Thermally throated ramjet targeted at two near-term projects led by US Air Force

Alliant Techsystems (ATK) has demonstrated a simplified hypersonic engine that could enable near-term development of a high-speed strike weapon. Tests included ground runs of a flight-weight, actively fuel-cooled engine at Mach 5.

After air or surface launch, the proposed missile would be rocket-boosted to hypersonic speed where the thermally throated ramjet (TTRJ) would ignite to maintain an M5 cruise, allowing the weapon to fly 650km (350nm) in 8min.

Designing for a single Mach number avoids the complexity of a variable-geometry inlet, while choosing a M5 cruise speed eliminates the need for exotic materials, supersonic combustion and complicated cooling and control systems, says Charlie Precourt, ATK Launch Systems vice-president of strategy and business development.

hypersonic engine    
The simplified engine could power a high-speed strike weapon like the one below
ATK's ALRJ-M5 engine uses available metallic materials, established manufacturing processes and conventional JP10 hydrocarbon fuel. Precourt says the fuel is used to cool the engine, but does not have to be pre-conditioned to burn in the combustion chamber. Ground testing has shown the capability to ignite on cold fuel, he says.

Compared with other's supersonic-combustion ramjets, the engine has a simplified flowpath, with a rectangular intake transitioning to a circular combustion chamber and exhaust. Airflow is slowed to below M1 for subsonic combustion by a shockwave at the throat of the engine.

high speed strike weapon

In a thermally throated ramjet, Precourt says, the positioning of the shockwave is determined by the back-pressure generated by combustion and controlled using the engine fuel flow. This avoids the complexity of controlling the flow using mechanically moveable inlet and exhaust vanes. The TTRJ is scalable to higher Mach numbers, with supersonic combustion, different inlets and advanced materials, he says.

While the fuel-cooled TTRJ combustor has accumulated more than 70min combustion time in direct-connect testing without deterioration, Precourt says, the fully integrated engine with closed-loop fuel system has reached thermal equilibrium in freejet testing. This simulated cruising at 85,000ft (26,000m) and M5.

The ALRJ-M5 is ready for flight testing, says ATK, which is targeting two near-term opportunities: as an alternative to the Pratt & Whitney engine in the Boeing X-51 scramjet demonstrator and the planned US-Australian HiFire hypersonic flight research programme (Flight International, 23-29 January). Both are being led by the US Air Force Research Laboratory.

ATK is fabricating an engine designed for the X-51, and which would be smaller and simpler than the P&W scramjet and which, says Precourt, could free up space in the vehicle for a recovery parachute, enabling the experimental waverider to be reusable.

Source: Flight International