<P> The effect of exhaust molecular weight on nozzle efficiency is most important for nozzles operating near sea level . High expansion rockets operating in a vacuum see a much smaller effect, and so are run less rich . The Saturn - II stage (a LOX / LH rocket) varied its mixture ratio during flight to optimize performance . </P> <P> LOX / hydrocarbon rockets are run only somewhat rich (O / F mass ratio of 3 rather than stoichiometric of 3.4 to 4), because the energy release per unit mass drops off quickly as the mixture ratio deviates from stoichiometric . LOX / LH rockets are run very rich (O / F mass ratio of 4 rather than stoichiometric 8) because hydrogen is so light that the energy release per unit mass of propellant drops very slowly with extra hydrogen . In fact, LOX / LH rockets are generally limited in how rich they run by the performance penalty of the mass of the extra hydrogen tankage, rather than the mass of the hydrogen itself . </P> <P> Another reason for running rich is that off - stoichiometric mixtures burn cooler than stoichiometric mixtures, which makes engine cooling easier . Because fuel - rich combustion products are less chemically reactive (corrosive) than oxygenated products, vast majority of rocket engines are designed to run fuel - rich, with at least one exception for the Russian RD - 180 preburner, which burns LOX and RP - 1 at a ratio of 2.72 . </P> <P> Additionally, mixture ratios can be dynamic during launch . This can be exploited with designs that adjust the oxidizer to fuel ratio (along with overall thrust) during the flight to maximize overall system performance . For instance, during lift - off thrust is a premium while specific impulse is less so . As such, the system can be optimized by carefully adjusting the O / F ratio so the engine runs cooler at higher thrust levels . This also allows for the engine to be designed slightly more compactly, improving its overall thrust to weight performance . </P>

What type of rocket fuel does nasa use