On Feb 26, 9:55=A0am, CharlesRCap...@[EMAIL PROTECTED]
wrote:
> On Feb 25, 4:29 pm, IsaacKuo <mech...@[EMAIL PROTECTED]
> wrote:
> > On Feb 25, 2:14 pm, CharlesRCap...@[EMAIL PROTECTED]
wrote:
> > Assuming you're radiating in the regime where gold is an efficient
> > reflector, of course. =A0Roughly speaking, if you want to radiate into
> > a smaller cone you'll generally want to be radiating at a higher
> > temperature to compensate.
> If gold is not an efficient reflector at the radiator temperature,
> Chromium as an 0.05 emissivity, not a significant difference in my
> design. It will cause me to have to re-juggle the numbers, but I was
> doing calculations based on reabsorbing up to 20% of the reflected
> energy with reasonable results, so reflecting 95% or 90% or even 80%
> can be made to work, but the remaining electricity for other things
> gets lower and lower.
You seem to be making some sort of fundamental mistake,
if reabsorbing 20% of the reflected energy gives reasonable
results. I suspect that this fundamental mistake is thinking
that you can recover some energy from it without emitting
radiation which is even more visible in the process.
> > Ah, here's a critical difference between your concept and
> > designs I tried to work out in the past. =A0I always assumed a
> > hull temperature of 3K. =A0In other words, all of the waste heat
> > had to be dumped into the directional radiator.
> I'm not going for invisible, just less visible.
I get the feeling that you're going for less visible, but you're
actually getting more visible. For a given power level, the
lower temperature radiator is actually more visible, since
it's spitting out lower energy photons (and thus, more photons).
> 10,000 M^2 at 50K is 3543 watts, at 25K it is 221 watts. It's a big
> hurdle, but it's what I'm aiming for.
I think this is where you're losing sight of the big picture.
The big picture is that you've got a certain number of watts
you've got to dump out there. You don't have some fixed
hull size and then you get to choose how much power
to radiate based on the hull temperature. You've got some
amount of power you've got to get rid of, and a lower
radiator temperature just means you've got to make the
radiator much bigger.
> > My gut feeling is that if a 50K hull temperature is deemed
> > acceptable, then the easiest method of dealing with waste
> > heat is to simply make a however big a 50K radiator you
> > need and forget about the directional radiator.
> Well, my design is considering a warship and not necessarily the OP's
> asteroid, so some slightly different issues are involved. The problem
> with such a big radiator has been covered elsewhere.
> Among other things:
> 1.) The heat addition to the system =A0from sunlight becomes a large
> problem.
You can limit this problem with an edge-on radiator and a flat
reflective sunshade. The sunshade reflects sunlight into a
narrow cone at an random angle.
You pretty much don't have a choice about this. Absorbing
sunlight would be a problem even with no on board power
generation at all.
> 2.) Your ship becomes more massive and larger target.
Well, I wouldn't worry about becoming a larger target, but
there are naturally limits to how big and massive something
is before you decide it's stupidly big and massive.
> 3.) From a detection standpoint having a larger cross-section
> countermands the benefits you get from having a lower temperature.
> (You are cooler but still emitting the same amount of total energy.)
You say this as if you had the choice of emitting less energy.
You don't.
> > Where is "the heat" coming from? =A0A fission reactor, perhaps?
> > In principle, you can make a very efficient reactor that converts
> > almost all of its heat energy into electricity. =A0Conveniently,
> > a heat engine is most efficient when the radiator temperature
> > is a small fraction of the reactor's temperature. =A0For stealth,
> > this seems like a win-win situation. =A0Your radiator is a lower
> > temperature, and you have less waste heat to worry about!
> I am assuming the use of a MITEE nuclear thermal rocket which can
> produce very little waste heat for its output. Further, I am assuming
> that the efficiency of that reactor can be adjusted downward so that
> more waste heat is radiated into the heat engine system if needed.
Oh, well then forget about any sort of stealth. The reason a
nuclear thermal rocket "produces little waste heat" is
because it actually produces lots of waste heat but most
of it is conveniently dumped into the exhaust. The rocket
exhaust is going to be rather visible; a Space Shuttle launch
is stealthy in comparison.
> > Presumably, the electricity is being used for "something useful",
> > like operating a (hopefully) stealthy mass driver rocket. =A0This
> > thread started with the idea of using a mass driver to propel a
> > stealthy impactor. =A0Unfortunately, this "something useful" might
> > not be 100% efficient, so it will generate extra waste heat which
> > must be dealt with somehow.
> I am assuming that you will only generate enough electricity while in
> stealth mode to run your life-support and electronics, so the amount
> of additional heat that needs to be dealt with is relatively small.
You're trying to put people on a stealthy rocket ship? Not
good, since people generate plenty of heat. For my "stealth"
attempts, I assume an unmanned drone with virtually no on
board power (other than propulsion). The main source of
heat is from sunlight, even though most of the sunlight is
reflected away with a mirror shade.
Isaac Kuo
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