From: neufeld@helios.physics.utoronto.ca (Christopher Neufeld)
Newsgroups: rec.arts.sf.science
Subject: Re: Lasers vs. Radiators
Date: 10 Apr 92 04:40:34 GMT
Distribution: usa
Organization: University of Toronto Physics/Astronomy/CITA

In article <rvutoINNnu6@agate.berkeley.edu> lizi@ocf.berkeley.edu (Cosma R. Shalizi) writes:
>In article <1992Mar13.205505.6345@helios.physics.utoronto.ca> neufeld@helios.physics.utoronto.ca (Christopher Neufeld) writes:
>>   The question was what sort of efficiency could you get by pumping a
>>laser for a photon rocket. I pointed out that you could get the same
>>efficiency as you could get by just running the electric current through a
>>heating coil behind a shadow mask on the back of the spacecraft, so the
>
>there _is_ a reason
>lasers are more efficent. Namely, all the light is of the same frequency
>(monochrome) and usually all in phase (polarized) so you don't get
>destructive interference.
>
   No, to paraphrase a Kentucky Fried Chicken commercial, "watts is
watts!"

>Setting a newspaper on fire with a 10W laser
>is trivial. Doing it with a 10W light bulb (non-polarized, polychrome
>light) requires substantial patience.
>
   Not if the spot size is the same for the two. If you put the light
bulb somewhere inside a spherical mirror cavity (not at the centre, for
practical reasons) and left a 1mm hole in it, then stuck a piece of paper
to the hole, then in the equilibrium configuration the cavity has to
radiate as much power as the light bulb is producing. If the reflectivity
of the walls is high, and the heat conductivity through them is low, then
you'll get essentially all the ten watts coming onto that small piece of
paper, and you'll ignite it as effectively as with a laser.

>   Besides, the problem with heat radiation is that you need surfaces
>perpendicular to the direction you want thrust
>
   Not necessarily, surfaces radiate heat essentially isotropically from
each element. An uncollimated radiating surface (say a plane exposed to
space, with heat radiating from one face) has a 50% efficiency in power
to momentum conversion, since half the energy is carried off in
transverse momenta which cancel out. I see you're a physics undergrad, so
if you want to do the integral don't forget the sin(theta) in the
Jacobian.

>You'd
>also need to rig it so its hottest _away_ from the body of your ship, other-
>wise it would act as a break (hey, that might be useful...).
>
   Well, the situation I was thinking of was very much like that
spherical cavity with the small hole in it, but with the added provision
that the hole have a collimating tube so that the heat is radiated
essentially radially, cutting down on the transverse momentum wastage.

>To do this,
>you need to expend more energy (the source of the heat you dump into the 
>radiator is in the ship) or the 2nd law goes into screaming fits.
>
   Sure you need to expend energy, but you also expend energy driving a
laser system, and since 90% of that energy is going to go into waste heat
(for current laser systems) that means you're going to need a huge laser
plus a radiator which has about 90% the capacity of a pure heat radiator
drive. So my question was, why bother with a finicky piece of hardware
like a laser when a small increase in radiator capacity can drive your
ship as efficiently.

>Cosma Shalizi (lizi@ocf.berkeley.edu)
>physicist in training & VP of Campus Crusade for Cthulhu (for which
>I _do_ speak)


-- 
 Christopher Neufeld....Just a graduate student  | alien: n. a being who
 neufeld@helios.physics.utoronto.ca    Ad astra  | travels great distances
 cneufeld@terranet.cts.com                       | to molest our cattle
 "Don't edit reality for the sake of simplicity" | and trample our grain