the original A-4b design had been changed at every point; the engine, airframe and navigation systems were now all new. New concept In September 1947 the US Air Force was split off from the US Army. As part of the split, the forces agreed to divide ongoing development projects based on range, with the Army taking all the projects with a range of or less, and the Air Force everything above that. MX-770 was well below that limit, but instead of handing it over to the Army's Ordnance Department who were working with von Braun on ballistic missiles, in February 1948 the Air Force instead requested that NAA double the range of the MX-770 to put it into the Air Force's domain. Examining the work to date, NAA abandoned the boost-glide concept and moved to the ramjet powered cruise missile as the primary design. Even with the more efficient propulsion offered by the ramjets, the missile would have to be 33% larger to achieve the required range. This required a more powerful booster engine to power the launcher, so the requirement for the XLR-41 Mark III was raised to . The N-1 INS system drifted at a rate of 1 mile per hour, so at its maximum range it would not be able to meet the Air Force's CEP. The company began development of the N-2 to fill this need and provide considerable headroom if greater range was requested. It was essentially the mechanism of the N-1 paired to a star tracker which would provide midcourse updates to correct for any accumulated drift. The Air Force assigned the missile the XSSM-A-2 designation, and then outlined a three-stage development plan. For Phase 1, the existing design would be used for technology development and as a testbed for various launch concepts, including the original booster concept, as well as rocket-track launches and air dropped versions. Phase 2 would extend the range of the missile to , and Phase 3 would further increase that to an intercontinental while carrying a heavier warhead. The design evolution ended in July 1950 with the Air Force of Weapon System 104A specifications. Under this new requirement the purpose of the program was the development of a range nuclear missile. WS-104A Under WS-104A, the Navaho program was broken up into three guided missile efforts. The first of these missiles was the North American X-10, a flying subrange vehicle