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Republic XP-72 Ultrabolt

Prototype High-Altitude Fighter / Interceptor [ 1944 ]

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.

Authored By: Staff Writer | Last Edited: 03/31/2019 | Content ©www.MilitaryFactory.com | The following text is exclusive to this site.

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.©MilitaryFactory.com
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Service Year

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United States

Development Ended.


National flag of the United States United States (cancelled)
(OPERATORS list includes past, present, and future operators when applicable)
Air-to-Air Combat, Fighter
General ability to actively engage other aircraft of similar form and function, typically through guns, missiles, and/or aerial rockets.
Ability to intercept inbound aerial threats by way of high-performance, typically speed and rate-of-climb.
X-Plane (Developmental, Prototype, Technology Demonstrator)
Aircraft developed for the role of prototyping, technology demonstration, or research / data collection.

36.6 ft
(11.15 m)
40.9 ft
(12.47 m)
16.0 ft
(4.88 m)
Empty Wgt
11,475 lb
(5,205 kg)
17,494 lb
(7,935 kg)
Wgt Diff
+6,019 lb
(+2,730 kg)
(Showcased structural values pertain to the base Republic XP-72 Ultrabolt production variant)
Installed: 1 x Pratt & Whitney R-4360-13 "Wasp Major" 28-cylinder, 4-row, air-cooled radial piston engine developing 3,450 horsepower and driving a four-bladed propeller unit at the nose.
Max Speed
490 mph
(788 kph | 425 kts)
41,998 ft
(12,801 m | 8 mi)
1,200 mi
(1,931 km | 3,576 nm)
5,000 ft/min
(1,524 m/min)

♦ MACH Regime (Sonic)
RANGES (MPH) Subsonic: <614mph | Transonic: 614-921 | Supersonic: 921-3836 | Hypersonic: 3836-7673 | Hi-Hypersonic: 7673-19180 | Reentry: >19030

(Showcased performance specifications pertain to the base Republic XP-72 Ultrabolt production variant. Performance specifications showcased above are subject to environmental factors as well as aircraft configuration. Estimates are made when Real Data not available. Compare this aircraft entry against any other in our database or View aircraft by powerplant type)
6 x 12.7mm Browning M2 Heavy Machine Guns (HMGs) in wings.

4 x 37mm Automatic cannons in wings.

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

Supported Types

Graphical image of an aircraft medium machine gun
Graphical image of an aircraft heavy machine gun
Graphical image of an aircraft automatic cannon
Graphical image of aircraft aerial rockets
Graphical image of an aircraft conventional drop bomb munition
Graphical image of an aircraft external fuel tank

(Not all ordnance types may be represented in the showcase above)
Hardpoint Mountings: 2

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.

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Images Gallery

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