guest blogger john hare
During the discussions on the TAN concept, it turns out that the original invention requires some seriously difficult injection into the supersonic stream to avoid shock losses and other difficulties. Jon confirmed that there were problems, just not the ones I was thinking of. There does seem to be a simple way of getting results as good or better that is accessible and affordable for newspace companies.
Cluster seven thrust chambers with the individual throats feeding a common expansion nozzle. At sea level, all of the chambers fire so that there is a low expansion ratio in effect. A ratio as low as 10 would allow decent performance from relatively low chamber pressures, though 20 would be better if possible. Even an expansion ratio as low as 5 would give the sustainer chamber a ratio of 35 in vacuum. The center chamber is pump fed from the thin walled core tank which goes all the way to orbit. The outer chambers are pressure fed from the drop tanks surrounding the core. As altitude is gained and ambient pressure drops, perimeter chambers are shut down to increase expansion ration and Isp from the remaining thrust chambers.
The outer drop tanks are pressure feed with cross feed to the sustainer tank. Tanks four and seven act as header tanks feeding the outer thrust chambers and replenishing the core propellants. The remaining four pressure tanks feed tanks four and seven two at a time before being dropped in pairs. This makes the vehicle four stage with just dumb tank drops. Or possibly two stage with the first stage dropped in pieces over a period of time.
With all of the expansion nozzle ‘charged off’ to the center sustainer chamber, the six outer chambers should be fairly high thrust to weight ratio affairs with the pressure feed not requiring pumps. Thrust vectoring could be done with differential throttling until the outer chambers are shut down, after which fluid injection, possibly with combustion, could be through the outer chambers. Roll control could be more challenging.
After the outer chambers are shut down, the center sustainer chamber has a very high expansion ratio for very high Isp vacuum operations. Since the sustainer chamber has to handle all of the cooling fluids for the expansion nozzle regenerative cooling as well as the center chamber, an expander cycle might be possible considering the heat flux to be handled. Expansion ratio is 140 if the initial ratio was 20. 85% of the propellant that was aboard at lift off was in tanks that were dropped before throttling down to the center chamber, so thrust to weight in vacuum should be good enough to avoid serious gravity losses. With high Isp, and a clean vehicle after the tanks are dropped, a quite substantial orbital payload should be possible from a vehicle with just one expansion nozzle.
As a side effect, a fairly high thrust to weight ratio should be available right off the pad, which reduces gravity losses. As velocity builds up aerodynamic losses become a factor. Throttle reduction to postpone transonic drag to a higher altitude is done by shutting down chambers which increases expansion ratio some in the middle altitudes.
Congratulations to Armadillo. You’ve earned it.
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