Aerojet is proposing development of a novel combined-cycle propulsion system for reusable hypersonic vehicles which packages current technology to achieve a seamless transition from a standing start to Mach 7 plus. Aerojet is a leading United States developer of ramjet and scramjet propulsion systems for missiles and is emerging as the leader in combined cycle propulsion for hypersonic cruise and space access applications.
Earlier work – Pyrojet was a kerosene burning turbine based combined cycle engine.
Pyrojet hypersonic engine
The TriJet concept is a near-term solution providing the Mach 0-7 propulsion capability which makes use of currently available Mach 2.5 turbine engines, supplemented by an ejector ramjet to bridge the thrust gap between the available turbine and dual-mode ramjet operation. A hydrocarbon-fueled (JP-10) 75-ft-long vehicle capable of Mach 7 cruise was selected into which to integrate these propulsion systems. Range was chosen as the figure of merit for comparison purposes. The results indicate that the range capabilities of the Pyrojet and Trijet are within 10% of each other.
The concept tackles key problems that developers face in trying to accelerate aircraft to high enough speed for a scramjet to begin operating. Although rocket boosters have been used to accelerate experimental scramjet-powered vehicles like the X-51A to the take-over point, this approach is not suitable for reusable platforms that would operate from a runway.
Major hurdles in the path to successful aircraft-like operation include producing sufficient thrust to punch through the high drag encountered at transonic speeds around Mach 1. Even if this can be overcome, designers also face a “thrust gap” between around Mach 2.5, where current turbine engine power falls off, and Mach 3.5-4, where the transition to a dual-mode ramjet/scramjet takes place. To date, all attempts to develop a viable high-speed turbine engine to bridge this gap have failed.
Aerojet’s TriJet concept builds on the advantages of two traditional air-breathing propulsion systems extensively studied for this role—the turbine- and rocket-based combined cycles (T/RBCC). However, in isolation, both have disadvantages. TBCCs require turbine engines that are often heavy and large, taking up space for fuel, and have poor transonic acceleration, while RBCCs have significantly lower fuel efficiency than turbine engine-powered concepts. The TriJet combines the two options into one by melding a turbine engine and rocket-augmented ejector ramjet (ERJ) with a dual-mode ramjet (DMRJ) to achieve the final push to hypersonic flight.
In a basic sense, the vehicle will take off using turbine engine power, which can be partially augmented by rocket-assisted thrust from the ejector and dual-mode ramjet. At higher speed, the turbine engine is shut down and closed off, with the ejector ramjet taking over. The exhaust from the ERJ is ducted through the nozzle of the DMRJ to form an aerodynamic choke. This enables the DMRJ to produce more thrust and accelerate the vehicle to greater speeds until the ERJ can be closed off, and the dual-mode ramjet converted into a scramjet.
The concept is attracting U.S. Air Force interest as a possible pathway to a high-speed intelligence, surveillance and reconnaissance (ISR) or strike aircraft, says Mel Bulman, Aerojet’s chief engineer of advanced propulsion and mission architecture. Revealing details of the TriJet concept, Bulman says: “We believe it’s mature enough to initiate a program, and a subscale demonstrator could be an option.”
For the challenging push from Mach 2.5 to Mach 4, Aerojet’s concept now reconfigures itself to get maximum performance from the DMRJ, which has no doors in the flowpath. The ramp doors of the ERJ partially close and the exhaust is directed into the DMRJ nozzle. The flow convergence in the nozzle forms a sustained aerodynamic choke, causing the flow through the engine to be wholly subsonic.
The fuel is injected by Aerojet’s core burning device, which was developed to improve combustion in inward-turning, or round flowpaths. In this device, three cantilevered fuel-injection struts are tied together at the center, allowing the pilot flame and the center of the combustion process to be kept away from the walls. The extended combustion flow area makes thermal balancing easier, and generates more thrust.
“Core burning turns the engine inside out. Flame propagates from the center of the engine, and only touches the wall at the back, which keeps the engine cool on the outside. It reduces the heat load by 40-50%,” says Siebenhaar.
The scramjet operates like two ramjets in this mode up to Mach 4.5, at which point the flow to the ejector ramjet is shut off and the vehicle relies solely on the scramjet, which is optimized for supersonic combustion to Mach 7 or 8.
Although Bulman acknowledges that the development of an ISR or strike platform remains a longer term option, he says the TriJet concept and specifically technologies such as the core burner can be applied to missiles.