The newly released Pentagon budget apparently cancels the General Electric/Rolls-Royce engine for the F-35 Joint Strike Fighter. This is the third year running that the DoD budget has cancelled the F136. The last two times, it has been reinstated by Congress. (See background here.) Will the third time be the charm?

The alternate engine program has been part of JSF from the start, for several reasons. One is the Pentagon's corporate memory of the late 1970s and early 1980s, when Pratt & Whitney's F100 engine was powering both the USAF's new fighters - the F-15 and F-16 - and experiencing a boat-load of problems, quite a few of them resulting in aircraft losses.

I remember how, every time one braved the alligators in the parking lot to talk to P&W in West Palm Beach, the next modification package was going to fix the problem once and for all. But the customer suspected that - like the alligators - P&W saw no pressing need to evolve, and that the problem only got fixed when the USAF showed that it was serious about developing the GE F110 for the F-16C/D.

The fact that the JSF was a single-engine airplane, and that the STOVL version would need as much thrust as it could get, drove the idea of an alternate engine. Avoidance of a combat-engine monopoly was important, too, as is the fact that the alternate engine is partially British: the F136 is potentially the second-largest source of UK jobs in the program.

But others have argued that the second engine costs a lot of money, and will continue to cost money in upgrades and support, and that no other critical component of the JSF has a back-up program in place. Loren Thompson has just weighed in with a detailed position paper making these arguments.

For the JSF program. too, canceling the second engine in the last two cycles has a book-keeping benefit: Projecting the costs of the second engine out to the end of the program allows them to save a lot of money, on paper. Selected acquisition reports have shown savings of as much as $7 billion including "learning-curve adjustments", which is a lot of money for a program looking down the barrel of a Nunn-McCurdy breakout.

However, there is a better reason for retaining the F136. Namely, gravity.

As this report from last spring makes clear, both the short take-off and vertical landing parameters for the Marine/UK F-35B variant are at risk (page 6); and as reported here over the last few months, the F136 design was completed after the 2003-04 weight crisis, and it takes advantage of provision for more airflow (and hence thrust) that was part of resolving the potential show-stopping problems at the time.

But there is another problem on the political side: thrust is a non-issue for the non-STOVL versions of the F-35. Sizing the engine for the STOVL version has left them with kilonewtons to burn. That may be why Thompson's report never mentions STO or VL.

Personal view: scrapping the alternate engine is premature. If STOVL tests - not due until late next year or early next - show that the P&W F135 can do the job, then arguments like Thompson's have to be considered. But no sooner than that.