Specific impulse of exhaust velocity depends on the temperature in the combustion chamber. Won't regenerative cooling lower the chamber temperature, causing the exhaust velocity to decrease?

  • $\begingroup$ The chamber temperature close to the walls is lower, but at the center of the chamber there is very little influence. But without cooling, a chamber for LOX and LH2 would survive only some seconds. $\endgroup$
    – Uwe
    Commented Jan 2, 2018 at 12:10

1 Answer 1


In a regeneratively cooled engine the heat extracted from the chamber and/or nozzle walls is absorbed by the fuel, which is later burned. So compared to e.g. an ablatively cooled engine, the reactants start combustion at a higher temperature and therefore finish combustion at a higher temperature, with more total energy available for conversion to velocity. Therefore regenerative cooling actually increases Isp compared to other cooling methods, by recapturing some of the energy that would otherwise be lost (radiated or ablated away) and returning it to where it is wanted.

  • $\begingroup$ Then as for liquid hydrogen & oxygen use on the space shuttle, won't there be a lot of losses in specific impulse compared to using them at room temperature? I know we need to cool them to sufficiently low temperature for storage but I feel like there's a lot of loss $\endgroup$
    – newbie125
    Commented Jan 2, 2018 at 14:49
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    $\begingroup$ We do, and it's called a regeneratively cooled engine..... $\endgroup$ Commented Jan 2, 2018 at 15:45
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    $\begingroup$ (if you meant in addition to that, where are you going to get the heat?) $\endgroup$ Commented Jan 2, 2018 at 15:46
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    $\begingroup$ @newbie125 I think you underestimate the amount of thermal mass represented by the fuel and oxidizer. Without even doing the math, I'd wager that using batteries to preheat the reactants isn't possible at all in any significant way. $\endgroup$ Commented Jan 2, 2018 at 17:11
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    $\begingroup$ What would the mass of batteries be to heat up 230,000 lb of cryogenic lh2? I'd say that idea is a non starter. $\endgroup$ Commented Jan 2, 2018 at 17:50

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