Re: Heat disposition in space

On Mon, Mar 17, 2003, Thomas Barclay wrote:

>I've also heard about heaters now that use 
>directed heat - they heat what they point at - 
>essentially an IR radiator or something of the 
>sort. Now, these are electrically powered. 
>
>But is it conceivable that the waste heat from 
>the ship could be usefully transformed into 
>emittable radiation (and directionally 
>controllable at that)? Or would this process 
>never be efficient enough to meet the major 
>need? I ask, because if this was the case, having 
>movable wings that could radiate heat would be 
>a good thing.... you could then radiate your 
>heat *away* from a direction you wanted to be 
>stealthy in (assuming you knew which direction 
>that was!). 

    The largest problem with this would be efficiency. In order to
radiate waste heat from an entire ship, the radiators have to be very
hot. Much hotter than the ship itself (to sync the rest of the ship
close
to background (~3K), let's say ~100K, would require the skin to be
cooled
200K colder than the interior (which is designed for habitability, and
thus fairly close to 300K (~27 Centigrade)), and considerably colder
than
any likely power source). In order to move heat to these radiators
(which
are hotter than the ship), requires energy for active transport (2nd law
of thermodynamics -- heat will move from hotter to colder, unless you
push it the other way, and that takes energy). This energy *can't* come
from the heat itself (1st/2nd laws combined -- you get energy by moving
heat from hot to cold, not the other way around).
    To get this additional energy to move the heat to the radiators, you
need more power. Thus your reactor generates additional power, and
additional waste heat. So you have to move *more* heat to the radiators.
So the radiators are even hotter than the rest of the ship. Thus, it
takes more energy to move *any* of the heat over a higher differential.
Thus, you need more power. Which generates more waste heat. Which has to
be moved to the radiators....
    This, of course, assumes that your radiators will never melt. It
also
assumes that the energy used to transport the heat is itself used with
100% efficiency (difficult when you have such a high heat differential).
It also assumes that you have an extremely efficient way to *move* heat
to such a hot surface, whether or not you have the energy available
(Thermodynamics will tell you how much energy it takes to move heat from
cold->hot under ideal conditions. It won't tell you how to actually *do*
that).
    All of this ignores the problem of parts of the radiator vaporizing
(and thus emitting high-temperature gasses which might be as detectable
as the ship itself), and assorted other difficulties which would
undoubtedly be encountered in moving from Physics to Engineering.

Is it possible? Potentially. Is it practical? Unlikely. Is it
economically feasible? Almost certainly not.

    -Brian Quirt

-- 
Brian Quirt <brianq@ncf.ca>
Proud Member of the Society for the Conservation of Angular Momentum
Visit the society web site at <http://www.ncf.ca/~cy856/bio/scam.html>
Or visit my web site at <http://www.ncf.ca/~cy856/>