Re: General Setting/Tech Musings

MacFrag...@xxxxxxxxxxxxxx wrote:
On 10 Jul., 01:59, Luke Campbell <lwc...@xxxxxxxxx> wrote:

If you want to remove heat quickly, you can use diamond (which has an
anomalously high thermal conductivity and can withstand withering
temperatures before it decomposes into graphite) and active heat
transport with a working fluid pumped through pipes and heat
exchangers.

Sounds fair enough. Concerning that fluid, would liquid Lithium be the
element of choice? I suppose that would mean you have to keep the
whole system on temperature at all times, as letting the Lithium get
cold and solid would clog the pipes and ruin the system.
Let's also talk about droplet radiators for a bit. What would be the
best coolant, also Lithium, or something else?

I am guessing you could use a staged set of coolants, perhaps water-
sodium-lithium. When everything is cold and solid except for the
stuff near the reaction bell and the lowest temperature coolant, you
use the low temperature coolant to distribute the heat to the next
higher temperature coolant until it melts and can flow through the
pipes. Then you use that to distribute heat to the coolant with the
next higher melting point, and so on. Lithium has a higher melting
and boiling point than sodium, hence my idea for water first melting
the sodium melting the lithium. Lithium boils at 1615 K, diamond
converts to graphite at 1973 K, so the points of failure of both
materials are similar (so, you don't quite get to run the bell as hot
as 3000 K like I used for my initial estimates). You do need to worry
about lithium reacting with the diamond to form lithium carbide (and
similarly sodium plus diamond to sodium carbide), so line the lithium
and sodium pipes with something unreactive with either alkali metals
and diamond (for example, not steel, because carbon dissolves in
iron). Perhaps quartz or corundum (a.k.a alumina and sapphire).
lithium will do the heavy lifting as far as removing heat, the other
stuff need not have as extensive of plumbing as long as it can get the
lithium flowing.

Other materials might work as well or better as a coolant than
lithium, I haven't explored all the possibilities by a long shot.
Perhaps some sort of molten salt.

For radiators, if you want to make your spacecraft look sleek and
sexy, what about gracefully swept radiator "wings" made out of diamond
conduits for sapphire-lined lithium heat pipes? If you run at 1600 K,
you radiate 370 kW per square meter. A spacecraft with 10 GW driver
beams for the torch running at 50% efficiency will need to get rid of
10 GW of heat, so it would need 27,000 square meters of radiator.
That's a square 170 meters on a side or a circle 90 meters in radius.

A lithium droplet radiator can't run as hot as contained lithium in
pipes, because at high temperatures some lithium will evaporate from
the surface of the droplets (in fact, in zero pressure, the droplets
will boil at much lower temperature since the only thing holding back
the vapor pressure is the surface tension of the droplets). The
Clausius-Clapeyron equation gives a vapor pressure of
P = (57235 atm) * exp(-(17692 K)/T)
for T the temperature and P the vapor pressure. Even at 1000 K, you
have a vapor pressure of 1/1000 of an atmosphere so you will be losing
a lot of your coolant from a droplet radiator (at 800 K, you've got
1.4E-5 atm, and at 600 K you've got 9E-9 atm, so 600 K seems pretty
reasonable). At 1000 K, you need 6.5 times more radiator area than at
1600 K, and at 600 K you need 50 times more area. This gives your
spacecraft long spider-arms for spraying and collecting droplets over
an area with linear dimensions 7 times larger than for a diamond/Li
radiator at 1600 K (assuming 600 K lithium droplets). This need to be
a bad thing, depending on the look you are going for.

I will also mention that the diamond/Li radiator could plausibly be
quite damage resistant. While individual kinetic impactors may punch
through bits, just seal off those coolant tubes and re-route through
undamaged areas. Use nanostructured toughened diamondoid rather than
perfect crystalline diamond to avoid having your radiators shatter
when struck. Instead, you get an effect more like shooting at a
cardboard target - you need to fire a lot of bullets downrange before
you destroy the target, even though each bullet punches a hole in it.

Once you have GW fusion reactors, you can light up an enclosed reactor
and use it to directly heat air. Use variable bypass turbo-air rams
to scoop up and compress air, heat it with the reactor, then shoot out
a jet at much higher velocities. <snip>

Sweet, thanks for the ideas. You still need of auxiliary engine to
attain the necessary speed for the ramjet to work, right? Or maybe a
bimodal turbine/ramjet engine, like a futuristic SR-71.

Yeah, that's what I was calling the variable bypass turbo-air ram.
Use a turbine at low speeds, gradually disengage it at high speeds to
use the ram effect.

problem is wicking the heat away from the laser crystals/
semiconductors/whatever to the radiators.

So the lasers will also need an active cooling cycle like the engines,
with the difference that you can't use a closed cycle with liquid
metal as coolant because the laser isn't going to produce heat all the
time.

Not to mention that the temperature of molten lithium will flat out
destroy the laser components (at least, if they are diode or solid
state lasers. Free electron lasers could plausibly take high
temperatures). Use water for a coolant to keep everything below 100 C
(373 K). A heat pump can shunt the heat from the water to the lithium
if you want to use the same radiators for both.

So you're limited to either just diamond heat conductors or
plumbing with some other, less awkward coolant. Liquid metal open
cycle cooling should work, however. Which means your laser needs ammo.
If (and I mean IF) you can vaporize the Lithium coolant and "charge"
it up to 3000K before ejecting it, you get rid of about 10GJ per ton.
That figure should also help us to determine workable laser powers.
Say how many tons of expendable coolant you're willing to lug around,
and how many shots you want to fire, the rest is doing numbers. For
instance, if you can spare 10t for laser coolant, and want to fire at
least 1000 0.1s-bursts, and have 50% efficiency, that's 100GJ waste
heat for 100s of fire --> you can have a 1GW laser (if the rest of the
technology plays along, like physical limits on the mirrors and the
like).
If you have to eject the Li right after it evaporates, that figure is
reduced to <6GJ/t at best.

One thing to consider - if you are using laser inertial confinement
fusion to ignite your torch drives, you need to have lasers that can
put out at least 1% of the torch output power and do so continuously
for days on end. This means a 1 TW torch automatically gets 10 GW
laser beams that can somehow deal with the heat issues. Furthermore,
they are lasers that can deliver nanosecond pulses of several
megajoules each. If focused so they are intense enough to flash-
vaporize a plasma layer, this will drive an impulsive shockwave with a
similar effect to the detonation of about a kg or so of TNT (4.184 MJ
per kg of detonating TNT). At 10 GW, this kg of TNT explosion would
be repeated about 2000 times per second. At close range (several
thousand km or more, I haven't worked out the threshold yet), these
things would be a buzz-saw rapidly blasting through anything the
spacecraft commander didn't like.

This was one reason for considering heavy ion beams instead of lasers
for igniting the torch (the other being that in theory heavy ion beams
are a bit more efficient). Ion beams are quite impractical for space
combat - they disperse way too fast. However, the same tech that
gives you heavy ion beams also gives you free electron lasers at about
the same efficiency. So even with ion beam drivers, you have high
power high efficiency lasers.

FWIW, I think that due to the military situation (or lack thereof) in
my setting, space lasers are currently produced for two purposes: one,
as point defense against incoming kinetic slugs, natural or otherwise;
and two, if required by the tech chosen, for power generation (as the
laser-induced fusion you mentioned).
There simply won't be powerful dedicated shipkiller deathrays because
there is no space navy. Earth will not be unified, rather arranged
into blocks/nations/alliances, but even in the event of war this will
take place on the ground. I believe planetary defenses will beat
spaceborne attackers every time, due to superior camouflage, power
supply, and above all, cooling.
By the time we enter the setting, the first colonies (Mars or
extrasolar ones) may feel a compelling drive to independence, but the
first independence war will be fought with refitted freighters and
couriers rather than dedicated warships. I also think that's going to
be much more fun.

Another thing to consider is that each and every one of your fusion
torch tramp freighters become a devastating weapon of mass
destruction. The Hiroshima bomb put out roughly 50 TJ of energy.
Thus, a TW torch will put out as much energy as a Hiroshima bomb every
50 seconds. The real kicker is to realize much of this energy is
being added to the kinetic energy of the spacecraft. At 900 km/s of
relative delta-V (the delta-V from 25 hours of 1G acceleration from
the D-He fusion torch analyzed earlier), a 20 ton spacecraft packs the
energy of 325 Hiroshima bombs, or about 3.9 megatons of explosive
force. A sacrificial freighter could use its full 1800 km/s of delta-
V to quadruple that, for about 15 megatons. Ram a robot freighter
into the atmosphere over the U.S. eastern seaboard and you can take
out New York, White Plains, and much of New Jersey (note that the
spacecraft will disintegrate in the upper atmosphere as soon as the
ram pressure overcomes its structural strength). Against this sort of
firepower, are you really worried about lasers?

Again, thanks a lot for your input, Luke, I appreciate it and try to
adopt as much as possible.

Glad I could help.

Luke

Luke
.

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