Technology promises to reduce size of missiles and offer high-performance combustion for hypersonic projects

Boeing is developing compact, high-performance combustors that it hopes will greatly reduce the size of ramjet-powered missiles and aid hypersonic projects and third-party engine developers.

The compact swirl-augmented thruster (CoSAT), developed by Boeing Rocketdyne and Simma Technologies, promises high-performance combustion in weight- and volume-limited propulsion systems.

The device, consisting of a system of guide vanes, swirls the air entering an engine's combustor. Boeing Rocketdyne Joint Strike Fighter (JSF) lift thrust augmentation (LTA) project engineer, Bob Pederson, says "the swirl pattern induces a recirculation zone in the combustor, eliminating the need for flame holders".

This promotes a stable flame, increases fuel mixing and turbulence levels, and enhances flame propagation. Pederson claims combustion efficiency is 90-99%.

The system was originally developed to provide additional lift thrust for Boeing's Short Take-Off and Vertical Landing (STOVL) X-32 JSF bid that lost to the rival Lockheed Martin X-35. Two LTAs provided 3,600lb (16kN) additional thrust during vertical lift operations, burning bleed air from the main engine's turbine with additional fuel, and delivered a fuel specific impulse of about 2,400s.

Boeing says the technology's reliable performance across wide throttle limits means engineers can easily scale the design for other applications, such as gas turbine engines, ramjets, and turbo-ramjet and rocket-ramjet combined cycle engines. Pederson says: "We're trying to get the technology known: we're working with the hypersonics team in the Phantom Works and briefing the US Air Force, Navy, NASA and DARPA [Defense Advanced Research Projects Agency]."

The performance of ramjet-powered missiles depends on their size and weight, Pederson says. "A typical long-range, rocket-ramjet powered, Mach 4 to M6 missile has a combustor length-to-diameter [l/d] ratio of five."

The length is driven by an inefficient conventional flame stabilisation and combustion propagation step-down mechanical flame holder, and the boost propellant packaging needed to accelerate the missile to ramjet takeover speed.

"Using CoSAT technology, an l/d ratio of 1.6 is possible, and we'd like to achieve less than one," Pederson says. The booster fuel would then be packaged separately and ejected as the ramjet takes over at M2 or M3.

In a turbo-ramjet combined cycle engine, turbojet compressor air could be burned by the ramjet using CoSAT technology to quicken the boost phase, as well as in the ramjet-powered phase at higher speeds.

Source: Flight International