René 41) and titanium shingles. This Shuttle TPS concept was rejected, because it was believed a silica tile-based TPS would involve lower development and manufacturing costs. A nickel superalloy-shingle TPS was again proposed for the unsuccessful X-33 single-stage-to-orbit (SSTO) prototype. Recently, newer radiatively cooled TPS materials have been developed that could be superior to RCC. Known as Ultra-High Temperature Ceramics, they were developed for the prototype vehicle Slender Hypervelocity Aerothermodynamic Research Probe (SHARP). These TPS materials are based on zirconium diboride and hafnium diboride. SHARP TPS have suggested performance improvements allowing for sustained Mach 7 flight at sea level, Mach 11 flight at altitudes, and significant improvements for vehicles designed for continuous hypersonic flight. SHARP TPS materials enable sharp leading edges and nose cones to greatly reduce drag for airbreathing combined-cycle-propelled spaceplanes and lifting bodies. SHARP materials have exhibited effective TPS characteristics from zero to more than , with melting points over . They are structurally stronger than RCC, and, thus, do not require structural reinforcement with materials such as Inconel. SHARP materials are extremely efficient at reradiating absorbed heat, thus eliminating the need for additional TPS behind and between the SHARP materials and conventional vehicle structure. NASA initially funded (and discontinued) a multi-phase R&D program through the University of Montana in 2001 to test SHARP materials on test vehicles. Actively cooled Various advanced reusable spacecraft and hypersonic aircraft designs have been proposed to employ heat shields made from temperature-resistant metal alloys that incorporate a refrigerant or cryogenic fuel circulating through them. Such a TPS concept was proposed for the X-30 National Aerospace Plane (NASP) in the mid-80s. The NASP was supposed to have been a scramjet powered hypersonic aircraft, but failed in development. In 2005 and 2012, two unmanned lifting body craft with actively cooled hulls were launched as a part of the German Sharp Edge Flight Experiment (SHEFEX). In early 2019, SpaceX was developing an actively cooled heat shield for its Starship spacecraft where a part of the thermal protection system will be a transpirationally cooled outer-skin design for the reentering spaceship.SpaceX CEO Elon Musk explains Starship's "transpiring" steel heat shield in Q&A , Eric Ralph, Teslarati News, 23 January 2019, accessed 23 March 2019 However, SpaceX abandoned this approach in favor of a modern version of heat shield tiles later in 2019. In the early 1960s various TPS systems were proposed to use water or other cooling