This system encompasses the main rocket engines, propellant feed lines, valves, and pressurization hardware required to achieve orbit. In the context of the Space Shuttle, it specifically referred to the three liquid-fueled engines on the orbiter and their associated cryogenic plumbing. Engineers monitor the system's pressures and temperatures continuously during the ascent phase to ensure nominal thrust.
Heard on the job
We had a slight pressure drop in the MPS liquid oxygen manifold right before main engine cutoff.