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In September, Pratt & Whitney said it had completed testing of an adaptive three-stream fan in an engine with an F135 core as part of the AETD program. An F135 is shown here on a production line. The development of a third stream provides an extra source of air flow to improve propulsive efficiency and lower fuel burn.

Three-stream engine architecture eyed for next-gen military aircraft

As fifth-generation fighter aircraft continue to fight their way into budgets and operation plans, there is already much work being done toward the development of various propulsion systems that may make it into future sixth-generation fighters, and potentially other military and commercial applications.

An example of such work formed the basis of the U.S. Air Force Life Cycle Management Center's Propulsion Directorate last summer issuing two separate $1 billion contracts to design, develop, fabricate, and test 45,000-lb thrust-class adaptive engines in the Adaptive Engine Transition Program (AETP). A particular focus of the AETP involves reducing associated technical and manufacturing risks. The contracts were awarded June 30, 2016, to GE Aviation and Pratt & Whitney.

Just this past September, P&W discussed testing it had completed on an adaptive three-stream fan in an engine with an F135 core as part of the earlier Air Force Research Laboratory's Adaptive Engine Technology Development (AETD) program on preliminary design and component tests. According to Matthew Bromberg, President, P&W Military Engines, “Preliminary data from the test indicates our three-stream fan met or exceeded expectations with respect to performance as well as the integrity of the turbofan machinery and fan module.”

Modern military turbofan engines have two airstreams—one that passes through the core of the engine, and another that bypasses the core. The development of a third stream provides an extra source of air flow to either improve propulsive efficiency and lower fuel burn, or to deliver additional air flow through the core for higher thrust and cooling air should the need for supersonic flight occur.

Being able to take advantage of a third stream of air that can be modulated to adapt the engine's performance across the flight envelope allows a fighter to have the best of both worlds by accessing an on-demand increase in thrust or to smoothly shift to highly efficient operations during cruise. This capability provides an optimal balance for combat scenarios requiring both high-end acceleration and increased range.

While P&W is demonstrating the efficacy of a three-stream architecture, it is also maturing other advanced propulsion technologies considered essential for high-speed and long-endurance performance requirements. This includes adaptive control systems as well as improved integrated power and thermal management capacity that can enable more sensors, data fusion, electronic warfare, and directed energy. The goal of the AETD program was to provide a 25% reduction in fuel consumption and a 10% improvement in thrust levels compared to today's fifth-generation combat aircraft engines.

Bromberg believes P&W has a definite advantage with the technology, having developed what it says was "the very first adaptive engine, the J58," which powered the SR-71 Blackbird, as well as having developed more recently, the F135 STOVL variant.

The AETD fan test was conducted at the Arnold Engineering Development Complex, located on Arnold Air Force Base in Tullahoma, TN. Later this year, P&W plans to conduct additional adaptive engine testing on a new high efficiency engine core developed under the AETD program.

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