Wednesday, September 19, 2012
Something I've been mulling over the last few days: The big problem with using light sail cloth as a reflector for solar electric propulsion is in holding the shape of the reflector when the spacecraft is accelerating. A parabolic dish reflector would be far superior than a "trough" shaped reflector, but as soon as you accelerate a flexible dish in any direction other than towards the Sun, that acceleration is going to have the effect of distorting the shape of the reflector, spilling much of the sunlight it intercepts. This to me seemed likely to severely limit the ideas practicality, but then it occurred to me that, if one wanted to travel quickly (delta v in the low hundreds of km/s) from any planet to any other planet most of the time a close pass to the Sun would be the optimum trajectory, the spacecraft would accelerate towards the Sun, make a close solar flyby choosing the track to send it to its target, turn as it passed the sun, and then again accelerate towards the Sun to slow its speed for planetary rendezvous. Material of only 0.1g/m^2 and suitable for solar sails now is being developed, so a km^2 of such material would mass only 100 kg, and could theoretically collect over a GW of power at 1 AU from the Sun, even at 20 AU (Uranus) a 400km^2 reflector of the same material could theoretically still collect a GW, and mass only 40 tonnes. Perhaps manufacturing imperfections would still be a problem in a large reflector, even though it's shape, only subjected to the constant forces of insolation, and the spacecrafts acceleration, should be stable in flight, in that case perhaps a system similar to adaptive optics, changing the temperature of parts of the reflector by reflecting light onto the back of it to slightly warm and expand over-tight sections of the reflector could be used. A GW of thermal power for 100kg or 40 tonnes compares rather well with near future nuclear systems, if an electric generation system to turn that thermal energy into electricity to power VASIMR (Ve ~200km/s) or similar electric propulsion could be built to produce 10 or even 20 KW/kg you'd have quite a ship, and one that could be built privately without the complications involved with nuclear energy systems, and because plenty of propellant is still aboard until deceleration starts there's still shielding available during the close solar flyby. Without using aerobraking a large fragile deep space reflector could be furled loosely behind a smaller more robust near Sun reflector, with the deep space reflector still useful as a true solar sail (not needing to hold near perfect shape) in the inner solar system - especially to accelerate the ship at an angle to the Sun to reach or decelerate from the various planets solar orbital velocities. Such a ship would be the ultimate in using ISRU, both the energy and propellant required is available throughout the Solar System, and extended missions without resupply and involving the rendezvous with several planets and smaller bodies would be possible, something even a nuclear system couldn't achieve, because even fission materials would mass too much to supply the TJ's of energy that would be required.