U.S. Supersonic Commercial Aircraft Assessing NASA's High Speed Research Program (1997) / Chapter Skim
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4 AIRFRAME
4 AIRFRAME
Pages 61-90
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From page 61... ...
This goal along with the additional design requirements and conditions encountered in the supersonic flight regime is driving the selection of material and structural concepts toward high risk, high payoff designs (Velicki, 1995~. These designs must have simultaneous improvements in material properties at elevated temperatures and in structural design efficiencies.
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From page 62... ...
(The radome will use special radar transmitting materials, and leading edges will use titanium alloys.) Skin temperatures are somewhat lower at lower cruise speeds: 250°F at Mach 2.2 and 210°F at Mach 2.0 (NRC, 1996; Johnson, 1994~.
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From page 63... ...
SELECTION OF MATERIALS This section discusses the HSR Program' s approach to developing advanced materials, followed by comments on aluminum alloys, titanium alloys, PMCs, structural adhesives, sealants, coatings and finishes, and the supplier base. Development Approach Materials and processes currently used by the aerospace industry cannot satisfy the performance and cost requirements of a Mach 2.4 HSCT.
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From page 64... ...
Specified deliverables in the area of materials, processes, and structures are as follows: · database of material properties, durability, fabrication processes, etc. · finite element models of airframe structures · test data on wing and fuselage components The HSR Program will use these deliverables to evaluate the feasibility of meeting the weight and performance goals of the TCA and to support development of a refined aircraft configuration (the TCn)
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From page 65... ...
In spite of the high strength-to-weight ratio of current titanium alloys, however, an all-titanium HSCT would not be economically viable because of excessive weight. Even so, titanium alloys are the prime candidates for wing and tail leading edge structures, the main wing box, foil for honeycomb sandwich core structures, and, perhaps, higher temperature fuselage structures.
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From page 66... ...
The cost reduction effort is exploring innovative fabrication technologies, such as forming, machining, joining, net-shape extrusions, metallurgical and adhesive bonding, laminated titanium alloy structures, and superplastic forming and diffusion bonding of structural honeycomb sandwich. The committee believes that the HSR Program' s titanium alloy and process development plan is properly scoped and does a good job of integrating work by NASA and the airframe manufacturers with work by materials suppliers and academia.
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From page 67... ...
, and they would be more compatible with lower-cost manufacturing methods, such as lamination, resin transfer molding, and nonautoclave processing. In summary, the HSR Program's focused effort to develop Mach 2.4 PMC materials, if successful, would produce high performance materials that could be used at temperatures from 200°F to 350°F.
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From page 68... ...
However, there is high risk associated with achieving the desired level of technology readiness within the current schedule, particularly with regard to titanium surface preparation.3 Sealants The HSR Program has accepted the difficult challenge of developing sealants (especially fuel tank sealants) that can survive environmental conditions associated with a Mach 2.4 aircraft.
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From page 69... ...
Currently available coating technology would not be economical for commercial applications at speeds above Mach 2.0. Thus, material and structural design decisions are currently being made based on a temperature profile that may not be achievable with the coatings and finishes that will be available.
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From page 70... ...
Finding 4-3. The adequacy of the materials supplier base could become a critical issue when industry considers whether to make an HSCT program launch decision.
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From page 71... ...
Currently, predictions of end-of-life properties for developmental materials are based on accelerated test techniques. Real-life testing to date has been limited and will not be able to validate end-of-life properties until many years after the HSR Program has selected the materials and structural design of airframe test articles.
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From page 72... ...
to develop predictive analyses. The ability of the HSR Program to develop accurate life prediction methodologies is limited because no actual service life data are available.
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From page 73... ...
Even so, it is incumbent upon the HSR Program to ensure that the materials technologies it is developing are compatible with affordable manufacturing processes. As noted earlier, airframe structural design, including economically feasible materials and manufacturing processes, will have more impact on HSCT affordability than any other technological area.
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From page 74... ...
Thus, widespread use of composites in an HSCT will be contingent upon the maturation of innovative, affordable composite manufacturing processes, such as advanced tow placement, resin transfer molding, resin film infusion, pultrusion, and nonautoclave processing (NRC, 1995~. Twenty years of experience with carbon-fiber PMCs indicates that the successful use of PMCs in commercial aircraft very much depends upon industry's ability to integrate PMCs efficiently into the aircraft design philosophy.
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From page 75... ...
will be used to fabricate major components of the HSCT wing and possibly the fuselage structure (see Figure 4-3~. =_ Fuselage ~: PMC Skin-Stringer Wing Strake PMC/TI-PMC Honeycomb sandwich An, I\ Outboard wing PMC Honeycomb sandwich ~;~` Main Wing Box Tl Honeycomb sandwich FIGURE 4-3 Materials and structures baselines for the TCA.
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From page 76... ...
In other words, the committee does not believe the proposed testing will determine if the structural designs are compatible with the manufacturing processes that will ultimately be used to build an HSCT. High temperature composites are notoriously hard to process, and industry has no experience using automated manufacturing processes with the candidate materials.
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From page 77... ...
The NASA and industry participants in the HSR Program should jointly place greater emphasis on the development of manufacturing technology and producibility demonstrations so the HSR Program can properly support the HSCT product launch decision. NASA and industry should develop an integrated manufacturing technology plan that enables the HSR Program's materials technology development efforts, which currently seem to be focused on nearterm component fabrication, to adequately consider overall, long-term manufacturing issues.
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From page 78... ...
The differences between the results produced by these two models become increasingly acute as structural concepts become more dependent on advanced materials and manufacturing processes. The specific goal of HSR structures technology development is to develop and validate structural designs for the TCA.
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From page 79... ...
Dependence on Materials Development Weight management techniques evident in the structural concepts being developed by the HSR Program include the use of advanced lightweight composite materials; the aggressive use of sandwich structure for large areas in the wing and, perhaps, in the fuselage; and the elimination of structural joints and fasteners through bonded integral construction. These innovative structural concepts are strongly dependent on the maturity and success of materials development.
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From page 80... ...
The performance of innovative structural concepts depends on successful development of the materials upon which they are based. It is virtually impossible to separate the structures design effort from the materials and manufacturing development effort.
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From page 81... ...
Finding 4-7. The use of multiple weight estimation methods has confounded weight tracking and obscured HSR Program successes in using innovative structural concepts to reduce vehicle weight.
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From page 82... ...
An iterative, feature-based, preliminary sizing tool is needed to permit quicker evaluations of the details of various structural concepts in a small portion of the structure (for example, going from honeycomb sandwich structure to skinstringer structure in the wing box) within an overall vehicle master model.
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From page 83... ...
Structural Test Program The airframe materials and structures effort currently plans full-scale component tests of a fuselage barrel section and the inboard wing box to validate the fuselage and wing structural designs, respectively (see Figure 4-6~. These tests are intended to develop confidence in proceeding from technology development to engineering and manufacturing development.
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From page 84... ...
Full-scale testing of large components, because of the cost and time involved, is more appropriate to the final structural validation of a specific vehicle point design. Large component tests, as currently planned, would not address critical structural issues for the full-scale vehicle or major structural joints.
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From page 85... ...
Subcomponents would be quicker and less costly to fabricate and test, thus allowing more tests for more in-depth investigations of fundamental issues, such as damage tolerance, repair, load interaction, environmental exposure, material variability, process repeatability, fabrication defects, etc. Subcomponent test articles are large enough to incorporate key structural concepts, address fabrication and handling issues, investigate some load interactions, and calibrate analytical models.
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From page 86... ...
Funds allocated for testing fullscale components during Phase II should be reallocated to achieve higher levels of technology readiness in the critical enabling materials and structures technologies, including the following: . materials characterization and life prediction methodologies · rapid, efficient design and analysis tools · robust structural concepts · technical criteria related to dynamic interactions among the airframe, propulsion, and flight control systems (APSE effects)
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From page 87... ...
. Achieving the HSR Program's performance goals requires a combination of low specific fuel consumption, very low structure weight fraction (on the order of 20 percent)
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From page 88... ...
Finding 4-15. SLFC has the potential to improve L/D by 10 to 15 percent, which could offset shortfalls in the attainment of predicted L/D, specific fuel consumption, or structural weight fraction, and would provide a margin for attaining performance goals in terms of aircraft range.
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From page 89... ...
The committee recommends that the HSR Program defer plans to produce full-scale components and provide additional funding during Phase II to mature enabling technologies for material and process development; structural design; validation of accelerated durability tests; and modeling tools for rapid preliminary sizing, thermal analysis, and analytical life prediction. Additional funding is also needed to investigate real-time, long-term durability, end-of-life properties, robust structural concepts, and fabrication techniques that could be used for full-scale production before full-scale test components are designed, fabricated, and tested.
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From page 90... ...
1996. Effects of Alternative Wing Structural Design Concepts on HSCT Life Cycle Costs, AIAA Paper 96-1381.
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