Anders Sandberg wrote:
> While we all seem to be in space colonisation mode, what about
> re-using solar sail material?
Just keep the sail in service- unloaded, it could get back to 2000 km
earth orbit rather quickly to pick up another payload. Since the
delta-V of sails are unlimited, they should be treated as slow cargo
haulers, not expendable ammunition. A dozen round trips in thirty years
would be good utilization.
> Would it make economic sense to send them back to Earth to receive new
> cargos (I guess they are not that useful for transporting propellant
> or other heavy products from the asteroid)?
Actually, the sail could deliver a large payload from the asteroid to an
aerocapture trajectory, then drop it off and change course before the
encounter. Often the delta-V to go from asteroid to simply
earth-crossing is quite modest, and the mass throughput can be high.
The payload must be able to aerocapture, though.
> Another use would be for the aerobraking part of the mission, where
> the fuel tanks are aerobraked for entry into LEO. The sails are
> definitely too flimsy to survive aerobraking in themselves, but I
> wonder if they might help provide some extra drag during the first
> pass. Any views on this?
Nope. To aerocapture, one must shed enough energy on one pass to go
from hyperbolic to elliptical. This typically requires several km/s in
a few hundred seconds and hits several gees. A sail would be destroyed
instantly and uselessly- consider a parachute made of conventional
aluminum foil tossed into a jet engine exhaust. Confetti.
A note of terminology- aerobraking assumes that the vehicle is already
in an elliptical orbit about the planet, and can use many pure-drag
passes at low acceleration to lower the apogee. Forces and heating
rates are low. Aerocapture converts a hyperbolic approach into a
captured orbit, with large forces, heating similar to atmosphere entry,
and needs significant hypersonic lift to do a pullup once the correct
amount of energy is dissipated. If the pullout is not done, the capture
could turn into a landing inadvertently, producing expensive fireworks
Aerocapturing thousand-ton payloads into earth orbit would be
politically difficult; witness the angst and hand wringing over
yesterday's Mir reentry of only a few hundred tons. A thousand tons of
propellant tanks would go over like... forgive me... a lead balloon.
-- Doug Jones, Rocket Plumber
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