Flight International – 6 August 2019

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PROPULSION

lethal if it extended its range. However, extra

range would be unusable if it comes at the

cost of the aircraft’s mission capability rate, an

area where the F-35 currently struggles.

Notably, despite several reassurances from

Lockheed over the past year, the F-35 will fail

to meet a required 80% mission capability

rate by September 2019, US defense secretary

Mark Esper said during his Senate confirma-

tion hearing on 17 July. Besides the F-35, the

goal also applies to other key combat aircraft,

including the USAF’s Lockheed F-16s and

F-22s and the navy’s Super Hornets.

“An important aspect of the adaptive en-

gine design involves reliability and sustaina-

bility,” says Stutzriem. “The threat environ-

ment involving peer competitors demands an

increase, not a decrease in overall reliability

and availability of engine components.” ■

burner, which “gives the F-22 exceptional

combat performance without compromising

mission range”.

So, as external tanks are not an option, the

USAF is looking at how the F-35 can do more

with its fuel capacity. One effort is the ser-

vice’s Adaptive Engine Transition Program,

which aims to create a novel engine with both

more power and better fuel economy.

Adaptive engines work by varying the vol-

ume of air flow that bypasses the turbine core

by opening a third stream when flying in

cruise mode. This third flow – in addition to

core flow and bypass flow – increases the en-

gine’s efficiency in producing thrust and also

creates a new heat sink within the engine, al-

lowing for further fuel efficiency gains from

increased core temperature. The result is an

engine that can play the turbofan’s efficiency

trick of running a large bypass ratio when

cruising – while retaining the raw power of a

turbojet in other situations.

The concept is not unique to the F-35 pro-

gramme. As revealed at June’s Paris air show,

Eurofighter is researching an enhanced Eurojet

EJ200 engine to improve thrust, range and per-

sistence on the Typhoon using “adaptive

power and cooling techniques”.

PERFORMANCE GAIN

The USAF believes adaptive engines could

provide 25% improved fuel efficiency, 10%

increased thrust, and significantly improved

thermal management. GE Aviation and P&W

were each awarded contracts worth $1 billion

in 2016 to develop adaptive engine demon-

strators by September 2021. GE is developing

the XA100 demonstrator and P&W the

XA101. If the programme is successful, the

F-35’s F135 engine could be swapped out for

one of the new adaptive units in the mid-

2020s.

For its part, GE announced the successful

completion of the XA100 detailed design pro-

cess in February 2019. The company believes

the engine could result in a 35% increase in

range and a 50% improvement in loiter time.

That improvement applied to the F-35C

would add 420nm – taking range for the

USN’s variant to 1,620nm.

“In the Pacific theatre of operations this is

significant, allowing either operations from

greater distances or more time to operate at the

combat edge,” says retired USAF Major Gener-

al Larry Stutzriem, now director of research

with the Mitchell Institute for Aerospace Stud-

ies. “This improvement could also allow F-35s

to drop from their tanker support earlier, keep-

ing those extremely vulnerable aircraft further

away from adversary defences.”

However, a 10% increase in thrust will not

be a game-changer for the F-35, he says.

“With respect to improved thrust, there is

nothing the F-35 will be enabled to do that it

can’t do now,” says Stutzriem. “That said,

many operating features will improve with

additional thrust. For example, take-off roll

can decrease marginally with additional

thrust, and that has positive benefits.”

Ultimately, the addition of an adaptive

engine could make the stealth fighter more

B-model variant has lowest fuel capacity, allowing for missions of no greater than 900nm

F-22’s powerplants mean it can fly supersonically without afterburners, maintaining reach

US Air Force

US Navy

“In the Pacific this is significant,

allowing operations from

greater distances or more time

at the combat edge”

Major General (retired) Larry Stutzriem

Director of research, Mitchell Institute for Aerospace

Studies

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