Reply To: General thought on old coal mines.

[vansig]

ok, these numbers are a little better.. they do not yet deal with heat management, but i’ve looked at other flight parameters, such as lift, drag, rate of climb, shock waves; at high mach numbers, air intake needs to be a large fraction of the forward facing area, (about half). at hypersonic speeds, leading surfaces will be attached to the shock wave, to stabilize flight and prevent air from becoming stagnant.

let me know what looks impossible here.

assume 100t mass; (47t empty);
lift/drag ratio 4.5; glide ratio 4.5 (about the same ratios as the space shuttle)
122m² underwing area

calculate, density-versus speed, given approx constant air inflow of 80 kg/s, 600 kN thrust, 15500 m/s exhaust velocity, and drag from 218 kN to 300 kN. the engine possibly cannot operate at below mach 2.4, but i have not proved that. particularly, thrust may possibly be doubled in dense atmosphere, with extra energy.. which would enable vertical take-off.

mach 2.4+ at 36 km altitude (density ~.01 kg/m³; drag ~218 kN);
mach 8 at 45 km altitude (density ~.003 kg/m³);
mach 24 at 54 km altitude (density ~.001; drag ~300 kN);
net forward propulsion to >100km

disregarding efficiency, *any* sort of propulsion needs to add a lot of kinetic energy to the incoming air
E_k = .5mv² = .5(80 kg)(15500)² = 7 to 10 GJ each second

if system efficiency were terrible, say 1%, then
decaborane consumption rate would be 14.5 g /s, and this makes
Isp = 600,000 kg m/s² / ( .0145 kg/s ) / 9.8 m/s² = 4.2e6 s

efficiency is probably not that terrible. like VASIMR, traditional pressure from heating becomes inefficient at the needed exhaust velocity, so the engine will rapidly ionize air and then accelerate it magnetically, before it thermalizes. I’ve not seen a good argument as to why VASIMR is supposed to be operated in vacuum only. However, Ad Astra does discuss formula for efficiency of their engine. http://www.adastrarocket.com/AIAA-2010-6772-196_small.pdf

finally, thrust bottoms out at ~2 kN in space proper, where alpha particles go directly to exhaust

Oh, and here’s a nice article, mentioning DPF and nuclear thermal rockets
http://quantumg.blogspot.com/2011/02/making-fusion-rockets-relevant.html