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The gas generator power cycle is an open cycle, with pre-burner exhaust vented after driving the turbine instead of contributing to thrust as in more advanced power cycles.

What percentage of the propellant is usually used to feed the pre burner?

Also, what percentage of the potential energy is "wasted"? That is, the energy that could have been used for thrust minus the energy that, while not used for thrust, is used for driving the turbines and therefore is not wasted.

Comparing Isp of staged combustion engines with gas generator engines, of course limiting to the same propellants, leads me to a result of around 5-15%, but there really are too many independent variables for a meaningful comparison.

I also found a mention of 2-7% on the Braeunig site, but unfortunately with no support or theory.

A few examples to get a ballpark figure will also suffice.

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    $\begingroup$ Note that in gas generator open cycle engines, the generator exhaust does leave the engine going fast in the right general direction -- in some engines such as the F-1, it's released along the nozzle walls as a relatively low-temperature curtain to protect the nozzle from the hotter primary exhaust, so the net effect is comparable to a staged engine running at a slightly lower temperature/pressure. I think there are probably too many design variables to give really concrete answers to your question. $\endgroup$ Commented Oct 14, 2016 at 3:05
  • $\begingroup$ But you don't have the nozzle's exhaust velocity boost effect, so I the Isp bonus of just venting the gas in the correct direction should be negligible. I think. :) $\endgroup$ Commented Oct 14, 2016 at 3:13
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    $\begingroup$ I guess the percentage will probably vary depending on the engine. So perhaps the question should specify an engine type, or instead make it clear that an example or two is an acceptable answer. $\endgroup$
    – Andy
    Commented Oct 14, 2016 at 7:57
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    $\begingroup$ It varies depending on the chamber pressure. At lower chamber pressures staged combustion and GG have (virtually) the same ISP. At higher pressures, the ISP in staged combustion rises (slightly), while the ISP in GG drops off, because of the wasted propellant. $\endgroup$ Commented Oct 14, 2016 at 15:12

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