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The X-35A, built to validate propulsion and flying qualities for the Joint Strike Fighter, takes flight in October 2000. (Lockheed Martin)

Weight Watchers

How a team of engineers and a crash diet saved the Joint Strike Fighter.

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The pre-World War II facility, formerly U.S. Air Force Plant Four, has seen the birth of tens of thousands of aviation legends, from the B-24 to the F-16. It has also witnessed expensive stillbirths like the A-12 Avenger. SWAT’s job was to keep the F-35 in the former category.

The cornerstone of the company’s future military aviation business, the F-35 is a complex undertaking: A worldwide network of partner contractors and subcontractors produce components that are assembled in a section of a mile-long building in Texas. Shared three-dimensional electronic design files are updated daily to keep each engineer working on the most current version. At the Texas plant alone, about 4,500 employees work on the three JSF variants, each with unique requirements and capabilities to suit the various needs of three finicky U.S. armed services and more than a handful of skittish international partners. For affordability’s sake, however, the variants must be largely—up to 80 percent or so—identical. Because of the high degree of commonality, modifications to the design of the portly problem child can be applied to the other two versions.

One challenge in designing stealth aircraft is that all stores—extra sensors, fuel tanks, and weapons—must fit internally. Anything hanging outside of the aircraft will increase the aircraft’s radar cross-section and thus diminish its stealthiness.

On the F-35 STOVL variant, the F-35B, the weapons bays must share internal space with an enormous lift-fan engine, which enables the vehicle to hover and land vertically, and with the engine’s ducts. The wide cavities demanded for these components contribute to weight gain because they compromise the best layout for the aircraft’s load-bearing structure. Creating an airplane around these systems is akin to designing a human skeleton after the organs have been installed. It forced the airframe team to adopt a heavier design.

The program’s initial focus on affordability also added weight. Off-the-shelf parts cost less but weigh more because they are not optimized for a fighter. To get bulk quantities of replacement parts for a lower cost might require using a heavier component. It soon became obvious that the plan to use common parts among the variants—a strategy that would lower costs and streamline future maintenance demands—was also bulking up the F-35.

Initial estimates of how much a part will weigh are based on its volume and material. But they are just estimates; the actual weight is another matter. A heftier hose, a wider screw, a thicker panel—in dribs and drabs, the weight steadily increases.

Even in a world of precision design tools, weight estimates still depend on data from previous aircraft. That turned out to be a problem as the crowded interior and the demands of the design translated into poundage. “Legacy estimating techniques just don’t work with this family of airplanes,” says R.J. Williams, Lockheed’s vice president of F-35 Air Vehicle Development.

Art Sheridan says that cost, not weight, was the most important measurement during the early history of the program. “The focus was very much on affordability at the time,” he says. “People realized there was a penalty to be paid, and that was included in the weight estimates. It was higher than we thought.”

No matter the reason, when weight became the enemy, the SWAT team concentrated its effort on reducing it, as well as reducing the bureaucratic hoop-jumping that can slow a redesign. “The number one commitment was to remove obstacles and make quick changes,” Sheridan says.

Instead of the typical series of boards that normally reviewed proposed design changes, SWAT consolidated the process into one review panel. Engineers were expected to come in with an idea, face detractors, and accept a decision in one sitting.

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