The failure of the Falcon 9 first stage landing during the Eutelsat/ABS mission was attributed to:
which indicate that engine thrusts are part of the telemetry data.
I can see how the thrust can be measured on a test stand. But how can they be monitored during the flight?
In the SSME the "thrust control loop's" independent variable was chamber pressure.
During the engine run phase, the MOV, MFV, and CCV are switched to run schedules, while the OPOV and FPOV are switched to closed-loop operations. The run schedules for the MOV and MFV cause them to simply remain fully open, whereas the run schedule for the CCV drives it between half open at 67 percent thrust (MPL) and fully open at 100 percent thrust (and above). This action maintains the appropriate flow relationships among the several parallel fuel flow paths as the HPFTP output pressure varies with thrust. During engine run, the OPOV and FPOV are used as control devices for thrust and mixture ratio. Manipulating these valves affects the output of the preburners, the speed of the turbopumps, and, therefore, the propellant flowrates. The FPOV is driven alone to maintain mixture ratio in the MCC, while the OPOV is driven with the FPOV to increase or decrease thrust while maintaining the mixture ratio. The control loops include the controller, the valve actuators, and the transducers that sense flowrates and MCC pressure; i.e., thrust.
from here, emphasis mine
And this which is a bit more explicit
If no malfunctions are discovered, the closed loop thrust control system is activated at 2.4 seconds. The MEC compares the measured MCC chamber pressure to a preprogramed chamber pressure ramp to RPL and modulates the OPOV in an attempt to zero out any differences.
from here
Acronymology:
MOV Main Oxidizer Valve
MFV Main Fuel Valve
CCV Chamber Coolant Valve aka Coolant Control Valve
OPOV Oxidizer Preburner Oxidizer Valve
FPOV Fuel Preburner Oxidizer Valve
MPL Minimum Power Level
HPFTP High Pressure Fuel TurboPump
MCC Main Combustion Chamber
MEC Main Engine Controller (SSMEC preferred because MEC also is Master Events Controller)
RPL Rated Power Level
They get a fair bit of telemetry from the engines. Over time, SpaceX has indicated they have instrumented their engines to greater and greater degrees.
Recall that a rocket engines thrust is mostly determined by the fuel (and oxidizer) flow. These are pumped by turbopumps, which SpaceX has innovated on in their engines.
A turbopump spins and moves fuel. When the fuel flow is lower than expected the turbopump is running lighter and can detect that difference.
Secondarily in this case, it is likely that the low thrust engine was one of the two side engines, in which case they would have gotten feedback that the stage was flying off-kilter due to unbalanced thrust.
