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Pratt & Whitney is working with Lockheed Martin on a long-term upgrade for the engine powering the F-35 Joint Strike Fighter that could substantially alter the configuration of the F135 powerplant, P&W military engines president Bennett Croswell says.

The US Air Force awarded $1 billion deals on 30 June to both GE Aviation and P&W for the adaptive engine transition programme (AETP), which aims to develop competing designs of a 45,000lb-thrust-class (200kN) fighter engine with a new fuel-saving cruise mode.

One of the potential applications for such an engine is the global F-35 fleet after 2021. Applying the AETP’s adaptive cycle technology in the F135 engine could reduce fuel burn by up to 20%, says Croswell, speaking to FlightGlobal in London on 10 July.

The adaptive cycle adds a third stream of airflow to a turbofan engine. In addition to airflows entering and bypassing the engine core, an adaptive cycle would open a second-stream of bypass airflow in cruise mode, to significantly improve fuel efficiency.

But adding a third stream of airflow requires the engine designer to slightly widen the engine, and that poses an integration challenge in the tight confines of the F-35 engine compartment, Croswell says.

“There isn’t a lot of extra space there,” he notes.

So P&W is already in discussions with Lockheed on ways to add space to the engine compartment in order to accommodate a future adaptive cycle variant of the F135, Croswell says.

In addition to adapting the bypass air flow, P&W also is considering other ways to insert adaptive cycle technology into the engine core, Croswell says. One example he cites are variable turbine vanes, not unlike the variable guide vanes in the compressor.

A suite of adaptive cycle technologies is being developed for the F-X fighter programme, which the US Air Force is planning to replace the Lockheed F-22 after 2030.