J.JPM@[36.21.0.13] (Jim McGrath) (02/14/86)
From: shawn@acc.arpa
While most of the talk of SDI is mostly over my head, I've also
been reading the messages about "laser powered" space flight. In
space flight, a laser is used to push a space ship from earth. In
SDI a laser is bounced off of an object that is amed elsewhere.
OK, so will the laser push the mirror out of its position with
each recoil and if so, would that push be directed by the angle of
the reflected shot?
This is not the problem you think it is. While any such mirror (which
is not an essential part of SDI by the way) would experience
acceleration due to the light pressure of a laser (much like a light
sail), the acceleration would be VERY low and controllable.
The confusion is due to a misconception of how the proposed laser
launch (earth to LEO) system would work. The payload would not be
"pushed" up by the light pressure of the laser. Rather the laser
would strike the interior of a shaped chamber and heat mass in that
chamber. The mass would then be expelled out the back, just as in a
conventional rocket. The savings is due to two factors. First, while
in the atmosphere the mass ejected could simply be heated air (the
ramjet principle), which is cost free. Second, the power plant
(laser) is back on earth, and you do not have to rely upon the (very
low) energies provided by chemical fuels.
The problems with this approach are threefold. First, you need
powerful lasers. Second, you need a good tracking system. Third, you
have to design the chamber right. The first two will be helped
greatly by SDI research, the last less so.
Jim
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