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Jun 13, 2023

The Republic P

The Jug spilleth over: the manifold relatives of the P-47 Thunderbolt If asked to name an American fighter of WWII chances are most people would say “why the hell are you asking me that?” but anyone

The Jug spilleth over: the manifold relatives of the P-47 Thunderbolt

If asked to name an American fighter of WWII chances are most people would say “why the hell are you asking me that?” but anyone with a passing interest in aviation would probably pick the P-51 Mustang, much to the chagrin of P-47 fans.

The Thunderbolt is very much the American Hawker Hurricane, a rugged, versatile and extremely important aircraft overlooked due to its sexier stablemate. Unlike the dowdy Hurricane’s relationship with the Spitfire however, the P-47 was arguably a better fighter than the Mustang, variations on the P-47 theme ended up proving faster and longer-ranged than their slender P-51 rival and it was always the more versatile design. The antecedents of the Thunderbolt had also been around since the dawn of the ‘modern’ stressed-skin monoplane age and the beginning of the Thunderbolt story starts in a very unexpected place, both conceptually and geographically, its development required two completely separate companies and it nearly joined the jet revolution. Join us as we take a look at the eclectic and somewhat confusing realm of the Thunderbolt Family:

Seversky SEV-3

The Thunderbolt came into being due to the work of two designers, Alexander Nikolaievich Prokofiev de Seversky and Aleksandre Kartvelishvili, known as Alexander Kartveli, both Georgian emigres, thus making the P-47 the most famous Georgian aircraft design in history. The dapper one-legged air-ace de Seversky (he shot down at least 7 aircraft during WWI making him the top ranking naval ace of Imperial Russia) had been selected in early 1918 as assistant naval attaché in the Russian Naval Aviation Mission to the United States and opted to stay in the US whilst Russia was torn apart by civil war. A talented engineer, Seversky became rich from the sale of the world’s first gyroscopically stabilised bombsight and set up the Seversky Aircraft Corporation in 1931 with Kartveli (possessed of two legs), who had spent time during the 1920s working for Bernard and Bleriot in France, as chief designer.

The first product of the new company was the Seversky SEV-3 of which around 35 were built. Despite being a three seat amphibian, this aircraft was the direct ancestor of the Thunderbolt. As may be seen, the SEV-3 possessed the elegant semi-elliptical wing that would find its way largely unchanged into the P-47 and it was an advanced, stressed-skin radial engine monoplane with a distinctly chunky aesthetic that would be carried over into all the fighter designs produced in the immediate future by Seversky and Republic.

The SEV-3 was a fascinating aircraft. In amphibian form, main wheels were housed in the floats but to allow the aircraft to alight on land, the floats had to tip nose down to allow clearance for the tailwheel. The aircraft was also produced as a dedicated landplane with large trouser fairings enclosing the conventional fixed landing gear. The amphibian was to prove the more popular version and on 15 September 1935, a Wright Cyclone-powered SEV-3 set a world record speed for piston engine amphibian aircraft of 230 mph (370.8 km/h) which stood for 49 years.

SEV-3 amphibians served in the Spanish Civil War with the Republican Air Force and with the Colombian Air Force but a planned landplane trainer designated the BT-8, of which 30 were ordered for the USAAC, proved appallingly underpowered and was quickly discarded in favour of the north American BT-9 (which would morph into the fantastically successful T-6 Texan).

Seversky P-35

During development of the SEV-3, the design was tinkered with slightly to produce a two seat fixed-undercarriage fighter called the SEV-2XP with a 735hp Wright R-1820 Cyclone engine replacing the 420hp Wright Whirlwind of the SEV-3. With all this extra horsepower, great things were expected of the new fighter but it was damaged en route to a fly-off against the new Curtiss XP-36. Kartveli took the opportunity to rebuild and rework the aircraft into a single seat fighter with retractable(ish) undercarriage called the SEV-1XP and the new rotund fighter flew for the first time in August 1935, slightly later than the Messerschmitt Bf 109 and slightly before the Hawker Hurricane, both of which were considerably superior to the new Seversky. Nonetheless, the XP-35 was judged superior to the XP-36 (prompting a frenzied and ultimately successful effort to improve the P-36 by Curtiss). 77 P-35s were ordered by the US to become America’s first all-metal cantilever monoplane fighter with retractable undercarriage and enclosed cockpit but the slow P-35 was obsolete by the time deliveries concIuded in 1938 – this placing it in the unenviable position of being at the vanguard of progress whilst simultaneously hopelessly outdated. Nonetheless the type managed to secure an export order from Sweden though not all were delivered due to the June 1940 arms embargo on all nations except the UK.

Curiously, the P-35 drummed up more contemporary interest as a civilian aircraft than a fighter. In 1937 Jackie Cochran flew a Seversky SEV-S1, a variant of the P-35 fighter, from New York, to Miami, Florida, in 4 hours, 12 minutes, 27.2 seconds thus breaking the record set by Howard Hughes in the H1 racer. Cochran also used the Seversky to set the women’s world speed record, managing too coax the aircraft over the magic 300mph mark and garnering her the Harmon International Aviatrix Trophy in 1938. The aircraft was also campaigned in the Bendix air race and was modified into the SEV-1XP ‘Executive’ as a fast business aircraft with a (cramped) cabin for a single passenger in the rear fuselage.

Meanwhile the military P-35s had been despatched to America’s Far Eastern colonial possession, the Philippines. Some of these were from the repossessed Swedish order, which arrived proudly wearing the three crowns marking of the Swedish Air Force. In the Philippines the P-35s unexpectedly found themselves facing the juggernaut of Japanese expansion and proved tragically wanting against modern Japanese fighters, a single air to air ‘kill’ may have been achieved by the P-35 but details are unclear, though one managed to sink a minesweeper. The sole survivor flew its last sortie on 3 May 1942. The P-47 would ultimately avenge this poor showing many times over but it would take a few more goes for Seversky and Kartveli to end up with the Thunderbolt.

Seversky 2PA/A8V1/AT-12 Guardsman

Hindsight is a glorious thing and all that but as if designing a fighter easy for the Japanese to shoot down wasn’t enough, Seversky compounded the issue by selling a very similar fighter to Japan, just to make sure they knew exactly what they were dealing with. This move would prove catastrophic for de Seversky, the Japanese sale made him something of a pariah to the US Army which ordered no further P-35s. Seversky was also rather better at spending money than making it and was estimated to have lost the company over half a million dollars during 1938. As a result, in April 1939, while de Seversky was on an international business trip, the board of directors of the company that he had founded and which bore his name voted him out of office as CEO. The name of the company was changed to Republic and Alexander Kartveli was appointed as vice president and technical director.

The Seversky 2PA, which de Seversky had flogged to the Japanese was a two seat development of the P-35 (itself of course a single seat development of a two seat fighter) intended as a long range bomber escort and dubbed a ‘convoy fighter’ by Seversky. Curiously, the addition of the second crew position under the extended canopy made the aircraft appear considerably sleeker, though admittedly it is difficult to think of any change being made to the P-35 that could possibly make it look less sleek. Japan received 20 of the 2PA-B3 model and issued them to the 12th Kokutai based near Nanking which used them operationally, albeit briefly, in the reconnaissance role during the second Sino-Japanese war. The development of superior indigenous fighters by the Japanese had rendered the A8V1, as it was designated by the Japanese, surplus to requirements and the aircraft had been withdrawn by the time America entered the Second World War, nonetheless the A8V was allocated the sniggerworthy reporting name ‘Dick’ in the expectation that it would be encountered in combat by Allied pilots.

Meanwhile the Japanese order had been noted by Sweden who then ordered 52 2PAs of their own which they designated B6. Only two were delivered before the US arms embargo was enacted, the balance of the Swedish order being taken on by the USAAF and named AT-12 Guardsman, ‘AT’ standing for ‘Advanced Trainer’ though it is unlikely any AT-12 was ever used as a training machine, most being allocated to squadron commanders as high speed courier aircraft. One AT-12 is kept in airworthy condition by the Planes of Fame museum in Chino, California. This is the only flyable Seversky aircraft in the world.

Seversky XP-41

The XP-41 started life as the last of the P-35s off the production line, it was then modified by Kartveli to attain the best possible performance that could be wrung out of the basic design. Externally, the most obvious change was a switch from the cumbersome rearward-retracting undercarriage to a far neater inward retracting design that closed flush when retracted. This was combined with a revised canopy and flush riveting throughout (a first for an American aircraft) resulting in a much more aerodynamic airframe. The engine was switched for a Pratt & Whitney R-1830 Twin Wasp with a two-stage supercharger delivering around 350hp more power than the Twin Wasp fitted in the standard P-35.

These improvements were sufficient to propel the XP-41 up to a creditable maximum speed of 323mph. However, by March of 1939 when the aircraft was first flown this was not all that impressive. The final nail in the coffin came when Seversky’s own AP-4, which could be considered something of a lash-up by comparison, proved to have a better performance than the highly refined and beautifully made XP-41, Kartveli abandoning the design to concentrate on development of the AP-4 into the P-43 Lancer.

Seversky AP-4

Seversky’s numbering system was a mess. The AP-4 was actually preceded by the AP-7, which was Seversky’s designation for the civil P-35s it produced. Seversky’s prototype production was a mess too. The AP-4 was developed concurrently with the generally similar XP-41. It was less aerodynamically advanced, consisting of little more than a stock P-35 with inward retracting undercarriage but critically it had been fitted with Alexander de Seversky’s latest wheeze: a turbosupercharger in the rear fuselage.

This was an unusual location for such a piece of equipment and necessitated extensive ducting to carry exhaust gases to the rear of the fighter and then return the compressed air to the engine. The payoff for all this added complexity was the transformation of the aircraft’s high-altitude performance, proving far superior to Seversky’s own XP-41 which suffered a rapid decline in performance (which wasn’t exactly stellar to begin with) over 15,000ft. At the Army’s Pursuit competition of 1939, the AP-4 proved to have the best performance of all the entrants, but the Curtiss P-40 was judged the winner, largely due to the massive industrial capacity of Curtiss, the Army figuring it was in serious need of a lot of fighters as quickly as possible – correctly, as events were to prove. The fact that the AP-4 caught fire in flight on 22 March 1939 and was destroyed after the pilot bailed out probably didn’t help either. The performance of the AP-4 could not be entirely overlooked however and Seversky received the consolation prize of an order of 13 test examples of a refined AP-4 to be designated the YP-43. By the time the order was placed however, Alexander de Seversky had lost his job and the Seversky Aircraft Corporation had become Republic Aviation.

Republic P-43 Lancer

Whilst definitely a better aircraft than the bulky P-35, the Lancer wasn’t that much better. It certainly looked the part far more effectively though and had a particularly impressive range capability that was to prove extremely useful, though not in its intended role.

The Lancer ditched the somewhat cumbersome canopy design that had remained largely unchanged from the P-35, replacing it with the distinctive narrow spine that extended right up to the hood and that would become a characteristic feature of the earlier ‘razorback’ Thunderbolts. The new aircraft also saw the carburettor air intake moved from the port wing root to a position under the engine resulting in the signature ovoid shape that would be carried over onto the Thunderbolt. The 13 YP-43 test aircraft impressed the Air Corps with their excellent high altitude performance and long range. Unfortunately, although pilot’s reported the aircraft to be generally pleasant to fly the maneouvrability of the Lancer was underwhelming. More serious however, given the reality of modern air to air combat, was the absence of any armour and the lack of self-sealing fuel tank, the wing itself being sealed to form a large fuel tank, a so-called ‘wet wing’ which conferred upon the P-43 its prodigious range. Nor could the wing be retrofitted with self-sealing fuel tanks either, being simply too thin to contain them.

By 1941 when the P-43 first flew it was clearly obsolescent and the Army was far more interested in Kartveli’s AP-10/P-47 design which appeared to offer considerably more development potential. Nonetheless 272 examples of the Lancer were built which made it the most produced Seversky/Republic design so far, although the orders were placed primarily with the primary intention of keeping Republic solvent until P-47 production could begin.

Despite the somewhat lacklustre reception given to the P-43 by the USAAF, the Chinese Air Force ordered over 100 examples of the Republic fighter. In operational service the wet wing gave trouble, the repeated stress of flight and landings caused leaks to appear around rivets and panel joints and the Lancer was prone to catching fire, especially if leaking fuel escaped onto the turbosupercharger. Several P-43s were lost to fire on routine ferry flights causing the deaths of several experienced pilots. On the other hand, the P-43 had a much superior altitude performance to any other Allied aircraft in China, potently demonstrated by ‘Flying Tigers’ pilot Robert Lee Scott Jr who filmed the summit of Mt Everest from a height of 44,000 feet. More relevantly to the war effort, this impressive altitude capability to intercept the Mitsubishi Ki-46 reconnaissance aircraft which was otherwise immune to interception. Gradually the leaky wing situation was improved and some armour added making the aircraft more combatworthy and the Lancer was used ever more as a reconnaissance machine, a role in which it proved extremely useful and serving into 1944 before being replaced by the F-5 Lightning.

Republic XP-44 Rocket

This one didn’t even exist but it came very close. The P-43 possessed obvious potential, especially when compared to the lumbering P-35, but wasn’t fully competitive with the world’s best and thus Kartveli schemed the P-44 ‘Rocket’. Essentially a re-engined P-43, the Rocket was to be powered by a 1400hp Pratt & Whitney R-2180 in a low-drag installation. Estimated top speed was 386mph, a useful increase over the P-43 but the armament was to remain the same quartet of 50 calibre machine-guns. The USAAC was keen and by September 1940 Republic possessed firm orders or letters of intent for more than 900 P-44s, a huge order by the standards of the day.

Nonetheless, the P-44 was destined never to be built. Reports of combat over Europe began to filter back to the US and Kartveli became aware that the AP-4 (and the AP-10 described below) was unlikely to prove a particularly effective fighter. More power, greater speed and better armament were obviously required and Kartveli undertook to rework the AP-4 into a larger, faster aircraft, no longer merely a warmed-over P-43 but an altogether more formidable machine.

Republic XP-47 and XP-47A

At last! It’s the P-47 itself. Or is it? Not exactly, for the first design to be actually designated the P-47 was, bizarrely, a small, lightweight fighter designed around the Allison V-1710 V-12 inline engine – a distinct departure from the huge, radial engine powered heavyweight we know and love (or are barely aware of and totally indifferent towards).

Kartveli proposed the AP-10 during the latter months of 1939 and the Army was keen, ordering two prototypes: an XP-47 with six gun armament and the reduced weight XP-47A with just four guns. The contract also covered tooling for an initial production run of the new fighter. Unfortunately development ran into difficulties, Kartveli found it impossible to keep the weight down whilst allowing for the necessary armament and equipment and at the same time doubt was being cast on the wisdom of using the Allison V-1710. This engine was also fitted to the P-38, P-39, P-40 and in due course, the P-51, thus making all US Army fighters reliant on just one engine type, with all the concomitant risk to the fighter fleet if supply were to be interrupted. With renewed interest in a radial powered aircraft, Kartveli ditched the XP-47/XP-47A and reworked the P-44 design into the AP-4L, or XP-47B as it was known to the Army.

Republic P-47 Thunderbolt

And here it is at last, Alexander Kartveli’s masterpiece, the P-47 Thunderbolt. Carrying over the best traits of the P-43 Lancer, the Thunderbolt was considerably larger and more powerful, indeed it was the largest and heaviest single engine fighter of the entire war. It was also extremely expensive (in 1945 a P-47 cost $83,000 compared to the P-51 at just shy of $51,000). Despite this, it became the most produced American fighter aircraft in history with 15,636 rolling off the production line between late 1941 and December 1945. Entire volumes have been devoted to its development and service, the following is merely a brief introduction to the basic Thunderbolt variants.

The Thunderbolt prototype, the XP-47B flew for the first time in May 1941. Despite superficially resembling the P-43, it was some 65% heavier and fitted with a Pratt & Whitney R-2800 Double Wasp rated at 2000hp, as also utilised by both the Vought F4U Corsair and Grumman F6F Hellcat. The XP-47B was lost in August 1942 by which time the first production P-47Bs were coming off the line. Unlike the turbosupercharger installation in the P-43, which was almost an afterthought, Kartveli set about designing the ideal installation for this equipment in the new fighter. Accepting that it was best located in the rear fuselage (some 22ft (6.7m) away from the engine) a complicated system of ducting took exhaust gases back to the turbine whilst cooling air was directed to the intercooler and further ducts brought compressed air forward to the carburettor. Despite its complexity, it performed reliably in service and proved surprisingly resilient to battle damage.

171 P-47Bs were built, all of which were retained in the US for training. These were followed by 602 P-47Cs which differed from the B primarily by having an eight inch extension to the forward fuselage to shift the centre of gravity forward and improve flying characteristics. The earlier P-47Bs are easily distinguished by the angled radio mast on the spine. The P-47C was the first Thunderbolt variant to see combat, with the first mission flown on the 10 March 1943. The classic P-47D differed very little from the C model and the two are indistinguishable externally, minor changes were made to the turbosupercharger and a different model of R-2800 engine was fitted. Huge orders were now being placed for the P-47 and Republic could not supply demand, despite a second factory being built to produce it. As a result, orders were placed with Curtiss-Wright to build the Thunderbolt, Curtiss built aircraft being designated P-47G and were essentially identical to the D-model.

It was around this time that the ‘Jug’ nickname was coined for the Thunderbolt. Some sources allege that this is because the aircraft resembles a classic milk jug in profile. However the fact that it requires quite a leap of imagination to visualise suggests that it is possibly a load of rubbish. An alternative theory is that when the 4th Fighter Group (the only fighter group operational with the 8th Air Force in Britain when the P-47 arrived) switched from flying the spitfire to the P-47C, the rotund seven ton Thunderbolt contrasted so starkly with the elegant and comparatively tiny Spitfire V that it acquired the soubriquet of ‘juggernaut’, quickly shortened to ‘jug’. Although the truth will probably never be known, the latter explanation seems (to me at least) more likely.

Following the ‘D’, Republic built the XP-47E which was fitted with a pressurised cockpit and the XP-47F which featured a laminar-flow wing. Both were modified P-47Bs and neither would enter production. Meanwhile, the Thunderbolt was going from strength to strength in combat. The habit of 8th AF pilots of strafing targets of opportunity when returning from escort missions saw the ground attack potential of this purpose-designed high altitude fighter being seriously explored and ultimately resulted in the P-47 becoming the most important USAF fighter-bomber of the war. The P-47 proved highly amenable to this change of role by dint of its heavyweight lifting capability and remarkable ability to absorb battle damage.

Complaints about rearward visibility saw Republic experimentally fit the bubble canopy from a Hawker Typhoon to a P-47D with a cut down rear fuselage. Known as the XP-47K, (a second bubble canopy conversion being designated the XP-47L) the result was so successful that the bubble canopy was introduced on the production line forthwith. Oddly, for such a seemingly major change, neither the P-47K or P-47L designation was used for production aircraft and bubble canopy Thunderbolts remained plain old P-47Ds.

The final iteration of the ‘standard’ P-47 was the ‘hot rod’ P-47M, developed specifically to combat the V-1 flying bomb with uprated engine for maximum speed at low level, though teething troubles prevented the aircraft from entering service until after the V-1 attacks had ceased. 130 P-47Ms were built, externally identical to bubble canopy P-47Ds, these representing the penultimate Thunderbolt variant to achieve series production.

TP-47G ‘Doublebolt’

Although several airframes were field modified with a second seat, the only factory-produced two seaters were a pair of P-47Gs modified on the Curtiss-Wright production line as prototypes of a trainer variant with a pupil’s seat directly in front of the normal cockpit in place of the main fuselage fuel tank. Further production did not go ahead.

Republic XP-47H

Deciding that the regular Thunderbolt wasn’t massive enough already, Republic thought shoving an untried inverted V-16 engine designed by Chrysler might be a good idea. As well they might, for the Chrysler XI-2220 was rated at a lusty 2500hp for take off, making it nearly four times as powerful as the P-35.

Despite its impressive output the XI-2220 possessed a commendably small frontal area and thus combined excellent streamlining with great power, promising excellent performance for the XP-47H. Republic took two P-47-D-15-RAs from the production line in August 1943 and set about modifying them for the new power unit. Unfortunately the modifications required were extensive and the first flight by an XP-47H only took place in July 1945. By then the USAAF was utterly fixated on jet power and the performance of the XP-47H was of largely academic interest. Expected to achieve a maximum speed in the region of 490mph, the top speed recorded in testing was 414mph, though more may well have been possible. Unfortunately, the test programme of the first aircraft was brought to an abrupt end in November after 27 flights totalling 18 hours flying time when the propeller shaft failed, resulting in a dead stick forced landing. The second XP-47H was flown briefly after the war but no further development occurred.

Republic XP-47J Superbolt

Despite the fact that it is obscure in the extreme today, the Superbolt recorded the fastest known speed in level flight of any propeller driven aircraft during WWII, achieving the colossal speed of 505mph (813km/h) on 4 August 1944. It was also decorated with a natty portrait of Superman brandishing a lightning bolt on the cowling, which probably didn’t make it go any faster but certainly ups its credentials as a cultural artefact.

In contrast to the XP-47H, the XP-47J was not a conversion but built from scratch, as such it incorporated a considerable amount of weight saving alterations to the structure. The aircraft was powered by the R-2800-57(C) which delivered 2800hp and was force cooled by an engine driven fan directly behind the propeller spinner. As a weight saving measure two of the machine guns were deleted leaving the XP-47J with six 50 calibre Brownings in the wings. The pilot also benefitted from an extra perspex panel behind the canopy hood which greatly improved the rearward view from the cockpit, though the unbuilt second prototype was intended to feature a bubble canopy.

The Superbolt flew for the first time on 26 November 1943 but didn’t fly again until March of the following year. At some point before August the aircraft was fitted with a General Electric CH-5 supercharger (not turbosupercharger) and became capable of the exceptional speeds that should have rendered it famous but ultimately didn’t.

So why didn’t the P-47 enter production? Mass production of the P-47J actually was seriously considered for a time but ultimately did not go ahead for several reasons. Most seriously, the XP-47J enjoyed only 30% commonality with the P-47D which was busy proving a highly successful fighter in Europe and the Pacific, a switch to the P-47J would have required a significant change to production tooling resulting in delays which could not be tolerated at this stage of the war – better to have a large amount of the extremely good P-47D now than wait several months for the slightly better P-47J. Furthermore, the lightened structure of the XP-47J resulted in a reduced fuel load and therefore a shorter range, at exactly the time when ever longer ranged fighters were desperately needed.

The final nail in the coffin for the P-47J though came from Republic themselves: applying the same engineering solutions to a new aircraft with the even more powerful new R-4360 Wasp Major promised even better performance than that achieved with the Superbolt. This aircraft would subsequently appear as the XP-72 and further development of the P-47J was abandoned.

Republic P-47N

The best of the various Thunderbolt variants and developments to actually enter service, the P-47N also saw the only major change to the basic structure of the P-47 to see series production, featuring a completely new wing.

Much P-47 development work had been performed in an attempt to extend the range of the aircraft and the P-47N, intended as a B-29 escort in the Pacific, took that work to its logical conclusion. With no more room available for fuel in the fuselage and no simple way to increase external tankage the only option for more fuel was to provide wing fuel tanks. With a greater fuel load then pushing up the weight, a strengthened wing with greater area was required and so a new wing was designed with a 22 inch greater span giving an increase of 22 square feet in area, with square cut wingtips fitted for increased roll rate. The maximum fuel load for the P-47N was an astounding 4792 litres (for comparison, the Spitfire Mk.I carried 386 litres). All this fuel pushed the loaded weight up to over nine tonnes and the P-47N was extremely sluggish at take-off until some of that fuel had been expended. Republic’s efforts certainly paid off though – the P-47N could outrange the famously long-legged P-51D and it was no slouch either, topping out at 460mph at 30,000ft, whilst the best the P-51D could manage was (an admittedly still impressive) 440mph at its best height. The big Thunderbolt also retained all the earlier P-47’s best traits such as its superlative damage resistance and exceptional build quality. As an all-round fighting machine, the P-47N has a pretty good claim to being the best Allied fighter of WWII to see production and actual operational service.

The first P-47Ns went into action during April 1945 in the vicinity of Okinawa and mostly flew bombing and strafing missions over Southern Japan until the end of hostilities. During the course of these operations, P-47N pilot Oscar F Perdomo became the last American ‘ace’ of WWII when he shot down five Japanese aircraft in a single mission a mere two days before the end of hostilities. With the end of the war production came to an abrupt end, contracts for 5934 P-47Ns were cancelled, the relatively modest total of 1806 P-47Ns was actually built.

Republic XP-72 Ultrabolt

Despite receiving an order for 100 production P-72s, the mighty Ultrabolt (or Super-Thunderbolt or Superbolt (again)) was destined never to be built in quantity, the USAAF decided the future lay with jet aircraft and changed the order to cover 100 P-84 Thunderjets instead. Given the performance of the early Thunderjet, they may have had cause to regret that decision as the P-72 was an absolutely sensational aircraft that bid fair to becoming the most formidable piston engine fighter ever built. Key to its spectacular potential was its engine, the Pratt & Whitney R-4360 Wasp Major, a ludicrous 28 cylinder four-row radial engine that delivered a whopping 3450hp as fitted to the XP-72 but which would ultimately exceed 4000hp in R-4360-51 VDT form intended for the B-36C. Today the Wasp Major enjoys a somewhat frenetic retirement as the power plant of choice for many of the highly modified Reno air racers. These aircraft are, tellingly, very, very fast indeed.

Schemed as a low-risk back up to Republic’s XP-69 (which would ultimately never be built) and seen as a more promising design than the XP-47J Superbolt, the USAAF was initially very keen on the XP-72 due to its obvious potential for chasing down V-1 flying bombs. Like its XP-47J predecessor, the close cowled engine was force-cooled by a fan and a supercharger was mounted in the rear fuselage, not a turbosupercharger, this being powered by a fluid coupling and shaft drive running back from the engine. Armament fitted to the prototypes was six 50 calibre Brownings, though this could be changed for the fearsome alternative armament of four 37-mm cannon.

Two prototypes were built, flying in February and June 1944 respectively. As completed the XP-72 was roughly the same size and weight as the P-47D but possessed about 50% greater power and was more aerodynamic, the resulting performance was fantastic, a 490mph top speed was recorded and developed versions of the R-4360 were expected to give production P-72s a speed of 540mph (an optimistic figure but indicates the uncharted realms of performance the P-72 was expected to be operating in).

Alas it was not to be. The war situation did not require the P-72 to enter production and the advent of promising new jet fighters suggested that its performance, spectacular though it was, would soon be outclassed by a whole new kind of aircraft. One airframe, without its engine, was given to a Long Island, New York chapter of the Air Scouts in August 1946 and its ultimate fate is a mystery. The other airframe was scrapped.

Republic ‘Jetbolt‘

The ultimate Thunderbolt development never made it off the drawing board but it is an intriguing concept. In 1944, the Army suggested that Republic might look at a jet powered Thunderbolt derivative, the large size of the fuselage was obviously amenable to house potentially quite bulky power units and the airframe was a known quantity with good high speed flying characteristics. Furthermore, with P-47 production in full swing, a jet fighter in the form of the ‘Jetbolt’ (or Turbobolt) might be able to be produced quickly and relatively cheaply without the need to produce substantial amounts of new tooling, an obviously attractive prospect.

Initially the General Electric J31 engine, a centrifugal flow turbojet based on the British Whittle W.2B was considered but even the portly Thunderbolt’s fuselage was not sufficiently large to satisfactorily accomodate this large diameter engine. Further development centred on the Allison J35, America’s first axial flow turbojet, which was slimmer. Removing the R-2800 piston engine freed up space in the nose for the jet intake and the proposed armament of six or eight 50 calibre machine guns giving a more concentrated firepower than the wing mounted guns of the standard Thunderbolt. The jet engine was mounted below the cockpit floor, necessitating a slightly deepened fuselage with the jetpipe taking up the space previously filled by the large turbosupercharger and its extensive associated ducting, which were obviously no longer required.

Ultimately the Jetbolt would progress no further, the P-47 was reaching the aerodynamic limits of its design, even in piston engine form, and jet propulsion promised only a marginal improvement. Kartveli was working on his first purpose-designed jet aircraft by this time and this was clearly a more promising line of development. This would appear in 1946 as the F-84 Thunderjet and brought to an end around a decade of continuous direct line development that all stemmed from the SEV-3 amphibian of 1933 but even this wasn’t quite the end for the P-47 line.

Republic AP-47

In 1948, Republic proposed a mixed-power ground-attack version of the P-47 to be powered by an R-2800 piston engine in the nose and with a Westinghouse 24C jet engine replacing the turbosupercharger in the rear fuselage. The cockpit was to be placed further forward to provide the pilot with a better view over the nose. Oddly for an aircraft featuring a jet engine, a tailwheel undercarriage was intended to be used and the aircraft featured the wing of the P-47N.

Despite some pretty impressive performance estimates, the mixed-power solution for combat aircraft was gradually falling out of vogue in the United States, with pure jet aircraft being developed to fill virtually all combat roles and the Ryan FR-1 Fireball became the only fighter example of this arrangement to actually enter service (with the US Navy). The USAF were not sufficiently interested in the AP-47 to order a prototype and the final iteration of the basic P-47 design remained on the drawing board.

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