(Image: Concept image of Lockheed Martin’s planned SR-72 hypersonic strike aircraft to serve as a replacement for the SR-71 Blackbird.)
DARPA recently selected Aerojet Rocketdyne to build a new type of aircraft engine that would allow flight at hypersonic speeds up to Mach 6, according to Aviation Week. Under DARPA’s Advanced Full Range Engine (AFRE) program, Aerojet Rocketdyne has been contracted to demonstrate the viability of a turbine-based combined cycle engine (TBCC), which uses a conventional turbojet for relatively low-speed flight and transitions to a ramjet and scramjet to accelerate from about Mach 3 to the target speed of Mach 6.
It is not yet clear what aircraft DARPA has in mind for the new engine, though the Department of Defense has been adamant in recent years about the need to develop hypersonic weapons systems. DARPA’s own hypersonic spaceplane, a joint enterprise with Boeing, will use modified space shuttle rocket engines. Lockheed Martin’s Skunk Works, on the other hand, is gearing up to fly an F-22-sized demonstrator in 2020 for the planned SR-72, an unmanned hypersonic strike and reconnaissance aircraft to serve as a replacement for the SR-71 Blackbird. Skunk Works plans to use TBCC engines on both the single-engine demonstrator and the twin-engine SR-72.
Previous efforts to build a TBCC engine involved designing a turbine from the ground up, but DARPA’s AFRE program will select an off-the-shelf turbine to modify with a dual-mode ramjet/scramjet (DMRJ). As Aviation Week points out, designing a TBCC will be difficult because turbojets can operate up to about Mach 2.5, but ramjets, which use pressure from the aircraft’s speed to compress air without an axial compressor, need to be traveling at about Mach 3.5 to operate effectively. (A scramjet, or supersonic combusting ramjet, engages combustion while the air is still moving at supersonic speeds, while a ramjet slows the air down to subsonic before combustion. The TBCC engine DARPA and Aerojet plan to develop will be able to switch between these two functions.)
To bridge the gap, Aerojet Rocketdyne will need to both increase the maximum operational speed of the turbojet and decrease the minimum operational speed of a ramjet. The program calls for an engine that can accelerate from a standstill to Mach 6 and then slow back down for a runway landing. The TBCC will use a common inlet and nozzle for the two engine types, and the turbine is designed to be “cocooned” during high-speed flight and restarted when the aircraft decelerates.
Integrating a turbine engine and a ramjet/scramjet will come with some significant engineering hurdles to clear. From Aviation Week:
Work on the TBCC is slated to begin in earnest next year. In 2018, Aerojet Rocketdyne and DARPA plan to start testing a full-scale DMRJ as well as the common inlet for the engine, build the common nozzle for the engine, and begin work to modify an off-the-shelf turbine. Once large components are tested independently, the engine will be assembled and high-speed ground tests can begin.
DARPA’s program to build a zero-to-Mach-6 engine is part of a larger trend in the aerospace industry to develop hypersonic capabilities, and develop them soon. The Defense Advanced Research Projects Agency says their TBCC engine could be used in a variety of operations, including high-speed strike and reconnaissance, as well as a two-stage launch system for sending craft to orbit.
Aerojet Rocketdyne has now been pulled in to build the engine. Lockheed Martin, Boeing, and Northrop Grumman are all researching hypersonic designs. It looks like it won’t be long until multiple new aircraft are screeching through the high altitude, breaking Mach 5 as they hurl around the world.
Source: Aviation Week