Following up to some questions asked in comments to the answer to this question: With pressure-fed engines, is any measurable thrust generated by venting the pressurant out the engine bell after flameout?

After Main Engine Cut-Off (MECO) there was ~3500 lbm of LOX trapped in the Main Propulsion System (MPS) plumbing and the Space Shuttle Main Engines (SSMEs). This prop was dumped through the engine bells shortly after MECO in a helium-pressurized "LOX Dump". How much thrust did it provide?


1 Answer 1


Please refer to this (modified) schematic diagram from the crew's Ascent Pocket Checklist for this discussion.

enter image description here

The blue shows the parts of the plumbing still filled with LOX after MECO. The green is the helium pressurization line.

Two minutes after MECO the onboard computers start the dump by opening the LO2 Manifold Pressurization Valve (1), the LO2 Prevalves (2), and sending a LOX DUMP command to the SSME Controllers (SSMECs), which respond by opening their Main Oxidizer Valves (MOVs)(3). (The Orbiter's hydrazine-fueled Auxiliary Power Units (APUs) must still be running to perform a nominal dump since the MOVs require hydraulic power to operate. This is a major reason for performing the dump soon after MECO so that the APUs can be shut down ASAP).

The subsequent flowrate and thrust profiles looked like this:

enter image description here

(Note these plots show total numbers, divide by 3 for per-SSME numbers).

The valves stay open for two minutes, then the onboard computers terminate the dump by closing the LO2 Manifold Repress valve and sending a TERM SEQUENCE command to the SSMECs, which respond by closing the MOVs.

Other References

1988 STS Press Reference Manual: Helium, Oxidizer, and Fuel Flow Sequence (See document page 238, pdf page 254)

Personal Notes

  • 3
    $\begingroup$ That works out, per engine, to around ~3kN spike, sustained 1kN. Specific impulse somewhere around 50-60 seconds if I haven't screwed up my units too badly? $\endgroup$ Commented Jun 23, 2018 at 17:35
  • $\begingroup$ Concur, that's in the ballpark. $\endgroup$ Commented Jun 23, 2018 at 18:09
  • 6
    $\begingroup$ And with a ballpark of 80t orbiter+payload, rounding the total thrust as 100s of 3kN sustained (initial spike and final drop-off canceling each other), we're getting 3.75m/s delta-V. $\endgroup$
    – SF.
    Commented Jun 23, 2018 at 18:53
  • $\begingroup$ In the early days when an OMS-1 apogee raising burn was done, this dump happened simo with the burn. The later "direct insertion" missions skipped OMS-1 and just did the dump by itself. $\endgroup$ Commented Jun 23, 2018 at 18:56

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