A 10- to 15-year look ahead into Pratt & Whitney’s vision for military engine technology reveals a very different kind of propulsion system.

It may not be enough for the motor powering the next fighter, bomber or airlifter to simply generate thrust and electric power for a given weight and fuel burn requirement. The future military turbofan could be a far more complex, adaptable machine.

As a throttle governs the amount of fuel flow in engines today, in future, new software and electronic controls will be able to modulate a motor’s bypass airflow and power off-take. Turbofan engines are adapting, even as combat fighters are evolving from short-range weapons trucks into long-range sensor and attack aircraft with a suite of kinetic and non-kinetic armaments.

If, say, a directed energy weapon is on board, the engine may need a way to modulate the bleed-air now used to pressurise the cockpit. That compressed air flow could instead be diverted momentarily to power a laser or a next-generation jammer.

Or the same engine could be reconfigurable in flight, by opening a third stream of airflow when rapid acceleration is not needed, to save fuel and extend range. That same extra stream of cool bypass air could also pull double-duty as a handy place to dump all the new heat generated by those high-power jammers and laser weapons, rather than allow the exhaust to betray the aircraft’s thermal signature by venting it directly offboard.

As the US military looks to field a new bomber, a sixth-generation fighter and improve the Lockheed Martin F-35 over the next 15 years, that highly flexible, endlessly adaptable propulsion concept is P&W’s vision for the next wave of military engines that could be introduced in the next decade.

The vessel for such new engine technology could take several shapes. P&W has previously revealed a concept engine in the 10,000lb (45kN)-thrust bracket called the PW9000, which combines the engine core of the highly-efficient PW1000G geared turbofan with the low-pressure spool of the F135 engine that powers the F-35. The F135 itself is a candidate as a mid-life upgrade in the 2025 timeframe. P&W is also developing new concepts for a sixth-generation fighter engine to power the aircraft that will replace the Lockheed F-22 and Boeing F/A-18E/F Super Hornet after 2030.

“One of the key issues you see and the reason why [adaptive technology] is so valuable in a military aircraft is the mission is tremendously variable. You fly high-altitude, you fly low-altitude. You fly fast, you fly slow. Now we’re seeing more and more things where I want to take power off the engine so that I can drive electronic warfare, or I can drive other kinds of directed energy capabilities,” says Jimmy Kenyon, P&W’s director of advanced programmes and technology. “And all these things are transient in nature and I need to have engine or propulsion capability that can do that. The more adaptive I can make my propulsion system, the better off I am.”

Elements of that vision have been known since 2006, when the US Air Force unveiled the adaptive versatile engine technology (ADVENT) programme. The public focus of that activity was always about the insertion of a variable third stream to increase fuel efficiency by 25% and combat radius of a retrofitted F-35 by 30%.

In recent interviews, however, P&W officials described a broader vision for incorporating adaptive technology – first in military aircraft engines, and later perhaps in other applications, such as supersonic business jets and commercial airliners.

The vision extends beyond an adaptive bypass ratio for the main propulsion system. It also includes inserting adaptive technology into the engine core, raising the possibility of modulating the pressure ratio in flight. P&W officials decline to confirm plans to develop a compressor with a variable pressure ratio, but it is clear that the company is thinking broadly about adaptive technology.

P&W’s public embrace of adaptable engine architectures comes slightly later than its competitors. In 2007, the US Air Force Research Laboratory (AFRL) awarded ADVENT contracts to GE Aviation and Rolls-Royce, allowing P&W to continue focusing on developing the F135. The AFRL later selected GE to develop the first ADVENT engine core demonstrator, which last year completed 60h of testing over a four-month period.

Although GE garnered the AFRL’s early contracts, P&W was selected along with it to compete for the adaptive engine technology demonstrator (AETD) programme; a follow-on to ADVENT to develop a production-representative fighter engine core. The next step is to develop a full-scale engine under the adaptive engine transition programme (AETP).

Since January, the AFRL has awarded GE and P&W indefinite delivery/indefinite quantity contracts worth up to $325 million. The awards are not specifically earmarked for AETP, and the competitors decline to clarify what the work scope of the contracts include. But the awards were made under the versatile affordable advanced turbine engines (VAATE) programme, which includes the ADVENT, AETD and AETP efforts.

On 15 May, the AFRL also awarded separate contracts worth $105 million to both companies to achieve a preliminary design review and an adaptive engine research design compatibility review.

Inserting a third stream of airflow has been the focus of each of the development programmes so far, but it might only be the beginning. Two years ago, P&W filed a patent application for an engine design featuring an adaptive bypass ratio and an adaptive compression ratio. Neither GE or P&W has so far been willing to comment about any development work underway with the USAF on adaptive compression technology. But P&W acknowledges that the scope of its development projects go far beyond adapting only bypass flows.

“AETD is really about adapting the low-spool,” says Kenyon, referring to the shaft connecting the fan and low-pressure compressor to the low-pressure turbine. “But what happens if I can make the core adaptive? We are looking at ways to do that. We are looking at ways we can take different air streams, whether it's bleed air or cooling air, and we can adapt that and we can modulate it so we can get more cooling when we need it and less cooling when we don’t need it. Looking at the different ways we can make engine architectures adaptive opens up whole new worlds.”

That new approach has forced P&W executives to also adapt some of their public views. Only three years ago, company president Paul Adams, who was then senior vice-president of operations and engineering, categorically dismissed the value of ceramic matrix composites (CMC) in aircraft engines. "Right now I don't see a path forward for large-scale integration of CMCs,” he said in 2012.

That position has changed as turbine inlet temperatures for next-generation engine concepts have climbed to 2,700˚F (1,480˚C), or 300˚F hotter than state-of-the-art engines today. Unlike GE Aviation, which has embraced CMCs in high-temperature applications for commercial and military engines, P&W had shown no public interest in the materials. In recent comments, however, company executives have been clear that CMCs will play a key role in future engine technology.

“For CMCs you need a fibre, coatings, the matrix. We have what we believe are clear pathways to doing matrix consolidation and matrices that are capable of 2,700˚F and maybe even beyond,” says Frank Preli, P&W’s chief engineer of materials.

As new materials and cooling systems allow core engine temperatures to rise, the next generation of military engines should be more and more fuel efficient. Although both GE and P&W are now committed to introducing CMCs in current and future engines, they are taking slightly different paths. GE’s commercial engines feature CMCs in non-rotating static components, such as a turbine shroud and combustor liners. GE also has tested CMC-based rotating parts in an F414 fighter engine demonstrator and considered manufacturing a CMC turbine blade for the GE9X commercial turbofan.

For P&W, however, CMCs will be reserved solely for applications in rotating parts.

“We’re not convinced that CMCs are the best material selection for a static part,” Preli says. “There’s a couple of issues with CMCs beyond the cost and manufacturability and that’s their thermal conductivity. Thermal conductivity is relatively low. In a non-rotating part you can take advantage of that very high thermal conductivity [of other materials] and actually get a part that is much more effective, with less cooling air that would be required for CMC.”

Source: FlightGlobal.com