On Feb 23, 11:16 pm, IsaacKuo <mech...@[EMAIL PROTECTED]
> wrote:
> On Feb 23, 8:42 pm, "dwight.thi...@[EMAIL PROTECTED]
"
> <dwight.thi...@[EMAIL PROTECTED]
> wrote:
> > On Feb 23, 8:17 pm, IsaacKuo <mech...@[EMAIL PROTECTED]
> wrote:
> > > On Feb 23, 7:56 pm, "dwight.thi...@[EMAIL PROTECTED]
"
> > > <dwight.thi...@[EMAIL PROTECTED]
> wrote:
> > > I've done numbers on various "stealth" radiators before, usually
> > > coming from the perspective of trying to design one (i.e. I was
> > > "pro-stealth"). I don't remember the specifics, but basically I
> > > settled on a design with a 60 degree radiation cone. I wanted
> > > to design one with a 15 degree radiation cone, but the numbers
> > > never came anywhere close to adding up.
> > Instead of saying this, why don't you just show the numbers?
>
> Because I don't feel like looking them up or redoing them. You
> don't like that? Well, deal with it. I simply don't feel like going
> through the effort.
>
> > > If you have a better design in mind, I'm all ears.
> > What? A radiator whose output is redirected by an advanced, actively
> > cooled optics system?
> > Something along the lines of a paraboloid with the radiating surfaces
> > at the focus?
> > There's not a whole lot more to say.
>
> That's the design which I had worked on before, and rejected
> it on the grounds that the amount of active cooling required
> exceeded the amount of power available. Well, I didn't
> "reject" it, exactly, I just wrestled with the numbers until I
> got something which worked about as well as I could make
> it (a 60 degree cone).
>
> If you have nothing more to say, then color me completely
> and utterly unconvinced.
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.
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.
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.)
[1] http://www.energyfromthorium.com/javaws/BraytonSim.html
[2] http://www.belarusembassy.org/science/275.pdf
[3]
http://web.mac.com/majumdargroup/iWeb/Site/Main_files/NANOTECH%20ALERT%20--%20JANUARY%2018,%202008.html#2
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.
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. So if we are only taking 5mw heat
into the system that leaves us with 1mw of thermal energy to be rid of
and 4mw of electricity to do it with. This isn't very farfetched,
there is a lot of talk about putting this into play for existing power
plants in the next few years if an efficient way to manufacture these
things can be found, which seems likely. The US DoD is investing into
this; they have plans to field diesel engines with thermoelectric
devices to scavenge heat by 2010 to 2014 IIRC. (I found a US DoE
PowerPoint that discussed this.) There is also another method of
scavenging heat that is also indicated to be 60% efficient if the
silicon version does not pan out, but it is only 60% efficient at much
higher temperatures. (800 Kelvin as opposed to power plant exhaust
temperatures which are roughly 400 Kelvin.)
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. Still this is a losing battle, since you still spend
more in electricity to pump and compress it than you regain by
scavenging (assuming my calculations are correct). This is that "Red
Queen's Race" again. Luckily we don't need to get the heat in the
system down to 0 watts per second; we just need to get it down to a
few thousand watts per second, which can be conveniently emitted
through the rest of the hull at 50 Kelvin. So we need to get 1000
kilowatts thermal down to about 3 kilowatts thermal. This seems
feasible to me and did pan out with the original calculations I did.
(I'm working on a new set that is more accurate however. See below.)
This of course ignores the remaining electricity in the system. Does
this electricity turn back into heat by running life support and
shooting weapons? Yes, but you aren't really stealthy if you are
shooting weapons, so you would abandon this clunky emission system and
start radiating through simple radiators and crank up the dial on your
reactor. While you are trying to remain stealthy you just keep the
dial on your reactor at just enough to generate for your life-support
and electronics and to reject the heat that is made in the process of
generating that electricity.
I'm still running numbers on a cycle that can do this. I had made a
program so that I could tinker around with the numbers, but since I
made some possibly bad assumptions in the calculations for various
stages, I could not say that it was within an order of magnitude in
accuracy. I'm still running them, making a new cycle that takes into
account the temperature of the helium at various stages so I know how
many compression stages I need to get the helium back up to operating
temperatures for the radiator system.
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.
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