The Mark 49 Mod 1 warhead was only different from the Mod 0 in that it incorporated an inertial environmental sensing device that would prevent arming of the warhead until it sensed a reentry environment. This modification was made due to concerns about accidental detonation caused by electrical or procedural malfunctions and errors, and the risk of sabotage. The request for this modification was approved in October 1958. The modification asked that the sensor be actuated as late as possible in flight and that it be located in the warhead in such a way that access to the device would be difficult and time consuming, as to prevent sabotage. Design release for both the Y1 and Y2 warheads was made in June 1959 and was retrofitted to Mod 0 warheads starting in October 1959.
