October 22, 2016

Next generation planes will either by large or they will use new engines that switch between performance and fuel efficiency

The United States Air Force is in the process of completing its initial research on a next-generation air superiority capability to replace the Boeing F-15C Eagle and Lockheed Martin F-22 Raptor fighters. Once such research is completed, the service will embark on an 18-month analysis of alternatives (AOA) starting this coming January to determine exactly what kind of capabilities it will need to gain and maintain control over the skies in the post-2030 threat environment. By then—in the year 2035—the stealthy F-22 will be 30 years old while most the F-15C fleet will be more than 50 years old.

Current generation fighters like the F-22 and the Lockheed Martin F-35 carry only a couple of missiles internally, which could be a limitation during future combat operations.

China and Russia would also be able to attack refueling tankers which would reduce the operating range of US fighters.

A future PCA might be a significantly larger aircraft that today’s fighters—designed to operate at far greater ranges while carrying a far greater ordnance load. Those requirements for range, persistence and payload will have to be balanced against the need for stealth, electronic warfare capabilities, speed, maneuverability and other traits.

Many of the Air Force’s potential future requirements might seem to be contradictory, but new technology might make such a plane technically feasible. Indeed, a very large fighter with a very large payload, huge range which is also extremely stealthy while being extremely maneuverable would be an extreme technical challenge with current technology. However, new technology such as adaptive cycle engines—which the Air Force is currently developing with General Electric and Pratt and Whitney—will likely solve many of those potentially contradictory requirements. “The bottom-line is it’s going to have to be a variable-cycle engine to meet those kinds of needs and not be a humongous airplane,” Jeff Martin, General Electric’s expert on sixth-generation fighter propulsion told me some time ago.

Adaptive Cycle Jet Engines

Thrust and fuel efficiency have always seemed destined to remain mutually exclusive – the higher the one, the lower the other – inevitably forcing jet engine designers to make calculated trade-offs between the two.

If the US Air Force's Adaptive Engine Technology Demonstrator (AETD) programme goes to plan, allowing future generations of aircraft to take to the skies that can switch from high-speed performance to maximum economy - and back again - as the need arises.

It is an ambitious goal, with a huge range of possible applications across the spectrum of fighters, bombers and tactical combat aircraft.

While it has all the makings of a potential game changer for the sector, the fundamental principle behind the idea remains fairly straightforward. Conventional jet engine designs are optimised either for range or speed primarily by reference to two key factors: the fan pressure ratio of the air pressure discharged from the fan relative to the input pressure, and the bypass ratio of the air flowing around the engine core relative to the air passing through it. Thus, commercial airliners and military airlifts have high bypass / low fan ratios to yield greater efficiency, while strike aircraft exhibit low bypass / high fan pressure ratios, sacrificing fuel economy in the interests of maximising thrust.

With adjustable fans and controllable air ducts, the thinking goes, you can increase the flow around the engine and raise the bypass ratio to improve cruising fuel efficiency, or force more air into the core to gain a burst of extra thrust, flexibly toggling between Grand Prix speed, or super-Mini economy.

SOURCES - National Interest, GE

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