fuel to the left or right wing tank. Radar cross-section reduction When the first overflights of the Soviet Union were tracked by radar, the CIA initiated Project Rainbow to reduce the U-2's radar cross-section. This effort ultimately proved unsuccessful, and work began on a follow-on aircraft, which resulted in the Lockheed A-12 Oxcart. Possible successor In August 2015, the 60th anniversary of the U-2 program, Lockheed Martin's Skunk Works revealed they were internally developing a successor to the U-2, referred to as the UQ-2 or RQ-X, combining features from both the manned U-2 and unmanned Northrop Grumman RQ-4 Global Hawk and improving upon them. Disclosed details say the design is essentially an improved U-2 airframe with the same engine, service ceiling, sensors, and cockpit, with the main differences being an optional manning capability (something Lockheed has proposed for the U-2 to USAF several times, but has never gained traction) and low-observable characteristics. USAF has no requirement or schedule for a next-generation High-Altitude Long Endurance (HALE) platform, but Lockheed sees a future need and wants something in development early. The company's last attempt to create a stealth unmanned aircraft was the RQ-3 DarkStar, which never made it past flight testing and was canceled. Plans for a U-2 replacement would not conflict with the development of the SR-72, another project by the company to create a hypersonic unmanned surveillance plane, as it would be suited for missions that require greater speed for time-sensitive targets. The company released a notional artist's impression of the TR-X aircraft at an Air Force Association conference in Washington on 14 September 2015. Its name was changed to mean "tactical reconnaissance" to reflect its purpose as an affordable peace and wartime intelligence, surveillance and reconnaissance (ISR) aircraft, distinguishing it from strategic, penetrating SR-71-class platforms; TR is a reference to the short-lived rebranding of the U-2 as the TR-1 in the 1980s. Size, and thus cost, is kept down by having less endurance than the Global Hawk at around 20 hours, which is still about the same time as a normal RQ-4 sortie even though it is capable of flying for 34 hours. The TR-X concept is aimed squarely at USAF needs and is not currently being marketed to the CIA or other government agencies. It would have increased power and cooling to accommodate new sensors, communication equipment, electronic warfare suites, and perhaps offensive or defensive laser weapons. TR-X