MPS
Initialism of main propulsion system (of the Space Shuttle).
MPS: the Space Shuttle's three-engine core
The main propulsion system, or MPS, refers to the trio of Space Shuttle Main Engines (SSMEs) mounted on the aft fuselage of the Space Shuttle Orbiter, burning liquid hydrogen and liquid oxygen from the external tank to provide primary thrust during ascent. Each engine produced 418,000 pounds of thrust at sea level and approximately 470,000 pounds in vacuum, giving the Orbiter's MPS a combined vacuum thrust of roughly 1.41 million pounds.
The three engines were mounted in a triangular arrangement with one engine centerline and two outboard engines on either side. This geometry allowed vectoring of the MPS thrust by gimballing individual engines up to ten degrees in pitch and yaw for attitude control during powered flight. The engines burned cryogenic propellants from the external tank at a combined mass flow rate exceeding 1,100 kilograms per second during main engine cut-off (MECO) calculations.
Maintenance and operational constraints
MPS servicing and inspection were labor-intensive operations. Each engine required extensive post-flight inspections including teardown, turbopump bearing checks, and hot-fire testing before reuse. The system incorporated automatic shutdown if any single engine failed during ascent, forcing an abort-to-orbit mode. Hydrogen leaks from the MPS plumbing posed explosive risks during ground operations; technicians used hydrogen detectors and inert-gas purging protocols as standard precautions.
The MPS differed fundamentally from the Orbital Maneuvering System (OMS), which used hypergolic propellants for in-orbit and deorbit burns, and from the Reaction Control System (RCS) of smaller thrusters. The Solid Rocket Boosters (SRBs) provided supplementary thrust during the first two minutes of ascent but were separate from the MPS proper. Understanding this partition was critical for launch and abort procedures.
The development of each SSME required decades of test firing and redesign. Multiple instances of engine failure, material fatigue, and turbopump cavitation shaped the final design. This heritage made the MPS one of the most heavily instrumented and monitored subsystems on the Orbiter, with redundant sensors feeding real-time data to flight control computers throughout ascent.