Adaptive technology engines promise great performance but come with technology, financial and interoperability risks

With over 700 aircraft operational the F-35 is becoming the centrepiece of air forces and navies across the globe. Not only is the aircraft itself a stunning success with its range of unmatched stealth and strike capabilities, but it is a remarkable exercise in international interoperability.


This article was contributed to the UK Defence Journal by James Maclaren. Maclaren is a freelance journalist and writer specialising in defence and security. He is a former UK military officer with command, staff, joint experience in a variety of roles and operational environments. He is based in London.

This article is the opinion of the author and not necessarily that of the UK Defence Journal. If you would like to submit your own article on this topic or any other, please see our submission guidelines.


Any potential adversary must reckon with the force multiplication effect of taking on combined forces with seamless operational and logistic capability.

It is no surprise that generals and politicians alike see the aircraft as a battle winner which with its beyond peer shoot and sensor capability will provide an air power lead for some years to come. The propulsion system is at the heart of the aircrafts stealth capability and in a platform as complex as the F-35 maintaining its edge over emerging Russian and Chinese competition means extending endurance performance and efficiency is a pre-requisite for the anticipated four decades the aircraft is expected to be in service.

However, while nothing about the F-35 can be said to lack ambition, a complete retrofit of the aircraft’s propulsion system was probably not envisaged. But this is what some senior leaders in Congress and the Pentagon are raising as the bold route to achieve future capability aspirations. It is a controversial proposal and has attracted disquiet for the risk such a complex and expensive change to the aircraft life cycle would have, particularly as a Pratt & Witney planned engine upgrade proposal goes a long way to achieving the necessary operational capability.

The proposal originates in a programme designed to support sixth-generation fighter technology the Adaptive Engine Transition Program (AETP). Both Pratt & Witney and General Electric have programmes to bring new technology propulsion into service, principally to meet the operational requirements of the next generation clean-sheet F-16 replacement. However, growing voices propose that this technology, still in development, could be retro fitted into the F-35 fleet, specifically the F-35A variant whose major customer is the USAF as well as allied air forces across the globe. The merging of AETP in theory would catapult F-35 endurance and performance to new levels.

Opponents of this idea point to the risk that a retro fit may just not be technically possible. There will be complex engineering and technology hurdles to overcome and rejecting current upgrade programmes in favour of untested technology risks a world beating fighter stagnating as its propulsion system waits for replacement. There is also the significant financial risk. No current budget for fleet replacement exists and procurement executives will view with anxiety attempts to bring AETP into the already hugely expensive F-35 programme. Whatever initial projections are, the reality is almost certain to be considerably more expensive. 

While the debate has so far been mostly confined to the technical media in the United States, almost certainly the initiative will have raised concerns in allied operators of the aircraft who will be conscious of the supply chain implications and wary of the cost implications of AETP which is likely to be passed on in one form or another.

The United Kingdom has a special interest, although it is currently keeping diplomatically quiet. It is the only Tier one partner in the F-35 program. Alongside the United States Marine Corps, and after considerable debate, it operates the F-35B, the short takeoff/vertical landing (STOVL) variant on its new carriers. This decision results in operation of a F-35 variant not currently believed suitable for AETP retrofitting and for which the planned production numbers are considerably less than either the F-35A and F-35C.

The danger for the United Kingdom and other F-35B operators is that this variant will wither, with programme resources directed towards AETP variants and with the commonality required for efficient supply chain management and operations reducing drastically. The RAF/RN and their counterparts in the USMC could end up with a poor cousin of an aircraft while the increased cost of an AETP equipped aircraft could have serious implications on the smaller operators of the F-35A.

A powerful advocate for the retrofit of AETP includes the Secretary of the Air Force Frank Kendall, who previously has been highly critical of F-35 cost over runs. As the single largest operator of the F-35 the single service drive towards AETP carries considerable influence to Congress. But the debate is finely balanced with many acknowledging that selecting AETP over the existing Pratt & Witney proposal exposes the programme to immense engineering, logistical and procurement risk. 

A major strength of the F-35 programme, in spite of its immense cost has been that it meets the operational requirements of ten militaries to date, with more buyers waiting in the wings. The pursuit of propulsion perfection may compromise that success. How much that will weigh upon the decision making of the US Department of Defense remains to be seen.

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Monkey spanker
Monkey spanker
10 days ago

Is this the same engine the USA are going to be developing for the US airforce 6th gen fighter? Are the navy planning on using the same engine. If the adaptive engine is being developed anyway for these aircraft then a refit to F35 could be adopted but I wouldn’t want to see all F135 engine development not done for what may turn out to be a lemon. Really comes down to how much of an advantage does it offer compared to the F135 engine upgrades and is it worth it or needed. I’ve not looked into why this B… Read more »

John Clark
John Clark
10 days ago
Reply to  Monkey spanker

In my option, it tells you where LM want to be with the F35 by 2030. I believe they really want to concentrate on an advance A version and move away from B and C. They have no intention of having the new engine hamstrung by having to adapt it to the B model, needing the B versions lift shaft and puffer jet bleeds. If they can drop the other two from production, they can double down on a really advanced A variant, with an avionics and engine refresh, hopefully keep the unit price down and tap up the USAF… Read more »

Spyinthesky
Spyinthesky
9 days ago
Reply to  John Clark

Not sure the Navy would accept not being kept up to date nor the Govt as it’s sponsors in reality the US Navy isn’t far behind the Airforce in importance espcially considering is hugely expensive carrier program but the B version is certainly not so secure. And one thing it won’t do is reduce costs as it will split the overall market and increase maintence costs of two engine types not to mention the cost of retrofitting a brand new engine as opposed to an updated one. No way would it keep unit price down it’s a matter of whether… Read more »

Watcherzero
Watcherzero
8 days ago
Reply to  John Clark

The issue is the new GE engine is longer, so it wont fit in the B version where the Fan occupies more internal volume and will require some adaptation in the C version as well as it already using the cavity space as well. P&W who also developed an engine under the AETP prefer upgrading the F35’s existing engine rather than replacing it with a new one, but again due to the extra length it would only fit the A & C though with less issues fitting it into the C.

DaveyB
DaveyB
5 days ago
Reply to  Watcherzero

I would also add that both of the new prototype engines have not been designed with the roll post bleed off air that the F135-600 engine has. Though, I think it may be P&W, who have said they may do this if there is enough interest? Secondly there is also the issue of the rotating nozzle to contend with. Both engines develop more power, though with the higher by-pass ratios the exhaust temperatures are not that much different. Except when using reheat. When using reheat there is more “colder” air from the by-pass air to mix with the fuel. This… Read more »

George Parker
George Parker
4 days ago
Reply to  John Clark

The F35C will benefit most from the new engine because it already carries more fuel than the A model. Combat range, loiter time, increased electrical power and thrust. Being even more important over the expanse of the ocean. Both A and C use the same engine plant now. The GE XA100 engine is being build from day one to be a “drop in” replacement for both A & C variants. If GE’s press releases and videos are to be believed.
https://www.youtube.com/watch?v=iJ1XDkGV3gk

Martin
Martin
10 days ago
Reply to  Monkey spanker

LM can’t even get block IV software in to the F35 inside a decade much less a new engine. If even a fraction of the nex gen air dominance program is to be beloved the air force will be looking to move on from F35 not spend billions created a new version.

Expat
Expat
10 days ago
Reply to  Monkey spanker

The problem is the F135 still has problems there’s no guarantee P&W won’t mess us the upgrades. GEs engine will go ahead anyway imo as adaptive engines are going to be far more efficient.

Spyinthesky
Spyinthesky
9 days ago
Reply to  Expat

Yes you and Martin frame the balance to be had very succinctly. The GE engine is promoted as being more reliable as well as more efficient than the troublesome P&W engine but then tech always is when on the drawing board, it’s how after all the whole F35 was promoted in the first place, a little different to eventual reality.

Bryanb
Bryanb
8 days ago
Reply to  Monkey spanker

Gen brown of us air force stated in senate testimony the adaptive engine changes make it a completely different engine for the ngad fighter. It is not for the f-35 in any way.

George Parker
George Parker
4 days ago
Reply to  Monkey spanker

There is an excellent YouTube video on this subject. Explaining the engineering principles and just how much of a leap fwd in performance the engine is. https://www.youtube.com/watch?v=Cba2REAjU-Y It seems to be the next logical step after super cruise. Jet engines are design optimised for either high thrust for supersonic flight or high efficiency/long distance endurance travel. Military fighter as opposed to civilian/military long haul. These new engines can do both. I forget all the figures but 30-35%+ range increase on internal fuel comes to mind and up to Mach 1.8. It also runs cooler; requires less maintenance per flight hour… Read more »

Monkey spanker
Monkey spanker
10 days ago

Thanks for the article it’s an interesting read

DanielMorgan
DanielMorgan
10 days ago

There is one serious problem for the UK. The ATEP engines being developed will not fit in the F-35B.
Adding New AETP Engine to F-35 Means Air Force Alone Would Pay for It – Air Force Magazine

Martin
Martin
10 days ago
Reply to  DanielMorgan

Not really a big issue at all, we can still fly our F35B and they will benefit for all upgrades to the platform. Eventually it will be replaced by drones long before we have to worry about a new engine.

DanielMorgan
DanielMorgan
9 days ago
Reply to  Martin

You miss the point.

Martin
Martin
10 days ago

The F35B is actually the second most produced variant not the F35C and it has been very successful in gaining foreign orders.

Supportive Bloke
Supportive Bloke
7 days ago
Reply to  Martin

Exactly

DaveyB
DaveyB
5 days ago
Reply to  Martin

Yep, there’s now over 500 either flying or on order.

DJ
DJ
2 days ago
Reply to  DaveyB

And more to come. UK has not finished ordering. Spain’s old model Harriers won’t last forever. South Korea is currently designing 2 x 45,000t STOVL carriers (so hasn’t ordered yet). Japan has only ordered 8 of its expected 42 F35B. Singapore is testing a handful (likely for land operations) & Australia has still not gone away (a follow on order for 28 fighters has not been decided yet, with both ‘A’ & ‘B’ on the list).

Expat
Expat
10 days ago

My understanding is the adaptive engine is actually at an advanced stage of development. It’s not new technology and in any case be used in 6th gen airframes so will mature. The biggest looser will be P and W, and there approach appears similar to the B52 re engine when they tried to convince the airforce that updated engines were better than a replacement. They eventuallypropsed an alternative when theyrealisedthat approachwas going nowhere. The F135 is far from perfect and has contributed to low availability of the F35 fleet.

Jon
Jon
9 days ago
Reply to  Expat

In order to take advantage of the upgrades to the F-35 beyond Block 4, more power is needed from the engines. But not that much more.

It seems obvious to me (although not to the US) that irrespective of what is decided for the A and C models, P&W’s incremental changes which have been ready to try for years, should be developed. They are the only game in town for the F-35B, and if they boost capability to a point where the adaptive engines aren’t justifiable for the other two models, so be it. Save everyone some dosh.

Watcherzero
Watcherzero
8 days ago
Reply to  Jon

P&W incremental upgrade wont fit the B either, adding the variable bypass to the existing engine makes it too long to fit the space available, though it would go into the C easier than GE’s new engine.

The main reason they want the new engine isnt really for the increased performance however thats just a bonus, its for the extra cooling capacity. The existing engine is pretty much already redlining on heat (contributing to its wear issues) and doesnt have the cooling capacity to cool more electrical equipment in the aircraft.

Jon
Jon
7 days ago
Reply to  Watcherzero

With great power comes great cooling, or it had better. That’s interesting, and not what I read into P&W’s comments regarding EEP fitting the B model. Even now their website claims the package is common for all three aircraft variants. Some arguments they have made regarding the F-35B mirror my own thinking. They are also claiming a 5% incease in vertical lift, so they must think it fits the B. To be honest, I thought the variable bypass was only part of the adaptive engine programme. In case we haven’t been talking about the same upgrades, I’ll add a link… Read more »

Last edited 7 days ago by Jon
Supportive Bloke
Supportive Bloke
7 days ago
Reply to  Expat

The main advantage being very much increased range due to massively improved fuel efficiency.

Expat
Expat
7 days ago

Yes but there’s also possibility of converting that efficiency into electrical power for direct energy weapons.

Supportive Bloke
Supportive Bloke
7 days ago
Reply to  Expat

There is that too.

If there is a viable DE weapon.

dan
dan
10 days ago

Where’s all this money going to come for new engines anyways? The cost of installing new engines on the B-52s has skyrocketed since it was signed. US debt is already over 31 trillion dollars and growing fast. Ugh

OkamsRazor
OkamsRazor
10 days ago
Reply to  dan

Do you have any evidence for that ridiculous statement?

Jonathan
Jonathan
9 days ago
Reply to  OkamsRazor

That is pretty much the level of US national Dept as reported by most analysis websites. ( slight variation between 29 to 30.5 trillion).

https://www.forbes.com/advisor/personal-finance/u-s-national-debt-surpasses-30-trillion-what-this-means-for-you/

OkamsRazor
OkamsRazor
9 days ago
Reply to  Jonathan

Wasn’t talking about US debt, but about B52 engines, that are supplied by RR.
With regard to debt, any A-Leve economist will tell you that the amount of debt is secondary to the cost of debt and who holds that debt. If servicing your debt is cheap the quantum is pretty irrelevant (see US & Japan)

Watcherzero
Watcherzero
8 days ago
Reply to  OkamsRazor

Basically its cost growth in the engine replacement program, but its not from the engines. Rather the US Airforce did its own unrealistically low internal estimate a decade ago on how much adapting the pylons to a new engine would cost and they now actually have a proper realistic estimate quoted by Boeing which is like three times as high.

Last edited 8 days ago by Watcherzero
OkamsRazor
OkamsRazor
7 days ago
Reply to  Watcherzero

So Boeing not RR, it may come as a surprise to some that Boeing are trying to extort mega profits from their military business after the cluster fuck of the aerospace business. And just like the T45 engine problems we don’t attribute blame where it lies, US contractors.

Monkey spanker
Monkey spanker
9 days ago
Reply to  dan

The costs of the B52 engine replacement are looking at increasing but it’s not from Rolls-Royce. It’s everyone’s favourite company Boeing saying the integration costs etc could be 50% higher. The engines will still cost roughly the same. As for the debt America could solve a lot of its deficit problems by getting taxes sorted out. Multi billion pound companies paying little or no tax. Millionaires paying little. 95% of the population would notice no changes but it’s probably not a vote winner. Also some of the things Congress spends money on is crazy. You have service users saying we… Read more »

nonsense
nonsense
9 days ago

Adaptive engines are very expensive technologies.

That is why some will argue that Britain must have it.

Just like little children want to have the most expensive toys in the department store.

How childish is the idea that you need the most expensive toy to be in awe of the children around you?

OkamsRazor
OkamsRazor
7 days ago
Reply to  nonsense

I presume someone will do a cost benefit analysis, but you would have to be pretty naive (to put it kindly) to believe that PW or GE won’t try to screw you on this.

DaveyB
DaveyB
5 days ago
Reply to  nonsense

It is not really that expensive to develop. As it’s the next evolutionary stage of jet engines, not a complete step change. After all the GE/RR F136 was a fore-runner of the adaptive cycle engine. But it got binned as they thought it was too complicated at the time.

Last edited 5 days ago by DaveyB
nonsense
nonsense
9 days ago

Some argue that you should buy a new, expensive, and powerful looking weapon. Even at the risk of reducing the number of soldiers. = Purchasing the latest weapons and maintaining the size of the military at the same time is impossible except in the United States. If the mod focuses on the minimal modern equipment needed while keeping the old and popular weapons directly from the mod, then there’s no need to downsize massively, or at least it’s trying to slow down the headcount. . It is no longer surprising that those who claim Britain’s strength and justice have no… Read more »

Jon
Jon
9 days ago

Is it really true that the F-35B will be bought in “considerably less” numbers than the F-35C?

I question that. Last I heard the Marine Corps alone was buying 353 F-35Bs, greater than all expected F-35C orders put together.

Jon
Jon
9 days ago
Reply to  Jon

Sorry. I see Martin made the same point.

Rob N
Rob N
9 days ago

There is a growing market for the F35B and so it makes sense to extend AETP to the B. My understanding is that AETP Can be modified to fit the B and I think some preliminary work has gone into its feasibility.

I think it is unlikely that the USA would just ignore the needs of the UK, Japan, Italy, Spain, possibly Australia in the future.

Do not forget that AETP for the B means dales.

Monkey spanker
Monkey spanker
9 days ago
Reply to  Rob N

I don’t understand why it won’t fit. I thought all models share the same engine bay? Looking at the 3 versions the one that would benefit the most from a power increase is the B model. More power for it means more load and bring back in stovl mode. Better efficiency also benefits it the most as it’s operators are less likely to have air to air refuelling available as much as the others. I really hope that tempest can be made to be great and not cost as much as this program. Not sure how but it must be… Read more »

Watcherzero
Watcherzero
8 days ago
Reply to  Monkey spanker

The way adaptive engines work is rather than a fixed bypass ratio they can vary the amount of air that bypasses the turbine and flow it around the outside of the turbine housing to add extra cooling. To do this you need more ducting which increases the length of the engine. While on the A this internal space is a empty cavity, on the B this intrudes into the space occupied by the liftfan, and on the C they dip into the space to accommodate extra gubbins.

DaveyB
DaveyB
5 days ago
Reply to  Watcherzero

From what I’ve read on the GE X100, it has a wider diameter than the current F135 engine. This is to account for a wider 1st stage compressor which will increase the by-pass air flow. The majority of fighter aircraft use low by-pass gas turbines that operate with a by-pass ratio below 0.5, for example the Typhoon’s EJ200 is 0.4. The F135 is closer to 0.6, the X100 is supposed to be higher still. It seems to split the by-pass air into two streams. The inner can be used within the engine, whilst the outer is used for engine and… Read more »

Spyinthesky
Spyinthesky
9 days ago
Reply to  Rob N

My gut feeling is that if this technology and new engine is chosen it will only do so if eventually at least it can be incorporated into the B, developing the PW engine just for the B while using the GE engine for the other two or just the A just seems like a whole load of hurt that even the US budget can’t really afford even developing the new engine for the 6th Gen aircraft and making it suitable for F35 will I suspect lead to complications and compromises and thus expense, so doing that AND still working on… Read more »

John Hartley
John Hartley
9 days ago

Some years back, there was talk of an F-35E, long range strike version to replace F-15E. The F-35E would have been based on F-35A, but with a bigger wing, bulged weapons bay doors & AETP engine. The hope was it would have F-111 combat radius (1200-1500 miles). Would also have been a great Tornado GR4 replacement.
For the UK, the P&W upgrade of the existing engine & RR liftfan, is the best we can hope for on new block 4 aircraft.

Watcherzero
Watcherzero
8 days ago
Reply to  John Hartley

The P&W engine upgrade wont fit in the B either. The extra ducting for both options increases the length of the engine and the B variant of the engine has a mechanical transmission/grearbox to the Liftfan.

Engine_of_F-35.jpg (3690×2470) (wikimedia.org)

Last edited 8 days ago by Watcherzero
John Hartley
John Hartley
8 days ago
Reply to  Watcherzero

Has it changed again? As some years back, the upgraded liftfan was part of the proposed engine upgrade.

Watcherzero
Watcherzero
7 days ago
Reply to  John Hartley

Its not ground up designed as an adaptive engine but has some of the new technology retrofitted in to the existing engine design.

Looking into it further theyve gone for a sawn-off option for the B that provides extra cooling and power available to the lift fan but no increase in thrust unlike on the A and C. The sawn-off option wasnt being considered when I last looked into it a couple of years ago.

F135 ENHANCED ENGINE PACKAGE – Pratt & Whitney (prattwhitney.com)

Last edited 7 days ago by Watcherzero
Rob N
Rob N
7 days ago
Reply to  Watcherzero

Having more power for the fan could be useful on landing. But extra thrust would be better….

DaveyB
DaveyB
5 days ago
Reply to  Watcherzero

You need to define, by what you mean by upgrade? The current F135 has two block upgrades as part of its life cycle. Due to the cracking they have found in the hot turbine section of the F135-600 in particular. Part of the Block 1 upgrade is being fitted when the engine goes through maintenance. This upgrade allows the engine to operate at a higher temperature for longer, especially useful when in the hover. The complete Block 1 upgrade is supposed to coincide with the aircraft’s block IV update. The Block 2 update for the engine is supposed to improve… Read more »

Gavin Gordon
Gavin Gordon
6 days ago

Currently, don’t see this as insurmountable, more scaremonger. ‘Worst outcome’, RR will stand in for UK. However, since war is looming, the political fallout for the Arsenal of Democracy would be too great.

DaveyB
DaveyB
5 days ago

I wonder if Rolls Royce could build a better engine for the F35B? There would definitely be a market for a better engine. As there are now over 500 F35Bs either flying or on order. Perhaps they could look at the lift fan design. Using their in-engine (within the engine’s main shaft) generator concept. Would it be practical to replace the lift fan gearboxes and drive shaft, with an AC motor? There would definitely be a space saving, not sure if there would be a weight saving though! Basically turning the lift fan into a giant electric ducted fan. Rolls… Read more »

Dave Wolfy
Dave Wolfy
15 hours ago

Just throwing this in because I can –

Old USN saying – if it says P&W on the engine it had better say Martin Baker on the seat.

geoff.Roach
geoff.Roach
7 minutes ago

Just got back so I’m a bit late. I don’t pretend to be a technical type but it seems that if this goes ahead we could be left standing. However the 48 B’s currently on order, perhaps with another ten would be enough for the Royal Navy. With the updates planned the B variant would still be a real handful at sea. We would also have a real fighting force available for the first time in decades, probably within the next five years.