Playing with fusion-rockets

[james_davis_nicoll]

Specifically with the table top fusion device.

Assume a probe has to be at least 5 tonnes to be interesting and that mass
ratios as high as e^3 are acceptable. Assume (since this thing doesn't produce
power) that we can dedicate a 10 gigawatt beam to powering this, of such a
wavelength that distance between the power plant and the probe isn't an insurmountable problem. In that case, I get an acceleration somewhere between
1/15 m/s/s and 1.33 m/s/s, with a final delta vee of around (obviously) 9,000 km/s. Does that look right?

If we are lazy, and I am, and we just use the 1/15 m/s/s value, then it takes
about 1560 days for the reaction mass to run out at which point the probe will be around 4000 AU out, moving at 3% the speed of light. Hmmm. 4000 AU suggests a short wavelength for the beam....

OK, it still takes so long to reach Alpha C that if such a probe were arriving now, it would have had to have been launched during the American Civil War. The probe does reach one light year in about a career's time, though.

Comments

[(Anonymous)]

I think I am missing a step. Not sure how the tabletop fusion source implies an exhaust velocity of 3000 km/s?

Carlos

[james-nicoll.livejournal.com]

That's the speed of the deuterium stream. Now, I wouldn't have thought to apply this to rockets, as the stream is fast but extremely weak, but the designer mentioned rockets as a potential application.

[(Anonymous)]

Ah. I will definitely have to add the paper to my journal run.

Carlos

[james-nicoll.livejournal.com]

Of course, now that I think about it what I should be thinking about is the energy of the 3He the process apparently produces.

I think that the Nature article must either have misquoted him or he doesn't understand rockets very well or I am misunderstanding something. To quote the article:

"Putterman also thinks that rocket propulsion could benefit. Space probes such as the European Space Agency's SMART-1, which recently arrived at the Moon, already use ion engines that eject a stream of charged xenon gas to produce a gentle forward thrust. The pyroelectric accelerator could produce a similar beam of ions moving at much greater speed, which would increase the thrust considerably, says Putterman."

No, it wouldn't. It would increase specific impluse. For a given power input, thrust would drop as ISP increases.

[(Anonymous)]

Could that [tabletop fusion] thing be useful for a thorium reactor? Or do the relative efficiencies preclude it?