IN FOCUS: Civil engines to drive P&W military upgrades

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When Airbus decided to power the A320neo family with the Pratt & Whitney PW1100 geared turbofan, few probably realised the immediate implications for the military engines market. However, its choice may have unwittingly initiated a potential new military engine derivative.

"With the neo launch we have yet another core we can explore," says Bennett Croswell, president of Pratt & Whitney Military Engines.

By invoking Airbus's re-engined single-aisle, Bennett was not confusing his civil and military programmes. P&W's future military engines will now follow the lead of commercial market needs.

In 2010, P&W officials disclosed the existence of the PW9000 mid-thrust combat engine concept based on the core of the geared turbofan selected to power the Bombardier CSeries, which occupies a smaller thrust range than the A320neo family.

The PW9000 concept is derived specifically from the core of the 21,000-23,000lb-thrust (93-102kN) PW1500 for the CSeries, which gives P&W a candidate to replace or compete against the General Electric F414 fighter engine.

With the PW1100 occupying the 24,000-33,000lb-thrust range, P&W is developing a new engine core in the thrust class spanning two of its major fighter programmes - the F100-229 powering the Boeing F-15 and Lockheed Martin F-16, and the F119 installed in the Lockheed F-22.

pw1500 

 Pratt & Whitney

The PW1500 was designed for Bombardiers for Bombardier's CSeries

However, the possibilities are purely theoretical. The US military seems largely content with its existing propulsion technology, including the GE F110 that competes against P&W's F100 for orders on F-15s and F-16s. Moreover, the Department of Defense has few requirements for new fixed-wing platforms needing an all-new source of propulsion.

The US Air Force, for example, has emphasised the need to leverage off-the-shelf technology as much as possible for the next-generation bomber programme, which is expected to yield deliveries of a maximum of only 100 aircraft. By contrast, the DoD still intends to buy 2,447 F-35s during a 30-year period.

"A lot of these new systems that are going to be built in the future - they're going to buy 80 of them or 100 aircraft," Croswell says. "They're not going to buy 2,000 of them or 3,000 of them."

As a result, P&W's strategists have turned to the civil market to seed the next generation of military propulsion technologies, such as the proposed PW9000 military engine family.

"How can we leverage the investment we made on the [geared turbofan] core and maybe even the low turbine from the next-generation product family and build a military engine around it?" Croswell asks.

There is nothing new in the movement of propulsion technology between the military and civil sector, but the direction has tended to be one-way: from military to civil programmes.

Indeed, the turbojet itself was invented by Germany's Hans von Ohain and the UK's Frank Whittle for military applications in the late 1930s. It was finally applied on airliners from the early 1950s.

Military funding also developed the first turbofan engine - the General Electric TF33 - which was another innovation quickly picked up by commercial customers. Even the most popular commercial turbofan - the CFM International CFM56 - was derived from the core of the GE F101 bomber engine.

More recently, the government-industry integrated high-performance turbine engine technology programme was aimed at improving the F100 and F119 fighter engine designs. P&W's engineers also applied the technology to the geared turbofan.

Although the independently-developed reduction gear tends to attract the most attention, P&W says the GTF core also features advanced technology, including a second-generation "super cooling" for turbine blades. That system improves on first-generation super-cooling originally developed for the F-35 Joint Strike Fighter's F135.

"We are working on turbine cooling technologies that are over and above what's in the F135 today," Croswell says.

The PW9000 series will not be the only beneficiary of GTF-derived technology. The F135 itself is poised to be eligible for a series of technology upgrades.

On 23 February, the Air Force Research Laboratory (AFRL) awarded P&W an $11.8 million contract to test an engine demonstrator. The contract pays for 50h of high-cycle fatigue tests on the XTE68/LF1 performance demonstrator, which is based on the short take-off and vertical landing version of the F135.

"We're putting some hot section technologies into the combustor and in the high-pressure turbine, and some [aerodynamic] technologies into the low-pressure turbine. Those are the technologies we are demonstrating," Croswell says.

The aerodynamic change consists of a new high-lift design for the stationary turbine vanes that reduces their numbers and lowers the engine's overall weight, improving specific fuel consumption.

More importantly, P&W is integrating a second-generation cooling system derived from GTF technology: a civil market feature being added to the world's most advanced military engine.

The new cooling system "will enable us to provide even lower pattern factors, so it's the profile and temperature end of the turbine," Croswell says. "So we have to design the turbine such that it can take the heat temperature profile. If we can flatten the profile out so there's no spikes, we could run even hotter engine temperatures."

One way to increase maximum thrust is to combust the fuel and pressurised air at hotter temperatures, but there are other benefits. Engine components that can survive at hotter temperatures can also last longer if thrust is not increased. Alternatively, the airflow extracted to cool the engine can be reduced, lowering specific consumption.

"The goal is to provide options. We have the temperature capability to go to more thrust if needed - or we could go to more durability or reduce cooling air and have better fuel consumption," Croswell says.

The AFRL and the US Navy are funding the XTE68/LF1 demonstration, but the F-35 programme has yet to commit to integrating the upgrades, even if they are proven to work.

"You have to divorce this from the JSF programme," Croswell says. "The JSF programme is aware of it and endorses it, but this is an AFRL/US Navy-funded technology programme."

The F135 is required to provide 43,000lb of thrust in afterburner mode. The XTE68/LF1 is demonstrating a thrust improvement between 5% and 10%, potentially raising the single-engined fighter's overall performance from more than 45,100lb thrust to 47,300lb thrust.

Before GE's alternative F136 programme for the F-35 was cancelled last year, P&W's rival made a talking point out of the existence of a thrust improvement programme for the F135. After all, the USAF's F-35A will not enter service until at least 2018, and its only engine supplier is already working on a thrust upgrade.

However, Croswell insists the XTE68/LF1 simply fits the historic pattern.

"It's kind of what we've done in the past. Through the technology programmes we demonstrate the capability, and then the JSF programme will decide if they want to transition," Croswell says. "We did that on the F100. We proved that technology. We increased the life by 50%. That's our [enhanced engine performance] package."

There is an exception, however, to the pattern of a steady stream of engine thrust upgrades. With the introduction of fifth-generation fighters, the aircraft are being introduced with more advanced capabilities built into the airframes, reducing space and the need for major performance upgrades.

"Quite frankly on the F119 they've never chose [to upgrade]," Croswell says. "We've got all this technology that's on the F-35 that's better than what's on the F119. We've offered those packages to increase the life.

"To date, the F-22 programme hasn't chosen to transition those capabilities. I guess they like the engine too much or they've got other things they want to do."

P&W eyes durability lift for stovl F135

PRATT & WHITNEY has quietly received approval to launch a second engine demonstrator focused on inserting durability improvements into the short take-off and vertical landing (STOVL) version of the F135.

The XTE69/LFU1 demonstrator aircraft will in late fiscal year 2014 begin testing upgrades, with P&W confirming that it entered production. The company's XTE68/LF1 demonstrator will enter testing in the fourth quarter of this year.

lockheed martin 

 Lockheed Martin

Durability has been an issue for early STOVL engines. Although the baseline version is performing as designed, unexpected heat or friction caused P&W to make three changes effecting the driveshaft, lift-fan clutch and roll-post actuator nozzles to successfully release the F-35B from probation last year.