Handbook of Turbomachinery Second Edition Revised - Ventech!

cycle known as staged combustion (some refer to it as a topping cycle).
Third, the engine was to be capable of throttling, or varying its thrust to
support the ascent trajectory requirements of the Space Shuttle vehicle.
From a design thrust level of (appropriately) 100% rated power (referred to
as RPL), the engine was to be capable of reduction in thrust to a minimum
of 65% of design. This is referred to as minimum power level, or MPL. Also,
thrust was to be capable of increasing to 109% of the design thrust. This
high thrust was to be used for emergency conditions. As such, it was referred
to as emergency power level, or EPL. Thrust variations were to be
controllable to increments of 1%. Another notable feature of the original
engine design was the requirement for a variable mixture ratio, or ratio of
oxidizer to fuel being consumed by the engine. This ratio was to be variable
from 5:1 to 7:1 as commanded from the Shuttle orbiter. This feature was to
be an attempt to ensure simultaneous depletion of both fuel and oxidizer
tanks. This combination of operating conditions placed demands on the
turbomachinery systems that remain unmatched 20 years later.
Before proceeding further, a short comparison of the SSME operating
cycle with those of its ancestors would be helpful. The Thor engine and its
descendants, along with the F-1 and J-2 engines, shared a common
thermodynamic cycle, the gas generator cycle. In this cycle, propellants
from the vehicle tanks reach the turbopump inlets under tank pressure.
Turbopump discharge ducting routes most of the fuel and oxidizer to the
thrust chamber inlet manifolds. A small amount of both propellants,
typically 2–3% of the total flow, is carried from the pump outlets to another
combustor, referred to as a gas generator. Usually quite small in
comparison to the main thrust chamber, the gas generator burns its
propellants at a mixture ratio to produce gas (usually fuel rich) to serve as
turbine drive fluid. Typically, these gases are in the 1000 to 1700 8F range.
Although it would certainly be possible to create turbine drive gas that is
oxidizer-rich rather than fuel-rich, the generally adverse chemistry
associated with this situation normally renders fuel-rich gas a better choice.
To this author’s knowledge, oxidizer-rich gas generators have not been
employed in any production engines produced in the United States,
although they have been (and still are) in use by other countries. Upon exit
from the gas generator, the drive gas is transported to the inlet manifold of
the turbine(s). Following expansion through whatever turbine arrangement
has been provided, the gas is typically either dumped to atmosphere
through an exhaust duct or is transported to the divergent section of the
engine exhaust nozzle, where it is injected into the supersonic flow stream.
Being relatively cool, this gas can provide what is referred to as dump
cooling of some portion of the nozzle divergent section. Incidentally, if drive
gas is simply ducted overboard, a restricting orifice is normally placed in
Copyright © 2003 Marcel Dekker, Inc.