Re: Slow Stealth

On Feb 25, 4:29 pm, IsaacKuo <mech...@xxxxxxxxx> wrote:
On Feb 25, 2:14 pm, CharlesRCap...@xxxxxxxxx wrote:

Assuming you're radiating in the regime where gold is an efficient
reflector, of course. Roughly 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.

Ah, here's a critical difference between your concept and
designs I tried to work out in the past. I always assumed a
hull temperature of 3K. In other words, all of the waste heat
had to be dumped into the directional radiator.

I'm not going for invisible, just less visible. If you can't be
detected at anything over 10 light seconds, that lends you strategic
and operational stealth. You may not have tactical stealth, but you
don't have that as soon as you start shooting anyway. I'm thinking 50K
is the upper limit, I'd like to get it closer to 25K if I can manage
it. (I think it's possible, but haven't calculated it yet.)

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.

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 from sunlight becomes a large
problem.
2.) Your ship becomes more massive and larger target.
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.)

So the combination of the two (you need to have a hull so why not use
it as a radiator) systems will provide the best compromise.

Where is "the heat" coming from? A fission reactor, perhaps?
In principle, you can make a very efficient reactor that converts
almost all of its heat energy into electricity. Conveniently,
a heat engine is most efficient when the radiator temperature
is a small fraction of the reactor's temperature. For stealth,
this seems like a win-win situation. Your 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.

Assuming that 98% gold reflector, the radiator will reabsorb only
about 2% of what it emits. However, NONE of this heat can be
scavenged for electricity. Assuming you want to keep the reflector
as cool as the rest of the hull, then there's no temperature gradient
downward to work with. The reflector is at 50K, and the rest of
the hull is also at 50K. There's no heat sink to run a heat engine
with.

If your hull is 0.99965 emissivity (from Dwight's link or just plain
old 0.98 of normal black carbon) at 50K you are radiating the same
energy that your 0.02 emissivity reflector is emitting at 133K so we
have a useful gradient there. (Particularly if you are radiating to
open space rather than back into a reservoir inside the ship. So would
the business end of those devices radiating into space be warmer than
the hull? It doesn't seem so, or if they are, it is a manageable
difference.)

This of course ignores the remaining electricity in the system. Does
this electricity turn back into heat by running life support and
shooting weapons?

Presumably, the electricity is being used for "something useful",
like operating a (hopefully) stealthy mass driver rocket. This
thread started with the idea of using a mass driver to propel a
stealthy impactor. Unfortunately, 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. I
will include it into the calculations. (And already did for my
previous set of calculations and it did not seem to push the system
out of balance. There was still a surplus of electricity that could be
reduced to 0 if you kept reducing the initial thermal input from the
plant.)

My biggest unknown is the calculations for electrical draw and heat
efficiency of compressing helium gas. Simply put, I'm making wild
guesses on how this would work. So my bad assumptions may push the
calculations out of an order of magnitude in accuracy.

Rather than getting bogged down into the details of a particular
heat engine, you can first look at things assuming a generic
efficient heat engine. To a first approximation, the inherent
thermodynamics limitations on a heat engine will be more
significant than the differences between specific heat engine
designs.

I have, assuming generic efficiencies; it works within an order of
magnitude. Now I'm getting bogged down in the details of calculating
the temperature gradient for the different stages of the system and
more accurate calculations on how the different pieces work to give a
more accurate answer on the feaseability of the system.
.

Relevant Pages

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  • Re: Slow Stealth
    ... temperature to compensate. ... heat you wish to move to compress helium back to operating ... heat out it costs you more electricity than heat remaining to pump it ... making more by cycling the heat from the reflectors to the radiator. ...
    (rec.arts.sf.science)
  • Re: Slow Stealth
    ... reflector, of course. ... temperature to compensate. ... had to be dumped into the directional radiator. ... and you have less waste heat to worry about! ...
    (rec.arts.sf.science)
  • Re: Mid-term Interplanetary Drives (long)
    ... I think it's less about the trail than the heat of your reactor that's ... Room temperature, 155 light-seconds. ... limited on an upper temperature for the radiator excepting the melting ... I'm thinking 50 Kelvin is fine but 20 Kelvin ...
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  • Re: Slow Stealth
    ... radiator directly: ... Note that your radiator's operating temperature must be lower ... details of specific heat engines and not looking at the big ... alloy) reflector set up to direct the radiation in a small cone. ...
    (rec.arts.sf.science)