The low-pressure oxidizer pump is a similar liquid-side design, but it is powered by an approximately 1800 hp, multistage hydraulic turbine driven by liquid oxygen drawn from the high-pressure oxygen pump discharge it intakes at 100 psia and discharges at about 430 psia.
The low-pressure fuel pump is a single-stage, axial-flow (propeller type) design driven by a two-stage reaction turbine powered by gaseous hydrogen drawn from the combustion chamber coolant circuit it raises the liquid fuel from 30 psia at the inlet to about 300 psia into the main pumps. Two low-pressure “boost pumps” at the motor inlets suppress cavitation in the high-pressure main pumps downstream. A functional flow schematic is shown in Fig. The motor can then use relatively high expansion-ratio nozzles (about 77:1) at launch as opposed to the about 10:1 nozzles on lower chamber pressure booster motors this provides significantly higher specific impulse at any operating altitude and the highest vacuum-specific impulse (454 sec) available from any United States motor. Because the shuttle stack uses SSME thrust from sea-level launch all the way into low-earth orbit, the overall system is enabled by operating these motors at very high (about 3300 psia) main combustion chamber pressure. Three of these large LO 2/LH 2 motors are used on each shuttle orbiter.
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