>>11733522There are legitimate reasons for why we'd want He-3 fusion reactors and engines, independent of D-D fusion being available.
The biggest reason is obviously the much reduced neutron flux per megawatt of thermal power created by the reaction. Neutrons are a shitty particle, they represent a large fraction of the energy released by any fusion reaction that produces them and they're very inconvenient and difficult to capture, direct, and/or contain. As a result of having no charge they pass straight out of the fusion plasma and get soaked up by any solid (or liquid) material in the area, which both causes those materials to become radioactive AND deposits a shitload of waste heat in a diffuse form.
Compared to neutronic fusion, anuetronic fusion offers much greater prospects in terms of utility as a rocket propulsion system, because despite being more difficult to ignite and sustain, aneutronic fusion engines could feasibly achieve a greater output thrust at higher efficiency than any neutronic fusion engine, because you can pretty much burn your He-3 reactor as hard as you want and you won't end up melting the inside of any solid panel thicker than a millimeter, as would start happening to D-D or D-T fusion engines being run at similar power outputs.
The downsides of He-3 fusion are that it's way harder to actually perform and it requires a fuel that we have no significant supply of. Luckily we don't really need He-3 for fusion engines to be a thing, it's just that we won't have anything resembling a fusion torch-drive until we crack high power aneutronic fusion, so that our engines only encounter thermal issues at power levels where the fusion gamma ray flux becomes a significant source of structural heating.