P6M SeaMaster
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The Martin P6M SeaMaster, built by the Glenn L. Martin Company, was a 1950s strategic bomber flying boat for the United States Navy that almost entered service; production aircraft had been built and Navy crews were undergoing operational conversion, with a service entry about six months off, when the program was cancelled on August 21, 1959. Envisioned as a way to give the Navy a strategic nuclear force, the SeaMaster was eclipsed by the Polaris submarine launched ballistic missile.
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[edit] Background
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, the USS United States, a large 'super carrier' to launch Navy strategic bombers from, having been a victim of budget cuts, the Navy chose instead 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 even special submarines, could be located closer to the enemy, and as mobile targets would be harder to neutralize.
The requirement, issued in April 1951, was for a seaplane able to carry 30,000 lb (13,600 kg) of bombs to a target 1,500 miles (2,400 km) away from its aquatic base. The aircraft had to be capable of a low altitude attack run at Mach 0.9 (1,100 km/h), which required an extremely capable aircraft. Both Convair and Martin submitted proposals, and the Martin one 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 planes.
Originally the plane was to have a Curtis-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 (downward slope to the tip) 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-flying "T" tail and a rotating bomb-bay—pneumatically sealed against seawater in the P6M.
[edit] Development
First flights of the XP6M-1 in 1955 showed that the engines were mounted too closely to the fuselage and scorched it when afterburners were used, leading to pointing the engines a little outward in subsequent aircraft. Flight testing was initially successful but on December 17, 1955 was marred by accident, a control system fault destroying the first prototype with the loss of all four lives aboard. Almost a year later, on November 9, 1956, the second prototype was also destroyed, thanks to an error in the tail control system design, but this time the crew ejected safely.
The first pre-production YP6M-1 was completed about a year later, with testing resuming in January 1958.
Five more were built in 1958 when the Navy announced that Harvey Point Defense Testing Facility in Hertford, North Carolina, would serve as the testing grounds for the fleet of Martin P6M Seamaster. These aircraft were fitted with test versions of the full combat equipment suite and were used for bombing, minelaying and reconnaissance evaluations.
This program proving successful, the first production P6M-2 was rolled out in early 1959. Changes included new, more powerful J75 engines, an inflight refuelling probe, improved avionics and a canopy with better visibility. A buddy refuelling drogue kit had also been developed to fit in the bomb bay. Three had been built by summer 1959 and Navy crews were moving them through operational conversion when the program was abruptly cancelled in August of that year.
The P6M-2 was certainly an impressive aircraft; its Mach 0.9 (1,100 km/h) performance 'on the deck' could be equalled by few aircraft of the time and not too many more now. A USAF B-52 can only reach Mach 0.55 (670 km/h) at low altitude. The planes were built incredibly tough, with the aircraft skin at the wing roots over an inch (25 mm) thick.
Although the technology was up-to-date, in the age of the ICBM and SLBM the manned bomber was considered an expensive, unreliable way to deliver nuclear weapons. The P6M program had already cost $400 million (about $2.5 billion in 2004 dollars) and could not be justified without the strategic mission.
All of the aircraft were scrapped. Some tail sections were retained for testing, and one of these is now in the Glenn L. Martin museum, along with some wingtip floats re-used by a Martin employee to build a catamaran.
Martin tried unsuccessfully to market the technology in the civilian market but there were no takers, and the company soon abandoned the aircraft business entirely to focus on missiles and electronics. The P6M was the final aircraft constructed by the Glenn L. Martin Company.
[edit] Specifications (P6M-2)
General characteristics
- Crew: 4
- Length: 134 ft 0 in (40.84 m)
- Wingspan: 102 ft 11 in (31.37 m)
- Height: 32 ft 5 in (9.88 m)
- Wing area: 1,900 ft² (180 m²)
- Empty weight: 91,300 lb (41,400 kg)
- Loaded weight: 120,000 lb (54,000 kg)
- Max takeoff weight: 176,400 lb (80,000 kg)
- Powerplant: 4× Pratt & Whitney J75-P-2 turbojets, 17,500 lbf (77.8 kN) each
Performance
- Maximum speed: 550 kt (630 mph, 1,010 km/h)
- Range: 1,700 nm (2,000 mi, 3,200 km)
- Service ceiling: 40,000 ft (12,000 m)
- Rate of climb: ft/min (m/s)
- Wing loading: 63 lb/ft² (310 kg/m²)
- Thrust/weight: 0.58
Armament
- Guns: 2× 20 mm cannon in tail turret
- Bombs: 30,000 lb (14,000 kg)
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