could be ready for service in the 2025 timeframe, with a fleet of 25–30 aircraft proposed to replace the nearly 40-aircraft mix of U-2s and RQ-4s.<ref>Malenic, Marina. "AFA 2015: Lockheed Martin pitches TR-X to replace U-2." janes.com', 17 September 2015. Retrieved: 7 December 2015.</ref> Lockheed Martin revealed more specifications about the TR-X concept at a 15 March 2016 media day, confirming the aircraft would be unmanned and air refuelable. Its maximum takeoff weight would be greater than either the U-2's or RQ-4's at around , with a payload and wingspan. It will use the same F118-101 turbofan and generator as the U-2, but thrust could increase to and power increased to 65–75 kVA; service ceiling would increase to with a second engine. The TR-X is meant to be "survivable, not unnoticeable", operating outside of enemy air defense bubbles rather than penetrating into them. Avionics Tech Refresh In 2020, the US Air Force awarded the Avionics Tech Refresh contract to Lockheed Martin for upgrading the U-2. In February 2020, the flight tests and the installation of new electro-optical reconnaissance systems were completed. SYERS-2C cameras manufactured by Collins Aerospace equip the entire U-2S fleet. The contract is valued at $50 million. The U-2S's ISR very high altitude mission requires changes for avionics suite for the U-2's onboard systems, a new mission computer designed to the U.S. Air Force's open mission systems standard and a new and modern cockpit displays (Primary Flight Display or PFD). The avionics upgrades are scheduled to be completed by 2022. Lockheed Martin then plans to refresh the U-2's sensors and other electronic systems., to act as a node in the Advanced Battle Management System (ABMS) now under development. Design The design that gives the U-2 its remarkable performance also makes it a difficult aircraft to fly. Martin Knutson said that it "was the highest workload air plane I believe ever designed and built … you're wrestling with the airplane and operating the camera systems at all times", leaving no time to "worry about whether you're over Russia or you're flying over Southern California". The U-2 was designed and manufactured for minimum airframe weight, which results in an aircraft with little margin for error. Most aircraft were single-seat versions, with only five two-seat trainer versions known to exist. Early U-2 variants were powered by Pratt & Whitney J57 turbojet engines. The U-2C and TR-1A variants used the more powerful