IN FOCUS Pratt & Whitney adapts to face military engine future

Washington DC
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Reducing fuel consumption by reconfiguring how the air flows through a jet engine during flight is the next frontier in military propulsion.

To some extent, Pratt & Whitney is playing catch-up in the adaptive engine race. In 2009, the Air Force Research Laboratory passed over P&W to award contracts to GE and Rolls-Royce for the adaptive versatile engine technology (ADVENT) programme. P&W re-entered the competition only a year ago, when the AFRL replaced R-R with P&W to compete with GE for the critical follow-on to ADVENT: the adaptive engine technology development (AETD) programme.

With the F135 propulsion system for the F-35 now in production, P&W is moving quickly to prepare for the next wave of military engine development contracts.

In September, the company held an initial design review on an adaptive engine concept based on a highly modified F135, says Jimmy Kenyon, general manager of P&W.

At the same time, P&W launched the evaluation of a test rig for an adaptive inlet fan, he says. The rig will inform the adaptive design as P&W performs a series of technology demonstrations over the next two years.

Adaptive engine technology is not profoundly new. As early as the 1960s, P&W adapted the airflow of the J58 engine to allow the Lockheed SR-71 to cruise well above Mach 2.0. More recently, Kenyon argues, the short take-off and vertical landing (STOVL) variant of the F135 is also adaptive, in the sense that it reconfigures the horizontal exhaust flow to vertical during flight.

This new interest in adaptive engine technology has a different focus. It arose after the price of jet fuel in the USA trebled within two years. At the time, Kenyon was overseeing the aerospace science and technology portfolio of the US military in the office of the secretary of defence.

The result was the launch of the ADVENT and AETD programmes. The goal is to preserve the raw thrust capability of an engine such as the F135, but improve fuel efficiency by 25%.

The only way to achieve such fuel savings is by adapting how the engine consumes air during flight. There are two air streams in a turbofan engine – one is funnelled into the core, compressed and combusted; the other is diverted around the core. As no combustion is involved, the latter bypass stream produces thrust most efficiently. The amount of bypass air flow, however, is limited by the smaller diameter of a fighter engine and the design imperative for rapid acceleration.

Opening a third stream of bypass air flowing around the engine core is the answer. The P&W concept for the AETD core increases the bypass flow by roughly a factor of two, Kenyon says. The third stream is fully variable throughout the flight envelope, he adds.

P&W is also taking new aerodynamic tools designed for commercial turbofan engines and using them to improve the compressor section of the F135 for the AETD core, Kenyon says.

Adaptive engine architectures could also include more advanced features. GE, for example, confirms its AETD engine core includes adaptive compression technology, meaning the engine can vary the overall pressure ratio of the airflow within the core.

P&W filed a patent application last June for an even more ambitious concept. The patent shows an engine with such features as adaptive bypass flow, adaptive compression flow, an intermediate pressure stage and a geared fan.

Kenyon, however, says adaptive compression is not one of the features planned for P&W.