Defense Intelligence Reference Document Materials For Advanced Aerospace Platforms

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PROPULSION SYSTEMS
Among propulsion systems, only reusable rocket engines are considered here because
they hold potential for significant progress that could reduce the cost of placing
payloads into orbit.
The basic concept of the space shuttle main engine is still viable, but the durability of
the materials used to make the hardware has been a major expense and source of
concern for NASA. Much of the concern is related to the effects of hydrogen on the Ni-
base alloys used in the turbo pumps. The turbo pumps are the heart of a liquid-fueled
hydrogen-oxygen rocket engine because they deliver the fuel and oxidizer to the thrust
chamber. The turbo pumps rotate at a very high speed-up to 35,000 rpm. Such
speeds create enormous centrifugal stresses in the rotating components, particularly
the disks that hold the air foils used to extract work from the hot gas stream. One side
of the turbo pump rotor operates in the hot gas stream created by the combustion of
the oxygen-hydrogen mixture; the other end is a cryopump that operates at cryogenic
temperatures, either in liquid oxygen (90 K) or in liquid hydrogen (20 K). The hot side,
a turbine, resembles the turbine rotor in a gas turbine engine and is made of many of
the same Ni-base alloys. The cryopump is made of Ti alloys.
The turbine essentially operates in a hydrogen-rich supercritical steam environment at
a maximum temperature of about 1,050 °Celsius. The turbine disk is a forged Ni-base
alloy, and the air foils are single-crystal investment castings. Neither material is well
suited to operate in a hydrogen-rich environment; however, no other material class can
withstand the operating temperatures and has better hydrogen tolerance-a situation
that still prevails today. The mechanism of hydrogen-induced cracking is much better
understood today as the result of extensive research over the past 25 years. Therefore,
it would be very useful to use this improved understanding to design a Ni-base alloy
that has improved hydrogen tolerance. If successful, a turbine that has improved
resistance to hydrogen cracking would greatly reduce the intermission refurbishment
time and cost.
Traditionally, the cryopump rotating parts have been made from forgings of the near-a-
phase Ti alloy Ti-SAl-2.SSn {Ti-5-2.5). This alloy has been chosen for its superior
notched tensile strength compared with Ti-6-4 when tested at cryogenic temperatures.
Hindsight suggests it is unclear that notched tensile strength is the best criterion for
selecting a cryogenic rotor material. The use of notched tensile strength originated with
steels. Here, the hydrostatic stress state at the notch root could trigger the onset of
brittle fracture of the type seen in smooth tensile tests below the ductile brittle
transition temperature (DBTT). Ti alloys do not exhibit a DBTT~ therefore, true fracture
toughness measured at the relevant temperature is a more accurate indication of the
fracture resistance of the rotor. An examination of the limited available fracture
toughness data for the two alloys shows no clear advantage in using Ti-5-2.5. Because
Ti-5-2.5 is more diffic~lt to produce than Ti-6-4, using Ti-5-2.5 adds cost to the
cryopump. Other, newer, higher strength Ti alloys-for example, several of the newer
J3-Ti alloys-may be even better suited than Ti-6-4 is for this application. The critical
question of how much toughness is really required cannot be answered at present
because the notched tensile ratio has been the deciding criterion. Therefore, a new
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