IN FOCUS: Record test run highlights F-35 engine growth path

Washington DC
Source: Flightglobal.com
This story is sourced from Flightglobal.com

Pratt & Whitney confirms that a testbed version of the Lockheed Martin F-35 propulsion system has run sustainably at possibly the highest temperatures ever recorded by a turbofan engine.

The test results are important because they illuminate a long-term growth path for the F135 engine and a potential solution to concerns about the F-35’s acceleration and fuel efficiency, and durability at higher thrust levels.

“We had a challenging problem. This is really hard stuff,” P&W general manager of next generation fighter engines Jimmy Kenyon says. “And we said we could get there, and we went out, and did the work and we nailed it.”

As the US military considers suppliers for a new bomber and, eventually, a sixth-generation fighter, the results on the F135 demonstrator also provide P&W’s unspoken rebuttal to claims by GE Aviation of achieving the hottest core temperature in tests earlier this year.

P&W is still working to validate the record with the US Air Force, but company officials believe the tests on the F135 demonstrator yielded the highest core temperatures achieved by any engine manufacturer.

“We know it ran hotter than any other Pratt & Whitney engine that’s ever run, and we believe it’s the hottest that’s ever been run in an engine in the world,” Kenyon says.

Increasing thermal capacity is a vital ingredient to improving the power and durability of jet engines. It creates options for the customer who buys the engine. Burning fuel at higher temperatures increases maximum thrust – up to an estimated 5% over the 28,000lb-thrust military (dry) power rating of the F135-PW-100 installed on the conventional F-35A and carrier-based F-35C variants. Alternatively, the operator can keep the engine at the same temperature and operate the same parts in the hot section for longer without damage.

“It gives you options, if you will, to improve the overall capability of the engine,” Kenyon says.

The demonstration on the F135 testbed – internally known as the XTE68/LF1 – was funded by US Air Force Research Laboratory's (AFRL) versatile affordable advanced turbine engine (VAATE) programme.

P&W is also funded by the US Navy’s task force on energy to improve the fuel efficiency of the F135 engine, which will lead to further test runs using the LF1 demonstrator. The navy-funded tests will focus on improvements to the high-pressure compressor of the F135 engine.

The goal is to make a “meaningful improvement in the high-[pressure] compressor at a meaningful cost – something that would be compelling”, Kenyon says.

By combining the technologies funded by VAATE and the navy’s energy task force, P&W hopes to have a suite of upgrades available for the F135 roughly by the time the programme is ready for a mid-life update around 2020.

The same technologies could then be applied to the next generation of engine technology for military aircraft. The USAF, USN and the Defense Advanced Research Projects Agency are each developing requirements and technologies necessary for a sixth generation fighter that will replace the Lockheed Martin F-22 Raptor, the F-35 and the Boeing F/A-18E/F Super Hornet.

For engine makers, the sixth generation fighter has focused on developing new adaptive propulsion technologies, such as varying the bypass ratio during flight to improve fuel efficiency and extend range.

But the improvements being developed for the hot section of the F135 under the VAATE and navy-funded programmes also could be re-used in a next-generation engine, Kenyon says. Such demonstrations are part of P&W’s strategy for remaining competitive with GE, which has benefitted from working with AFRL under the adaptive versatile engine technology (ADVENT) programme since 2009.

In the meantime, the hot section improvements are the first major design changes proposed for the F-35 propulsion since the programme was launched in 2001.

The F-35 has been criticised in a series of government reviews for missing range targets and being slow to accelerate through the transonic range.

Solving those problems may also require weight reductions in the airframe, but improving the power capacity and efficiency of the engine can help, Kenyon says.

Such improvements could also offset any new weight growth on the F-35, as the aircraft is fielded and capabilities are added.

“I wouldn’t be comfortable giving you those numbers today, but we are looking at those sorts of analyses,” Kenyon says.

The first step to introducing the F135 upgrades requires the demonstration tests on the LF1 engine. For the tests, P&W used standard components of the F135 in the compressor, and modified only the first two stages of the high-pressure turbine.

To extract maximum power from the combustion process, the core section – including the high-pressure compressor, combustor and high-pressure turbine – already runs hotter than the melting point of all but the most exotic materials.

Not surprisingly, the holy grail for an engine maker is to find ever-more sophisticated systems to cool key elements of the hot section or to invent new materials that can survive at higher temperatures, at an affordable price.

While GE focuses on material solutions by replacing metals with ceramic matrix composites (CMCs), P&W says it takes a more “holistic” view to the thermal problem.

“The right answer is always some combination of the two,” Kenyon says.

In the recent tests, however, P&W focused mainly on the cooling system design.

The hottest part of any jet engine is the turbine inlet, where exhaust gases from the combustor flow into the first spinning wheel blades of the high-pressure turbine – also known as the stage one turbine.

Each blade features an intricate set of internal passageways. Inside, cooler air extracted from the compressor flows through the passages, then blows over the surface, cooling the outside and inside of the blade to just below the melting point of the nickel-alloy-based metal.

To improve the blade’s thermal tolerance, P&W developed a proprietary new casting process to create the passageways inside each blade, allowing the same amount of compressor bleed air to provide more cooling, Kenyon says.

The new blades are also coated with a new thermal barrier material, he says. The coating helps the blade resist higher temperatures, but Kenyon says that was secondary to the new improved blade design.

“The real driver was cooling technology,” he says.

The metallic shrouds surrounding the first and second turbine stages were not modified for the demonstration. GE has started replacing such metallic shrouds in its most advanced engines with CMC-based materials. P&W, however, does not expect to make the same material change.

“I don’t know [if] we would need to make a fundamental material change like that to get [to the higher temperatures],” Kenyon says.

The test runs proved that the redesigned turbine blades exposed to the hottest temperatures in the engine could survive at higher thrust levels, he says.

“We took the engine up [to the record temperatures], we stayed at that condition for a pretty decent amount of time, and then we stayed there a bit longer,” he says.

Importantly, the test verified that the redesigned blades were not damaged. A post-test inspection revealed that all of the modified components emerged in a “pristine” condition, Kenyon says.