An advanced compressor, combined with turbine, form core of proposed F135 upgrade

Originally published by Guy Norris for Aviation Week & Space Technology on June 22nd 2016

As Pratt & Whitney nears completion of the extensive F135 system development and demonstration (SDD) program for the F-35 Joint Strike Fighter engine, the company is revealing new details of a proposed upgrade that could cut fuel burn by as much as 7% on aircraft entering service by the early 2020s.

The upgrade package builds on a fuel-burn reduction technology effort driven by the U.S. Navy and an improved compressor developed by Pratt. Initial test results have been positive and are “building a lot of confidence” that the combination could be introduced as a relatively straightforward “drop-in” upgrade package, says Jimmy Kenyon, senior director of advanced programs and technology at Pratt & Whitney.

However, Pratt stresses that the potential upgrade, which would offer extended range and engine life to all F-35 variants, will have to be sold to the program before it becomes a reality. This not part of the JSF program, Kenyon says. “It is a Navy technology demonstration, but right now we are talking to the Joint Program Office, the services and the partners [about how to transition the technology] into some sort of modernization program that would follow-on from SDD. There’s a long way to go,” he cautions.

Parts for Pratt's proposed F135 upgrade will be designed for drop-in replacement during depot visits. Credit: Pratt & Whitney

Parts for Pratt’s proposed F135 upgrade will be designed for drop-in replacement during depot visits. Credit: Pratt & Whitney

The fuel-burn reduction program, initially demonstrated on the modified F135 test engine XTE68/LF beginning in 2013, “has made a lot of progress,” says Kenyon. “The Navy is trying to demonstrate on the test stand a 5% improvement in fuel burn, and when we transition it to the final product there are some other things [with the compressor and turbine] we can do to integrate it into the aircraft that will yield 5-7% projected fuel-burn improvement,” he adds. Working with the Navy, Pratt is taking the turbine-cooling technology tested on XTE68/LF “and maturing that design. Instead of pushing the temperature, we are buying margin, allowing us to improve the efficiency of that part of the cycle.”

The improved design is focused on changes to the aerodynamics of the six integrated bladed rotors that form the high-pressure compressor module; it “takes advantage of the advances in aero design capability that have happened since the start of the F135 program,” Kenyon says. The revised configuration was rig tested at AneCom AeroTest’s facility in Wildau, Germany, in 2015. “We got the performance we expected and which we need out of that compressor to enable the overall engine improvement. So that becomes a big risk reduction for the program and that compressor design now is going into the engine demonstrator,” says Kenyon. The integrated test is expected to take place early in 2017.

The Navy, meanwhile, has “put hard constraints” on the extent of the changes to simplify eventual integration of the upgrades into the existing engine, says Kenyon. “We can’t mess with the diameter because it has to be retrofitable and variant-common. So whether it is increasing thrust, or fuel-burn reduction, or if the Marine Corps needs additional powered lift [for the F135-600-powered F-35B short-takeoff-and-vertical-landing variant] this gives you that capability.” Maintaining commonality with the existing footprint of today’s turbine and compressor sections minimizes cost and reduces disruption. “This is a big deal,” says Kenyon. “It allows you to do this at the first depot interval.”

The concept is designed for “downward compatibility,” says F135 Vice President Mark Buongiorno. “By the time it cuts into production, if there are 1,000 aircraft, then you get the opportunity to bring [all of them] up to that current standard. That’s a significant number, and it is therefore important to keep the support of the international partners, some of which will have the majority of their aircraft delivered by then. That’s how the program will really continue to progress.”

Although initial runs of the combined engine upgrade package will begin as a technology demonstrator next year, Pratt is realistic about the time line for potential introduction. “You still have to qualify it on the airplane,” says Kenyon. “The sooner you start it, the sooner it becomes available. As you look at opportunities to get that started, you are probably looking at early to mid-2020s.”

In the nearer term, Pratt is positioning to improve the performance of the first stage of the F135 three-stage fan via a production change. The company is switching to a new linear friction-weld manufacturing process that will see the current hollow design replaced with a solid blade. “It’s a more efficient design and thinner,” says Buongiorno. The machine, which is one of the largest of its type in the world, is undergoing commissioning in Pratt’s Compressor System Module Center in Middletown, Connecticut, and more than doubles Pratt’s capacity to do friction welding.  The change will be available in 2017.

This article was first published for Aviation Week & Space Technology subscribers on June 16, 2016.