the possibility of hybrid submarines that could operate as hunter-killers while carrying eight Polaris missiles, but McNamara noted that this would be inefficient, as twice as many submarines would need to be on station to maintain the deterrent, and cautioned that the effect of tinkering with the US Navy's 16-missile layout was unpredictable. The Treasury costed a four-boat Polaris fleet at £314 million by 1972/73. A Cabinet Defence Committee meeting on 23 January 1963 approved the plan for four boats, with Thorneycroft noting that four boats would be cheaper and faster to build. A mission led by Sir Solly Zuckerman, the Chief Scientific Adviser to the Ministry of Defence, left for the United States to discuss Polaris on 8 January 1963. It included the Vice Chief of the Naval Staff, Vice Admiral Sir Varyl Begg; the Deputy Secretary of the Admiralty, James Mackay; Rear Admiral Hugh Mackenzie; and physicist Sir Robert Cockburn and F. J. Doggett from the Ministry of Aviation. That the involvement of the Ministry of Aviation might be a complicating factor was foreseen, but it had experience with nuclear weapons development. Mackenzie had been the Flag Officer Submarines until 31 December 1962, when Le Fanu had appointed him the Chief Polaris Executive (CPE). As such, he was directly answerable to Le Fanu as Controller of the Navy. His CPE staff was divided between London and Foxhill, near Bath, Somerset, where Royal Navy had its ship design, logistics and weapons groups. It was intended as a counterpart to the United States Navy Special Projects Office (SPO), with whom it would have to deal. The principal finding of the Zuckerman mission was that the Americans had developed a new version of the Polaris missile, the A3. With a range extended of , it had a new weapons bay housing three re-entry vehicles (REBs or Re-Entry Bodies in US Navy parlance) and a new W58 warhead to penetrate improved Soviet anti-missile defences expected to become available around 1970. A decision was therefore required on whether to purchase the old A2 missile or the new A3. The Zuckerman mission came out in favour of the new A3 missile, although it was still under development and not expected to enter service until August 1964, as the deterrent would remain credible for much longer. The decision was endorsed by the First Lord of the Admiralty, Lord Carrington, in May 1963, and was officially