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Weighing the F-35

March 16, 2008 (by Eric L. Palmer) - Weight in the design of aircraft has always been a curse to aerospace engineers. The F-35 is no different. Here are a few points to consider when the topic of F-35 weight comes up.

Weight and see

What are the different kinds of aircraft weight? Peter Goon, an Australian flight test engineer has some thoughts:

"One can track the weight growth of the JSF by going back and looking at the design IOC target weights for all variants for design configurations 240-1. 240-1, 240-3, 240-4 .

These are design target weights and there is likely a 3% margin on top of these.

Are these the Design Empty Weight of each variant……or are they what is called the Basic Empty Weight for each variant (which includes the unusable fuel and undrainable oil, survival equipment, etc) …..or are they the Operational Empty Weight for each variant which is the Basic Empty Weight + weight of crew, weapon racks, ejectors, rack adaptors, gun and everything else for the operational mission except the actual weapons and fuel?"

The table below [1], points to some of the early weight history of the F-35 program.

F-35 Target Weight*240-1 CY2002 (pounds)*240-2 CY2003
*240-4 CY2006 (pounds)Change since 2002

Weight may be one of the easiest factors of combat aircraft design for a non-engineer to understand. It has significant effect on how much fuel, weapons, and mission equipment the aircraft can carry. This means that when excess weight appears in the design, the aircraft will be limited on how far it can fly, how much agility it will possess, how many targets can be hit with what effect and how well the aircraft can detect threats and defend itself.

The 2004 event addressed by the SWAT (STOVL Weight Attack Team) was the result of discoveries that the STOVL (Short Take-Off Vertical Landing) variant of the JSF known as the F-35B was at risk of not meeting its weight requirements. So much so that this variant would not be able to accomplish its mission unless there was a serious reduction in aircraft weight.

The SWAT event was mostly solved by reducing the F-35B STOVL mission requirements. All three variants had a design specification to carry two-each internal air-to-ground weapons in the 2,000 pound class. A weight savings on the F-35B STOVL was achieved by changing the internal carry of this variant. SWAT proposed having the STOVL variant to carry two 1000 pound air to ground weapons. This produced a weight reduction of 2000 pounds. The main “customer”, the U.S. government representative and coordinator for F-35 worldwide customers known as the Joint Strike Fighter Program Office was told that either some design compromises would have to be agreed upon to allow the aircraft to fly or the STOVL variant may fail. The customer agreed. Another 700 or more pounds of weight were reduced by changing the design of numerous items on the aircraft. These combined efforts of SWAT also reduced weight from the other two F-35 variants by about 1300 pounds.

The result of the SWAT effort, while saving the STOVL to fight another day in the design process had some negative effects too. The above mentioned design changes had a great effect on the production line process.

So from all that, a weight problem on one variant, changed the production process, produced huge delays and caused a rise in the total cost of the program.

While the 2004 SWAT episode may be the signature F-35 weight reduction event, there were signs of these problems a year before. Back in 2003, Lockheed Martin announced that an earlier weight reduction event had shed 1000 pounds off the design. The down side is that "quick-mate joints", appliances that made aircraft assembly efficient, were removed from the production line procedure. This would add several days time to the production cycle. Tom Burbage, Lockheed Martin's director of the Joint Strike Fighter Program stated: "That was the trade-off we had to make to get the weight down". [2]

When the AA-1 test vehicle F-35 was rolled out in 2006, one Lockheed Martin official stated: “The weight of this F-35A is greater than what was originally projected, but not so high that the aircraft does not meet key performance parameters. The margins would be very tight—they are not wide, even with the redesign—but it would have made it. Every F-35A that follows will be lighter.”[3]

Weight growth will be critical to making the whole F-35 program sink or swim. The conventional take-off and landing (CTOL) variant the F-35A offers the least weight resolution problems. Like an F-16 or F-15 or other similar conventional runway fighters it’s launch and recovery issues seem easy. What won’t be easy is that the other two designs have to demonstrate a short-take-off and vertical landing (F-35B) and do all the difficult launch and recovery tasks associated with the demanding carrier aviation environment (F-35C).

While Lockheed Martin is hoping to contain weight growth to 3% of the design, other experts have chimed in and stated historical growth in past combat aircraft designs have been as much as 6%. [4]

On March 14, 2008, the U.S. Department of Defense announced a $70 million dollar contract to develop weight reduction for one of the F-35 engines: The Pratt & Whitney F-135. [5]

With two of the three F-35 variants having demanding take-off and landing requirements, weight-growth containment will be the nemesis for F-35 design engineers for the foreseeable future.

End Notes-

[1]Peter Goon,Notam No.1, The Biggest Loser,, Air Power Australia, April 4, 2007, Retrieved March 15, 2008,

[2]Bob Cox, Team Seeks Weight Loss for F-35 Joint Strike Fighter, Fort Worth Star-Telegram, June 19, 2003

[3] Joe Pappalardo, Weight Watchers, Air and Space Magazine, October-November 2006, Retrieved March 15, 2008, "Design and assembly changes, mostly related to the SWAT recommendations, have cost about $4.8 billion—part of a $6.2 billion replanning to accommodate the additional design cycle required to make the improvements. The replanning forced an 18-month slip in F-35 deliveries. According to a 2006 Government Accountability Office report, since inception, the development costs of the JSF program have increased 84 percent and its timeline slipped by about five years. The STOVL’s final delivery deadline has been extended two years, to 2012."

[4] U.S. Government Accounting Office (GAO) -, GAO-08-388- Joint Strike Fighter: Recent Decisions by DOD Add to Program Risks, (Table 6), - March 11, 2008 Report, ,Retrieved March 15, 2008

[5]DOD Contract, March 14, 2008, retrieved March 15, 2008,