I used to dream about a partially airbreathing engine, a mixed air/LOX oxidizer. The idea here is that at "low" (35,000 feet) altitudes, there's plenty enough air to run the engine, but as the vehicle climbs, there's less air available. So you add LOX to make up. You lose Isp, of course, but you keep thrust at the same level. LOX also cools and compresses the air, accelerating it to more like the velocity of the vehicle before it enters the combustion zone.<br /><br />When air is suddenly compressed, it's temperature goes up. If it's compressed enough, nitrogen starts combining with oxygen to form NO, a chemical reaction that absorbs energy. So the temperature stays at that level until all the oxygen has combined with the nitrogen, then the temperature of the NO/N2 starts rising again. If more oxygen is available, then more heat can be absorbed by this chemical reaction. This energy is released by combustion, so it's not lost.<br /><br />It's chemically cooling the air, which is in addition to the evaporation of the LOX.<br /><br />mlorrey, do you know anything about such engines? I think you mentioned one in a post some time back. Something about an exisiting jet engine modified for this. I would think you would need a clean sheet of paper; this partially airbreathing engine is neither a jet nor a rocket, it's a whole different animal.