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When a bi-propellant liquid rocket engine receives a command to shut down,

  1. will the fuel gets cut off first and then the oxidiser - a fuel lean cutoff?

  2. Will the oxidiser gets cut off first and then the fuel - a fuel rich cut off?

Which shut down mode is preferred and for what reasons(like safety, damage to engine,etc)?

Will the mode differ between static engine testing vs during flight?

Will the mode of cut off depend when a turbine is used to induce propellant flow vs a pressure fed case?

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  • $\begingroup$ Reason to ask about static firing is because we are planning to perform a static testing of a small engine in our lab. We need some concrete reason to follow which mode to follow for safety reasons! $\endgroup$
    – karthikeyan
    Commented Jul 20, 2018 at 16:17
  • $\begingroup$ If the small engine is cooled by the fuel, cooling should be continued during engine cutoff to prevent damages by heat. If the hot engine may be damaged by oxidation through excess oxidiser, a fuel rich cut off may be preferred. Do you want to reuse the engine after static tests as well as after flight? $\endgroup$
    – Uwe
    Commented Jul 20, 2018 at 16:42
  • $\begingroup$ @Uwe probably after a static test but not after flight. $\endgroup$
    – karthikeyan
    Commented Jul 20, 2018 at 16:46
  • $\begingroup$ A restartable engine should survive a static test undamaged. If the engine may not be reused after a static test, numerous tests for certification will be a bit expensive. $\endgroup$
    – Uwe
    Commented Jul 20, 2018 at 22:07

1 Answer 1

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For liquid hydrogen / liquid oxygen rocket engines, O2 rich cutoff is disastrous. Extra H2 was loaded into the shuttle's external tank to ensure that small performance problems or analysis errors would not result in an O2 rich cutoff. This was referred to as the "fuel bias". On a nominal day, this extra fuel was carried to orbit, wasting payload capability, so you can see how important it was to prevent a fuel depletion cutoff.

Important Safety Note: if propellant depletion occurs, it must occur first on the oxygen side. If the hydrogen runs out first, the last sputters at the turbine will be much closer to stoichiometric, and, well, bad things wil happen. Did occasionally happen early in ground testings. Big mess in the bottom of the flame trench at Stennis. Not what anybody wanted in flight.

source

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  • $\begingroup$ Thanks, first of all! Is a fuel bias basically a shift in equivalence ratio towards stochiometric and a corresponding temperature rise?! $\endgroup$
    – karthikeyan
    Commented Jul 20, 2018 at 16:35
  • $\begingroup$ What they called "fuel bias" was ~1000 lbs of extra LH2 added to make totally sure that you didn't have a fuel depletion cutoff. It didn't affect the engine mixture ratio, it was just a pad to make sure that if you ran out of prop, it would be O2 first. The price was paid in loss of payload to orbit. $\endgroup$
    – Organic Marble
    Commented Jul 20, 2018 at 16:36
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    $\begingroup$ What's the reason O2 rich cutoff was disastrous? $\endgroup$
    – Heopps
    Commented Jul 20, 2018 at 16:56
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    $\begingroup$ From the previous paragraph in the linked source: "The reason for such a wretchedly low mixture ratio is that the closer the MR is driven toward stoichiometric, the hotter the fire. The turbine blades in the turbines that power the pumps feeding the engines can’t take a much hotter fire than results from 6.02. Blades would melt, casings too, bad things indeed would happen." That's only for these particular turbines, though. It's unclear whether other LOX/LH2 engines would have a problem with a stoichiometric mixture. $\endgroup$
    – user2357112
    Commented Jul 20, 2018 at 20:01
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    $\begingroup$ @Heopps: an O2 rich cutoff will leave hot engine parts exposed to a high concentration of oxygen at a pressure above 1 bar. This will damage or destroy the hot parts. There is no fuel left for cooling those parts and protecting them by a thin film of fuel against oxidation. $\endgroup$
    – Uwe
    Commented Jul 20, 2018 at 22:02

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