P&W gears up for next-generation military engine

Not too long ago - October 2007, to be precise - engine maker Pratt & Whitney seemed painted into a strategically perilous corner.

New requirements were emerging for next-generation bombers, sixth-generation fighters, tactical airlifters and surveillance aircraft. These demanded a new kind of jet engine, one that could make the next leap in fuel efficiency by reconfiguring and squeezing the air flow at higher levels than ever before.

P&W seemed to be caught on the back foot. The US Air Force Research Laboratory (AFRL) had selected GE Aviation and Rolls-Royce in 2007 to demonstrate next-generation engine technologies under two contracts - the adaptive versatile engine technology (Advent) and the highly efficient embedded turbine engine (HEETE). P&W had submitted bids for both programmes, but lost. The company's grip on the Lockheed Martin F-35 programme with the F135 engine was secure, but GE and Rolls-Royce seemed poised to leapfrog the military's dominant engine supplier for the next wave of combat aircraft.

© Pratt & Whitney The geared Turbofan's core and an F135 style low-spool are to combine in the PW9000

Much has changed, however, since the AFRL bypassed P&W for Advent and HEETE. The debate about the F-35 alternate engine is likely to linger on for years, but the GE/R-R-supplied F136 is officially off the books. Congress this year finally complied with the Department of Defense's five-year-old campaign to sever funding for the alternate engine, although supporters continue to push the cause.

REQUIREMENTS CHANGE

Meanwhile, the US Air Force's requirements have changed since 2007, and the service is no longer looking for a next-generation engine to power the next-generation bomber. Instead, service officials aim to leverage off-the-shelf technology as much as possible. That is a good sign for the P&W F135, the only programme of record in the 40,000lb-thrust (178kN) class and which is scheduled to be in full-rate production when the new bomber arrives.

Most importantly, P&W has revealed its own next-generation jet engine aimed squarely at the Advent and HEETE class of 20,000-35,000lb thrust. Company officials declined to be interviewed about the internally funded PW9000 engine for this article. It is not clear if P&W ever intended to reveal its existence, as it has been discussed publicly on a few occasions and only when specifically prompted by questions from journalists.

Details about the engine's configuration and some underlying technologies can be discerned from statements by company officials and documents, including recent patent applications.

Before Warren Boley, president of Pratt & Whitney Military Engines, left the company recently, he briefly described the PW9000 in February, when he defined the project as a combination between the F135 and the company's latest commercial engine, the PurePower PW1000 geared turbofan. "You can take an F135-style low-spool with the GTF core - we call that the PW9000 and people are very excited about that," Boley said. "Now you're talking about significant fuel-burn improvements."

A broad range of applications for the PW9000, which can be scaled down to the 10,000lb-thrust class, is also envisaged.

"There are hundreds of decisions down the line," Boley explained. "An unmanned bomber. A sixth-generation [tactical aircraft]. There is something that comes after the [Lockheed] F-22 for air dominance. The [US] Navy and all the airframers are looking what the sixth generation is."

The navy is seeking a next-generation air dominance fighter to replace the Boeing F/A-18E/F Super Hornet after 2025. The so-called F/A-XX fighter would be an attractive candidate for an engine in the PW9000-class between 20,000lb and 30,000lb thrust. Boeing has already displayed a model of a manned or optionally manned tailless fighter to replace the Super Hornet in about 15 years.

Winning a jet engine contract for such an aircraft would be a historic coup for P&W. The F/A-18E/F is powered by the GE F414, a turbo­fan a notch below the F135/F136 size class. One decade after witnessing the termination of the F136 alternate engine contract, P&W could drive GE out of the fighter engine business for good with a PW9000 powering the fighter that replaces the Super Hornet.

© Pratt & Whitney Overall pressure ratio indicates a step change

P&W officials have always been coy about identifying the technologies embedded in the GTF core. Much of the public interest on the GTF has justly focused on the gear itself, but it is clear the engine core's performance relies on more than decoupling the speed of the fan and low-pressure turbine.

In late March, P&W officials revealed almost offhandedly that the overall pressure ratio (OPR) of the GTF core begins at 45:1, and rises to 50:1 in the Airbus A320neo application. By contrast, the OPR of the F135 powering the F-35 is reportedly around 30:1, indicating that P&W has achieved a step-change increase in fuel efficiency.

OPR is a key indicator of engine fuel efficiency. It compares the pressure of the air exiting the high-pressure compressor with the air entering the fan. As the ratio grows higher, specific fuel consumption generally declines. The trade-off with raising the pressure of the airflow is the need to manage hotter temperatures, especially as the air exits the combustion chamber at the rotor inlet.

P&W apparently believes it has solved this challenge with a technical breakthrough called microcircuit cooling and tip blowing. In a two-year-old patent, P&W engineers Francisco Cunha and Jason Albert claim that the microcircuit approach is "the most advanced cooling technology in existence today". In AFRL documents dated 2008, microcircuit cooling is credited as an outgrowth of the F135 programme.

The PW9000 is likely to benefit from both the higher OPR of the GTF compressor and the microcircuit cooling approach pioneered by the F135, among other critical improvements.

However, it is not clear how the PW9000 compares with the technologies in development by GE and Rolls-Royce for Advent and HEETE. Whereas the GTF is credited with a top OPR of 50:1, GE has claimed that the objective of its HEETE design is to achieve ratios up to 70:1.

Moreover, one of the key objectives of the Advent programme is to introduce a third duct for bypass air around the engine core. Increasing the ratio of air bypassing the engine core also reduces specific fuel consumption at high speeds. However, P&W officials have argued that the benefits of a third bypass stream have been over-hyped.

As government-funded research, data gathered during Advent and HEETE demonstrations by GE and Rolls-Royce would be made available to other bidders anyway, says P&W.

All three companies are now waiting for requests for proposals from the AFRL to launch the third phase of the Advent and HEETE programmes. These technology maturity contracts could be awarded to P&W, despite the company's losses in previous rounds of Advent and HEETE.

"It's the next evolution," Boley said in February. "You take the next technologies from Advent - the adaptive technologies from Advent - the compressor technologies from HEETE, and now you go to the next full-scale development phase.

"When the techmat [technology maturity] programme is let when those RfPs come out we will not have enjoyed the US government technology programmes of Advent and HEETE, but we will have done our own company-funded technology development that powers the geared turbofan," Boley added. "So beyond the focus of the fan and the gear there's a high-efficiency core that we would use to go to techmat."

Source: Flight International