Last August, DARPA conducted the second test flight of its hypersonic technology vehicle, the Falcon HTV-2. The test ended when the vehicle sent itself into the Pacific Ocean nine minutes into the flight. At the same time, the reasons for the abort were unclear and frustrating. The project's program manager, Maj. Chris Schulz, USAF, said: "We'll learn and try again. That's what it takes." To help figure out what it takes, DARPA enlisted the aid of an independent engineering review board comprised of government and academic experts to evaluate the data collected during the flight. The vehicle was built to demonstrate the technology, and as a data-gathering platform. Thus, the ERB had plenty of data telling the story of what happened. The goal of the program is to develop a vehicle that can reach any location in the world within an hour, requiring hypersonic speeds. Apparently, the test flight successfully achieved stable, aerodynamically controlled speeds up to Mach 20 for the first 3 minutes. Then, the vehicle appears to have experience shockwave disturbances that were 100 times more intense than it was designed to withstand, eventually losing control. The ERB conclusion this happened because of "unexpected aeroshell degradation, creating multiple upsets of increasing severity that ultimately activated the Flight Safety System," which triggered a controlled descent and ocean ditch of the vehicle. Vehicle engineers knew there would be a "gradual wearing away of the vehicle skin as it reached stress tolerance limits." However, more of the skin separated than was expected. DARPA say the next step for the program is to improve models for "Characterising the thermal uncertainties and heat-stress allowances for the vehicle's outer shell." In the US, a multi-university and industry partnership is working on the problem under the umbrella organisation, National Hypersonic Science Centre for Materials and Structures. A recent paper (doi:10.1063/1.3657835) by a research team at the University of California, Santa Barbara (one of the partners) describes a method for measuring strain at high temperatures, noting that the development of such materials requires the ability to reproduce conditions in the laboratory, which is not trivial. To read more, please visit: www.nhsc-ms.net
