San Diego's long-range maritime patrol jet seaplane designs for the Navy

The invention of the jet engine undoubtedly had a profound impact on the performance of a brand new generation of American bomber aircraft coming of age after World War II by offering increased speed at higher altitudes occupied by the piston-powered B-29, B-36, and B-50. Although the exclusive focus of the US Navy's efforts to make the gradual transition to jet aircraft in 1945 involved deploying its first jet fighters, the Martin P4M Mercator land-based patrol aircraft flown in 1946 had a unusual combo of both piston and jet engines like the Ryan FR Fireball fighter, with two auxiliary turbojet engines at the rear of the piston engine nacelles, and archival documents demonstrate that the aerospace industry in San Diego explored the idea of a jet-powered seaplane even before the end of World War II. This blog post will give an overview of Convair's jet-powered patrol flying boat designs for the US Navy that never were, including an early jet patrol flying boat design by Convair and a giant seaplane running on nuclear power.

An artist's rendering of the Convair High Speed Flying Boat project from May 1945

Convair began investigating the benefits of jet propulsion for a patrol aircraft in December 1944 when it analyzed three different design approaches to a maritime patrol seaplane design powered by two Pratt & Whitney R-4360 Wasp Major radial piston engines and two auxiliary General Electric J35 (TG-180) turbojets. One design involved a patrol flying boat similar in hull design to the XP5Y and R3Y which had two R-4360s and two J35s situated at the rear of the engine nacelles housing the Wasp Majors, and which had a gross weight of 85,000 lb (38,555 kg). A second design approach featured a twin-float seaplane which had the same gross weight as the flying boat design but had a slender fuselage and the J35s below the engine nacelles for the R-4360s, and two iterations of this design configuration were proposed, only differing in size and wing loading. The third design approach involved an unorthodox single-float seaplane whose tail empennage was supported by a small fuselage boom and which had the crew and all military equipment carried in the float. By May 1945, Convair moved beyond its design studies for a compound propulsion patrol seaplane and undertook design of a pure-jet maritime patrol flying boat, officially known as the High Speed Flying Boat. This proposal was 104 feet (31.7 meters) long with a wingspan of 113 feet (34.44 meters), a gross weight of 90,000 lb (40,823 kg), and floats at the wingtips. Power was supplied by six 3,000 lb (1,360 kg) thrust Westinghouse J34 turbojets buried in the wing roots, and fuselage design of the High Speed Flying Boat was influenced by the company's XB-46 strategic bomber, which had only been envisaged a few months earlier. The High Speed Flying Boat would have had tremendous performance advantages over the PBY, PB2Y, and PBM, but the US Navy showed little interest in approving the design for full-scale development.  

Left: Company artwork of the Convair Model 52 HSML jet flying boat (August 1952 design)
Right: Three-view company drawing of the ultimate Convair Model 52 HSML design conceived in August 1952   

Convair took a fresh new shot at jet flying boat design in late 1951 when it envisaged a high-speed jet-powered flying boat under the company designation Model 52 after the US Navy issued the OS-125 specification for a High Speed Minelayer (HSML) seaplane capable of mining enemy harbors at low altitudes at speeds of over 600 miles per hour. The initial Model 52 design was a flying boat with wings sweptback 35 degrees, a length of 129 feet 7 in (39.5 meters), a wingspan of 93 feet 6 in (28.5 meters), a gross weight of 188,500 lb (85,502 kg), and four Wright J67 turbojets (American copy of the Bristol Olympus turbojet) buried in the wing roots, and it carried a crew of five. Armament consisted of 32,000 lb (14,515 kg) of mines carried in two bomb bays and two 20 mm machine guns in a remote-controlled tail turret. Convair eventually submitted a revised Model 52 design in August 1952 with a wingspan of 105 feet 6 inches (32.16 meters), a length of 127 feet (38.7 meters), a gross weight of 173,750 pounds (78,812 kg), and the twin bomb bays replaced by a single central bomb bay accommodating 20,000 lb (9,072 kg) of mines. In the end, the Navy declared the competing Martin Model 275 design the winner of the HSML contest in late 1952, assigning it the designation P6M and official name Seamaster. The P6M Seamaster flew on July 14, 1955 and Martin built a total of 16 P6Ms before the deployment of the Polaris SLBM and 1st-generation American ballistic missile submarines like the George Washington- and Ethan Allen-class submarines led the Navy to cancel the P6M program on August 21, 1959, six months before the operational deployment of the Seamaster was to have taken place.

Selected designs of the Convair Model 23 nuclear-powered seaplane: Model 23A delta-wing version (left) and Model 23B design with rear-mounted nuclear turbojets and backswept wings (right)

Selected designs of the Convair Model 23 nuclear-powered seaplane: Model 23A delta-wing version (left) and Model 23B design with rear-mounted nuclear turbojets and backswept wings (right)

Convair's design work on jet flying boats was not confined to conventionally-powered aircraft. In 1951, the Aircraft Nuclear Propulsion (ANP) program was established, and in May 1953 Convair received a study contract from the US Navy in May 1953 to investigate the possibility of nuclear power for large flying boats given that nuclear power itself offered the advantage of hunting down and tracking enemy submarines over the oceans and seas for an indefinite flight time. Convair's first nuclear-powered flying boat design, envisaged in early 1955, was a subsonic flying boat with a T-tail design which measured 171 feet (52 meters) long with a wingspan of 131 feet 6 in (40.08 meters) with four Pratt & Whitney J75 turbojets fueled by a Pratt & Whitney nuclear reactor. One proposal devised in early 1956, dubbed the 6-Engine Nuclear Powered Attack Seaplane by Convair, was a backswept wing behemoth 274 feet 4 in (83.62 meters) long with a wingspan of 155 feet (47.24 meters) and a wing area of 5,700 ft² (529.54 m²). Power was provided by six Pratt & Whitney NJ-2B turbojets fueled by a nuclear reactor inside the fuselage, and the turbojets were clustered around the reactor aft of the shoulder-mounted swept wing. The 6-Engine Nuclear Powered Attack Seaplane had a crew of five carried in a heavily shielded cockpit near the nose, and it would have carried bombs or mines in a bomb bay and an air-to-surface missile designed by Convair's Forth Worth division mounted atop the forward fuselage. Convair began design studies in mid-1956 for a supersonic nuclear-powered seaplane under the Model 23 designation. The baseline Model 23A was a sleek, delta wing aircraft with a length of 153 feet 10 in (46.88 meters), a wingspan of 76 feet 3.6 in (23.26 meters), and a crew of three, with power provided by one nuclear-fueled General Electric turbojet and one booster rocket. The Model 23A-1 iteration was powered by non-nuclear Pratt & Whitney JT9 turbojets, whereas the Model 23A-3 had backswept wings and the engine inlets moved to the top of the fuselage, while dispensing with the booster rocket and hydroski. The Model 23B was a backswept wing design with a length of 204 feet 2 in (18.95 meters), a wingspan of 115 feet (35 meters), and a crew of five. It had four Pratt & Whitney NJ-2B nuclear turbojets housed in a nacelle above the rear fuselage and fueled by a nuclear reactor inside the rear fuselage, and it could carry bombs and mines in weapons bay situated ahead of the reactor. A Model 23B iteration conceived in June 1956, the Model 23B-1, replaced the nuclear turbojets with non-nuclear JT9s, and one variant of the Model 23B proposed in August, also called Model 23B-1, differed from the Model 23B sans suffixe in having two General Electric turbojets fueled in two nuclear reactors in the engine nacelles and the engine inlets extending beyond the wing's leading edge. The top speed of the Model 23A and 23B was Mach 2, and the Model 23C and 23D designs retained the backswept wings of the Model 23B but were designed for subsonic speeds and had the engine nacelles straddle the vertical stabilizer. The Model 23C had a wingspan of 100 feet (30.5 meters), while the Model 23D had slightly wider engine nacelles and spanned 120 feet (36.58 meters), slightly greater than the wingspan of the Model 23B design. None of Convair's designs for nuclear-powered patrol jet seaplanes went past the design phase because the US Navy, realizing that nuclear powerplants required bigger aircraft than the Convair Model 23 and rival Martin Model 331, gave up on all nuclear-powered seaplane development in December 1959. The ANP program continued a little over a year until it was terminated on March 26, 1961, after an expenditure of over $1 billion.

[EDIT: Thanks to a Convair document from 1957 containing study plan views of the Convair Model 23 designs and other aircraft used for size comparisons, I can now discern some dimensions of the Model 23C and 23D designs, namely the wingspan.]

References:

Bradley, R., 2010. Convair Advanced Designs: Secret Projects from San Diego 1923-1962. North Branch, MN: Specialty Press.

Buttler, T., 2010. American Secret Projects: Bombers, Attack, and Anti-Submarine Aircraft 1945-1974. Hersham, UK: Ian Allan Publishing.

Lowther, S., 2023. US Supersonic Bomber Projects, Volume 2. Horncastle, UK: Tempest Books.