grounded, the timetable for the development of a photo-reconnaissance version of the Victor, known as the SR.2, was accelerated. A prototype (XL165) was flown for the first time on 23 February 1965, and the first aircraft (XL230) was delivered to No. 543 Squadron on 18 May 1965. The Victor SR.2s carried out extensive photographic survey work, which assumed increased importance after the bombers switched to low-level operations. No. 543 Squadron was disbanded on 31 May 1974, but a flight of four remained until 30 March 1975 to participate in French nuclear weapons testing in the Pacific. Responsibility for the reconnaissance role passed to No. 27 Squadron, which had been re-formed in November 1973, and operated the Vulcan SR.2. No. 27 Squadron was disbanded in March 1982. Aerial refuelling In addition to the roles for which they were designed, all three V bombers served as aerial refuelling tankers at one time or another. The Valiant was the RAF's first large-scale tanker. The probe and drogue system for aerial refuelling was developed by Sir Alan Cobham, but the Air Ministry doubted its value so long as Britain maintained bases around the world. However, on 8 January 1954, the Air Staff decided that the V bombers should be capable of both aerial refuelling and acting as tankers, and an Operational Requirement (OR3580) was issued in 1956 for an electronic positioning system to facilitate aerial refuelling. Initially, there were no aircraft to perform the role, but two new types of Valiant were ordered. Fourteen B(PR)K.1 versions were produced. These were a tanker variant of the photo-reconnaissance model, with a hose drum unit (HDU) in the bomb bay. The final production model of the Valiant was the BK.1 version, which had a fuel tank in the front of the bomb bay and an HDU in the rear. Some 44 were built. No. 214 Squadron was selected to carry out tanker trials, while retaining its bombing role, in February 1958. The trials were successful. In August 1961, a second Valiant squadron, No. 90 Squadron, was ordered to begin training in the aerial refuelling role. Nos 90 and 214 Squadrons became full-time tanker squadrons on 1 April 1962. In a demonstration on 20/21 June 1962, a Vulcan B.1A from No. 617 Squadron flew non-stop from RAF Scampton to Sydney in 20 hours and 5 minutes, refuelled four times by tankers from No. 214 Squadron. They served in