Anonymous@inmet.UUCP (07/12/83)
#N:inmet:7600002:000:1689
inmet!Anonymous Jul 11 12:01:00 1983
Someone mentioned David Brin's book "Sundiver" a while back. I read
it and liked it, but am a little unsure of the physics behind it.
Perhaps someone here can say if his machinery would work.
The premise is that people have built a ship to go exploring the sun.
To do this Brin had to think of some way to keep the ship from vaporizing. If a
refrigeration system keeps some area cooler than its environment, it has
to make some other area hotter. The heat removed from the ship's hull
has to go somewhere. Ordinary radiators wouldn't work, because they
would have to be hotter than the 6000 degree plasma outside in order
to dissipate the heat. They would instantly vaporize, and you would back
where you were. The heat could be dumped into some fluid that would
then be ejected from the ship (this is how rocket combustion chambers
keep from burning up), but whatever put the heat into the fluid would
again have to be hotter than the outside.
Brin's solution is to dump the excess heat into an X-ray laser
(xaser?). The laser beams the energy out through the surrounding gas.
A blackbody that emitted its peak energy in the x-rays would have a
temperature in the millions of degrees. The laser, then, is 'hotter'
then the surroundings and so can radiate to them.
This sounds kind of plausible, but my thermodynamics isn't solid enough to
be sure. Can you really cool things off by making them lase? The book has a
lot of other nice ideas (eg. in order to join the galactic federation a species
has to bring another species to sentience. Intelligent chimpanzees and
dolphins figure into the plot.), but I'm not too sure of its physics.
John Redford
DEC HudsonKING@KESTREL@sri-unix.UUCP (07/14/83)
From: Richard M. King <KING at KESTREL> The physics is wrong. The output of a laser is virtually entropyless. It is possible to have a heat-pumped laser at any frequency, but said laser has to dump heat at some lower temperature. The best we can hope for is Qi/Qo=Ti/To, where Qi is heat input to laser, Qo is heat output of radiator's output, Ti is heat source temperature, and To is radiator temperature. HOWEVER, with one supplemental idea this would be a tremendous help. Suppose the ship carried a supply of water. Further suppose that the ship went to the 1 atmosphere level of the sun. For every calorie the laser absorbed at 6000K it might only have to dissipate .06 calories at 373K. This would tremendously reduce the need for water. The laser need not be an X-ray laser. The output of ANY laser is virtually entropyless (effectively at infinite temperature, like electricity in a wire). To convince yourself of this, remind yourself that the light of an IR laser was frequently used to burn holes in razor blades in the early days of lasers (and is now used to burn holes in materials that can't easily be cut in any other way). The key point is that the light is coherent and can be focussed arbitrarily tightly. Incoherent light can only be focussed enough for the target to reach the temperature of the source. The laser might work as follows: heat the working material (CO2?) to 6000K. The molecules go to a high energy state. Expand the gas (supersonic nozzles) which cools it, so the high energy state is dis- equilibrious and therefore there is a population inversion. Lase. Cool the gas to a lower temperature and recompress it. There is no need for a laser. A garden variety turbogenerator or MHD device would do as well, and have the additional advantage that power would be generated and that there would be no constraint on temperatures or working fluid. The bulk of the electricity has to be sent somewhere; a radio transmitter comes to mind. (They certainly need one anyway). The most important points are YOU NEED THE WATER and THE X-RAY-ness OF THE LASER or even the use of a laser AT ALL is spurious. Corrections welcome. Dick -------