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Republic XP-72 Ultrabolt Prototype High-Altitude Fighter / Interceptor

The Republic XP-72 Ultrabolt would have been an exceptional fighter but the Allied advance in both Europe and the Pacific derailed its promising development.

 Updated: 12/26/2016; Authored By Staff Writer; Content ¬©

The Republic XP-72 Wasp Major-powered "Super Thunderbolt" (or "Ultrabolt") was being developed in the latter part of World War 2 (1939-1945) as the ultimate incarnation of the war-winning P-47 Thunderbolt fighter/fighter-bomber (also produced by Republic). The XP-72 incorporated largely the same airframe with the massive, experimental Pratt & Whitney R-4360 "Wasp Major" series engine rated at 3,450 horsepower. As promising as the high-altitude fighter was becoming, the changing nature of the war ultimately found little need for such an aircraft. No more than two prototypes were produced.

As an expediency, the XP-72 made use of both the proven fuselage and wings of the P-47 before it. Some alterations to accommodate the large Pratt & Whitney were made to the airframe and this included a revised lower cowling to allow for proper aspiration of the engine. A large-diameter, four-bladed propeller unit was affixed to the powerplant. The aircraft was indeed considered the "Super Thunderbolt" when compared to the original P-47 offering.

Developed as soon as the P-47 was entering service, the Republic team focused on two preliminary designs that were to take the beneficial design qualities of the existing P-47 and improve upon it wherever possible. The United States Army Air Force (USAAF) accepted both initial designs, one being an XP-69 mated with a Wright R-2160 radial engine and the other the XP-72 with its Pratt & Whitney. As the designs progressed, the XP-69 design - complete with high-altitude features including a pressurized cockpit and armament of 2 x 37mm cannons and 4 x 12.7mm machine guns - was dropped on May 11th, 1943, in favor of the more promising XP-72. Two XP-72 prototypes were then ordered on June 18th, 1943 with the air war in full swing over both the Pacific and Europe.

The first XP-72 prototype flew on February 2nd, 1944 (with a basic four-blade propeller) and development was very much unlike other aircraft of the period in that the program actually exhibited few delays as it progressed. The first prototype flew through the air with ease - even with the turbocharger off - at a very exceptional speed of 480 miles per hour (though neither XP-72 prototype would ever exceeded 500 miles per hour in flight testing). The aircraft proved quite responsive at the controls and pilots commented on it as being "a joy to fly". With the XP-72 achieving success in its early development, the aircraft was envisioned to rival the high-speed, high-altitude performance found in the latter series of British Supermarine "Spitfire" fighters while at the same time offering the operational range that the Spitfire lacked. Indeed, the XP-72 might have been a good candidate for V-1 rocket interception duty over the South of England than the Spitfires already charged with the task.

The second prototype was given an Aero Products contra-rotating propeller arrangement (a pairing of three-bladed units to a single engine, equaling six total blades) and an estimated maximum speed of 550 miles per hour. Primary armament was to become an array of 6 x 12.7mm (.50 caliber) machine guns mounted in the wings (a reduction from the 8 x 12.7mm machine guns in the original P-47 Thunderbol). An alternative weapons loadout built into the contract was to allowed for 4 x 37mm autocannons in place of all of the machine guns. As secondary armament, the aircraft was to carry high-explosive rockets, 2 x 1,000lbs bombs, or 2 x fuel droptanks as needed at hardpoints under each wing.

Despite the performance gains and relatively easy development process, the conditions in the latter years of the war were such that all-new high-altitude fighters / interceptors were in little demand as existing types were more than enough to fulfill the role. The simple truth became that the XP-72 was no longer a needed commodity in the grand scope of the war - particularly with prop-powered aircraft reaching their technological apexes and turbojet technology beginning to take hold. As such, the two XP-72 prototypes were dropped from further development and scrapped by war's end along with cancellation of a production contract for 100 of the type. The XP-72 "Super Thunderbolt" would go down as just another of World War 2's "what-if" programs that failed to materialize any tangible fruits by war's end - leaving to the imagination what impact, if any, such powerful aircraft would have had if promptly adopted into operational service.

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Republic XP-72 Ultrabolt Technical Specifications

Service Year: 1944
Type: Prototype High-Altitude Fighter / Interceptor
National Origin: United States
Manufacturer(s): Republic Aviation - USA
Production Total: 2

Structural (Crew Space, Dimensions and Weights)

Operating Crew (Typical): 1
Overall Length: 36.61 feet (11.16 meters)
Overall Width: 40.91 feet (12.47 meters)
Overall Height: 16.01 feet (4.88 meters)

Weight (Empty): 11,473 lb (5,204 kg)
Weight (MTOW): 17,491 lb (7,934 kg)

Installed Power and Standard Day Performance

Propulsion: 1 x Pratt & Whitney R-4360-13 28-cylinder air-cooled radial piston engine developing 3,450 horsepower.

Maximum Speed: 490 mph (788 kph; 425 knots)
Maximum Range: 1,200 miles (1,931 km)
Service Ceiling: 41,998 feet (12,801 meters; 7.95 miles)
Rate-of-Climb: 5,000 feet-per-minute (1,524 m/min)

Armament / Mission Payload

6 x 12.7mm machine guns in wings

4 x 37mm cannons in wings

2 x 1,000lb bombs, Rockets, OR Fuel Droptanks under the wings.

Global Operators / Customers

United States (cancelled)

Model Variants (Including Prototypes)

P-47 "Thunderbolt" - Production Series Designation of base P-47 design on which the XP-72 was built upon; 8 x 12.7mm machine guns.
XP-72 "Ultrabolt" - Prototype Series Designation; two examples constructed; 6 x 12.7mm machine guns; 1 x Pratt & Whitney R-4360-13 Wasp Major engine of 3,450 horsepower.