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Is the engine gimbal check done before or after ignition?

Also curious: How many seconds does the check take? What does it consist of? Do they gimbal each engine separately, or do they gimbal all engines simultaneously?

No worries if you don't have references handy. If the answer depends on the type of rocket, feel free to answer for the ones you know. Thank you!

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  • $\begingroup$ Here's a Space Shuttle Gimbal check. It happens 3 minutes before ignition. Blue Origin's New Shepard also does a gimbal check before ignition. I don't think rockets with many engines mostly do a gimbal check. As far as i know, the Saturn V (5 engines) did not do a gimbal check, and I don't think Falcon 9 (9 engines) does it either (i have to look into that). $\endgroup$ – Star Man Jan 4 at 5:12
  • $\begingroup$ Related: space.stackexchange.com/questions/42023/… $\endgroup$ – Star Man Jan 4 at 5:14
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    $\begingroup$ To do a gimbal test after ignition, you either have to hold the rocket on the pad while performing the test (wasting propellant and putting a lot of stress on the rocket and launch structure), or you do the test while flying, which a) will alter the thrust vector and thus the trajectory and b) is not the time to discover a problem. $\endgroup$ – John Bode Jan 4 at 17:39
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    $\begingroup$ On Apollo 11 the SPS gimbal check was carried out an hour before launch: the PAO reported, "T minus 61 minutes on the Apollo 11 countdown, and all elements are Go at this time. Astronaut Neil Armstrong has just completed a series of checks on that big Service Propulsion System engine that sits below him in the stack. We want to assure ourselves before lift-off that that engine can respond to commands from inside the spacecraft. As Neil Armstrong moved his rotational hand controller we assured ourselves that the engine did respond by swivelling or gimballing." $\endgroup$ – GordonD Jan 4 at 22:13
  • $\begingroup$ @GordonD sounds like an answer. $\endgroup$ – Organic Marble Jan 4 at 22:25
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It is unlikely that gimbal checks would be done on a running engine (at least in flight) due to the resultant effects on the vehicle.

Shuttle:

In addition to the famous prelaunch gimbal test of the main engines prior to launch as described here, gimbal checks were also done on the Orbital Maneuvering Engines (OMEs) and the Solid Rocket Boosters (SRBs) thrust vector control systems.

  • Orbital Maneuvering System

The OMEs were 6000 lbf pressure-fed engines burning nitrogen tetroxide and mono-methyl hydrazine. They were gimballed using electromechanical actuators.

On some missions, a prelaunch gimbal check was performed on the OMS at T-7 hours 30 minutes. The criteria were as follows

OMS GIMBAL PROFILE IS ONLY REQUIRED ONE TIME PRIOR TO LAUNCH MAKING THE REPEAT OF THE TEST OPTIONAL FOR A RECYCLE LAUNCH COUNTDOWN. DO NOT PERFORM THIS TEST IF IT HAS BEEN PERFORMED IN THE PREVIOUS COUNTDOWN UNLESS THERE IS CONCERN OVER ACTIVITY IN THE GENERAL LRU AREA OR CONCERN OVER POSSIBLE TREND DATA OR CONCERN OVER THE TIME ELAPSED SINCE THE LAST TEST.

(Apologies for all caps, that is how it is written in the OMI S0007 Shuttle Countdown)

In flight, a gimbal check was performed by executing an ITEM 34 on the Maneuver display.

enter image description here

The gimbal check flag caused the engines to be commanded to plus 7° yaw and 6° pitch, then to minus 7° yaw and 6° pitch, and back to zero° yaw and pitch.

On a nominal mission, these gimbal checks were performed twice, once after the OMS-2 circularization burn using procedures in the Ascent Checklist, and once before the deorbit burn, using procedures in the Entry Checklist. The Entry Checklist procedure is shown.

enter image description here

If problems were suspected with the thrust vector control system, gimbal checks could be done as required.

  • Solid Rocket Boosters

The shuttle Solid Rocket Boosters thrust vector control system was a hydraulic system pressurized by two hydrazine-powered Hydraulic Power Units (HPU) in each booster. The HPUs were started at T-26 seconds and the gimbal test was started at T-21 seconds and nominally completed by T-16 seconds.

References:

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  • $\begingroup$ @Organic_Marble Thank you! This is huge help. Thanks also for the link to the shuttle crew operations manual. $\endgroup$ – user36480 Jan 4 at 19:16
  • $\begingroup$ @Alex you're welcome. I am looking for more details on the SRB test. $\endgroup$ – Organic Marble Jan 4 at 19:34

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