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Saturn V would automatically initiate launch abort if:

  1. Launch vehicle rates exceeded a threshold (4 deg pitch/yaw, 20 deg roll)
  2. Two or more booster engines dropped below 90 % rated thrust

I'm assuming a drop below 90% rated thrust would indicate an engine failure.

And I want to just say "got it, monitor thrust and trigger launch abort if two below 90 %."

But in practical terms... how would you monitor engine thrust? Just take your accelerometer's reading and multiply it by the current mass estimate? It's one way, but how was it actually done? Anybody?

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    $\begingroup$ I don't know how it was actually done. Acceleration is a good parameter to use. Chamber pressure can be used as a proxy for thrust. It has the added advantage that you get a reading for each chamber, while acceleration is an aggregate value. $\endgroup$ – AJN May 31 at 4:03
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    $\begingroup$ You can just measure the elastic deformation of the engine mount, which is linear to the force on it, with a strain gauge. I think this is how all electric scales works now. $\endgroup$ – user3528438 May 31 at 4:43
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    $\begingroup$ @AJN Acceleration can work nicely for tiny vehicles such as Mini AERCam. But for a sizable vehicle, detecting RCS failures using acceleration is just a very bad idea. Been there, done that, with a vehicle that was eventually cancelled solely because the vehicle's budget matched nicely with an ISS funding shortage. Solving the noise-to-signal ratio problem for that vehicle was quite challenging. I called it a noise-to-signal ratio problem because the signal-to-noise ratio was less than unity. $\endgroup$ – David Hammen May 31 at 11:44
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    $\begingroup$ @DavidHammen By RCS, do you mean the smaller engines (used in final stages or for attitude control) ? You may be right. But I think OP is asking about the powerful booster engines whose failure will definitely put a dent on the acceleration signal IMO. Usually the rockets will have a very accurate accelerometers for GNC. $\endgroup$ – AJN May 31 at 11:53
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    $\begingroup$ Your question on the clamps reminded me that engine performance need to be monitored even before lift off. Accelerometer won't help while the rocket is clamped to earth! $\endgroup$ – AJN May 31 at 11:55
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Apollo-era engines had redundant THRUST OK chamber-pressure-actuated switches that toggled to ON at a certain value of the chamber pressure (for the F-1, it was 90% of nominal).

1-130. Engine mainstage is that period of engine operation that is initiated when the engine has attained 90 percent of its rated thrust. Mainstage is signalled by the actuation of the thrust OK pressure switches.

F-1 Training Manual

This oft-referred-to diagram shows the redundancy (green arrows)

enter image description here

From this answer

Modern engines have engine controller computers that monitor a number of parameters. As David Hammen writes in his answer it would be wise to pick a variety of parameters to use for the shutdown decision. The only in-flight engine failure shutdown in the shuttle program was caused by bad sensors. In that case there were multiple sensors, but all checking the same parameter, and all with the same design flaw.

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Just take your accelerometer's reading and multiply it by the current mass estimate?

That is not a good idea. I'll start with launch, where using sensed acceleration wouldn't work at all. Launch vehicles are held down by hold-down clamps until engine ignition is confirmed. Most launch vehicles shut themselves down if ignition is not confirmed. An accelerometer is useless for this because the hold-down clamps prevent the vehicle from accelerating upward.

The solution is to use multiple sensors. Flow rate sensors, combustion chamber pressure sensors, combustion chamber temperature sensors, ... The key problem here is that sensors can and do fail. The combustion chamber in particular is a very hostile environment. The Space Shuttle experienced multiple Redundant Set Launch Sequencer (RSLS) aborts, at least one of which was caused by sensor failures rather than effector failures.

Once a vehicle has launched, accelerometer readings can provide clues regarding main engine failures, but only clues. The propulsion system / thruster sensors that are need to confirm ignition provide better clues, so long as those sensors haven't failed. The solution is to use reliable sensors that are unlikely to fail, and redundant sensors in case one does fail.

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