FIGURE 4-1 Predicted equilibrium skin temperatures for a Mach 2.4 HSCT. Source: Johnson, 1994.

The skin of a high-speed aircraft is heated during flight by friction with the atmosphere. However, the relationship between temperature and cruise speed is not linear; skin temperature increases more rapidly at higher speeds. Figure 4-1 shows predicted equilibrium skin temperatures for a Mach 2.4 HSCT configuration. Except for the nose (radome) and leading edge structures on the wing and tail, the maximum effective skin temperatures estimated for the primary airframe structure on the fuselage, wing, and tail are 320°F. (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).

Two types of materials are generally available for airframe structures: composites, such as polymeric matrix composite (PMC) resin systems using carbon fibers; and metals. The estimated thermal stability of potential HSCT structural metals and polymeric matrix composite (PMC) resin systems is shown in Figure 4-2 (Smith, 1996).¹ As indicated, the basic polymer systems available for HSCT applications above 250°F are more limited than at lower temperatures. The availability of suitable adhesives, sealants, and paints follows the same pattern (Smith, 1996). Thus, the goal of developing technologies compatible with a cruise speed of Mach 2.4 critically affects development related to airframe materials, structures, and