The ULMS ‘decisions’ of the early 1970s, and particularly the final design of the submarine, were made on the assumption of the eventual development of a large Trident II missile. Although other factors – particularly Rickover's desire to build new large reactors and ‘the Navy's’ (not SSPO's) desire to justify new FBM submarines – drove the larger size of the Trident submarines, they were justified on the basis of the need to carry the large Trident II, which itself was characterized as a continuation of the Strat-X missile concept. There was little doubt that a Trident II would make full use of the extra launch tube volume, but what remained to be decided was exactly when, and what capabilities the missile would possess. In the original ULMS conceived in Strat-X, long range had been considered an important attribute and originally Trident II was to have a 6000 mile range, as opposed to the 4000 miles of Trident I. In the late 1960s and early 1970s extra range provided an uncontroversial way of justifying the new missile.