# Engine failure: how to detect?

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?

• 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.
– AJN
Commented May 31, 2021 at 4:03
• 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. Commented May 31, 2021 at 4:43
• @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. Commented May 31, 2021 at 11:44
• @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.
– AJN
Commented May 31, 2021 at 11:53
• 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!
– AJN
Commented May 31, 2021 at 11:55

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)