Pratt & Whitney will offer a drop-in thrust or fuel efficiency upgrade for the Lockheed Martin F-35’s engine as the fighter’s joint programme office develops options for the Block 4.2 upgrade package now scheduled to enter service in late 2023.

The Growth Option 1.0 proposal from P&W would improve the 43,000lb-thrust F135 engine’s acceleration force by 6 to 10% depending on the aircraft’s flight condition or reduce fuel consumption by 5% to extend the range of the tactical stealth fighter, P&W Military engines president Matthew Bromberg tells FlightGlobal in an interview.

The unique proposal is shaped as P&W’s response to the Defense Department’s call for accelerating modernisation programmes by breaking up large upgrade packages into smaller increments. In the software field, F-35 JPO executive Vice Adm Matthias Winter is working on a proposal to break up the decade-long series of Block 4 software updates into smaller subsets of applications. Those that have no impact on the fighter’s airworthiness – such as sensor updates -- might be delivered sooner.

Unlike, software updates, the opportunity to accelerate engine upgrades is limited by physics and the rigours of airworthiness testing, Bromberg concedes. Instead of updates on a yearly or even semi-annual basis, P&W is proposing to offer new performance improvements as each F135 engine enters a depot maintenance period, which is generally timed in five- to seven-year intervals.

Growth Option 1.0, which P&W claimed to validate in a ground test of a full engine earlier this year, proposes to update hardware and software controls in only the power module of the F135, which contains the high-pressure compressor, combustor and high-pressure turbine, Bromberg says. P&W’s proposal calls for the JPO to fund an engineering and manufacturing development programme, but the production hardware would cost no more than the existing parts, he says. The upgrade also can be completed by the government’s depot workers.

As P&W discusses the proposed upgrade with the JPO, the company is working internally on Growth Option 2.0 package using a similar philosophy, Bromberg says. The second package also would be focused on delivering performance improvements, but stop short of inserting “third-stream” fan stage technology, Bromberg says.