In a typical, modern satellite launch, what triggers the cutoff of the orbital insertion stage's rocket engines?
I can think of three basic possibilities:
- Compute how much fuel you need for the trajectory, load exactly that much, and burn all the fuel
- Bring a little extra fuel and cut off the engines at a specific time
- Bring a little extra fuel and cut off the engines when telemetry indicates you're at the desired altitude and speed
The first two seem like they'd be susceptible to small variations in atmospheric conditions, engine performance, and so on.
A historical example: Apollo 13's second stage. The center engine cut off early due to pogo oscillation, when the fuel pressure dropped below a shutdown threshold which was intended to make sure the engine stopped cleanly when the stage started to run out of fuel (which itself implies that option 1 is a little uncertain; not all the fuel is going to get burned).
To compensate, they simply ran the second stage longer. Was that a natural result of consuming all the remaining fuel through four engines instead of 5 (implying option 1), or did they have to explicitly command a later cutoff time (implying option 2), or was the compensation completely automatic (option 3)?
Note that I'm asking about the initial launch and orbital insertion phase, not about fine-tuning the orbit and/or rendezvousing with a target via multiple incremental burns.