Engines for JSF

**AK Fan** · 13 Oct 10,, 12:28

I just wonder what is the history regarding two alternative engines for JSF, how all this started and developed?

**gunnut** · 13 Oct 10,, 18:55

Pratt & Whitney F135 - Wikipedia, the free encyclopedia

General Electric/Rolls-Royce F136 - Wikipedia, the free encyclopedia

Not much, but I guess you can start with the wiki article.

**JRT** · 11 Nov 10,, 08:22

Originally Posted by gunnut

Pratt & Whitney F135 - Wikipedia, the free encyclopedia

General Electric/Rolls-Royce F136 - Wikipedia, the free encyclopedia

Not much, but I guess you can start with the wiki article.

If the numbers at those links are accurate, one interesting comparison is size. There isn't a big difference, but is the difference enough to make the F136 easier to retrofit to exisitng planes?

Pratt & Whitney F135

Length: 220 in (5.59 m)
Diameter: 51 in (1.29 m)

General Electric/Rolls-Royce F136

Length: 221 in (5.6 m)
Diameter: 48 in (1.2 m)

.

**highsea** · 11 Nov 10,, 17:26

Originally Posted by JRT

...but is the difference enough to make the F136 easier to retrofit to exisitng planes?

I assume you mean could you put the F136 in a frame that was built for the F135?

No. They aren't interchangeable. The F-35 has to be ordered with the engine the customer wants. The lift fan will work with either engine of the STOVL variant, but the engine bays of the jet are different depending on Pratt or GE configuration.

**Stitch** · 11 Nov 10,, 17:36

Actually, the Department Of Defense has a history of "hedging their bets", if you will, and initiating competing engine programs for most of their aircraft contracts; I believe this started back in the '80's with the competing P&W F-100 project, and the GE F-110 AFE (Alternative Fighter Engine) for the F-16. Part of this is so no one company has a monopoly on high-performance fighter engines, and partly this is because the DoD recognizes that competition between GE and P&W will, hopefully, keep the costs down on these engines. The same situation happened when GE and P&W squared off in the F-22 program with their competing F-119 and F-120 engines; in the event, the more conventional P&W F-119 won, but both engines were developed further, with the F-119 forming the basis for the F-35's F-135 engine, and the F-120 forming the basis for the F-136 engine, which will also be used in the F-35. Again, this is the DoD's way of "hedging their bets", and making sure no one engine company has the market cornered on high-performance fighter engines.

**outofshdw** · 15 Nov 10,, 12:03

Originally Posted by Stitch

Actually, the Department Of Defense has a history of "hedging their bets", if you will, and initiating competing engine programs for most of their aircraft contracts; I believe this started back in the '80's with the competing P&W F-100 project, and the GE F-110 AFE (Alternative Fighter Engine) for the F-16.

The F-16 has been the exception, not the rule. In every other fighter program over the past half century, the competition ends when the Defense Department awards a contract to the winning team.

There was only one engine selected for the F-4. Only one engine for the F-104. Only one engine for the F--5. The F-18C and the larger F-18E also had only one engine offered. The list goes on and on. Once a design is down-selected maintaining two separate engine production lines only add cost. It means that the Pentagon has to pay a premium for two manufacturing supply chains, has to maintain two sets of Technical Orders, has to train its maintenance personnel to service two entirely different engines, and so on. It is an enormous logistical cost burden - which is why the Pentagon has attempted repeatedly to shut down the "alternate engine" program. If they don't have the spare cash to develop a separate airframe that meets the USAF, USN and USMC's competing requirements - then they sure don't have the extra cash to support two separate engine programs.

The "Great Engine War" that created the F100/F110 rivalry developed in response to a specific set of circumstances - when the original version of the F100 failed to live up to its customer's expectations. That's why you didn't see two separate engines for the F-18 or the F-22. Even on the F-14, where the USN was unhappy with the performance of the TF30, you didn't see the Navy paying to install a version of the F110 and to install another batch of aircraft with a navalized F100 (a program which the Navy actually cancelled in the 1970s, only to realize that it still needed a replacement for the TF30 during the 1980s). Once the Navy picked the design it wanted, the competition was over - winner take all.

The same could be said for the F135 today, which has reached its Initial Service Release having met all of its customer's performance requirements. The F135 today has over 17,000 hours of test and operational experience under its belt. The F136 hopes to achieve 1,000 hours by the end of this year. That's 16,000 hours of learning curve that the Pentagon would have to pay for, to catch up with a design that they're already happy with. That's why the DoD has recommended canceling the alternate engine program year after year. It's a luxury that they cannot afford in this economy, and which provides the warfighter with no added value. The only thing that keeps the alternate engine going is politics - and the Congressmen with GE manufacturing plants in their districts.

**JRT** · 16 Nov 10,, 15:48

Originally Posted by highsea

I assume you mean could you put the F136 in a frame that was built for the F135?

No. They aren't interchangeable. The F-35 has to be ordered with the engine the customer wants. The lift fan will work with either engine of the STOVL variant, but the engine bays of the jet are different depending on Pratt or GE configuration.

No, that is not what I meant. I think the engine competition topic is already in current discussion/debate. And I realize that most aircraft are designed to accept only one engine, would need modification to accept another.

The question was toward the possibility, utility, viability, economics, etc., of retrofitting non-STOVL variation of the F136 to older existing designs for enhanced performance. Toward using them in either new or exsting airframes. F-18 might be out of the question with its much smaller engines, but maybe in F-15, F-16, maybe also in F-14 if enough usable airframes still exist. Using that F-14 example, GE's F136 is smaller than the TF30 that was used in F-14A, but produces much higher dry and wet thrust than the F110 used in F-14B/D.

Please note that these are questions, not suggestions. I do not have the knowledge and experience in this to making any suggestions. I see a lot of discussion on old planes, and a lot of discussion on new planes, but not a lot of discussion on the possibility, utility, viability, economics, etc., of extending older designs.

.

**highsea** · 16 Nov 10,, 17:46

Originally Posted by JRT

The question was toward the possibility, utility, viability, economics, etc., of retrofitting non-STOVL variation of the F136 to older existing designs for enhanced performance. Toward using them in either new or exsting airframes. F-18 might be out of the question with its much smaller engines, but maybe in F-15, F-16, maybe also in F-14 if enough usable airframes still exist. Using that F-14 example, GE's F136 is smaller than the TF30 that was used in F-14A, but produces much higher dry and wet thrust than the F110 used in F-14B/D.

Well, ignoring the problems of retrofitting the engine bays, weight and balance issues, airflow issues, plumbing and wiring, etc., you are really putting a lot more thrust on the airframe than it was designed to take. These airframes are largely designed around the engines. It's a lot easier to upthrust the existing engines than retrofit something else.

Again, you're limited by what the airframe can handle. 10-15% is about the upper limit in most cases. You're talking about going from ~29,000 to ~43,000 lbs, that's quite a bit more than any airframe that was designed around an F110 can deal with.

**Stitch** · 16 Nov 10,, 18:25

The main problem with the longevity of any given design is the airframe; an airframe only lasts so long until it becomes fatigued and, therefore, unsafe. All airframes have a projected lifetime, usually measured in flight hours; to go beyond the airframe's projected lifetime is inviting disaster. Sometimes, depending on the design, there are ways to extend the service life of a given airframe (the A-10 SLEP comes to mind) but, for the most part, the utility of any given aircraft is limited by the airframe's durability (or lack thereof). Sure, you can put new engines and new avionics in an airframe, but that won't do you much good if the wing falls off, or the tail separates in flight; now you've just made a very expensive hole in the ground with the brand-new engines and the brand-new avionics and a 30-year old airframe.

I agree it's a good idea to upgrade an airframe, if possible, to get more utility out of it but, unfortunately, most of the time that's just not possible. I wouldn't mind seeing the F-15E re-engined with the F135 or F136 but, economically, it doesn't make much sense; there wouldn't be that much of a performance gain to make it worthwhile. The F-14 proved to be an exception, as they were able to retrofit the F110 into the Tomcat with minimal modifications, while the airframe still had a reasonable amount of fatigue life left on it.