Reply To: General thought on old coal mines.

[vansig]

zapkitty wrote:
… but energy density is still the problem. Even with the most optimistic shielding estimates you still have to have over 4.2 metric tons of water with each power module in addition to the FF cores, caps, onion etc… and both safety and cooling mean that you will not be able to squeeze all the cores into one module.

not quite. the power modules don’t need to be shielded from each other; so really there is just enough shielding to protect the payload and crew. i haven’t worked the numbers through, for EHD, as i’m not yet optimistic about it, for different reasons. but regardless, it seems the biggest problem to manage is heat, after all.

for 10 GWt of heat rejection,
keeping airfoil surfaces below glowing red hot, or around 800 kelvin, requires either a radiative surface area of 43 hectares,
or expulsion of coolant mass. ( 800 K radiates at most 23.226 kW/m²; and 10 GW / 23.226 kW/m² = 4.30e5 m² )
the situation is improved if very high temperatures can be tolerated. for 1600 K, 2 hectares would do; and for 3200 K, 1681 m² would do.
Ta4HfC5 melts at 4488 K, but is not exceptionally resistant to oxidation; layered hafnium carbide/silicon carbide seems the most resistant to ablation in air, (ablation rate of 5 µm/s measured at 595 kW/m², which corresponds to T=1800K).

it would be nice if all of the heat could somehow be carried away by air flowing over the wings and through the engine, but to accomplish that, i am told, we’d have to avoid stagnant air anywhere in the system.