that purpose. The Vulcan and Victor were armed with British-built bombs Blue Danube, Red Beard, Violet Club, and Yellow Sun of both the Mk 1 and Mk 2 versions. Project E modifications to Valiants commenced at RAE Farnborough in February 1956. Crew training was carried out with American instructors at RAF Boscombe Down. The planned V bomber force was reduced to 144 aircraft, and it was planned to equip half of them with Project E weapons. The first 28 Valiants were modified by October 1957; the remaining 20 Valiants, along with 24 Vulcans, were ready by January 1959. Under the Project E Memorandum of Understanding, US personnel had custody of the weapons. That meant they performed all the tasks related to their storage, maintenance and readiness. While the bombs were at the same bases as the bombers, they were stored in Secure Storage Areas (SSAs) that British staff were not permitted to enter. It was therefore impossible to store British and American bombs together in the same SSA. Bomber Command designated RAF Marham, RAF Waddington and RAF Honington as bases with US SSAs. Another three sites had British SSAs. US custody created operational problems. The procedure for handing over the bombs added an extra ten minutes to the bombers' reaction time, and the requirement that US personnel had guardianship of the weapons at all times meant that neither they nor the bombers could be relocated to dispersal airfields as the RAF desired. Initially, 72 Mark 5 nuclear bombs were supplied for the V bombers. They had a yield of up to . The successful British development of the hydrogen bomb, and a favourable international relations climate caused by the Sputnik crisis, led to the United States Atomic Energy Act being amended again in 1958, resulting in the long-sought resumption of the nuclear Special Relationship between Britain and the United States in the form of the 1958 US–UK Mutual Defence Agreement. The United States now agreed to supply the V bombers with megaton weapons in place of the Mark 5, in the form of Mark 15 and Mark 39 nuclear bombs. The Treasury immediately inquired as to whether this meant that the British megaton bomb programme could be terminated. The answer was no; the operational restrictions imposed by Project E "effectively handed the US government a veto over the use of half of Britain's nuclear deterrent". With sufficient British bombs on