Under the F135 Block Upgrade Plan, a series of initial ‘Block 1’ improvements might potentially be available for new-production examples and for retrofit into existing ones from as early as 2018. Then a second series of AETD project-derived, more substantial ‘Block 2’ improvements could be put “into production in the very early 2020s”, depending on “funding and the requirement pool”, according to Kenyon.Although Kenyon points out this plan is “not part of the F135 programme proper”,
he said “it is an opportunity looking ahead to demonstrate fuel-burn cost savings” to F-35 operators – and particularly to the US Navy, with which P&W is working to try to get the Block 1 F135 upgrades into production within the next few years.
The Block 1 upgrades rely partly on development work that P&W carried out for a demonstration in autumn 2013 for the navy’s XTE68-LF1 project. This focused on demonstrating a capability for higher operating temperatures in the F135’s turbine and introduced a variety of new technologies into the engine’s high-pressure turbine (HPT) module. These improvements, which Kenyon described as “advanced cooling technologies”, included new casting technologies for metal- alloy parts; new, highly temperature-resistant HPT materials; new thermal barrier coatings for HPT blades and vanes; more temperature- resistant oils for lubricating and cooling; and a new main shaft bearing. He said P&W’s XTE68-LF1 demonstration in autumn 2013 was “tremendously successful” – so successful that it recorded “the hottest-ever temperature in a production engine”.
P&W has parlayed this successful demonstration into another F135 development initiative on which it is working with the US Navy, called the Fuel Burn Reduction (FBR)programme.
FBR has married the HPT technologies from the XTE68-LF1 demonstration with a series of improvements to the F135’s six-stage, all-blisk compressor to produce an engine offering a fuel-burn reduction of “about 5% – and we’ve identified another couple of opportunities to give [another] 1%-2% of fuel-burn improvement”, said Kenyon. However, saying the XTE68-LF1 and FBR technologies would just reduce fuel burn only in production F135s would be inaccurate: the technologies would also offer other improvements. “TheJPO(F-35Joint Programme Office) and navy are both focused on [engine] life-cycle cost and they need to have the technology working reliably,” said Kenyon. An important feature of the XTE68-LF1 and FBR advanced cooling technologies is that “you need less air to do the cooling [in the HPT] and you can use it more to do other things, with the cooling capability already in the engine. We’re using low-lying fruit to get better thermal management,” he said.
“Right now I’m going to insert this nice cooling technology into my turbine because it helps with [life-cycle] cost [by making the engine more durable], but I could use it to generate more thrust” – perhaps up to 10% more, making the F135 capable of meeting any foreseeable F-35 thrust-growth requirement.
There is no doubt about this. During its original ground-testing effort for the F135, P&W ran an unimproved engine at thrust levels of up to 51,000lb in uninstalled configuration (ie without any accessory gearboxes or drives drawing power from the engine). This suggested that, even on an installed basis, the F135 had several thousand pounds of additional thrust available if necessary, if run at high temperatures.
Kenyon pointed out that, although the F-35 airframe has had heat-retention issues, “right now, there are no thermal restrictions with the engine”. In-service F135s are operating within the specifications required by the JPO and presumably could be operated at a higher maximum temperature as long as the F-35 airframe could withstand and dump the additional heat burden.
F135 Block Upgrade
Programme: Block 2
Pratt & Whitney’s proposed Block 2 upgrades for the F135 would be more complex than those in Block 1. The manufacturer can see a potential path by which it could introduce into the F135 engine, for production from the early 2020s, some of the advanced technologies it has developed for the compressor and turbine in its high-pressure AETD core.
The F135 has the same dimensions as the AETD demonstrator upon which P&W is now working. But while that is a three-stream engine, at this juncture P&W does not view redesigning the F135 into a three-stream, adaptive-cycle engine – which may be theoretically possible.
However, Kenyon said there is an “exciting opportunity we’ve identified, which is unique to Pratt & Whitney’s position, to take some of the key technologies of the three-stream AETD architecture into the F135 to get lots of benefits”.
This opportunity could rely on a potential evolution of the company’s engine-control software for its AETD demonstrator.
Kenyon said: “For example, if you can modulate the [existing] bleed air using the three-stream control laws, rather than using an actual third stream of air,” this could produce substantial benefits from an enhanced capability to use cooling air flexibly at different places in the engine’s hot section. “The trade is really around the re-use of current learning,” he said.
In addition to incorporating a series of AETD- derived hardware and software upgrades, as well as other cooling and design technologies,a future Block 2 upgrade of the F135 might also incorporate the improvements already provided under Block 1. P&W isn’t sure yet of the scale of the fuel-burn reduction and thrust increase a Block 2 upgrade would offer, but they would certainly be more substantial than those provided in Block 1. It’s possible Block 2 modifications could produce benefits even greater than the AFRL’s AETD project targets of a 25% fuel-burn improvement and 10% thrust increase over today’s F135 engine.
The future of adaptive-cycle engines for US fighters, and even the future development of the existing F135, relies greatly upon Congress approving President Obama’s AETP budget request. If approval comes at all, it will likely come this year. Pratt & Whitney is now beginning to flesh out a two-pronged strategy to try to retain its dominant position in current- generation US fighters and win a prominent position on US next-generation fighters. It will hope Congressional approval for AETP is prompt and generous – and that the AFRL likes P&W’s AETD design.