The Thermal Paradox of Hypersonic Cruisers

High–speed transport vehicles require the use of high temperature materials to withstand the high thermal fluxes during flight. Moreover, the long duration of the mission requires a thermal protection system able to cope with the accumulated heat load during flight while considering that part of this heat needs to be evacuated after touch–down. Thus, both the operating temperatures and the exposure time need to be evaluated in order to assess the feasibility of the hypersonic cruiser for a given mission. The heat fluxes on the aeroshell during short duration trans–atmospheric missions, e.g. launch and re–entry, are typically approximated by the cubic power of the free–stream velocity. However, a simplified zero–dimensional analysis showed that this approximation is no longer appropriate for hypersonic cruisers. In fact, the heat flux during sustained flight at Mach 8 is only 25% to 35% higher than at Mach 5 for the same aircraft configuration and cruise range. Furthermore, the reduced flight time at Mach 8 results into a 20% to 25% lower heat load to the airframe with respect to the Mach 5 cruise. This implies that the heat accumulated during cruise decreases with the flight Mach number. To assess these trends throughout the full trajectory, the study was extended to a one–dimensional analysis coupled with an on–board thermal–energy management system. The detailed airframe architecture addressed the interfacing areas between the different subsystems: tanks, cabin, propulsion plant and the aeroshell. Distinct aeroshell panels were considered to better represent the geometry and the various loads across the pressure and suction sides of fuselage and wings. The thermal loads to the airframe across the aeroshell were then evaluated for two possible vehicle missions with cruise phases at Mach 5 and Mach 8. The airframe heating which resulted is in close agreement with the prediction by the zero–dimensional model.