Post by n2maniac on Dec 23, 2016 22:15:04 GMT
Going to re-suggest aiming for a radiator operating at 2100K to 2600K. The math/plot is here.
Other than a few details (helium boils at 4K and absorbs almost no heat upon boiling) open cycle would work very well for a reactor that needed to operate for less than, say, 5 minutes. There is another thread on that topic elsewhere, but a larger delta-T heat engine would improve its case.
You wouldn't actually go to 4k for liquid helium, you'd go to liquid hydrogen (~20k) which has a decent specific heat of almost 10kj/kg.
Obviously, you can't run your reactor 24/7, but long enough to destroy any target of your main laser? Perfectly viable.
Shamelessly self-quoting from the heatsink thread as it seems relevant (there was probably one more thread on this, can't seem to find it):
...sublimators, such as those used in manned spacecraft.
Doing some math on this, it would take ~800 tons of water to cool my 3GW (thermal) reactor on my 8kt frigate for 10 minutes (main assumption: 2250 kJ/kg enthalpy of vaporization does all of the cooling). My 15cm thick SiC radiators on the ship (which also weigh about 800 tons total) could be replaced with very thin ones (~1.5cm or less) and just be retracted as combat begins. This gets even easier on the stock ships (1 has 1GW, 2 have 500MW, and the rest are under 300MW). This seems a worthwhile option. Methane (~20% as mass effective but common on ships as a propellant) and sodium (common on ships as a coolant and ~2x as mass effective as water) would also be good candidates.
This seems even more valuable for running lasers that don't work very effectively above 1200K whose radiators 1) outweigh the 2400K reactor radiators and 2) only operate during combat. Flushed coolant would be "ammo".
Both resistojets and nuclear thermal rockets are big heat exchangers to vented gasses and could be modelled as the heat exchangers (indeed, they are essentially focused heat sinks).
...
And that was with a naive "lets just run the output temperature the same and boil off coolant at a wildly different temperature" assumption. I would have some concerns on reactor complexity to actually utilize that cold of a sink temperature effectively (would be a lot of stages?), but pretty dang viable for combat timeframes. Or, you know, that instant your radiators get shot off and you know you have nothing left to lose.