Re: Slow Stealth

On Feb 25, 2:14 pm, CharlesRCap...@xxxxxxxxx wrote:

Not to be lumped in with Dwight, I find his tone abrasive as well, but
unless my numbers are completely messed up I have a way to deal with
the mirror heating.

Cool! Like I admitted, I'm too lazy to write up my own
"stealth" designs right now. My current efforts are in
fiddling with free electron laser design.

I make a couple of assumptions.

1.) I assume that a Helium Turbine design can be pushed to 50%
efficiency. This is not much of a stretch. The linked simulation
software[1] seems to indicate that you can push the GT-MHGTR design to
50.11% with two intercoolers per reheat.

2.) I assume a reflector of 98% efficiency with blackbody radiation.
Polished gold has an emissivity of 0.02, so I assume that would
translate to 98% efficiency.

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.

3.) I assume a 60% efficient[2] Rough Silicon Nano-Wire Multi-Stage
Electrothermal Device[3].

4.) Said Rough Silicon Nano-Wire Multi-Stage Electrothermal Devices
will be on par with roughly 1.5 grams per watt generated. This is
roughly equivalent to modern aircraft engine power to weight ratios,
which since the linked PDF[2] mentions replacing aircraft engines with
ones based on these devices I think that's plausible.

5.) I also assume that it takes electricity equivalent to 110% of the
heat you wish to move to compress helium back to operating
temperatures. (With 10% of that becoming new heat in the system.)

Now, I'm still working the numbers to take into account new issues
that come up, but so far it still looks workable. You end up using a
lot of electricity getting the heat out in that small cone, but you
still have some left over. The problem is that if you want to pump the
heat out it costs you more electricity than heat remaining to pump it
out. So you end up in a "Red Queen's Race" that Isaac mentioned in
another thread. You can never get rid of all the heat because you keep
making more by cycling the heat from the reflectors to the radiator.

However you don't have to get to 0 watts thermal energy in the system.
You only need to get to a few thousand watts thermal energy in the
system and then send the rest to the cold hull radiating at 50 Kelvin.
So you just need to have enough electricity to keep ahead of the heat
until it gets to that level.

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.

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.

The first step is to get as much of the heat turned into electricity
as possible before it enters the radiator system. A 50% efficient
helium turbine combined with a 60% efficient Electrothermal device
leaves only 20% heat in the system.

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!

Unfortunately, the design constraint isn't just efficiency, it's
also power/weight ratio. The lower the radiator temperature,
the larger the radiator must be for a given power level. For
a 50K hull, you should be able to get decent levels of power
even without worrying about the directional radiator (which
I suspect will be more trouble than it's worth). For a 3K
hull, it doesn't matter how big the hull is--a zillion square
meters multiplied by zero watts per square meter is still
zero watts.

Next we have a radiator design that can put out the heat in a narrow
cone. (I'll leave the actual physical design to other minds or to the
imagination of the reader.) If the radiator reabsorbs less than 50%
(hopefully much less) of what it emits then scavenging the heat for
electricity with another electrothermal device makes it a little
easier to get rid of what's left in the system by eating some of that
heat and then providing electricity to pump the remainder back through
the system again.

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.

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.

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.

Isaac Kuo
.

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