Handbook of Turbomachinery Second Edition Revised - Ventech!

pressure ratio that is required and also lowers the pump discharge pressure
required.
Turbine efficiency also becomes a significant problem with which the
designer must be concerned. In gas generator cycle engines, turbine
efficiency had relatively little effect on engine performance, since turbine
flow was small, and pump discharge pressure was set (practically speaking)
only by thrust chamber injector inlet pressure requirements. In a staged
combustion cycle engine, however, the turbine pressure drop is in series
between the thrust chamber injector and the turbopump discharge. Reduced
turbine efficiency requires a higher turbine pressure ratio, which in turn
increases the needed pump discharge pressure. It is therefore desirable to
maximize the turbine efficiency in order to meet the main combustion
chamber pressure requirement with the lowest possible pump discharge
pressure.
During the engine definition effort, a fundamental design decision was
made to drive fuel and oxidizer pumps by separate turbines rather than by a
common turbine. Since the engine required not only throttling capability
but active mixture ratio control, it would be necessary to vary the speed of
fuel and oxidizer pumps independently. This would necessitate a separate
gas generator (referred to as a preburner) for both turbopumps, with an
oxidizer flow control valve in series with each preburner for turbine gas
temperature control. The early design studies for fuel and oxidizer pump
turbines focused on basic configuration issues. Studies were performed for
turbines with from one to three stages, with diameters from 10 to 12 in., with
blading varying from impulse to substantial degrees of reaction. The final
choices made were for both fuel and oxidizer turbines to have two stages of
reaction blading, with diameters of 10 in. Although this was not absolutely
optimum from a turbine performance standpoint, it optimized the
turbomachinery system for size, weight, and efficiency when integrated
with the overall engine system. This is a good example of where
thermodynamic performance of a component is intentionally compromised
to bring about net gains in overall engine system performance or suitability.
In the case of these turbines, a 12-in. diameter would have been desirable for
higher machine efficiency. However, the turbines are enclosed in a hot gas
manifold structure, which is comprised of relatively thick sections of hightemperature
material. Reduction of the turbine diameter to 10 in. resulted in
adequate turbine performance, while reducing engine weight significantly.
The early design selection effort for the SSME fuel and oxidizer pumps
brought about some interesting choices and a novel configuration. Let us
focus on the fuel pump system first. The fuel turbopump system of the
SSME has the task of displacing about 150 lb/sec of LH2 and raising its
pressure from about 35 psi (set by allowable propellant tank pressure) to in
Copyright © 2003 Marcel Dekker, Inc.