The Martin SeaMaster & Convair Sea Dart

v1.0.8 / 01 jun 17 / greg goebel

* 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 history and description of the SeaMaster and the Sea Dart.

Martin XP6M-1 SeaMaster



* In the post-World War II period, the US Air Force built up the Strategic Air Command (SAC), a nuclear strike force of long-range bombers. US Navy brass 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 a Navy own nuclear strike capability to keep 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 mine-laying. 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 soon led 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 and head of the Martin advanced design department; J.D. Pierson, a hydrodynamicist; and J.L. Decker, an 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 that engine suffered development problems, 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; the J35 was used on the Republic F-84 Thunderjet, being 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 wing had a strong anhedral droop to ensure the floats made contact with the water. 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 -- in fact, early Martin concepts that led up to the SeaMaster had envisioned a seaplane derivative of the XB-51. 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 and fire-control radar.

* 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. 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 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 landing ship docks, two seaplane tenders, and the submarine USS GUAVINA were sent to shipyards to refit 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.

Martin YP6M-1 SeaMaster

* 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, featuring large overhead panels for a better field of view; 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, allowing the SeaMaster 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.

Martin XP6M-1, P6M-2

* 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 canceled 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.

Incidentally, Martin performed design studies early on for a nuclear-powered derivative of the SeaMaster, nuclear-powered aircraft being something of a fad in the atom-crazy 1950s. The final conclusion that nuclear power demanded a substantially larger aircraft to handle the reactor, with further investigation showing that the entire concept of nuclear-powered aircraft was impractical.



* 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.

Convair Sea Dart

After the end of World War II, there was substantial interest by the US military and aviation industry for a jet-powered seaplane fighter, since it would be able to operate from forward areas without need of airfields. Following up studies performed from 1946, in response to a 1948 US Navy requirement, Convair came up with a subsonic seaplane fighter design known as the "Skate" -- something along the lines of a swept-wing seaplane version of the Douglas Skyknight night / all-weather fighter, with a side-by-side canopy for pilot and radar operator, twin Westinghouse XJ40 turbojet powerplants with intakes just below and behind the canopy, and four 20-millimeter cannon in the wing roots.

During the same timeframe, the US Air Force was, somewhat surprisingly, also interested in a seaplane fighter, with the Edo Corporation -- a manufacturer of floats for floatplanes -- coming up with a design designated the "EDO 150". It was a single-seater with swept wings, a vee tail, an extendable "hydro-ski" on the belly, wingtip floats, twin Allison J35 turbojet engines fed through an intake on the back, and armament of four 20-millimeter cannon in the wing roots. The hydro-ski allowed a flying boat to dispense with the boat hull, improving aerodynamics and reducing weight.

Convair Skate, Edo 150

The Edo 150 never advanced beyond the model stage, though a Grumman Goose seaplane was used to trial the hydro-ski concept, which turned out to work very well. The Convair Skate never happened either, since in 1950 the Navy revised the seaplane fighter requirement to specify a supersonic machine. The Navy wasn't certain at the time that a supersonic aircraft could be operated from a carrier of any reasonable size and felt a seaplane fighter might be preferable.

Convair came up with a proposal, which 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 take-off and landing. Incidentally, the Skate program was not discontinued immediately, with Convair continuing studies on variations for a time, building a 1/7th-scale model for hydrodynamic tests.

The Sea Dart had a vee-shaped hull, the aircraft's internal spaces being 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 somewhat antique-looking windscreen with twin oval glass panels in a metal frame. Apparently the pilot field of view 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 fitted with an Aero 13E fire-control system with AN/APS-50 radar, and 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. Navy brass were 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.

* Since the first Sea Dart prototype was finished before the XJ46 engines were ready, the aircraft was fitted with twin Westinghouse J34-WE-32 engines providing 15.11 kN (1,540 kgp / 3,400 lbf) maximum take-off 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 take-off and landing, though a redesign effort helped reduce this problem. It would also turn out, also to no great surprise in hindsight, that the salty sea air was hard on engines.

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, with the lack of engine power and poor fuel economy high on the list of pilot complaints against the "Gutless", as it was known.

At this point, the Navy began to rethink the Sea Dart program. The second prototype was canceled, 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.

Convair F2Y-1 Sea Dart

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 break Mach 1. However, since 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 in full view of a press tour. The accident essentially killed the program as well. The Navy was no longer particularly fearful of operating supersonic aircraft off carriers, and despite improvements in the hydro-ski design, the Sea Dart still suffered from serious vibration on take-off and landing.

The Navy had begun cutting back the program in December 1953, before the delivery of the YF2Y-1, canceling 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 take-off 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; after some initial problems, the single-ski scheme proved remarkably successful, allowing safe take-offs 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 surviving aircraft are now in museums. Incidentally, Convair conducted a number of design studies for bomber-sized supersonic flying boats, with configurations ranging from machines that looked somewhat like scaled-up Sea Darts to machines that looked like improved SeaMasters; nuclear power was envisioned for some of the designs. Of course, none of them ever came close to happening.



* The Sea Dart was not the only, and not 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 blessed the concept by issuing a specification designated "E.6/44" to fit the proposal.

The reality was that 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 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 Armstrong-Siddeley Sapphire turbojet, with the exhausts in the upper fuselage behind the wing.

Saunders-Roe SR.A/1

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 17.1 kN (1,745 kgp / 3,850 lbf) each. The aircraft was fitted with straight wings and tail, featuring 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.

SaRo P.121

Saunders-Roe also became interested in the hydro-ski scheme, releasing a proposal for an improved seaplane fighter, the "P.121", in late 1950. The P.121 was a swept-wing fighter, with a Sapphire engine fed by a back-mounted intake, four 20 millimeter cannon, and a single hydro-ski extending from under the front hull. However, the project failed to acquire any momentum, and was canceled in early 1952.



* As concerns copyrights and permissions for this document, all illustrations and images credited to me are public domain. I reserve all rights to my writings. However, if anyone does want to make use of my writings, just contact me, and we can chat about it. I'm lenient in giving permissions, usually on the basis of being properly credited.

Convair Sea Dart

* 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 
   v1.0.1 / 01 nov 03 / Review & polish.
   v1.0.2 / 01 nov 05 / Added P.121.
   v1.0.3 / 01 nov 07 / Review & polish.
   v1.0.4 / 01 oct 09 / Added Skate & EDO 150.
   v1.0.5 / 01 sep 11 / Added Convair bombers.
   v1.0.6 / 01 aug 13 / Review & polish.
   v1.0.7 / 01 jul 15 / Review & polish.
   v1.0.7 / 01 jun 17 / Review & polish.