Jacob Samorodin said the following:

>..Since it's unlikely that Helium-3 [either lunar source; or Earth
>source] will ever end up as a commercially viable fuel for fusion
>power-reactors on Earth, could Helium-3 possibly end up being >used as fuel for fusion-propelled interstellar spaceprobes in the
>next 100-200 years instead?

Jacob is probably thinking about the Daedalus spacecraft developed
by the British Interplanetary Society (I've been a Fellow of the B.I.S.
for over 20 years). Daedalus is a brilliant concept and a tribute to British engineering. The propulsion system for Daedalus is based upon inertially confined nuclear fusion coupled with a magnetic mirror acting as a nozzle. The Nuclear Ignition Facility (NIF) at Lawrence Livermore Labs could be thought of as a prototype for the Daedalus propulsion system. The He-3 fuel cycle was selected for the Daedalus fusion reaction because He-3 does not produce any neutrons (same feature makes He-3 attractive for terrestrial power generation). Neutron production is a bad thing for interstellar spacecraft because they represent a significant waste of energy as well as causing damage to the vehicle. The limitations of the rocket equation are brutal when relativistic effects are included.

Anyway the B.I.S. designers sort of glossed over the problem of getting the He-3 to ignite. I think(?) they proposed relativistic particle beams but my memory could be wrong. Getting He-3 to light up is very difficult. It maybe technologically impossible. IMHO, the basic Daedalus concept is sound but the fuel cycle would have to be based upon deuterium with the adverse neutron implications accepted. This probably represents an order-of-magnitude hit on the top speed that the vehicle could reach, i.e. it would take 400 years to get to Alpha Centuri rather than 40 years. It was assumed that the vehicle's autopilot would be self aware. Just have it carry more spare parts and allow the autopilot to reset its processor clock from 2 gigahertz to 20 hertz for when things are slow (I suspect even a computer could get bored doing nothing but no-op loops for 50 years).

Gary

p.s. Really "clean" nuclear weapons might be possible based upon He-3, e.g. blow away a major city and then immediately occupy it because there's no fallout.