Chair Force Engineer

Friday, September 25, 2009

The Other Engine

Whenever I check my gmail account, I see these odd adds from General Electric telling me that I should petition my congressman to continue funding for their F136 turbofan engine. While appeals to the public rarely have the desired effect in defense acquisition, it does raise an interesting point. The F136 program is an unprecedented development in military aviation acquisition: it's the first time that a major weapons system (the F-35 Lightning II) has been authorized with two separate engines before the first flight.

General Electric makes its argument based on their experience producing the F110 replacement engine for the F-14 Tomcat and F-16 Fighting Falcon. Yet the F110 was the product of unique circumstances that are not present in the development of today's F-35 fighter jet and its F135 engine, produced by Pratt & Whitney.

The F110 story actually begins before 1964, when Pratt & Whitney produced the first low-bypass turbofan engine for supersonic fighter aircraft. The TF30 was eventually fitted to the F-111 supersonic medium bomber, F-14 Tomcat (a carrier-based fighter,) and the A-7 subsonic light bomber. All three of these aircraft suffered from engine reliability problems. The F-111's problems were least severe, and the aircraft still flies (in the Royal Australian Air Force) with its original engines. The A-7's performance with the TF30 was so poor that Allison Engines produced a license-built version of the Rolls-Royce Spey to replace it during future production models of the A-7. For the Tomcat, which suffered most from compressor stalls, a replacement engine would have to wait until the late 80's.

The situation was not much better when Pratt & Whitney designed the F100, a fighter turbofan that was well ahead of its time when it first powered the F-15 Eagle in 1972. The engine's reliability problems were less pronounced in a twin-engined aircraft like the F-15, but they became much more critical when the engine was used in the single-engine F-16.

General Electric came to the rescue with a derivative of the F101 turbofan designed for the B-1 supersonic bomber. With some modifications, the F101 was adapted into the F110 fighter engine. The new engine became the powerplant of choice for future F-14 and F-16 production, and was retrofitted to older F-14's. Interestingly, both the F100 and F110 will power South Korea's version of the Strike Eagle (F-15K.)

Having an alternative engine waiting in the wings was a great blessing to both the F-14 and F-16. General Electric reasons that they will be the savior for the F-35 program too. The problem for GE is that the two situations are very different. The TF30 and F100 were designed when supersonic turbofans were still in their infancy. By contrast, the F135 baseline engine for the F-35 draws on mature propulsion technologies developed for the F119 engine in the F-22 Raptor. The chance of F135 becoming a dud like the TF30 or F100 are far slimmer. While the F136 ensures two engine vendors for the F-35, it's a very expensive option for the defense department to retain. It seems like an expensive bailout of GE to keep two major fighter engine manufacturers in business, more than anything else.