David.Smith@CMU-CS-IUS.ARPA (06/19/84)
I think there is a flaw in that argument based on the rocket engine box diagrams. I agree that with the box bottom closed, there is no net thrust. There is also no flow of propellant into the box, if the pressure is to remain the same. The second box, with the bottom removed, requires a certain propellant flow into the chamber in order to keep the pressure at 100 psi. Fine. Now the third box is in the 15 psi atmosphere, so that the pressure differential is 85 psi. The thrust is now 85 pounds instead of 100. OK, I'll buy that. ------ BUT ------ The 85 psi pressure differential causes less loss of combustion products out the bottom. Therefore, maintenance of the 100 psi chamber pressure requires less propellant input to the engine. So all we have proven is that burning less fuel in the atmosphere produces less thrust than burning more fuel in space. Alternatively, if the mass flow is the same, the chamber pressure will rise above 100 psi. David Smith