JRS: In article , seen in
news:sci.space.science, nafod40 posted at Wed, 11
Feb 2004 08:30:18 :-
wrote:
I think the fuel danger is not excessive. The fuel containers could be
kept at a considerable distance from the station, on a wire heading
toward the earth. Tidal forces would keep the wire taught. That might
be necessary if they were solid fuel, which could explode if hit by a
micro-meteorite.

OTOH, if the fuel was two components, say oxygen and hydrogen, then
neither oxygen tanks not hydrogen tanks are explosive on their own. To
keep them cold in space mainly requires sheilding from sunlight.

We could send up water, then just let solar array-powered electrolysis
slowly do its magic to make the fuel. Two years for a bag of fuel? No
problem, no rush.

That way no volatile components in the launch. You could freeze the
water, and use it as part of the structure of the launch vehicle to
reduce weight. Alternate launch techniques such as rail guns? The
payload would certainly tolerate the G's.

To do a reasonably useful mission requires, give or take an order of
magnitude or so, enough fuel to accelerate a ton by 10 km/s, putting 0.5
* 1000 * 10000^2 = 5E10 joules into the payload; given the way a rocket
works, the energy in the fuel must be significantly greater - say
double, giving 1E11 joules.

The solar constant is about 1400 W/m^2; assume solar panel efficiency
about 35% which means 5E2 W/m^2 available, 5E2 J/s/m^2. A year is 3E7
s, giving 1.5E10 J/yr/m^2.

On that basis, a modest 6 m^2 panel allows for a mission per year.

That's well-enough placed with respect to the ballpark to justify a
calculation using better figures. The efficiencies above are
intentionally generous.

Load momentum is 1E7 units; to get that in a year needs a thrust of
about 0.3 N. What Isp can an 0.3 N H-O rocket give? I think of the
electrolysed gases being fed directly to an engine, so producing a
continuous-thrust mission running on sunlight and water.

--
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