hand, operational issues, and the concept of an independent nuclear deterrent came to the fore. The Air Council decided on 7 July 1960 that Project E weapons would be phased out by December 1962, by which time it was anticipated that there would be sufficient British megaton weapons to equip the entire strategic bomber force. Project E weapons were replaced by British Yellow Sun bombs at RAF Honington on 1 July 1961 and Waddington on 30 March 1962. Problems encountered in the development of the Red Beard bomb meant that the replacement of kiloton weapons took longer. The UK-based Valiants at Honington and Wittering were withdrawn in April and October 1962, and the last Valiants were retired from the V bomber force in July 1965. The final practice loading at RAF Marham—with the Mark 43s—was in January 1965, and the last US personnel left the base in July. Low level strike The prospect of bombers being able to avoid the Soviet air defences dimmed with the appearance of the Mikoyan-Gurevich MiG-21, which Nigel Birch, the Secretary of State for Air and senior RAF officers saw at Tushino Airfield on 24 June 1956. There was no immediate concern as Soviet designs often took several years to deploy; but its ceiling of posed a clear threat to the V bombers. So too did the new SA-2 surface-to-air missiles, which appeared in 1957. One of them shot down an American Lockheed U-2 piloted by Francis Gary Powers over the Soviet Union on 1 May 1960. In 1957, the supersonic Avro 730 bomber was cancelled. This freed up funds for the Blue Streak missile programme, but it too was cancelled, on 24 February 1960. To extend the effectiveness and operational life of the V bombers, an Operational Requirement (OR1132), was issued on 3 September 1954 for an air-launched, rocket-propelled standoff missile with a range of that could be launched from a V bomber. This became Blue Steel. The Ministry of Supply placed a development contract with Avro in March 1956, and it entered service in December 1962. By this time, it was anticipated that even with Blue Steel, the air defences of the Soviet Union would soon improve to the extent that V bombers might find it difficult to attack their targets, and there were calls for the development of the Blue Steel Mark II with a range of at least . Despite the