Handbook of Turbomachinery Second Edition Revised - Ventech!

exposure may be subjected to impingement by cryogenic air with
unpredictable results. Finally, the unusually low fuel temperature creates
an external heat leakage problem that is usually impossible to ignore. The
result is that most liquid hydrogen propellant systems contain large amounts
of thermal insulation, and sometimes protective shells to prevent damage to
the insulation.
Another problem unique to LH 2 concerns its low density. Since
changes in head and pressure are proportional to each other by the
reciprocal of the fluid density, lower-density fluids require higher developed
heads to impart a given pressure rise. In the case of hydrogen near
saturation conditions, head rises required to develop a given pressure rise
from a pump are an order of magnitude higher than that required for denser
propellants such as kerosene, liquid oxygen, and others. Pump designs for
LH2 service, therefore, typically require comparatively large amounts of
input power, have near-maximum impeller tip speeds, and are often
multistage machines.
The Rocketdyne Division of North American Aviation was the builder
of the J-2 engine under contract to NASA. As it happened, the company
had already been engaged in the development of a liquid hydrogen pump for
a nuclear rocket engine project not related to the J-2 program. The LH2
pump, called the Mark 9, worked to performance requirements similar to
those of the J-2. The Mark 9 was an axial-flow pump with six main stages,
plus an inducer. Although not a common design for a rocket pump, the
axial-flow configuration had advantages in weight, packaging, efficiency and
was relatively easy to multistage. For these reasons, as well as the lack of a
throttling requirement (the J-2 was a fixed-thrust engine), the J-2 fuel pump
was designed to a configuration very much akin to the Mark 9, although a
seventh main stage was added to provide satisfactory stall margin. This
pump (known as the Mark 15-F) went on to see a great deal of operational
service in the upper stages of the Saturn 1B and Saturn V vehicles.
In the late 1960s, the U.S. government decided that the next large
launch vehicle development effort would center around a reuseable vehicle
to ferry astronauts and cargo to and from low earth orbit. This vehicle was
to become the Space Transportation System (STS) or Space Shuttle, which
is currently operating. The system went through many incarnations during
definition, the details of which are beyond the scope of this text. When, in
1972, the vehicle configuration was chosen, the resulting propulsion system
requirements were unlike any previously stipulated. First, the STS’s main
propulsion system (later named the Space Shuttle main engine, or SSME)
was to use LOX/LH 2 propellants and operate at a chamber pressure of
3,000 psi. This was three times higher than the highest pressure production
engine prior to it. Second, the engine was to operate on a thermodynamic
Copyright © 2003 Marcel Dekker, Inc.