This question is about use of high energy propellants on a rocket engine designed for low energy propellants. In any engine, which is currently working on RP1+LOX, if we change the fuel from RP1 to LH2, and off course make other necessary changes to suit the requirements of the new fuel (e.g storage tanks, temp., changes in propellant ratio etc. - keeping the overall mass flow rate of propellant same - if required), will it provide the same thrust? or will it change.. for better or worse? This might sound similar to "what if petrol is used instead of diesel" or vice-versa, - in an automobile. But there the working principles are different. Unlike so in a rocket engine, hence the question.
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4$\begingroup$ You also have to change the pumps and the other plumbing. So it is not so straight forward. "make other necessary changes to suit the requirements of the new fuel". This is not so simple. It will become a totally new engine. Then you might as well just compare the ISP at that point. If you compare just ISP, then, larger ISP propellant for same mass flow rate will give higher thrust (by definition) $\endgroup$– AJNCommented Jun 7, 2021 at 14:59
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2$\begingroup$ Agree with AJN, not that straightforward, but if your question is simplified to: Do different propellant combinations produce the same thrust at equal total mass flow? The answer is simply no $\endgroup$– RubenCommented Jun 7, 2021 at 15:02
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3$\begingroup$ Just like gasoline and diesel run on different "cycles", rocket engines can also run on different cycles. A cycle suited for LH2LOX may not be the best for RPLOX combination. So the gasoline/diesel analogy may still hold. $\endgroup$– AJNCommented Jun 7, 2021 at 15:04
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$\begingroup$ Most obviously there comes a point where an engine designed for low-energy propellants will burn out when fed with high-energy… Before that if you change the fuel and make the necessary changes to suit the new requirements, why might it not provide the same thrust? $\endgroup$– Robbie GoodwinCommented Jun 8, 2021 at 0:24
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1 Answer
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Thrust is equal to mass flow rate times specific impulse.
If you maintain the mass flow rate, then if the specific impulse is increased (almost a certainty if you change from RP-1 to hydrogen) the thrust will increase proportionally.
In practice, maintaining the mass flow rate would require a totally different turbopump and combustion chamber, because hydrogen is something like 12 times less dense than kerosene, so it couldn't really be said to be "the same engine".
The LR87 of the Titan II is one engine that was adapted from dense fuels to hydrogen; despite the fact that specific impulse increased around 50% in its experimental hydrogen-fueled version, thrust was significantly lower, because the mass flow rate of the original couldn’t be maintained.
Besides the LR87, the RL10 is the only other example I know; this is a small hydrogen engine that has been experimentally fired with methane and propane, presumably with increased thrust and decreased specific impulse, but I don't know anything about its performance in those tests.
answered Jun 7, 2021 at 15:29