So, the Federation of American Scientists just published the Pentagon's latest 53-page Selected Acquisition Report for the F-35 Joint Strike Fighter program published in December of 2010. This trove of info is produced annually to give lawmakers the DoD's latest estimates for the plane’s cost and development performance.
This report shows that Pentagon officials estimate the F-35A Air Force variant of the JSF will to meet its minimum combat radius requirement of 590 nautical miles. Granted, it misses the target by only six miles, but still, it's missing the minimum performance metric. Program officials had estimated that the plane would have a combat radius of 690 nautical miles. This puts the Air Force version of the jet in between the Marines’ short take-off and vertical landing F-35B and its combat radius of 469 nautical miles and the Navy/Marines' F-35C carrier variant which has a radius of 615 nautical miles.
This is troubling because some of the combat scenarios Pentagon officials think about the most involve operations in the Pacific theater where the "tyranny of distance" is a major factor.
Apparently, the range shortfall is caused by:
Based on updated estimate of engine bleed, the existing Conventional Takeoff and Landing Variant's Combat Radius prediction of 584 nautical miles (nm) is below the threshold of 590 nm. The current prediction is based on estimates for bleed usage, aircraft performance, and fuel capacity that are not yet fully known. Current estimates have built-in margin that may not be realized. Non-material (analysis and test) measures continue to reduce key performance parameter (KPP) uncertainty. Realistic aircraft modifications to add fuel capacity exist to recapture the KPP. These design modifications are being matured to sufficient level to allow for a program decision on incorporation if the current estimate remains below threshold as uncertainty is reduced. This estimate is based on preliminary data. The Program is still in the data analysis stage.What's all that mean? Basically, the plane's engine and avionics are running hotter than expected which requires "bleed air" from the F135 engine to be fed into the airframe to cool it down, as Steve Trimble at Flight Global points out. This reduces engine efficiency and therefore combat range. Additionally, the aircraft has less fuel capacity than planned and its stealth targeting pod is causing more drag than expected. All this has reduced the jet's range.
As the SAR says, the program is looking at "aircraft modifications to add fuel capacity" to get the jet back to at least a 590 nautical mile combat radius.
Trimble's sources told him:
One simple change under review is a software tweak that would maximise the amount of fuel taken onboard during in-flight refuelling. Another relatively simple fix is to raise shut-off valves higher inside the fuel tank to create slightly more capacity, a source said, adding: "That gets you back a lot of the fuel that you need to recover" to meet the range mandated by the contract.
A more complex solution also being considered is to install new fuel tanks in a small number of hollow spaces within the aircraft's structure.
But programme officials are also debating whether to change how the range of the F-35A is calculated, the source said. The equation does not include a buffer margin of 5% of fuel capacity, which is intended to be preserved through the end of the flight test period in 2016. Eliminating the buffer margin adds another 72.4km to the aircraft's combat radius, the source said