Handbook of Turbomachinery Second Edition Revised - Ventech!

excess of 6,000 psi for consumption by the engine. This equates to a head
rise of approximately 200,000 ft. Although past design practices would have
suggested the use of an axial-flow pump (and one was considered), the need
to throttle over nearly a 2:1 range of thrust requires the fuel pump to operate
over a range of inlet flows that axial machinery would have difficulty
supporting. The throttling requirement was a major factor that dictated a
centrifugal pump configuration. Also, since the head rise requirement was
clearly beyond the capability of a single stage; a multistage machine would
be required. Centrifugal pump configurations with from two to four stages
were considered, with a three-stage configuration being selected. Designed
for a shaft angular velocity of 37,000 rpm, it was the best compromise for
weight, complexity, efficiency, impeller speed, and hydrodynamic stability
over the required operating range.
Another outcome of the early fuel pump design process is worth
special mention. Initial pump sizing efforts showed that the propellant tank
pressure (approximately 35 psi) was much too low to support operation of a
fuel turbopump that was optimized for size and weight. While a single pump
to perform this job could certainly be designed, its bulk and performance
would be clearly undesirable. However, if a large inducer stage was placed
upstream of the main fuel pump inlet, and provided approximately 4% of
the required total head rise, a much more compact and higher-performing
main fuel pump could be devised. As it happened, such a large inducer stage
(referred to as a pre-inducer) had been the object of some previous
development work for an experimental version of the J-2 engine. This preinducer
took the form of an entirely separate low-pressure fuel turbopump.
Driven by warm gaseous hydrogen extracted from elsewhere in the engine,
the low-pressure fuel turbopump generates a discharge pressure of
approximately 250 psi. With this available inlet pressure, the main pump
(referred to as the high-pressure fuel torbopump) can operate under much
more favorable conditions. This machine has gone on to become what is
perhaps the most visible single piece of turbomachinery engineering in the
world today.
Like the fuel turbopump system, the oxidizer turbopump system of the
SSME is a somewhat novel configuration that is a direct result of the unique
demands placed on it. This system must move approximately 1,000 lb/sec of
LOX, which is ingested at a propellant tank pressure of approximately
100 psi. For reasons analogous to the fuel pump system, a separate lowpressure
oxidizer turbopump was contrived to enable the high-pressure
oxidizer turbopump to be considerably smaller and lighter than would be
possible otherwise. Approximately 90% of the engine LOX flow is destined
for the main combustion chamber, which requires a pump discharge
pressure of 4,000 psi. The other 10% is required to operate the two
Copyright © 2003 Marcel Dekker, Inc.