The Martin P6M SeaMaster was an experimental strategic bomber flying boat built by the Glenn L. Martin Company for the United States Navy that almost entered service; production aircraft were built and Navy crews were undergoing operational training, with service entry expected in about six months, when the program was cancelled on 21 August 1959. Envisioned as a strategic nuclear weapon delivery system for the Navy, the SeaMaster was eclipsed by the Polaris submarine-launched ballistic missile (SLBM). Due to the political situation at the Pentagon and weapon system choices made amid budgetary constraints, the Navy promoted the P6M primarily as a high speed minelayer.
Design and development
In the immediate postwar defense climate, the United States Air Force's Strategic Air Command was the linchpin of the United States' security as the sole means of delivery of the nation's nuclear arsenal. The Navy saw its strategic role being eclipsed by the Air Force and knew both its prestige and budgets were at stake.
Its first attempt to address this came in the form of the , a large supercarrier intended to launch Navy strategic bombers. This was cancelled in 1950 shortly after her keel was laid down, a victim of budget cuts and US Air Force interference. In response, the Navy chose to create a "Seaplane Striking Force", useful for both nuclear and conventional warfare, including reconnaissance and minelaying. Groups of these planes supported by seaplane tenders or special submarines could be located close to the enemy, and being mobile, they would be hard to neutralize.
The requirement issued in April 1951 was for a seaplane able to carry a warload over a range of from its aquatic base. The aircraft was to be capable of a low altitude dash at Mach 0.9 (1,100 km/h). Both Convair and Martin submitted proposals, and the Martin proposal was chosen as more promising. An order for two prototypes was issued which was projected to lead to six pre-production aircraft and a projected twenty-four production aircraft.
Originally the plane was to have a Curtiss-Wright turbo-ramjet engine, but this ran into problems and a more conventional Allison J71-A-4 turbojet was employed, fitted in pairs in overwing pods to keep the spray out of the intakes. Wings swept at 40° were used; they displayed a notable anhedral and were designed with tip tanks that doubled as floats on the water. Many features of Martin's XB-51 bomber prototype were used, including an all-moving T-tail and a rotating bomb bay—pneumatically sealed against seawater in the P6M.
Operational history
thumb|right|YP6M-1 in flight
thumb|right|P6M-2 on beaching gear
thumb|right|P6M-2 illustrating takeoff spray pattern
The first flight of the XP6M-1 came on 14 July 1955, but early tests showed that the engines were mounted too close to the fuselage and scorched it when afterburners were used, leading to angling the engines slightly outward in subsequent aircraft. Flight testing was initially successful, but, on 7 December 1955, a control system fault destroyed the first prototype with the loss of all aboard. The first prototype, BuNo 138821, c/n XP-1, disintegrated in flight at due to the horizontal tail going to full up due to a control malfunction, subjecting the airframe to 9 g stress as it began an outside loop, crashing into the Potomac River near the junction of St. Mary's River, killing four crew members.
Eleven months later, on 9 November 1956, the second prototype, BuNo 138822, c/n XP-2, first flown on 18 May 1956, was also destroyed, due to a change made in the horizontal stabilizer control system without adequate evaluation before test flying the design. The crash occurred at 15:36 near Odessa, Delaware due to a faulty elevator jack. As the seaplane nosed up at ~ and failed to respond to control inputs, the crew of four ejected. The airframe broke up after falling to before impact.