The Martin SeaMaster & Convair Sea Dart

v1.0.1 / 01 nov 03 / greg goebel / public domain

* The 1950s was an era of aggressive innovation in aircraft design, not all of which was successful. Two of the more interesting exercises were a pair of advanced seaplanes, the Martin "SeaMaster" flying boat and the Convair "Sea Dart" hydro-ski fighter. This document provides a short history of these two aircraft.



* In the post-World War II period, the US Air Force built up the "Strategic Air Command", a nuclear strike force of long-range bombers. The US Navy realized that the strategic nuclear mission was now of overwhelming importance, all the more so because defense budgets were being cut, and wanted to build up their own nuclear strike capability to prevent them from being overshadowed by the Air Force / SAC.

Proposals to build a "super carrier", the USS UNITED STATES, as a floating base for Navy strategic bombers were shot down in 1949, and so the Naval Bureau of Aeronautics came up with another scheme, the "Seaplane Striking Force (SSF)". The SSF envisioned a fleet of big, jet-powered seaplanes that would not only be capable of long-range nuclear strike, but would also be useful for conventional bombing, reconnaissance, and mining. Laying mines was seen as particularly important, since to reach the open seas the Soviet Navy had to pass through a number of "bottlenecks" that could be blocked by mining. The seaplanes would be able to operate from advanced areas, supported by a seaplane tender or even a submarine.

The Navy issued a request to industry in April 1951. The SSF seaplane was to carry 13,600 kilograms (30,000 pounds) of warload to a target over 2,400 kilometers (1,500 miles) from the seaplane's aquatic "base". The aircraft was to be capable of a Mach 0.9 dash at low altitude.

Convair and Martin submitted proposals, with Martin winning the competition. On 31 October 1952, the Navy awarded Martin a contract for two prototypes, with the company designation of "Model 275" and the Navy designation of "XP6M-1", plus a static test article. This initial order would presently lead to further contracts for six pre-production service evaluation machines, with the designation of "YP6M-1", and up to 24 full-production machines, with the designation of "P6M-2".

Martin gave the aircraft the name "SeaMaster". Apparently the company had run out of names starting with "Mar".

The Martin design team was led by George Trimble, an aeronautical engineer who as head of the Martin advanced design department; J.D. Pierson, a hydrodynamicist; and J.L. Decker, a aerodynamicist. Using the P5M Marlin flying boat as a starting point, they developed a revised hull design, with a length-to-beam ratio of 15:1, which was felt to offer the best efficiency in both air and water. The XP5M-1 Marlin flying boat prototype was rebuilt to test the new hull design, with this test aircraft designated the "Martin Model 270".

The original powerplant was supposed to have been a Curtis-Wright turbo-ramjet engine, but the engine development program ran into trouble, and so the decision was made to fit the SeaMaster with four Allison J71-A-4 turbojet engines with 57.87 kN (5,900 kgp / 13,000 lbf) afterburning thrust each, mounted in pairs in nacelles above the wing near the wing roots. The J71 was a derivative of the J35 axial-flow turbojet, used on the Republic F-84 Thunderjet, and originally developed by General Electric as the TG-180 but passed on to Allison for full production.

The wings featured a sweepback of 40 degrees and ended in wingtip tanks that served as floats. The wingtip floats were also fitted with gear to help dock the aircraft. The SeaMaster was to have a pressurized cockpit and crew of four, including pilot, copilot, navigator / radio operator, and flight engineer.

The SeaMaster leveraged off Martin's advanced XB-51 attack bomber design, with features such as an "all flying" tee tail and a rotating bombbay. The bombbay flipped over in flight to expose munitions or camera payloads, and was pneumatically sealed to keep it watertight. The sole defensive armament was a remote-controlled tail turret with twin 20 millimeter cannon.

* The first SeaMaster prototype was rolled out in secret on 21 December 1954, and performed its first flight on 14 July 1955, with Martin test pilot George Rodney at the controls. The flight test program revealed only one serious flaw: the engines scorched the rear fuselage, and so the use of afterburner had to be limited.

The Navy publicly announced the SeaMaster in November 1955, inviting the press to witness the rollout of the second XP6M-1 prototype. Unlike the first prototype, the second prototype was fitted with operational navigation and bombing gear.

The test program continued smoothly until 7 December 1955, two days after the death of Glenn L. Martin. During a routine check flight for the first Navy pilot, the initial SeaMaster prototype crashed into Chesapeake Bay, killing all four aircrew on board.

The post-mortem revealed a control-system fault that caused the aircraft to pitch nose down, bending its wings down and ripping them off. The second SeaMaster prototype was refitted with new flight instrumentation and ejection seats. Test flights finally resumed in May 1956. Unfortunately, the second prototype went out of control on 9 November 1956 during a flight test of a modified tail configuration. The aircraft broke up, but the crew were able to eject safely. The problem was traced down to an an error in the design calculations for the tail control system.

* Despite the loss of both prototypes, the Navy still remained enthusiastic about the SeaMaster. A beaching cradle was designed to allow SeaMasters to taxi in and out of the water, and two LSDs (landing ship docks), two seaplane tenders, and the submarine USS GUAVINA were sent to shipyards to fit them as SeaMaster support vessels. A home base was set up at Naval Air Station Harvey Point, near Elizabeth City, North Carolina.

The first pre-production YP6M-1 was rolled out in November 1957, with flight tests resuming in January 1958. It featured afterburning Allison J71-A-6 engines, which were visibly "toed out" to reduce the effect of exhaust blast on the rear fuselage. The engine inlets were also moved back from the leading edge of the wing, presumably to reduce water ingestion. Five more YP6M-1s were built in 1958 and participated in an extensive flight test program, performing practice drops of conventional and (dummy) nuclear munitions, and evaluating day and night photoreconnaissance pallets.

* The first production P6M-2 was rolled out in early 1959. The production SeaMaster featured more powerful non-afterburning Pratt & Whitney J75-P-2 turbojet engines with 77.89 kN (7,940 kgp / 17,500 lbf) max thrust each, providing a total increase of 53.36 kN (5,440 kgp / 12,000 lbf) thrust, and permitting a substantial increase in gross weight. The engine installation was visibly different: the engine exhausts in the XP6M-1 and YP6M-2 had been staggered, but they were parallel in the P6M-2.

   _____________________   _________________   _______________________
   spec                    metric              english
   _____________________   _________________   _______________________

   wingspan                31.37 meters        102 feet 11 inches
   wing area               176 sq_meters       1,900 sq_feet
   length                  40.84 meters        134 feet
   height                  9.88 meters         32 feet 5 inches
   empty weight            41,400 kilograms    91,285 pounds
   max loaded weight       80,000 kilograms    176,400 pounds

   maximum speed           1,010 KPH           630 MPH / 550 KT
   service ceiling         12,200 meters       40,000 feet
   range                   3,200 kilometers    2,000 MI / 1,740 NMI
   _____________________   _________________   _______________________

The increased gross weight meant the production SeaMasters sat lower in the water, and so the wing anhedral was eliminated. The P6M-2 was fitted with a new canopy with large overhead panels for improved visibility; solid-state Sperry navigation and bombing systems; and a mid-air refueling probe. A probe-and-drogue tanker kit was also developed that could be plugged into the SeaMaster's bombbay, allowing it to be quickly converted into a tanker.

The SeaMaster was a futuristic aircraft, and its performance demonstrated that it wasn't just a pretty toy. The wings were built very strong for low altitude operation, with aluminum 2.5 centimeters (an inch) thick at the wing roots, and the SeaMaster was able to attain the Mach 0.9 requirement for "on the deck" flight. In contrast, the Boeing B-52 was only capable of Mach 0.55 at low altitude.

* Three production P6M-2s had been completed by the summer of 1959, with all-Navy crews moving them through operational conversion for service introduction in early 1960. Five more were in construction. However, the Navy had been steadily cutting back the number of production aircraft, from 24, to 18, and then to 8, and then on 21 August 1959 cancelled the SeaMaster program completely.

There were loud protests, since the program had cost about $400 million USD and the machine was certainly whizzy, but in truth the SeaMaster was an obsolete concept. The Navy was already moving full steam ahead to a much more effective nuclear deterrent capability in the form of the Polaris ballistic missile submarine.

Martin tried to promote other seaplane designs, such as an eight-engine airliner version of the SeaMaster that was informally called the "SeaMistress", but the writing was on the wall. Martin formally abandoned the aircraft business to focus on missiles and defense electronics. The SeaMasters that had been built sat idle for over a year and were then scrapped, and sadly only bits and pieces of them survive.



* The Martin SeaMaster remains something of an icon of Disney World of Tomorrow "gee-whiz" 1950s technology. It was complemented by another unusual aircraft of the era, the Convair XF2Y "Sea Dart", a delta-winged jet fighter that could take off and land on water.

The Sea Dart grew out of a 1948 request for proposals by the US Navy for a supersonic interceptor seaplane. Although operating from the oceans would allow such an aircraft to operate from forward areas, there was another reason for wanting to build such an aircraft: the Navy wasn't certain that a supersonic aircraft could be operated from a carrier of any reasonable size.

Convair's proposal won the competition on 19 January 1951. The contract specified two prototypes of a single-seat delta-wing fighter, to be designated the "XF2Y-1 Sea Dart", that took off and landed on water using two retractable "hydro-skis". The engines were mounted on the back of the aircraft, with the intakes well up above the wings to prevent water ingestion during takeoff and landing.

The Sea Dart had a vee-shaped hull, and its internal spaces were organized as multiple watertight compartments to keep it afloat if battle damaged. It had twin dive brakes on the lower rear fuselage that could be also be used as water brakes or rudders. Flight controls were hydraulic. The Sea Dart could not take off or land on a runway, but each of the hydro-skis had a small wheel at the end, and a third small wheel was mounted near the rear of the aircraft to allow it taxi onto or off of a seaplane ramp. The cockpit canopy pivoted up as a single unit, and featured a rather antique-looking windscreen with twin oval glass panels in a metal frame. Apparently pilot visibility was not very good.

The Sea Dart was originally planned to be powered by twin Westinghouse XJ46-WE-02 engines with 26.68 kN (2,720 kgp / 6,000 lbf) afterburning thrust each. The XJ46 engine, an afterburning derivative of the Westinghouse J34 axial-flow turbojet, was expected to give the aircraft a top speed well in excess of Mach 1. The aircraft was to be armed with four 20 millimeter cannon and a pack of 70 millimeter (2.75 inch) folding-fin air rockets (FFARs), though in fact no Sea Dart would ever be armed. The Navy was so enthusiastic about the Sea Dart that even before it flew, the service ordered a total of four "YF2Y-1" service evaluation aircraft and 16 "F2Y-1" production aircraft.

* As the first Sea Dart prototype was finished before the XJ46 engines were available, it was fitted with twin Westinghouse J34-WE-32 engines with 15.11 kN (1,540 kgp / 3,400 lbf) maximum takeoff thrust each. Taxi trials began in San Diego Bay in mid-December 1953, with test pilot Sam Shannon at the controls, leading to first official flight on 9 April 1953.

The Sea Dart was, to nobody's surprise, badly underpowered with its J34 engines and remained solidly subsonic. The hydro-skis turned out to give an extremely rough ride on takeoff and landing, though a redesign effort helped reduce this problem.

The XJ46 engines were installed in the prototype later that year, but they failed to meet their designed thrust levels. The detestable Vought F7U Cutlass carrier would use production J46 engines, and the lack of engine power and poor fuel economy would be high on the list of pilot complaints against the "Gutless", as it was known.

At this point, the Navy began to rethink the program. The second prototype was cancelled, with development moving on to the first service evaluation YF2Y-1, fitted with J46 engines, although the Navy was seriously looking for a better powerplant. The YF2Y-1 was similar in appearance to the XF2Y-1 but had a longer, redesigned exhaust, and the little beaching wheels were removed from the hydro-skis and the fuselage, meaning it had to be fitted with external beaching gear to be brought up on shore.

The YF2Y-1 began test flights in 1954, and on 3 August 1954, Convair test pilot Charles E. Richbourg took the machine through Mach 1 in a shallow dive. The Sea Dart is believed to be the only seaplane to ever achieve Mach 1. However, as it had been designed before the new "area ruling" scheme was introduced, its supersonic handling characteristics were poor.

The YF2Y-1 was lost in a crash during a low-level demonstration on 4 November 1954, killing Richbourg. This accident essentially killed the program as well. The Navy was no longer particularly frightened of operating supersonic aircraft off of carriers, and despite improvements in the hydro-ski design, the Sea Dart still suffered from serious vibration on takeoff and landing.

The Navy had begun cutting back the program in December 1953, before the delivery of the YF2Y-1, cancelling ten of the production aircraft. The other six were killed off in March 1954, well before the fatal accident. Following the accident, the program was further scaled back to a test exercise, and plans to produce an "F2Y-2" with area ruling and a single Pratt & Whitney J75 turbojet with 66.71 kN (6,800 kgp / 15,000 lbf) thrust were abandoned.

The XF2Y-1 was then refitted with twin J46 turbojets and a single-ski configuration in hopes that would solve the takeoff and landing problems. The fit was strictly experimental. The ski was not fully retractable and the wells for the old twin skis were not faired over. The new single ski had a pair of retractable beaching wheels at the end, allowing the aircraft to beach itself.

The modified XF2Y-1 first flew in late December 1954, and after some initial problems the single-ski scheme proved remarkably successful, allowing safe takeoffs and landings even in fairly rough seas.

The second YF2Y-1 performed its first flight in March 1955. It was powered by twin J46 turbojets and had a modified twin-ski system, with pivoting beaching wheels at the end of each ski. The twin-ski system didn't work much better than before, and the aircraft was put into storage at the end of April 1955, never to fly again.

   _____________________   _________________   _______________________
   spec                    metric              english
   _____________________   _________________   _______________________

   wingspan                10.26 meters        33 feet 8 inches
   wing area               52.30 sq_meters     563 sq_feet
   length                  16 meters           52 feet 7 inches
   height, skis extended   6.33 meters         20 feet 9 inches

   empty weight            5,725 kilograms     12,625 pounds
   loaded weight           9,750 kilograms     21,500 pounds

   max speed at altitude   1,325 KPH           825 MPH / 720 KT
   service ceiling         16,700 meters       54,800 feet
   range                   820 kilometers      510 MI / 445 NMI
   _____________________   _________________   _______________________

   Performance values are estimates.
The original XF2Y-1 was used for further tests of various ski systems until the fall of 1957, when it was finally withdrawn. Two more YF2Y-1s were built but never flown, and all four aircraft are now in museums. One YF2Y-1 serves as a "gate guard" on a pylon in front of the the San Diego Aerospace Museum at Balboa Park, and certainly provides a distinctive and unusual display.



* The Sea Dart was not the only, and not actually even the first, seaplane jet fighter. The British Saunders-Roe ("Saro") firm had actually built three prototypes of a seaplane jet fighter, the "SR.A/1", in the late 1940s.

The idea for a British seaplane jet fighter arose in 1943, when the Allies were considering how to take the offensive against the Japanese in the Pacific. Saunders-Roe officials believed that a seaplane fighter would be useful in this environment, since it would be able to use any atoll or other protected anchorage as a ready-made airfield. Using the new jet engines for such an aircraft had particular advantages, since the lack of a propeller eliminated the need for long, draggy floats and allowed the fighter to be designed in a more streamlined configuration.

Saunders-Roe submitted a proposal for an aircraft along these lines designated "SR.44" to the British Air Ministry, and in May 1944 the Air Ministry gave their blessing by issuing a specification designated "E.6/44" to fit the proposal.

However, in reality the Americans were able to build enough carriers to support intensive forward air operations in the Pacific, and in the absence of any real need the SR.44 stayed at low priority. Engine development problems contributed to delays, and the first SR.A/1 prototype didn't fly until 16 July 1947. A second prototype took to the air on 30 April 1948, followed by the third, and last, on 17 August 1948.

The SR.A/1 was arguably one of the uglier aircraft ever built, with a classic flying boat hull and jet intakes in the nose, giving it a tubby and "pig-nosed" appearance. It was powered by twin Metropolitan Vickers F2/4 Beryl axial-flow engines, an ancestor of the better-known Rolls-Royce Sapphire turbojet, with the exhausts in the upper fuselage behind the wing.

The first two prototypes were fitted with development versions of the Beryl, but the third prototype was fitted with fully rated engines, with a thrust of kN (1,745 kgp / 3,850 lbf) each. The aircraft was fitted with straight wings and tail, with wing-mounted stabilizer floats pivoting inward into underwing recesses, and was armed with four Hispano 20 millimeter cannon in the nose above the air intake.

   _____________________   _________________   _______________________

   spec                    metric              english
   _____________________   _________________   _______________________

   wingspan                14.02 meters        46 feet
   wing area               38.60 sq_meters     415 sq_feet
   length                  15.24 meters        50 feet
   height                  5.11 meters         16 feet 9 inches

   empty weight            5,108 kilograms     11,262 pounds
   loaded weight           8,635 kilograms     19,035 pounds

   max speed at altitude   824 KPH             512 MPH / 445 KT
   endurance               2.4 hours
   _____________________   _________________   _______________________

Despite the SR.A/1's uninspiring appearance, its performance was surprisingly good, but in the postwar period there was no serious interest in the type. The last of the three prototypes was retired in June 1951.

* The SeaMaster was also not the only large jet flying boat to be built. The Soviet Beriev OKB (experimental design bureau) designed a number of large jet-powered flying boats, but it seems more appropriate to discuss them in a separate document that surveys all of the Beriev OKB's aircraft.



* I have seen the Sea Dart in front of the San Diego Aerospace Museum. Unfortunately, I didn't have a camera with me at the time. Hopefully I'll get back there and get some digital imagery of it one of these years.

* Sources include:

The material on the Sea Dart owes a good deal to a very detailed document written by aviation enthusiast Joe Baugher. The material on the SeaMaster was originally derived from notes published by the Glenn L. Martin Aviation Museum.

* Revision history:

   v1.0.0 / 01 nov 03 / gvg
   v1.0.1 / 01 nov 03 / gvg / Minor cosmetic update with some fixes.