DOW-UAP-D120, AAWSAP DIRD, Materials for Advanced Aerospace Platforms, January 2010

This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed. This DIRD surveys the structural materials needed for advanced aerospace platforms and argues that material choice cannot be separated from overall vehicle design, because launch vehicles, space vehicles, reusable systems, and propulsion hardware each face different temperature, durability, weight, and manufacturing constraints. The report reviews a range of candidate material classes, including advanced aluminum alloys; polymer-, aluminum-, and titanium-matrix composites; titanium alloys; nickel-base alloys; refractory metals; ceramic-matrix composites; carbon-carbon composites; and titanium aluminides. It emphasizes that balancing performance against practical limits such as fabrication methods, cost, inspection, contamination control, and service environment remains a major challenge. Its overall conclusion is that many promising materials exist, but their usefulness depends on application-specific tradeoffs and on closer integration of material selection with design and manufacturing, with some systems judged to be more valuable targets for development than others.

UNCLASSIFIED/f FQR 8FFI@Itltt tJ91! ertt I
Defense
Intelligence
Reference
Document
Acquisition Threat Support
12 January 2010
!COD: 1 December 2009
DIA-08-0912-008
Materials for Advanced
Aerospace Platforms
UNCLASSIFIED/I f8R 8fFIEilAI: W6& Qtlb¥

UNCLASSIF/IPEeDl\O/ P PICIUASLEO NLY
MaterifaolArsd vancAeedr ospaPclea tforms
Preparbye:d
AcquisiStuipopnoD rivtsi ino (DW0-3)
Defensew arninOfgfi ce
DirectofroaArnt ael ysis
DefenseI ntelliAggeennycc e
Author:
AAP Peorn s66
AdministrNaottiev e
COPYRIWGAHRTN INFGu:r tdhiesrs emionfta htpeih oont ogriantp hhipssu bliciasnt oiatou nt horized.
Thipsr oduicsot n ei na s erioefas d vancteedc hnorleopgoypr rtosd uicneF dY2 009
undetrh eD efenIsnet elliAggeennccey, DWeafrenniOsnfegf iAcdev'asn cAeedr ospace
WeapoSny stAepmp lica(tAiAoWnSsPA rograCmo.m menotrqs u estipoenrst aitnoi ng
i
thidso cumesnhto ubleda ddrestsoMe Pd P ers1o n AAWSAP rogram
!
ManagDeerf,e nIsnet elliAggeennccAeyT ,T NC:LA R/DW0-B3l,d6 g0 00W,a sihngton,
DC2 0340-5100.
ii
UNCLASSIF/IliEODOA/ F FIOIliA9L1!!e rttI

UNCLASSIFIED//FOR OFFI@IAl W&lii 9PU.Y
Contents
Introduction ...........................................................................................................iv
Launch Vehicles ................................................................................................. 1
Reusable Crew Modules...................................................................................... 9
Reusable Single-Stage-to-Orbit Vehicles ...................................................... 10
Advanced Al Alloys ....................................................................................... 11
Polymer Matrix Composites.......................................................................... 11
Al Matrix Composites.................................................................................... 12
Ti Alloys ....................................................................................................... 12
Ti Matrix Composites.................................................................................... 14
Ni-base Alloys .............................................................................................. 16
Refractory Metal Alloys ................................................................................ 18
Ceramic Matrix Composites .......................................................................... 18
Carbon-Carbon Composites .......................................................................... 19
Titanium Aluminides .................................................................................... 20
Propulsion Systems.......................................................................................... 21
Summary and Recommendations ..................................................................... 22
Figures
1. Schematic Diagram of Friction Stir Welding ....................................................... 2
2. Specially Modified 747 Transporter Unloading a Boeing 787 Composite
Fuselage Barrel Section ..................................................................................... 4
3. Micrograph Showing Phase Formation at the Surface of a Ti Alloy That has
Been Exposed to Air at Elevated Temperature ................................................. 13
4. Cross-Section Micrograph of a Ti Matrix Composite.......................................... 15
Tables
1. Potential Materials by Use Temperature Regime and Property ......................... 10
2. Example of Properties of Ti Matrix Composites ................................................ 14
iii
UNCLASSIFIED//FOR 0FFI€1Ak Uili ,u1k¥

UNCLASSIFIED//FOA OFFI€1Ak Ulilii ONkY
Materials for Advanced Aerospace Platforms
Introduction
"Advanced aerospace platforms" is a broad topic that can be divided into
several narrower subtopics to enable a more concise discussion of materials
advances, challenges, and opportunities. Consequently, this document
discusses the areas of launch vehicles, space vehicles, and space propulsion
systems separately because their key requirements are often application
specific, which affects materials selection decisions. In addition, single-use
and reusable boosters have different durability requirements that directly
impinge on design and materials selection. Furthermore, current engineering
practice has evolved to the point that design synthesis must integrate the
structure and construction materials to achieve optimum product performance.
For example, the space shuttle was designed to meet customer-imposed
mission requirements (range, payload, empty weight, landing capability, and
so forth) without significant real-time consideration of materials capability.
This approach led to significant compromises at later stages in the shuttle's
development and maturation. (Arguably, the shuttle could be designed as a
more efficient vehicle today.) In the extreme, a spectacular engineering
failure was the National Aerospace Plane (also dubbed the Orient Express),
which was launched as a military project and was intended to be a mach 12
reusable strike vehicle. This project rapidly became materials limited and was
canceled in 1993, after about $750 million in federal R&D expenditures and a
substantial private sector investment. The point is that any "clean sheet of
paper design" must start with an assessment of the requirements for
construction materials and be accompanied by a realistic assessment of the
capability of currently available materials to meet these needs. If these two
assessments indicate a gap between requirements and existing materials
capability, a risk assessment and a risk-mitigation plan must be developed
before expending engineering hours and funds.
Since the inception of manned space flight, the approach to design has
changed to include the concept of damage tolerance. This shift in design
philosophy was prompted by the (eventual) recognition that complex
structures cannot be designed and produced with zero defects. With the
maturation of fracture mechanics and means of reducing these concepts to
practice, the transition from zero defects to defect tolerance became the norm.
This new approach in turn led to recognition that high-performance materials
required not only high specific strength and stiffness but pacing increases in
strength with simultaneous improvements in fracture toughness and fatigue
crack growth resistance. The introduction of damage tolerance was
accompanied by a renewed emphasis on nondestructive inspection capabilities.
This latter thrust was driven by the need to demonstrate the capability to
reproducibly locate small flaws that could become failure initiation sites,
either because of static or because of cyclic loading conditions. In the case of
atmospheric flight, the U.S. Air Force has introduced standards for airframes
(the Aircraft Structural Integrity Program) and propulsion systems (the
Engine Structural Integrity Program) that tie structural life and reliability to
iv
UNCLASSIFIED//FOA OFFl&IAI:: U&li &Ptl::lf

UNCLASSIFIED//FOR OFFl@IAL l:ISl!!! 9HtY
this demonstrated inspections capability. Implementation of these standards,
starting with the B-1 bomber and the F-100 and F-110 engines, has
dramatically reduced (but not eliminated) the incidence of catastrophic
failures of critical components that endanger crews, vehicles, or both.
Taking these changes into account, this document outlines the current
situation regarding the design and production of high-performance structures
for aerospace platforms, including launch vehicles, space vehicles, and
propulsion systems for transporting space vehicles (and payloads) into orbit.
V
UNCLASSIFIED//FOR. OFFISIAI: YSE OHL\«

UNCLASSIFIED//FOR 8FFI@IAL t:191!! l>flt I
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 resistance 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.
1
UNCLASSIFIED/,'FOR 8FFI&i1Ak Uili ODIL\'

UNCLASSIFIED/} FOR OFFICil<L l:191!!! 8HL1/
The newer Al alloys also can be specially processed to render them superplastically
formable. This capability opens a realm of possibilities to replace structures that, in the
absence of this capability, are machined from thick plate. Very large structures are
produced in sections that must be joined. Conventional fusion welding techniques do
not work for high-strength Al alloys such as 7075, 7050, 2024, or 2050 because either
the welds lead to cracks or the welds made under conditions that avoid cracking have
greatly reduced tensile properties. Because these alloys are not amenable to welding,
heavier, fatigue-prone mechanically fastened joints must be used. Recently, scientists
developed a joining process that permits joining of Al alloys such as 7050. This process,
called friction stir welding (FSW), allows joint designs in a variety of configurations that
were not considered possible when fusion welding was the only alternative.
In FSW, a rotating steel tool is inserted into the seam between the two Al alloy pieces
to be joined. As the rotating tool is driven forward, the friction between the tool and the
work piece generates enough heat to soften the Al alloy without melting it. A schematic
of this process is shown in Figure 1.
Tool
/
retreating
side
Figure 1. Schematic Diagram of Friction Stir Welding
The combined action of the rotation and the traversing of the tool essentially kneads
the two pieces together, leaving a mechanically sound joint. Although the weld
properties may be somewhat inferior to those of the base metal, they are good enough
that a relatively small increase in thickness at the joint position can compensate.
Although substantial development of the FSW process is ongoing, FSW already has
been put into practice. For example, the current external propellant tank on the space
shuttle is made from an Al-Li alloy fabricated through FSW. The weight advantage of
using welded as opposed to bolted joints in a large structure such as a launch vehicle is
considerable. With earlier high-strength alloys such as 7075, concerns about fracture
toughness in conjunction with monolithic structures would have caused a welded
2
UNCLASSIFIED//FOR. OFFl&IAI:: W&li &Ptl::lf

UNCLASSIFIED//POlt OPPl@IAL YSI!! 8HLY
construction to be considered too risky. Today, the combination of higher toughness
alloys and FSW opens up the possibility of greater design flexibility resulting in lighter
large structures with equal or greater reliability than earlier ones.
In sum, metallic, nonreusable (at least nominally so) launch vehicles made from
advanced Al alloys and fabricated through FSW constitute an incremental but significant
improvement over earlier versions.
In recent years, PMCs have matured significantly. For many components that are not
exposed to elevated temperatures, PMCs provide a degree of design flexibility not
readily available in metals. Consequently, PMC materials have begun to supplant Al
alloys in the construction of commercial subsonic aircraft. The use of PMCs in the
empennage of the Boeing 777 was one of the first examples of Al alloys being
displaced. Subsequently, the new Boeing 787 has more structure made from
composites than from metallic materials. Once PMCs are introduced into a structure in
significant quantities, a constraint related to galvanic incompatibility between the PMC
structure and any adjoining Al alloys also is introduced. When a PMC structur