In the Autumn of 1942, Republic Aviation was informed that the Army Air Force was no longer interested in the firm's latest fighter design. Chief designer, Alexander Kartveli, had designed the AP-18 fighter to meet the USAAF's Type Specification XC-622 for a high-altitude pursuit. But the resulting AP-18 - tentatively designated XP-69 by the military - was a very large and complex design. [1] With America's entry into WW2, it had become apparent that it would take too long to develop the AP-18. Instead, the USAAF elected to focus on developments of Republic's existing P-47 Thunderbolt turbocharged pursuit. The AP-18/XP-69 was dropped.
The military designation for the abandoned AP-18 was shifted to a new design - the AP-21 low-altitude fighter-bomber. For this new XP-69A, the USAAF envisioned a P-47 derivative optimized for low-level work. But Alex Kartveli knew that the Thunderbolt's fuselage had effectively been designed around the P-47's massive turbocharger system and its bulky duct work. Instead, he proposed a heavily revised design using only the wings and tailplane of the Thunderbolt. This was agreed to by the USAAF and detail designs were drawn up. Kartveli and his assistant, Murray Burkow, produced an initial AP-21 design by March of 1942. It was effectively a 'turbo-less' Thunderbolt with large fuel tanks in the rear fuselage.
After review, a USAAF panel recommended a smaller, lighter airframe. Another suggestion was shifting from the high-demand Pratt & Whitney R-2800 to the Napier Sabre H-24 engine which was just about to enter production in the US. [2] Most of that Sabre production was for British end-use but the USAAF also saw an opportunity to take some pressure off of Pratt & Whitney. The scale of Kartveli's revised AP-21B was reduced by eliminating the wing fuselage attachments and simply joining the wing panels along the fuselage centreline. The tailplane remained standard P-47 components but the fuselage was redesigned to both shorten its length and accommodate the new, liquid-cooled powerplant. This AP-21B design was accepted for service as the P-69A Reaper.
Bottom Republic XP-69A Reaper prototype (AP-21B-01) when employed on USAAF armaments tests at Wright Field in late September 1942.
The first Republic XP-69A prototype was completed in late August 1942, flying from East Farmingdale, NY, on the power of an imported Napier Sabre IA. Production US H-2240-NK-1 'Saber' engines were not received from makers Nash Kelvinator until the middle of September. Because of the re-use of some Thunderbolt parts and components, production was acheived quickly but, due to P-47 priority, most of the early model P-69 Reapers were actually license-built by Curtiss in Buffalo, NY. The majority of Republic-built P-69As went to the Army Air Corps as trials aircraft. No cannon-armed P-69B-RE-2 Reaper would ever be completed.
The first service Reapers were P-69C-CU-4 models armed with an M4 37 mm Automatic Gun and a .50-cal Browning machine gun in each wing. The latter had 320 rounds per gun, the cannons only 40 rpg. The 37 mm cannon was potentially hard-hitting but its rounds had a totally different trajectory from the .50-calibre machine guns (complicating aiming). In the field, results with the P-69Cs were indifferent. Some units swore by the autocannons, others replaced the big guns with 20 mm British Hispanos where available. At home, production quickly shifted to the P-69D-RE-6 and Curtiss-built P-69G-CU-8 - both eight-gunned fighters powered by 2,250 hp Nash Kelvinator H-2240-NK-3 Sabers.
Top A UK-based Curtiss-built P-69C-CU-1 Reaper armed with twin 37 mm Automatic Gun, M4 cannons. The RAF-style yellow surround to the fuselage star was a recognition feature. The Insignia Yellow swastika markings on the forward fuselage represent ground attack missions (rather than aircraft 'kill' markings).
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[1] The AP-18/XP-69 was to be an enormous fighter - 15.70 m long, with a 15.75 m wingspan. A 2,500 hp Wright R-2160-3 Tornado - a 7-bank, 42-cylinder engine - would drive contra-rotating propellers. Empty weight was to be half again that of the Thunderbolt.
[2] When Napiers ecountered problems producing Sabre sleeve valves, several US Sundstrand centreless grinders were diverted to the UK. All of these high-value machines were lost to U-Boats. It was then decided to simply machine and harden the Napier sleeves in the US. Eventually, that scheme escalated to full-scale production of the Napier engines in Detroit.