Supermarine Spitfire Mk IX

By early 1942 the Focke-Wulf Fw 190 clearly had the measure of the Spitfire Mk Vb, and the RAF needed an answer in months rather than years. The key component already existed: in September 1941 Rolls-Royce had fitted a two-stage Merlin 60 — designed for a high-altitude bomber — into the Mk III prototype N3297 at Hucknall. Jeffrey Quill called the improvement a quantum leap over the Mk Vb. Since the Mk VII and VIII were more than a year from production, the Air Ministry ordered Mk Vc airframes converted to the new engine. The first Mk IX was exactly that, a modified Mk Vc, AB505, tested by the Air Fighting Development Unit in April 1942; the report noted that above 20,000 ft the aircraft was outstandingly better than the Mk V, that it climbed easily to 38,000 ft and that it could still manoeuvre and fight at 40,000 ft.

The longer engine pushed the nose out by seven inches, and the intercooler needed a radiator of its own, so the starboard radiator was halved and a new fairing under the port wing housed a square oil cooler alongside the other half matrix. A four-blade Rotol propeller of 3.27 m turned inside a pointed spinner, with six fishtail exhaust stubs per side. The C wing was standard, but mid and late production aircraft took the E wing, with two Hispano cannon and two .50 in Brownings replacing the four .303s. Flush riveting, an enlarged pointed rudder, the Mark II gyro gunsight of 1944 — which doubled the effectiveness of RAF gunnery — and, on the last aircraft, a cut-down rear fuselage with a bubble hood, all followed.

The Merlin 61 gave way early in 1943 to the Merlin 63 and 63A. During the second half of that year F Mk IX production yielded to the Merlin 66-powered LF Mk IX, by far the most numerous sub-variant with over 4,000 built, good for 650 km/h at 21,000 ft on 1,720 hp. The HF Mk IX, with the high-altitude Merlin 70 of 1,710 hp, entered service in the spring of 1944 in comparatively small numbers. Endurance was always the weak point: 85 gal internally, slipper tanks of 30, 45 or 90 gal, and on late aircraft rear fuselage tanks that allowed a ferry range of more than 1,900 km at the cost of handling so poor that the pilot's notes warned against instrument flying.

Specialised versions followed: unarmed PR Mk IXs with two vertical cameras, one of which photographed the Ruhr dams the day before and the day after Operation Chastise; armed FR Mk IXs with a single oblique camera, painted pale pink, which covered the Arnhem area during Operation Market Garden; and, postwar, the two-seat T Mk IX and TR 9 exported to India and Ireland. In all 5,656 Mk IXs were built, 561 by Supermarine and 5,095 at Castle Bromwich, until April 1945. Combined with the Mk XVI, this "compromise" was the most heavily produced Spitfire of all.