Liquid-fueled rocket engines almost always seem to have lower specific impulse at sea level than in a vacuum. I know of one reason: 1. Nozzle expansion. Many engines are optimized for higher-altitude performance, so their nozzle is over-expanded for lower altitudes. I would say more, but I think most people familiar with the topic are rather familiar with this one: Underexpansion - the nozzle doesn't expand far enough and the gas plume exerts force on the air instead of the nozzle, wasting energy from the combustion. Good expansion - all of the exhaust is going directly to the right in the picture, so the change in momentum is maximized. Overexpansion - the nozzle goes too far, and ends up re-compressing back to ambient atmospheric pressure. As a result, some momentum is going inward and rightward (right in the picture) motion isn't maximized. But here's what I'm curious about: 2. Atmospheric Pressure So, largely going off of physics guesswork here, but I imagine the fact that there's air exerting a pressure into the combustion chamber will reduce the efficiency of the engine. If the combustion chamber pressure is 1 atmosphere, for example, then the exhaust won't flow out and you'll have zero efficiency, heh. If it's only 2 atmospheres, then it will barely leak out. If it's 200, then it will blast out and create plenty of thrust. So I was wondering, can you reclaim most, if not all, of the reduced sea-level efficiency by using a higher chamber pressure?