One of the challenges when designing sharp leading edged vehicles is that at a given velocity, the temperature at the tip of the leading edge is inversely proportional to the square root of the leading edge nose radius. In other words, as the radius of curvature at the tip of the wing leading edge decreases, its surface temperature increases. Vehicles with sharp leading edges, that is: leading edges with a nose radius on the order of inches (or smaller) versus feet found on the current blunt bodies such as the shuttle orbiter, will therefore, as mentioned previously, require higher temperature thermal protection materials.

In an attempt to bridge this temperature gap, NASA Ames is developing a subset of UHTC materials consisting primarily of Hafnium and Zirconium Diboride (HfB₂ and ZrB₂) ceramics. The diborides have extremely high melting temperatures (>3000°C) and have relatively good resistance to oxidation in simulated reentry environments. Recent work at NASA Ames is focused on developing improved manufacturing methods for these materials, characterization of the materials' mechanical and thermal properties and evaluation of the materials' performance in simulated reentry environments produced in NASA Ames' Arc Jet facilities. Figure 1 shows a variety of models that have been tested in the arc jet, including nose cones and wedge models of similar geometry and scale as anticipated for use on an actual vehicle. Figure 2 shows an image of a UHTC nose cone during arc jet testing. The surface temperature during this test exceeded 2000°C.

Although the UHTC materials are considerably more dense than RCC, it is anticipated that only relatively small amounts of UHTC will be used along the vehicle's leading edges so the total UHTC mass is minimized. Also, the UHTC mass is located forward on the vehicle, helping to balance the vehicle's center of gravity and offset the relatively high mass anticipated for the engines at the rear of the vehicle. Some trade studies have indicated that most, if not all, of the UHTC mass is offset by the reduction in ballast required to balance the vehicle's center of gravity.

The UHTC's discussed here are monolithic ceramic materials composed primarily of HfB₂ or ZrB₂ with SiC additives. These represent a small portion of the UHTC family of materials and are only one of a series of potential material types that will enable the development of sharp leading edged vehicles. Other options to the UHTCs include carbon-carbon materials with higher temperature coatings and carbon fiber reinforced UHTC matrix materials.

Sharp leading edges manufactured from higher temperature materials will allow significant improvements in total vehicle safety. During reentry, a sharp leading edged vehicle will have significantly increased cross range thus providing the capability for the vehicle to reenter from virtually any point during its orbit and land safely. In contrast, the shuttle orbiter has a narrow window in which it can safely reenter, due to its lower cross range capability. Also, during launch, system studies have shown that a sharp leading edged vehicle has a relatively large window during which it can abort during ascent and perform a safe landing. This ascent abort window is much smaller for a blunt bodied vehicle such as the shuttle, thus resulting in an abort in the ocean.

Ames is continuing work in the area of materials for sharp leading edges for future generations of hypersonic reentry vehicles. On-going work includes studying the effects of composition and processing methodologies on the behavior of UHTC materials in simulated reentry environments and developing design methodologies to determine how best to integrate the UHTC's onto a wing leading edge.