Pratt & Whitney has unveiled a development pathway for thrust growth, fuel-burn reduction and other improvements in the F135 engine powering the F-35 Lightning II Joint Strike Fighter (JSF). Itcould result in the U.S. armed forces fielding improved versions of the basic F135 for land-based, STOVL and carrier-based F-35s as early as 2020.

The company sees its F135 development pathway as essentially a two-step program, which would result by the mid- to late-2020s in an entirely new engine employing a third airstream, which would allow both an adaptive-cycle compressor and an adaptive-cycle turbine and also would allow weapons-system cooling technologies.

However, Steven Burd, P&W’s chief engineer for advanced programs and technology, said an additional intermediate step could allow further improvements to be made—beyond the first upgrade step—to the substantially modified F135 engine-and-lift-system combination that powers the STOVL (short take-off, vertical landing) F-35B flown by the U.S. Marines and the British Royal Air Force.

Pratt & Whitney has formalized its proposed two major F135 development steps as Growth Option 1 and Growth Option 2. The company is informally calling the intermediate technology-insertion option for the STOVL F135 “Growth Option 1A,” according to Burd, who declined to provide details of the additional technological improvements this upgrade would incorporate.

The manufacturer recently displayed  a developmental F135 to reporters visiting P&W’s West Palm Beach, Florida, engine test center. P&W has verified—at its own cost—a suite of compressor, turbine and software-controlled airflow-modulation technologies on the test engine. These new technologies offer a 5 to 6 percent fuel-burn improvement and a thrust bump of 6 to 10 percent over the F135’s 40,000-pound validated maximum-thrust, throughout the F-35’s flight envelope. Matthew Bromberg, president of Pratt & Whitney Military Engines, declined to quantify the respective proportions of the overall performance improvements each of the different technological upgrades provides, saying this information was both classified and proprietary.

P&W has verified the performance improvement, which could also be used to produce hot-section durability and reliability improvements, in extensive ground-testing of the development engine on a test stand at the West Palm Beach facility. Displaying the specially modified engine (which P&W calls its FBR, or Fuel Burn Reduction, engine and has labeled FX701-01) at its static display (C2), Burd said FX701-01 incorporates two separate sets of technological upgrades.

One set comprises the compressor adaptations developed and tested under the company’s FBR research program for the U.S. Navy, according to Burd, who said the second set of upgrades in the FBR engine are turbine-cooling technologies tested by P&W under the U.S. Air Force’s CAESAR (Component And Engine Structural Assessment Research) program.

“The [modified] compressor is about driving better efficiency and the turbine about more effective … cooling” for the F135’s high-pressure turbine blades, said Burd. These technologies could well be useful for the F135 in the next few years, because “modernization of the F-35 is putting demands on the propulsion system,” he said.

Pratt & Whitney is packaging all of the test-verified technological improvements in the F135 FBR engine into Growth Option 1, which would potentially be available for all F135s, whether new-production or already-installed engines. Growth Option 1 would require replacing the F135’s core (or “power”) module, and for any already-installed engine the module replacement could be performed during a routine depot visit, according to Burd. No other modifications would be necessary: as a customer, “you can’t see the upgraded parts,” he said.

“If we were given the green light, we would launch a short program and would have it [ready] by 2020,” in time to incorporate into new F135s under the Block 4 production contract, said Bromberg. (Interestingly, Bromberg’s career before he joined P&W’s parent United Technologies in 2002 included serving as a Goldman Sachs investment banker and as an officer on one of the U.S. Navy’s nuclear submarines.)

The “green light” P&W would need to proceed with full development and validation testing of Growth Option 1 would be authorization and funding from the JSF Joint Program Office (JPO) to embark on an engineering manufacturing and development (EMD) program for the upgrade. The JPO could use any of several acquisition strategies to provide the upgrade, according to Bromberg: “For the procurement cost of the motor, it depends on how the program office wants to do it. If they do it on an attrition basis, it will be cost-neutral,” other than the purchase cost needed to provide a new module for the damaged module being replaced.

Should the JPO want to retrofit the Growth Option 1 upgrade to all in-service F-35s, new power modules would have to be purchased to replace the still-functional modules in F135s sent for depot maintenance. Providing Growth Option 1 modules for new-production F135s from 2020 would also be cost-neutral, according to Bromberg, who noted that P&W has at this stage reduced its purchase-price per engine by more than 50 percent, compared with early-production F135s.

(By the end of May, P&W had delivered 325 F135s, and production is continuing to ramp up, according to Bromberg. He said P&W delivered 42 F135s in 2015 and 72 in 2016. It will deliver 80 this year and 110 in 2018. F135 mission readiness and capability is now above 98 percent, a higher rate than P&W’s original target for 2020, Bromberg added.

P&W’s F135 Growth Option 2 is an entirely new engine, which P&W reckons it could size to fit the F-35 and/or any sixth-generation fighters ordered by the U.S. Air Force. This engine would be available “four to eight years down the road after Growth Option 1,” according to Bromberg. It would provide “a step change” in fuel-burn, thrust and durability benefits over those offered by a Growth Option 1 or Growth Option 1A F135. 

Growth Option 2, or XA101 in U.S. Air Force Research Laboratory (AFRL) nomenclature, is the production engine which ultimately will result from P&W’s adaptive-cycle R&D under the AFRL’s Advanced Engine Transition Program (AETP). AETP is the EMD risk-reduction program under which the AFRL has contracted both P&W and GE Aviation to develop production-capable fighter engines employing the adaptive-cycle fan technologies they separately developed and tested under the AFRL’s previous Adaptive Engine Technology Development (AETD) research program. (GE Aviation is also working on an AETP engine, which the AFRL has labeled XA102. This engine could see service instead of, or as well as, P&W’s XA101.)

“Adaptive [-cycle] is simply changing the performance configuration at different parts of the envelope,” said Bromberg, noting that the F135—particularly in its STOVL incarnation— already has adaptive-cycle characteristics. The term ‘adaptive-cycle’ can be applied to four different types of technology, he added: operation of the engine core; employment of a third airstream in the engine, which can be deployed in different ways and in different parts of the engine to optimize its performance in every area of the flight envelope; software control of engine parts and airflows to provide modulated cooling of the engine’s hot section; and—to date little-discussed publicly—even having the engine provide modulated cooling of the aircraft’s weapons systems when necessary.

Noting that P&W will also be able to back-insert technologies from its XA101 AETP research into other engines—including the F135—if required, Bromberg declared that the company’s AETP effort is going well. “We’re on time with the contract and we’re hitting all the key development milestones,” he said. “We’re on budget … [and] we can speed it up if they ask us to.”

Indeed, P&W may already have run its three-airstream AETP adaptive-cycle fan and core in a demonstrator engine. When asked if P&W had already done so, Bromberg declined to say ‘yes’ or ‘no,’ but, he added, “We will be announcing the results of tests in the near future. We feel very good about where we are.”