The last scheduled activity on the public demonstration was a takeoff, flyby, and landing by the Convair YF2Y-l Sea Dart by veteran test pilot Richbourg. The Sea Dart made a spectacular takeoff run from the bay, and Richbourg retracted its skis immediately after liftoff. He then flew east of San Diego and turned back to perform a westerly flyby over the bay. The Sea Dart had reached about 500 knots over San Diego city hall when Richbourg fired the after- burners. The aircraft suddenly disintegrated, enveloped by a huge fireball, and plunged inverted into the bay near Convair rescue boats. Richbourg was killed by the impact and his body immediately recovered by frogmen. As a result of the disaster, all Sea Dart operations were temporarily suspended until a Navy accident board completed its investigation. In December 1954, the board concluded that the accident had been caused by pilot-induced longitudinal pitch oscillations and not any unique design deficiencies in the Sea Dart itself.
Even before the public fiasco at the YF2Y-1 flight demonstration, the Navy had been gradually losing interest in the Sea Dart project. Even with more powerful engines, the aircraft could not achieve supersonic speeds. Continuing problems with saltwater intrusion plagued the jet engines, and excessively vibrating water skis could not be corrected. As a consequence, the Navy canceled 10 of the 16 production aircraft in December 1953. All remaining six production F2Y-1s were canceled in March 1954. The fatal crash by Richbourg later that year, with the attendant bad publicity, put the quietus on further development, and the Sea Dart program was relegated to test status only. Operational testing of all Sea Darts ended in 1957.
Flying Boats: The Tradewinds and the SeaMaster
The SSF featured the innovative Martin P6M SeaMaster flying boat as its principal strike weapon, one designed to operate from forward, mobile bases at sea, free of costly airfields or aircraft carriers. Two additional aircraft types were required to support the SeaMaster in the new weapons system. They included protective defensive fighters, as epitomized by the Convair XFY-1 Pogo and F2Y-1 Sea Dart. In addition, large, fast seaplanes acting as transport, supply, and refueling aircraft would be vital to support the SeaMaster strike element and mobile base components. The Convair R3Y Tradewind was developed to meet this demanding requirement.
R3Y Tradewinds were a derivative of the postwar XP5Y patrol flying boat, two of which were built by Convair in San Diego for the Navy. The XP5Ys featured a high-aspect-ratio wing and four complex turboprop engines driving six-bladed contra-rotating propellers. Delivered in 1950, these predecessor aircraft included a laminar flow wing mounted high on a sleek fuselage with a single-step hull. One of the two experimental test models crashed at sea in July 1953 from presumed engine failure. Shortly after this incident, the Navy terminated XP5Y tasking for maritime patrol and switched its mission to cargo and troop transport for the SSF.
The first of five sleek R3Y-1 Tradewinds, successors to the XP5Y flying boat, made its initial flight in February 1954. All armament and tail-plane dihedrals were deleted from the predecessor design. The new cargo and transport version had a cargo hatch 10 feet wide on the port side of the hull aft of the wing, and its engine nacelles were reconfigured for new Allison T-40-A-10 turboprop motors. These complex engines, driving two contra-rotating propellers through a gearbox, proved to be an Achilles heel for the R3Ys. The Tradewinds had a conventional two-step flying boat hull, without bulkheads above the cargo deck, thus opening up a vast interior storage space that could be configured in various ways. The R3Y-1 could seat 80 combat-equipped troops in rear-facing seats, carry 72 litter patients plus 12 attendants, or haul 24 tons of cargo—all in air-conditioned, pressurized comfort. In February 1955, one of the five R3Y-1s set a seaplane record that still stands; it flew from the West to the East Coast at an average speed of 403 miles per hour.
Over the next two years, six improved R3Y-2 aircraft were delivered to the Navy; they featured a clamshell cargo door on the front of the fuselage. This earned them the appellation of “Flying LSTs” because they included the same high-speed roll-on, roll-off cargo-handling capability employed by the Navy’s Landing Ship Tank. A serious operational problem arose with the clamshell front door version of the R3Y-2 aircraft. Pilots reported that it was almost impossible to hold the aircraft steady with only engine power while it was loaded and unloaded. This was a crucial shortcoming, as failure to hold steady might cause the aircraft to broach catastrophically in the surf. Three of the R3Y-1s and one R3Y-2 were later modified to become aerial tankers, essential for the fighter aircraft incorporated in the SSF. The converted R3Y-2 achieved fame in August 1956 by refueling four F9F Cougar fighter jets simultaneously, the first time such a feat had been accomplished.
In March 1956, all the R3Y-ls and R3Y-2s were placed under operational control of Navy transport squadron VR-2 at Naval Air Station Alameda, California. Apparently insoluble problems with the Allison turboprop engines continued. In-flight separations of the gearbox and propeller afflicted two different R3Y aircraft during test flights in May 1957 and January 1958. Financial constraints and repeated failures of the Allison turboprop engines resulted in the aircraft’s termination after only 11 had been delivered to the Navy. Transport squadron VR-2 was disbanded in April 1958. All remaining P5Y and R3Y aircraft were grounded later that year.
By the late 1950s, only the centerpiece of the SSF, the Martin P6M SeaMaster, remained under development. It too was experiencing severe problems with test-flight accidents, cost overruns, and seemingly interminable delays. The key to the SSF would be its own nuclearbomb-carrying strike aircraft, a jet-powered, fast, technologically advanced seaplane. Accordingly, it issued specifications for such an aircraft in April 1951. Design requirements for the new flying boat were stringent. To achieve them, a seaplane would require a performance equal to that of a land-based jet. The aircraft would need a bomb capacity of 14 tons, be able to attack targets 1,500 miles from its mobile base, and achieve speeds of 650 miles per hour during low-level attacks. The Navy selected Martin to build two prototype aircraft to these rigid specifications in October 1952, to be identified as XP6M-1s.
The two XP6M-1 prototypes were fitted with four Allison J71-A-4 turbojet engines mounted in pairs within four nacelles above the wing near its roots. Known as SeaMasters, the two aircraft had anhedral drooped wings, featuring 40 degrees of sweepback that ended in wingtip fuel tanks that also served as floats. The wingtip floats contained equipment that helped dock the aircraft. The SeaMaster had a pressurized cabin and a crew of four: pilot, copilot, navigator, and flight engineer. Its sole defensive armament was a pair of 20mm cannons mounted in a remote-controlled tail turret.
Testing the SeaMaster
During flight testing in 1955, the initial prototype SeaMaster quickly revealed one obvious weakness. Its jet engines had been oriented parallel to the hull so that exhaust gases exited over the rear fuselage, thus scorching it in that area and limiting use of afterburners. Corrective action was taken on later P6M-1 and P6M-2 models, which mounted their four turbine nacelles in a toed-in manner so that jet exhausts were directed outboard of the rear fuselage. Other problems encountered by the first experimental XP6M-1 were unexplained vibrations throughout the hull, plus rear turret and rotary bomb rack malfunctions.
By late 1955, most problems with the XP6M-1 were determined to be curable, and the Navy assigned an evaluation team from its nearby Naval Air Test Center in Maryland to work with Martin during further development. In December 1955, a mixed crew of Martin and Navy personnel took one XP6M-1 up for a routine test flight. While descending at full power from 8,700 feet, the test aircraft suddenly exploded and disintegrated in the air, killing all four occupants.
The Navy immediately instituted an exhaustive accident investigation into the loss of the XP6M-1, concluding that the plane had experienced longitudinal divergence that tore the engines loose and caused the wings to fold entirely under the airplane before they broke away. The investigation could not ascertain the cause of the divergence but suggested that it might have been the result of a failure in the activator for the horizontal stabilizer. The Navy’s continued confidence in the SeaMaster program drove further development of the aircraft, and there was no cancellation of the contracted six YP6M-1 service evaluation planes. With surprisingly little delay, the remaining XP6M-1 resumed testing in May 1956. It was modified to include new flight instrumentation, plus ejection seats for all four crew members. During a flight test in November 1956, the aircraft again broke up in the air, although this time all crew members ejected safely. An investigation traced the cause to an error in the design calculation for the tail control system.
Throughout 1958, the YP6M-1s tested their mine-laying, bombing, navigation, and reconnaissance systems. The Navy proceeded with 24 production versions of the P6M-2s, the first of which was delivered by Martin early in 1959. These aircraft were powered by more powerful non-afterburning Pratt & Whitney J75-P-2 turbojet engines that permitted a substantial increase in gross weight for the aircraft. Since this meant the SeaMasters sat lower in the water, their wing anhedral was eliminated. The P6M-2s were also fitted with improved navigation and bombing systems, plus midair refueling probes. In this production version, the SeaMaster was an impressive weapon. It achieved the specified 650 miles per hour for on-the-deck attacks. But the aircraft also evidenced some unpleasant flight characteristics, such as rapid changes in directional trim, severe buffeting, and wing drop requiring high control inputs to counter. These defects were traced to larger engine nacelles required by the J75 engines. Other problems also became evident as testing continued, such as tip floats digging into the water during choppy seas and engine surges.