Fastest Piston Engine Fighter Ever; The Republic XP-47J Superbolt

February 15, 2023

With the P-47 Thunderbolt, the United States Army Air Force (USAAF) got themselves a winner. Though a big brute the P-47, with its huge Double Wasp engine and firepower of eight 0.5-calibre machine guns that could mincemeat just about anything when in entered service in 1942, was arguably the first American fighter of the Second World War capable of matching those of other nations on a better than equal footing.


Sure, the P-40 held its own, no denying that, but it was more a case of being good enough rather than excelling. And the P-38 was a good long-range, high-altitude fighter, but it could have problems down low in a dogfight until hydraulically boosted ailerons were fitted later in the war.

But the P-47 was the first of the American wartime fighters that could not just hold its own, but happily go toe-to-toe with Axis aircraft across most altitude ranges, shred enemy ground forces with its formidable attack capability and suck up stupid amounts of punishment.

Certainly, the Thunderbolt had places that it could be improved, but that too was recognized. If the big “Jug” was to retain its edge, it needed its performance to continue to improve as the fighters of enemy countries also got better.

I my previous article on the XP-47H I discussed how the USAAF was keen to see what could be wrung out of the Thunderbolt airframe by fitting a liquid cooled engine. But even before this effort, an attempt was made to see what could be done by making modifications to the existing powerplant and lightening the airframe.  In fact, this was proposed in November 1942, the very same month the baseline P-47 entered service, when Republic suggested to the USAAF that they build a lightened experimental P-47.

Obviously, everyone was rather busy at the time, but the Air Force signed a contract the following June for the construction of two prototypes, with the aircraft receiving the designation of XP-47J.

The most notable visual difference between the XP-47J and the standard Jug was the alterations to the engine mount and nose. The engine was changed out for a Pratt & Whitney R-2800 57(C) radial engine that had a standard rated power output of 2,100hp but in war emergency setting could produce 2,800hp. But in contrast to the original P-47 with its radial engine cooled by direct air flow, the P-47J had a tight cowling fitted to enclose the engine and cooling was achieved by an intake fan and a new chin scoop.


Inlets were also fitted to provide air for the turbosupercharger, and the exhaust system altered so that it vented out of ventral ports that would provide the aircraft with additional boost.

Because the aim was to see what could be achieved with the P-47, other measures were taken to improve the aircraft’s performance. Armament was reduced to six Browning machine guns, and ammunition cut down to 267 rounds per gun. The rear fuel tank and the radio equipment was also removed and, to keep the airframe as aerodynamically clean as possible, the XP-47J had no provision for external ordnance or drop tanks.

Initially it was hoped to fit a contra-rotating propellor to the aircraft, but delays with this meant that the idea was shelved, and it was decided to use the second prototype to test this. It was also intended for the second aircraft to also be fitted with a bubble canopy, which was soon to become standard on the production Thunderbolts.

As the idea was that the design would provide an improved aircraft off the basic P-47 frame, which in turn meant that hopefully it could go into production on the existing lines with minimal disruption and complications, things moved quickly. Because it was basically a converted P-47D of “razorback” type taken straight off the line, the XP-47J first flew in November 1943, just five months after the contract for its building was issued.


And the modifications certainly achieved their basic intentions, with the XP-47J shaving around 1,000lbs (c.300kgs) off the flat weight of the standard P-47D.

This made it fast! In July 1944, Republic decided to see just what they could get out of the aircraft and fitted it with a new propellor and a General Electric CH-5 turbocharger. With these alterations and flying over a calibrated course in August 1944, the Republic test pilot reportedly measured a top speed of 505mph (812.7km/h).

It also had a heck of a climb rate, capable of reaching 30,000 feet (9,144m) in six minutes and forty-five seconds. This all would make the XP-47J the fastest piston engine fighter of all time and justified its nickname as the “Superbolt”. However, it should be pointed out that these results were never replicated and when the USAAF took possession of the aircraft that December and began their own test program, they only ever achieved a top speed 484mph (779km/h) – which to be fair is not bad, but was achieved at the cost of the exhaust manifold failing.

Indeed, the Superbolt suffered a number of issues during its development, not really surprising considering the aircraft was pushing the envelope, but the engine had to switched out due to the amount of wear it suffered after only ten hours of flight testing. Plus, the second prototype ultimately never got built, limiting the XP-47J to just the single aircraft.

This was partially due to the fact that Pratt and Whitney never sorted out the issues with the specialist contra-rotating propellor system that was intended to be fitted, but also to the fact that Republic itself had rapidly lost interest in the idea of the P-47J.

For starters the hope had been, as said, that the type could be built on the existing P-47 production line with minimum disruption. But it turned out that only thirty percent of the aircraft was the same as the P-47D, and as a result switching to the P-47J as the production standard would require a large amount of retooling just as the requirement for fighters was at its height.

And after all, as history has shown, the P-47D was more than adequate for the job at hand, so why mess around with things.

Plus, Republic had even grander schemes in mind. In July 1943, just a month after they had received the contract to build the XP-47J prototypes, they issued a report saying that, actually, the XP-47J likely wouldn’t be worth the effort and that they would be better off devoting their resources, and the USAAF’s money, to their next generation of fighter, the XP-72 Ultrabolt.


The USAAF agreed with their assessment, and subsequently cancelled the second prototype. This probably was the right decision, and the Ultrabolt would fly only three months after the XP-47J first did and likely had more development potential. But ironically, the XP-72 also never saw service as jets were to rapidly spell the end for the piston-engine fighter era.

But to return to the Superbolt, it remained essentially a testbed. Though the efforts expended on it certainly don’t seem to have been wasted, as they seem to have been running side by side with another effort that Republic was working on – the P-47M.

This unofficial test program also looked at modifying standard the P-47 with the R-2800 C-Series engine used by the -47J, but with less structural changes. This would pay off, along with Republic’s experience in working with the engine and CH-5 turbo in the Superbolt, when an urgent requirement for an extremely fast interceptor was issued to deal with a new threat – the V1 Flying Bombs that started hitting the UK in June 1944.

Republic were able to turn out 130 P-47Ms in reasonably short order, though not soon enough for them to see use against the “Doodlebugs”, and they would be the fastest piston-engine aircraft to see service with frontline units during the Second World War.

So, while the XP-47J never lived up to its potential and would have been eclipsed in short order even if it had gotten into service, it does deserve note for being the fastest piston engine fighter ever built and helping in the development of a sibling that would hold the same accolade as a service aircraft.


The XP-47H; Shark-Nosed Thunderbolt

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The Republic P-43 Lancer – A Giant’s Stepping Stone

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