made by Thorneycroft on 10 June 1963. The choice of the A3 created a problem for the Atomic Weapons Research Establishment (AWRE) at Aldermaston, for the Skybolt warhead that had recently been tested in the Tendrac nuclear test at the Nevada Test Site in the United States would require a redesigned Re-Entry System (RES) in order to be fitted to a Polaris missile, at an estimated cost of between £30 million and £40 million. The alternative was to make a British copy of the W58. While the AWRE was familiar with the W47 warhead used in the A2, it knew nothing of the W58. A presidential determination was required to release information on the W58 under the MDA, but with this in hand, a mission led by John Challens, the Chief of Warhead Development at the AWRE, visited the Lawrence Livermore Laboratory from 22 to 24 January 1963, and was shown details of the W58. The Zuckerman mission found the SPO helpful and forthcoming, but there was one major shock. The British were expected to contribute to the research and development costs of the A3, backdated to 1 January 1963. These were expected to top $700 million by 1968. Skybolt had been offered to the UK at unit cost, with the US absorbing the research and development costs, but no such agreement had been reached at Nassau for Polaris. Thorneycroft baulked at the prospect of paying research and development costs, but McNamara pointed out that the United States Congress would not stand for an agreement that placed all the burden on the United States. Macmillan instructed the British Ambassador to the United States, Sir David Ormsby-Gore, to inform Kennedy that Britain was not willing to commit to an open-ended sharing of research and development costs, but, as a compromise, would pay an additional five per cent for each missile. He asked that Kennedy be informed that a breakdown of the Nassau Agreement would likely cause the fall of his government. Ormsby-Gore met with Kennedy that very day, and while Kennedy noted that the five per cent offer "was not the most generous offer he had ever heard of", he accepted it. McNamara, certain that the United States was being ripped off, calculated the five percent on top of not just the missiles, but their fire control and navigation systems as well, adding around £2 million to the bill. On Ormsby-Gore's