sounds like custom-made isotopes selected for specific missions. So, the underlying physics seems well in-hand, but the selection of radioactive isotopes is limited by physics, and it would be interesting to hear which ones they are thinking of using for various missions.
The other device that "seeks to trap fusion ions in electrostatic fields" is basically envisioned as a fusion reactor with an open end to make it a rocket. In that sense, it is easier to achieve than a contained fusion reaction. And, it seems to be continuously using power to force the fusion, so it would need to be able to tap some of the fusion-generated power to keep going - that seems to be part of the plan. See
https://www.space.com/fusion-powered-spacecraft-could-launch-2028.html .
But, that story says the developers have not yet achieved fusion in their device. And, looking at
https://en.wikipedia.org/wiki/Helium-3 , I see this:
"The appeal of helium-3 fusion stems from the aneutronic nature of its reaction products. Helium-3 itself is non-radioactive. The lone high-energy by-product, the proton, can be contained by means of electric and magnetic fields."
And, that containment process might be used to also make electric power. But the proposal for this rocket engine seems to be to use the heat in a Brayton Cycle to drive a generaor, instead.
But, as the Wiki link states:
"Because of the higher Coulomb barrier, the temperatures required for 2H + 3He fusion are much higher than those of conventional D-T fusion. Moreover, since both reactants need to be mixed together to fuse, reactions between nuclei of the same reactant will occur, and the D-D reaction (2H + 2H) does produce a neutron. Reaction rates vary with temperature, but the D-3He reaction rate is never greater than 3.56 times the D-D reaction rate (see graph). Therefore, fusion using D-3He fuel at the right temperature and a D-lean fuel mixture, can produce a much lower neutron flux than D-T fusion, but
is not clean, negating some of its main attraction." (I added the emphasis.)
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