Republic XP-47J Superbolt – Fastest Piston Engine Fighter Ever

The P-47 certainly made its mark, but it isn’t so well remembered that one example was the fastest piston engine fighter ever built – the XP-47J.

In mid-1942, Republic Aviation Corporation initiated a design study to lighten their P-47 Thunderbolt fighter for improved performance. The Thunderbolt had been steadily gaining weight as the design matured, while comparative enemy aircraft, like the Focke-Wulf FW 190A, were much lighter. Republic officially proposed a light-P-47 to the Army Air Force (AAF) on 22 November 1942. On 1 April 1943, the AAF gave Republic a letter of intent to purchase two light-weight P-47s, and the contract was officially approved on 18 June 1943. This new aircraft was designated the XP-47J.

As with all P-47s, Alexander Kartveli was the main designer of the XP-47J, and he was assisted by Murray Burkow. The XP-47J was similar in appearance to a P-47B, but it was a completely new aircraft. The XP-47J had a close-fitting cowl installed around its Pratt & Whitney (P&W) R-2800 engine of increased power output. A large spinner was added, along with a fan to aid engine cooling. The turbo-supercharger’s intake had been refined, and the flow of its exhaust was directed to provide additional thrust. Two of the .50-cal machine guns were deleted (leaving six) in the XP-47J’s lightened wing, and the rounds per gun were reduced to 267. Other weight-saving measures were the omission of radio equipment and the aft fuel tank. To keep the aerodynamics clean, the XP-47J had no provisions for external stores under the wings or fuselage.

Originally, the AAF wanted the XP-47J to have contra-rotating propellers and a bubble canopy, like a late P-47D. To expedite the XP-47J, the decision was made for the first prototype to be a razorback version and the second prototype would include a bubble canopy. The six-blade contra-rotating propeller was test-flown on a P-47B but showed no increase in performance. This, combined with delays at P&W on the R-2800-61 engine with a contra-rotating gear reduction, resulted in the substitution of a R-2800-57 engine with a standard gear reduction. Both the -57 and -61 engines were rated at 2,100 hp (1,566 kW) and had a War Emergency Power (WEP) rating of 2,800 hp (2,088 kW). The -61 engine would be supplied later, when it was available. The engine, cowling, and cooling fan were installed and test flown on a P-47D-15.

The XP-47J had a wingspan of 40 ft 10 in (12.4 m) and a length of 33 ft 3 in (10.1 m). The aircraft had an empty weight of 9,663 lb (4,383 kg) and a design gross weight of 12,400 lb (5,625kg). Its service ceiling was 45,000 feet (13,716 m). The XP-47J had a range of 765 mi (1,231 km) at a cruising speed of 400 mph (644 km/h) and a range of 1,070 mi (1,722 km) at optimum cruising speed. Fuel capacity was 287 US gal (1,086 L).

An early image of the Republic XP-47J before the Superman nose art was applied. Note the cooling fan vanes around the spinner inside the cowling

On 31 July 1943, Republic issued a report comparing the estimated performance of the XP-47J with the P&W R-4360-powered XP-72 that was under development. The report concluded that the Republic XP-72 had more potential and recommended resources be focused on that aircraft. In addition, 70% of the P-47 production line needed to be re-tooled in order to produce the P-47J. Republic called for the cancellation of the second XP-47J prototype but encouraged the completion and testing of the first prototype. The AAF approved Republic’s plan and cancelled the second XP-47J.

The sole XP-47J prototype (serial number 43-46952) was completed in mid-November 1943 and made its first flight on the 26th of that month. The aircraft was quickly dubbed Superbolt and wore nose art on both sides of the cowling of Superman holding a lightning bolt. After about 10 hours of flying time, the R-2800-57 engine was making metal and was replaced by another engine of the same type in February 1944.

A short time later, P&W informed the AAF and Republic that the R-2800-57 engine was not compatible with the 2,800 hp (2,088 kW) WEP rating. A water-injected R-2800-14W was substituted in the XP-47J in April. Water injection is a system that sprays anti-detonation fluid (a mixture of alcohol and water) into the induction system to lower the charge temperature and prevent detonation in the cylinders. This allowed the -14W to produce 2,800 hp (2,088 kW). In March, P&W informed Republic that it was still having difficulty with the R-2800-61’s contra-rotating gear reduction and did not know when the engine would be available. This effectively put an end to the possibility that the XP-47J would have contra-rotating propellers anytime in the near future.

On 11 July 1944 and equipped with a General Electric CH-3 turbo-supercharger, the XP-47J achieved 493 mph (793 km/h) at 33,350 feet (10,165 m). Although the engine was producing 2,800 hp (2,088 kW), Republic believed the aircraft had more potential. At its own expense, Republic installed a CH-5 turbo-supercharger and a larger 13 ft (3.96 m) Curtiss propeller. The propeller was an experimental unit with 2 in (51 mm) added to its trailing edge to increase its width. With the changes, the engine producing 2,730 hp (2,036 kW), and 400 lb (1.78 kN) of jet thrust from the exhaust, Mike Ritchie flew the XP-47J over a calibrated course at 34,450 (10,500 m) feet on 4 August 1944* and achieved 505 mph (813 km/h). This is the highest speed recorded in level flight by any propeller-driven aircraft during World War II.

The XP-47J was handed over to AAF and arrived at Wright Field, Ohio on 9 December 1944. During flight tests, the AAF was unable to get maximum power from the R-2800 engine. The AAF recorded a speed of only 484 mph (779 km/h) at 25,350 feet (7,727 m) and with the engine producing 2,770 hp (2,066 kW). Near the end of flight testing, the exhaust manifold system had a serious failure while the aircraft was at 36,000 ft (10,943 m). The cause of the failure was the increase in pressure and temperature from the CH-5 turbo-supercharger acting upon the unstrengthened exhaust system. The Official Performance Summary report states the XP-47J had a max speed of 507 mph (816 km/h) and a 4,900 fpm (24.9 m/s) initial rate of climb. Republic’s Test Report No. 51 (27 January 1945) lists the max speed as 502 mph (808 km/h).

While the XP-47J Superbolt had remarkable performance, it was overshadowed by other aircraft, like the XP-72, that were under development. Work on the XP-72, which first flew on 2 February 1944, was not far behind that of the XP-47J, but as the war progressed and with the advent of jet fighters, neither aircraft were needed.

*Mike Ritchie’s report recording the 505 mph (813 km/h) speed was dated 5 August 1944, but it is believed the flight actually occurred on 4 August 1944.

Sources: Old Machine Press; YouTube

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