AAWSAP DIRD, Materials for Advanced Aerospace Platforms, January 2010

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Materials for Advanced Aerospace Platforms
LAUNCH VEHICLES
For the purposes of this document, launch vehicles are defined as the structure that
supports and/or encloses the propulsion system, the fuel supply, and the crew or
payload module. Launch vehicles today are either single use or multiple use after
recovery and extensive refurbishment. This approach adds considerably to the cost of
transporting a pound of payload into earth orbit, regardless of whether an unmanned
satellite or a manned orbiting crew module that must withstand the temperatures and
loads associated with safe reentry to earth. Furthermore, the larger the payloads are,
the greater are the reaction forces the launch vehicle must withstand during launch.
With the total weight of the payload, the empty weight of the launch vehicle, and fuel
all needing to be lifted initially, fuel-efficient propulsion and lightweight launch vehicles
are essential to maximizing the payload. Except in the area around the propulsion
system exhaust, the temperatures experienced by launch vehicles during launch are not
demanding. Therefore, advanced, high-strength aluminum (Al} alloys and polymer
matrix carbon fiber composites (PMCs) are prime candidates for the parts of the
structure that experience aerodynamic loads and where aerodynamic heating does not
exceed about 125 °Celsius. One class of advanced Al alloys is the lithium (Li)-bearing
alloys, such as Al alloy 2090. This alloy contains enough Li to reduce its density by 8
percent while increasing the elastic modulus (E) by 10 percent. Other, newer advanced
Al alloys, such as 7050 and 2050, have been developed to have improved damage
tolerance. These alloys have excellent specific strength at or near room temperature
and experience no major loss of ductility at cryogenic temperatures. The newer variants
of the 2000 and 7000 Al alloys also have substantially improved resistance to most
types of corrosion, including exfoliation and stress corrosion cracking. This can be
important in a reusable vehicle.
Perhaps the most important aspect of the improved Al alloys is their higher fracture
toughness, accomplished through a combination of alloy composition control and
improved processing. In alloy composition control, the concentrations of the residual
elements iron (Fe), chromium (Cr), manganese (Mn), and silicon (Si) are reduced at
the ingot stage. These elements combine with Al to form hard, brittle intermetallic
compounds known as constituent phases. The advanced alloys contain fewer, smaller
constituent phases, leading to improved fracture resista nce and higher fracture
toughness values. In applications such as body skins for commercial aircraft, this
improved toughness has enabled an increase in the spacing of the circumferential
fuselage frames, or "hat sections," that serve both as stiffeners and as crack stoppers
to prevent a catastrophic failure during pressurization. For any given operating stress­
in this case the pressurization stress-the spacing of the frames is directly related to
the critical crack size of the body skin . Higher toughness alloys have larger critical crack
sizes, and the frames can be spaced further apart without increasing the risk of
catastrophic failure. The increased spacing ultimately allows a fuselage design that
requires fewer frames. Consequently, the airplane benefits from a commensurate
reduction both in weight and in manufacturing cost. Similar possibilities exist for the
design of a fail-safe launch vehicle that has a lower empty weight. Clearly, the
advanced Al alloys offer intrinsic improvements over the alloys used in the Saturn
launch vehicle and introduce the prospect of new, more efficient launch vehicle designs.
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