A team of aerodynamics researchers at the University of Texas at Arlington have begun a three-year project to help NASA break through one of the biggest obstacles to air-breathing hypersonic flight - maintaining a combustible mix of fuel and air at high Mach numbers.
Armed with a $640,000 grant from NASA, the Mach 3 windtunnel and a new hypersonic shock tube at UTA's aerodynamics research centre, plus a lot of computer power, researchers Luca Maddalena and Luca Massa hope to determine the optimal design for fuel injectors and their ideal configuration inside a combustion chamber. But however impressive the tools at their disposal, the scale of the problem is at least as great. When vehicles reach hypersonic speeds - beyond Mach 5, or about 5,600km/h - the fuel and oxygen at typical speeds inside an engine do not have enough time to mix and ignite effectively.
Or as Massa puts it: "The reactive mix gets blown out of the system."
Maddalena, the experiment leader, and Massa, who is responsible for the project's computational aspect, believe the way forward lies in detailed study of how vortices react with turbulent air flow. They expect to spend the coming year studying non-reactive air flows before beginning to examine how chemical reactions between fuel and air affect the system. After that, the aim is to present NASA with a guide to injector configuration and placement.
Initially, says Massa, the goal is to achieve M5-plus hypersonic vehicle speeds without having to slow air flow within a combustion chamber down to subsonic levels. But ultimately, to achieve the M15 vehicle speeds NASA is targeting for the air-breathing phase of a single- or double-stage to orbit flight will demand an injector system that can cope with M8 air inside a combustion chamber, says Massa.
But however successful Maddalena and Massa are over the next three years, a broader look at NASA's hypersonics propulsion and airframe development timetable points to the scale of the challenge of hypersonic flight. Small vehicles suitable for use as weapons could reach technology readiness level 6 - implying readiness for prototype testing, possibly in a laboratory environment - in 2020. Depending on budget constraints, NASA expects TRL6 for commercial cruise flight in 2030, and for space access in 2040.