Riding the Wave of Scramjet-Powered Hypersonic Flight

  • Published
  • By Tom Brown
  • Propulsion
The initial launch of Air Force Research Laboratory's X-51A Waverider flight test vehicle broke aviation records as the longest-ever supersonic combustion ramjet (scramjet)-powered hypersonic flight. The recently completed event--an unqualified success culminating 6 years of collaborative effort between AFRL, the Defense Advanced Research Projects Agency, and industry--entailed a 200+ sec burn by the engine, an air-breathing scramjet that accelerated the vehicle to Mach 4.8, or nearly 4,000 mph. Before this milestone achievement, which marks the first use of a practical hydrocarbon-fueled scramjet in flight, the longest scramjet burn achieved in a flight test was the National Aeronautics and Space Administration X-43 vehicle's 12 sec run.

Normally defined as Mach 5 (five times the speed of sound) and beyond, hypersonic flight presents unique technical challenges related to heat and pressure. These extremes render conventional turbine engines impractical; they also pose problems in producing thrust via scramjet--problems akin to lighting a match in a hurricane and keeping it burning. To overcome these issues, engine developer Pratt & Whitney Rocketdyne worked with systems integrator Boeing Phantom Works to incorporate a fuel-cooled engine design. The design serves both to heat the JP-7 fuel to optimal combustion temperature and to assist the engine in enduring the extremely high operating temperatures of long burns.  In addition to demonstrating scalable scramjet propulsion, the X-51A configuration validates thermal protection system materials, airframe and engine integration, and high-speed stability and control.

The remaining three of AFRL's four industry-built Waveriders will likely fly in the fall, depending on data review results and available funding. To control costs, the program team has leveraged proven technologies wherever possible and will continue to do so. Likewise, the Air Force intends to employ virtually identical flight profiles for the trio of future launches, building knowledge with each successive flight. Accordingly, the ample data collected by onboard sensors for this flight test alone--between 140 and 170 seconds' worth--will play a cumulative role in transforming a breakthrough propulsion technology into actual warfighting capability. Rapid engagement of long-distance targets, coupled with a capacity to capture and burn oxygen in the thin atmosphere (eliminating the need for oxidizer tanks such as those carried by the space shuttle and other rockets) are just two of the new scramjet engine's distinct advantages.

In successfully demonstrating the viability of scramjet engine technology for propelling an aerospace vehicle, the success of this effort greatly increases the likelihood that more Waverider vehicles will be built and, further, that the engine technology itself--which represents a leap equating to the post-World War II jump from propeller-driven aircraft to jet engines--will ultimately contribute to the creation of game-changing space access and hypersonic weapons platforms.


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