Lockheed as been working with rocket propulsion specialists Aerojet for several years on the project, using company funds. Although the design could lead to a Mach 6 unmanned strike aircraft, Lockheed Martin has dubbed it the SR-72, after the company’s SR-71 Blackbird manned strategic reconnaissance aircraft that reached Mach 3 but was retired in 1997.
The engine design modifies a standard military turbine such as an F100 or F110 and couples it to a supersonic combustion ramjet (scramjet), using a common inlet and nozzle. Aerojet has been publicizing its Trijet combined cycle concept for some time already, noting that it would bridge the so-called “thrust gap” between turbines that reach Mach 2.5 and scramjets that work above Mach 3.5. Aerojet was merged with the former Pratt & Whitney Rocketdyne business to form Aerojet Rocketdyne earlier this year.
Lockheed Martin said that the design is “affordable” and could be operational by 2030.
A series of wind-tunnel tests have proved that the design is controllable from takeoff through hypersonic speeds. Subscale ground tests of the combined cycle engine, intakes and nozzle have taken place, as have scaled tests of other components. Details of how the thrust of the turbine is augmented and how the scramjet can work at lower speeds than hitherto remain proprietary, Leland added. He said that work on a single-engine, F-22-size demonstrator could start in 2018, with a first flight in 2023. The full-scale SR-72 is 100 feet long.
The Skunk Works developed the rocket-launched Hypersonic Technology Vehicle 2 (HTV-2) using funding from the Defense Advanced Research Projects Agency (Darpa) Falcon program. Lockheed Martin said that the SR-72 design incorporates lessons learned from the HTV-2, which flew at Mach 20.
Leland did not fully explain why ADP is revealing the project now. He linked the move to the U.S. Air Force high-speed strike weapon (HSSW) missile project. The service’s hypersonics roadmap envisions the HSSW as a stepping stone in 2020, to an SR-72-type vehicle by 2030. But Leland acknowledged that hypersonics are perceived as expensive and exotic, despite the recent success of the Boeing X-51A Waverider project.
The aircraft would take off on turbine power, then ignite the ejector ramjet to push through the transonic drag rise and accelerate to the take-over speed for the dual-mode ramjet. But the elegance of the trijet concept is how the operating envelopes of the three engines overlap to provide seamless propulsion.
It starts at the inlet which feeds all three engines. While the dual-mode ramjet has an unobstructed flowpath, the turbojet and ejector ramjet are concealed behind doors that open and close depending on the phase of flight.
From zero airspeed to above Mach 1, the inlet doors to the turbine and ERJ engines would be fully open and both engines would be operating to produce enough thrust to punch through the transonic drag peak. Above Mach 1, the doors to the turbine engine gradually close until, at Mach 2.5, the turbojet is shut down, purged and cocooned. The ERJ continues to provide thrust to Mach 4+ before both inlet doors and nozzle flaps are closed.
On its own, the DMRJ (dual mode ramjet) begins to provide thrust around Mach 3. But it can be made to produce thrust down to Mach 2.5 by operating it in "sustained aerodynamic choke" (SAC) mode, Aerojet says. Opening the nozzle flaps causes the ERJ (ejector ramjet) exhaust plume to "choke" the flow through the DRMJ (dual mode ramjet). Upstream of the choke, flow is subsonic, allowing the DMRJ nozzle to be used as a ramjet combustor. Pumping fuel into this much larger space allows both the ERJ and choked DMRJ to produce thrust from M2.5 to M4. Above that speed the ERJ nozzle flaps are closed and the DMRJ provides all the thrust.
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