DOW-UAP-D120: Materials for Advanced Aerospace Platforms - Why Material Choice Cannot Be Separated from Vehicle Design

Source file: DOW-UAP-D120_AAWSAP-DIRD-Materials-for-Advanced-Aerospace-Platforms-January-12-2010.pdf Originating agency: Defense Intelligence Agency (DIA), Defense Warning Office, Directorate for Analysis; produced under the Advanced Aerospace Weapon System Applications (AAWSA) Program Document type: Defense Intelligence Reference Document (DIRD), "Acquisition Threat Support" series; control number DIA-08-0912-008 Date: 12 January 2010 (ICOD, the information cutoff date: 1 December 2009) Classification: UNCLASSIFIED//FOR OFFICIAL USE ONLY, with the "For Official Use Only" caveat struck through on every page; publicly released in 2026 Page count: 27 VIRIN: 260918-D-D0360-1109 PURSUE Release: 6

Summary

This DIRD is a practitioner's survey of the structural materials that a new generation of launch vehicles, reusable spacecraft and reusable rocket engines would need. It was prepared by the Acquisition Support Division (DWO-3) of the Defense Warning Office in DIA's Directorate for Analysis. The author appears in the released version only as "AAP Person 66", and an administrative note directs comments to "AAP Person 1", the "AAWSA Program Manager", at DIA, "ATTN: CLAR/DWO-3, Bldg 6000, Washington, DC 20340-5100" (page 2).

The paper has one organizing idea, stated in the introduction and repeated in the closing section: materials selection has to be treated as part of design from the first sketch, not bolted on afterwards. Around that idea it walks through launch vehicles, reusable crew modules, a hypothetical reusable single-stage-to-orbit vehicle (which the paper abbreviates "SSO"), and reusable rocket engines. It contains four figures (a friction stir welding schematic, a photograph of a modified 747 unloading a Boeing 787 composite fuselage barrel, a micrograph of "α case" on titanium, and a cross-section of a titanium matrix composite), two tables and six textbook-style references.

Nothing in it concerns UFOs or UAP. The paper serves the "materials" and "configuration and structure" technical areas of the AAWSAP Statement of Objectives (DOW-UAP-D110), and its last section touches propulsion hardware.

Research Article

An AAWSAP paper without the exotic

AAWSAP was a DIA-administered program (2008-2012) whose contractor was Bigelow Aerospace Advanced Space Studies (BAASS) in Las Vegas. Several sibling DIRDs in this release deal with warp drives, wormholes and negative mass. This one does not. Every material it discusses was in production or in advanced development by 2009, and its reference points are the Space Shuttle, the SR-71, the Boeing 777 and 787 and the GE90 engine.

The cover carries the banner "Acquisition Threat Support", but the text offers no threat assessment, names no foreign program, and says of its own scope: "With the exception of military applications, which are outside the scope of this document, empty vehicle weight is a critical metric" (page 15). The administrative note calls the paper one of "a series of advanced technology reports produced in FY 2009"; the cover date, 12 January 2010, actually falls in fiscal 2010, and the 1 December 2009 cutoff date suggests the text was finished late in 2009. The paper also carries a copyright warning: "Further dissemination of the photographs in this publication is not authorized."

The founding argument: design and material together

The introduction (page 4) opens with a criticism of the Space Shuttle, which was designed to meet mission requirements "without significant real-time consideration of materials capability," leading to "significant compromises at later stages." The author adds: "Arguably, the shuttle could be designed as a more efficient vehicle today." The extreme case is the National Aerospace Plane, nicknamed the Orient Express, "intended to be a mach 12 reusable strike vehicle." According to the paper, it "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 prescription follows: any clean-sheet design must begin by assessing the materials it requires against what current materials can deliver, and if there is a gap, "a risk assessment and a risk-mitigation plan must be developed before expending engineering hours and funds." The second theme of the introduction is the shift from "zero defects" to damage tolerance, driven by fracture mechanics and better nondestructive inspection. The author credits the Air Force's Aircraft Structural Integrity Program and Engine Structural Integrity Program, applied from the B-1 bomber and the F-100 and F-110 engines onward, with having "dramatically reduced (but not eliminated)" catastrophic failures (pages 4-5).

Launch vehicles: aluminum, composites and titanium

Because a launch vehicle stays relatively cool except near the exhaust, the paper recommends advanced aluminum alloys and polymer matrix composites (PMCs) wherever aerodynamic heating stays below about 125 °C. The lithium-bearing alloy 2090 cuts density by 8 percent while raising the elastic modulus by 10 percent (page 6). Newer alloys such as 7050 and 2050 gain toughness because the iron, chromium, manganese and silicon impurities that form brittle "constituent phases" are reduced at the ingot stage. Friction stir welding (Figure 1), in which a rotating steel tool softens the metal without melting it, makes it possible to weld alloys that fusion welding cracks; the paper notes that the Shuttle's external tank was already made from an aluminum-lithium alloy joined this way. Its verdict: "an incremental but significant improvement" (pages 7-8).

On composites, the author follows the transition from the Boeing 777 tail to the 787, whose structure is mostly composite, and explains why titanium fasteners are used where carbon composite meets metal (galvanic corrosion), "directly analogous to the plastic bushing a plumber puts in the joint between copper and iron piping" (page 8). The paper describes three fabrication methods: hand layup of pre-preg, automated tow placement (Figure 2) and resin transfer molding. It lists resin temperature limits of about 125 °C for epoxies, 175 °C for bismaleimides and 300 °C for PMR polyimides (page 11), and warns of "compression after impact," hidden delaminations from dropped tools or hail. It also points to a commercial gap: new resins come from small SBIR-funded startups, while large companies such as BASF, DuPont and GE Plastics "typically are not interested in materials with small annual sales volume" (pages 11-12).

Titanium is preferred above about 200 °C. Ti-6Al-4V has been in use "for more than 40 years", and the landing-gear truck beam of the 777 is made of Ti-10V-2Fe-3Al, up to 6 inches thick in places (page 12). The limit is set by numbers: beta titanium alloys can exceed 1,300 MPa in tensile strength, but their fracture toughness then drops to about 40 MPa·m^0.5, so at a design stress of two-thirds of ultimate strength the critical crack size is only about 4 millimeters, a serious problem for field inspection of a reusable vehicle (page 13).

The Shuttle, Columbia and the metallic heat shield

In the section on reusable crew modules the author describes the Shuttle as a "cold structure" of aluminum alloy 2219 under an insulating thermal protection system (TPS): ceramic tiles underneath and carbon-carbon composites on the wing leading edges. Then comes the sharpest line in the paper: "Hindsight shows that C-CCs 'age' and lose much of their fracture toughness during repeated thermal exposure. (This loss of toughness was a prime factor in the Columbia disaster. Had a metallic heat shield that included the wing leading edges been used, this disaster arguably could have been avoided.)" (page 14). This is the author's own judgment, flagged by the word "arguably", and the paper gives no source for it.

The historical case is the DynaSoar glider, whose engineers studied the refractory alloy Mo-0.5%Ti ("Moly half Ti") and "Columbia-based" alloys (the paper's spelling, evidently for columbium, the older American name for niobium), protected by a MoSi2 coating formed in a fluidized bed. The paper's comment: "it is now unclear how a large structure could have been successfully coated in this manner" (pages 14-15).

Table 1 and the ten material classes

For a reusable single-stage-to-orbit vehicle the paper argues that metallic TPS and hot structure are unavoidable, and that "every additional pound of empty weight reduces the payload by the same amount." It adds that a real design can only come from detailed mission requirements. Table 1 (page 15) sorts candidates by three temperature regimes (ambient to 250 °C, 250 to 550 °C, above 550 °C) against five properties: specific strength, specific stiffness, fatigue resistance, fracture toughness and creep resistance. The paper counts "essentially 10 distinct classes" (page 16). Highlights from the review:

• Titanium alloys and α case. Above about 550 °C titanium reacts with air and forms a harder, less ductile oxygen-rich surface layer (Figure 3). "The industry standard for α case has essentially been one of zero tolerance," and the author argues that this should be revisited, listing six open questions. It notes that the Air Force Materials and Manufacturing Directorate is starting a project on the question, motivated by hypersonic flight, and cites the SR-71, which flew above Mach 3.2 "for 34 years (1964-1998)" with skin temperatures above 300 °C on the beta alloy B-120 VCA (Ti-13V-11Cr-3Al) with no known α case problems (pages 18-19).
• Titanium matrix composites (TMCs). Table 2 gives 1,902 MPa tensile strength, 226 GPa modulus and a density of 4.43 g/cm3 at a fiber volume fraction of 0.39, more than twice the strength and stiffness of the matrix "with no increase in density" (pages 19-20). Then comes a cautionary tale in nine bullets: the SCS-6 silicon carbide fiber was made only by Textron Specialty Materials in Lowell, Massachusetts; the company chose to sell components rather than fiber; highly visible test failures followed; Japanese Nicalon and British Sigma fibers were sometimes barred from federally funded programs; a mid-1990s study priced TMCs at about $500/lb (1995 dollars) if about 10,000 lb a year could be used; and work stopped after "about $500 million of U.S. government funds." The lesson: new materials from laboratories "should be approached with caution and never be put on the critical path of product design" (pages 20-21).
• Nickel-base superalloys. They are used above half their melting point thanks to Ni3Al (γ') precipitates, whose strength rises with temperature up to about 1,000 °C. The low-expansion alloy Inco 903 was undone by a form of cracking named "stress-assisted grain boundary oxidation (SAGBO)," but the author thinks the class deserves another look for TPS (pages 21-23).
• Refractory metals. Niobium is the most attractive of the group, but "at 1,250 °Celsius, a breach in the coating will cause immediate, catastrophic failure," so a sensible design rule would exclude such metals from critical hot structure (page 23).
• Ceramic matrix composites (CMCs). Fibers raise SiC-SiC toughness to as much as 10 times that of the bare matrix, with service to about 1,400 °C, but "there currently exists no means of estimating the reduced toughness as a function of service life." Even so, they hold "the greatest promise" of beating the temperature limits of metals (pages 23-24).
• Carbon-carbon and gamma titanium aluminide. Carbon-carbon has a silicon-rich, self-healing coating that is good to 1,200 °C but vulnerable at intermediate temperatures. Gamma titanium aluminide took "more than 30 years" to mature and had just entered service in GE's low-pressure turbine blades for the 787 and 747-8, at up to about 750 °C (pages 24-25).

Reusable rocket engines: the hydrogen problem

The propulsion section considers only reusable rocket engines. Space Shuttle main engine turbopumps spin at up to 35,000 rpm, with a turbine running in "a hydrogen-rich supercritical steam environment at a maximum temperature of about 1,050 °Celsius" at one end and a pump at 90 K (liquid oxygen) or 20 K (liquid hydrogen) at the other. The nickel alloys of the turbine are not well suited to hydrogen, and the author proposes using 25 years of research on hydrogen cracking to design a more tolerant alloy. The author also questions why the cryogenic pump is made of Ti-5Al-2.5Sn: it was chosen for notched tensile strength, a criterion inherited from steels, although titanium has no ductile-brittle transition temperature (pages 26-27). The paper's ranking is explicit: lighter combustion chambers of the NARloy-Z type (Cu-3%Ag-0.5%Zr) matter less than hydrogen-resistant turbine materials.

Significance

The paper's value to the AAWSAP record is that it shows how wide the program's scope was. Besides the speculative physics papers, the program commissioned a straightforward engineering baseline of the kind any aerospace materials specialist would recognize. The most concrete lessons in the paper are about cost and the industrial base rather than physics: the $750 million National Aerospace Plane, the $500 million titanium matrix composite effort and the gap between laboratory innovation and production supply.

The caveats are clear. The author says the paper deals with the subject "with little technical detail"; many figures, including the Columbia judgment, have no specific source; and it is a snapshot of late 2009, so descriptions of the 787 and 747-8 are in the present tense of that time. The paper does not discuss exotic or anomalous materials (topics taken up by siblings such as DOW-UAP-D117 on metallic glasses and DOW-UAP-D141 on metamaterials), makes no forecast in years, and never mentions UAP. As the official blurb warns, it reflects its framing at the time of writing and does not imply current validation.

Key People

Locations

Key Concepts

Notable Quotes

"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." -- page 4

"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." -- page 4

"This example supports the unwritten rule that the pathways leading to high-performance, high-value structures typically are not technology dependent but are cost intensive, no matter what technology is employed to meet the requirements." -- pages 10-11

"(This loss of toughness was a prime factor in the Columbia disaster. Had a metallic heat shield that included the wing leading edges been used, this disaster arguably could have been avoided.)" -- page 14

"In truth, the real issue was one of quality and not the fundamental viability of the TMC material concept." -- page 21

"New material concepts originating from nonproduction sources such as national laboratories or research universities should be approached with caution and never be put on the critical path of product design." -- page 21

"Nevertheless, CMCs are the material class that holds the greatest promise of defeating the temperature limits of current metals." -- page 24

"While a lighter combustion chamber would be welcome, developing hydrogen-resistant turbine materials would be a much better use of available rocket engine alloy development resources." -- page 27