The annual Pentagon Director of Operational Test and Evaluation report has become available this week–and sheds some light on last February’s Joint Requirement Oversight Council’s decision to relax some of the Lockheed Martin F-35 Joint Strike Fighter’s key performance parameters. At the time, it looked like it was mostly the aircraft’s range that was impacted. Now looking at this report, it is clear there was much more than that.
The F-35′s sustained turn rate requirements have been slashed as have its transonic acceleration requirements. Most impacted is the Navy’s F-35C, which has had more than 43 seconds added to its Mach 0.8 to Mach 1.2 acceleration times. But this wasn’t exactly unexpected, as almost exactly one year ago Lockheed’s Tom Burbage told me this when I was still at Defense News:
“Based on the original spec, all three of the airplanes are challenged by that spec,” said Tom Burbage, Lockheed’s program manager for the F-35. “The cross-sectional area of the airplane with the internal weapons bays is quite a bit bigger than the airplanes we’re replacing.”
The sharp rise in wave drag at speeds between Mach 0.8 and Mach 1.2 is one of the most challenging areas for engineers to conquer. And the F-35′s relatively large cross-sectional area means, that as a simple matter of physics, the jet can’t quite match its predecessors.
“We’re dealing with the laws of physics. You have an airplane that’s a certain size, you have a wing that’s a certain size, you have an engine that’s a certain size, and that basically determines your acceleration characteristics,” Burbage said. “I think the biggest question is: are the acceleration characteristics of the airplane operationally suitable?”
Some of the backstory, according to an industry source is that originally the designers had intended the F-35 to be somewhat longer and more slender–in keeping with the principles of the Whitcomb area rule. Back then, the weapons bays were placed one behind the other–AMRAAMs in one bay, JDAMs in another. Apparently, the tail-end of the jet started to get heavy, and Lockheed had to change the configuration as a result–which is how we got the current weapons bays. They were kinda squished together–to use a technical description. As a result of that design change, there was never any chance that the F-35 was going to be able to match the transonic acceleration of a Block 50 Viper (Lockheed Martin F-16 Fighting Falcon) armed only with two wing-tip AIM-120s.
Here is the full DOT&E report. It’s long and the F-35 has a lot of problems, and there is a lot of work ahead for Lockheed and Joint Program Office. Nonetheless, as an engineer source tells me: “My general thought is that there is not a fundamental problem (i.e., a “show-stopper”) on any of the three F-35 types/variants. Many of the issues being resolved are typical of development aircraft.”