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A fly in the ointment for eventual development of a hypersonic reconnaissance/strike vehicle is the need for a propulsion system able to take the aircraft from a standing start to beyond Mach 6 and back to a runway landing.Adherents to aircraft-like air-breathing operations have long favoured the turbine-based combined cycle (TBCC) concept, bringing together a high-Mach turbojet with a dual-mode ramjet/scramjet. But efforts to develop a high-Mach turbojet have failed, leaving designers with a problem -- how to bridge the gap between the M2.5 maximum speed of today's turbine engines and the M3-3.5 take-over speed of a dual-mode ramjet (DMRJ) capable of accelerating the vehicle to M6-7.Failure of the U.S. Air Force Research Laboratory's HiSTED program to produce a small turbojet capable of speeds up to Mach 4 helped doom DARPA's Mach 6 Blackswift. Now AFRL is proposing a follow-on TBCC demonstrator that would be built instead around an off-the-shelf fighter engine. That raises the issue of how to bridge the thrust gap.TBCC concept (Art: Lockheed Martin)As revealed in this week's issue of Aviation Week & Space Technology, Aerojet thinks it has a solution in the conceptually elegant, if aerothermomechanically mind-bending, TriJet TBCC engine. This bridges the gap between an off-the-shelf turbojet and a DMRJ by introducing a third engine -- a rocket-augmented ejector ramjet (ERJ). Essentially, 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 concept is how the operating envelopes of the three engines overlap to provide seamless propulsion.All graphics: AerojetIt starts at the inlet (above, in a twin TriJet installation), 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 above, a view through the TriJet looks like this:From the side, it looks like this: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 M1, the doors to the turbine engine gradually close until, at M2.5, the turbojet is shut down, purged and cocooned. The ERJ continues to provide thrust to M4+ before both inlet doors and nozzle flaps are closed.Here's where it starts to get mind-bending. On its own, the DMRJ begins to provide thrust around M3. But it can be made to produce thrust down to M2.5 by operating it in "sustained aerodynamic choke" (SAC) mode, Aerojet says. Opening the nozzle flaps (above) causes the ERJ exhaust plume to "choke" the flow through the DRMJ. Upstream of the choke (see below), 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.The combined result of all this techno-trickery is a propulsion system that has sufficient thrust in all flight phases to accelerate the vehicle rapidly to its Mach 6+ hypersonic cruise, yet the flexibility to take off from a runway, refuel in flight, return to a powered landing and even execute a go-around. And Aerojet says the technology is mature enough to demonstrate in flight. Now all we need is the money -- and the will.
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