27 JUNE, 2016 BY: STEPHEN TRIMBLE WASHINGTON DC
A turbine-based combined cycle (TBCC) propulsion system to enable routine hypersonic flight by a vehicle that can take-off and land from a runway is back on the agenda at the Defense Advanced Research Projects Agency (DARPA) after a five-year hiatus.
The experimenting agency has set a “proposers day” on 13-14 July for potential bidders of the Advanced Full Range Engine (AFRE) programme, which is scheduled to launch as a new-start effort in Fiscal 2017.
Combining a turbine engine with a ramjet in the same vehicle has been a dream for the aerospace industry since the early 1950s, when the US Air Force proposed adapting Republic’s concept for the XF-103 fighter with a ramjet to intercept Soviet bombers at speeds up to Mach 5.
But TBCC concepts are limited by a propulsion gap between the Mach 2.5 top speed of a turbine engine and the Mach 3-3.5 minimum speed for a ramjet engine.
In 2009, DARPA attempted to bridge that gap with a high-speed turbine and a low-speed ramjet under the Mode Transition (MoTr) programme, but the project was cancelled two years later. By 2013, Lockheed Martin’s Skunk Works organisation appeared to lobby for a revival of the research effort by releasing a concept for a Mach 6.0-capable SR-72 for high-speed surveillance missions, which was based on a similar TBCC propulsion system.
The AFRE programme now seeks to pick up where MoTr left off, leading to a ground demonstration of a fully integrated propulsion system capable of taking-off from a runway and accelerating beyond Mach 5. The system will include an off-the-shelf turbine engine and a dual mode ramjet/scramjet capable of operating with subsonic or supersonic airflows. Both engines share a common inlet and exhaust nozzle, but transition from turbine to ramjet power at a certain speed over Mach 2.5.
“This won’t be the first time that ambitious engineers will attempt to combine turbine and ramjet technologies. But with recent advances in manufacturing methods, modeling, and other disciplines, we believe this potentially groundbreaking achievement may finally be within reach,” says Christopher Clay, DARPA programme manager.
The programme could benefit from other recent experiments, including the Boeing X-51 Waverider programme funded by the Air Force Research Laboratory. The X-51 completed the first flight tests of a ramjet powered by hydrocarbon fuel, which also served as a coolant. The X-51, however, required a disposable rocket — a booster stage from the Army Tactical Missile Systems (ATACMS) — to accelerate to Mach 4.0, where the ramjet took over.
The Republic XF-103 was an American project to develop a powerful missile armed interceptor aircraft capable of destroying Soviet bombers while flying at speeds as high as Mach 3 (2,300 mph; 3,700 km/h). Despite a prolonged development, it never progressed past the mockup stage.
Mach 3 performance in the 1950s was very difficult to achieve. Jet engines work by compressing the incoming air then mixing it with fuel and igniting the mixture, with the resulting expansion of gases producing thrust. The compressors generally can ingest air only at subsonic speeds. To operate supersonically, aircraft use advanced intakes to slow the speed of the supersonic air to a usable figure. The energy lost in this process heats the air, which means the engine has to operate at ever-higher temperatures in order to provide net thrust. The limiting factor in this process is the temperature of the materials in the engines, in particular, the turbine blades just behind the combustion chambers. Using materials available at the time, speeds much beyond Mach 2.5 were extremely difficult to achieve.
The solution to this problem is the removal of the turbine. The ramjet engine consists mostly of a large tube, and is relatively easy to air-cool by forcing extra air around the engine. Experimental ramjet aircraft of the era, like the Lockheed X-7, were reaching speeds as high as Mach 4. There are numerous problems with the ramjet engine, however. Fuel economy, or thrust specific fuel consumption in aircraft terms, is extremely poor. This makes general operations like flying from one airbase to another expensive propositions. More problematic is the fact that ramjets rely on forward speed to compress the incoming air, and only start to operate efficiently above Mach 1.
Specifications (XF-103, as designed)
- Crew: one pilot
- Length: 77 ft (23.5 m)
- Wingspan: 34 ft 5 in (10.5 m)
- Height: 16 ft 7 in (5.1 m)
- Wing area: 401 ft² (37.2 m²)
- Empty weight: 24,949 lb (11,317 kg)
- Loaded weight: 38,505 lb (17,466 kg)
- Max. takeoff weight: 42,864 lb (19,443 kg)
- Maximum speed: Mach 3 (as a turbojet) / Mach 5 (ramjet-only)
- Service ceiling: 80,000 ft + (24,390 m +)
- Rate of climb: 19,000 ft/min (5,800 m/min)
- Wing loading: 96 lb/ft² (470 kg/m²)
- Thrust/weight (jet): 0.57:1 (afterburner only); 0.95:1 (afterburner and ramjet)
- Combat radius: 245 mi (394 km)
- Ferry range: 1,545 mi (2,486 km)
- 36 2.75 in (70 mm) FFAR rockets
- 6 GAR-1/GAR-3 AIM-4 Falcon
The WaveRider destroys targets by simply crashing into them at hypersonic speeds. But the technology in this remarkable missile may have wider applications, including ultrafast planes and new space vehicles. Designed by Boeing and Pratt & Whitney for the Air Force Research Laboratory, the X-51 uses just one moving part—the fuel pump—to hit Mach 5, or 3600 mph.
|Rocket booster The X-51 is carried to 45,000 ft. by a B-52 bomber or a fighter jet, then released. A rear-mounted Army Tactical Missile Systems rocket kicks in to propel the 1600-pound missile to Mach 4.5 and 100,000 ft. The rocket then drops away and the X-51’s engine takes over.|
|Internal inlet The missile’s sharp nose funnels shock waves produced at hypersonic speeds into a rectangular opening on the craft’s belly. The shock waves compress the air, eliminating mechanical parts that normally do this.|
|Isolator This component adjusts airflow—which can reach 2500 pounds per square foot—to a stable pressure for the combustor. Slowing airflow increases drag on the vehicle, but allows for more complete combustion.|
|Combustor Thrust is created when the compressed air mixes with a mist of JP-7 jet fuel and is ignited. Because hypersonic speeds generate sustained temperatures of up to 4500 degrees, the propellant also acts as a coolant—and prevents the X-51’s engine walls from melting.|
|Airflow PM consulted NASA to estimate the fluid dynamics for external airflow around the nose, engine, stabilizers and tail of an X-51 traveling at Mach 5. The rear contour illustrates the engine exhaust plume shape.|