RSE #34 Engine Pressure

Rocket Science for Earthlings # 34, CAN'T STAND THE PRESSURE!!!!

A rocket engine converts the chemical energy of the fuel and oxidizer, into the pressure energy of hot gas in the combustion chamber, which is then converted into kinetic energy by the Laval nozzle. So, you need pressure to make thrust, but how much, and is more always better. I've heard somewhere that you need at least 26 PSI to get to supersonic velocities in a convergent nozzle, so that's about the lowest pressure you can have. The German V2 ran at about 200PSI. The Germans studied higher pressures but thought it wasn't worth the effort. US rocket engineers wanted to do better than the Germans, so they started out at about 500 PSI and have worked their way up to the 3000 PSI chamber pressure of the Space Shuttle Main Engine. A Russian rocket engine runs at 4000 PSI chamber pressure. There's a lot of one-ups-manship in engine pressure levels.

For a turbo pumped rocket engine, higher pressures give you a smaller combustion chamber, and slightly higher specific impulse, (efficiency). But the relationship is one of diminishing returns. Higher pressures place great demands on turbine driven pump systems, which can mean very expensive engines.

For the pressure fed engine systems of Minimum Cost Design launch systems, higher pressures mean heavier tanks and a lower mass ratio. So,... higher pressures give somewhat higher efficiencies, but can hurt overall performance due to weight, and heavier tanks also cost more, but you need pressure to produce thrust, so is more pressure better? Running the numbers it turns out that very low pressure engines, somewhere below 200 PSI, give the best performance in Big Dumb Booster designs, which is really kind of surprising and very gratifying. I probably need to find better data for low pressure engines, but these rockets keep getting dumber and dumber every time I calculate some aspect of their cost performance. They seem to be pushing the limits of stupidity. Very interesting!