Supermarine Spitfire (1940): the Merlin interceptor

· THE STORY

The shape of the battle

The Spitfire is the aircraft everyone can draw: a thin elliptical wing, a long nose over a Rolls-Royce Merlin, a bubble of a canopy. In the summer of 1940 it was the fighter the RAF used to go after the German fighters while the more numerous Hurricanes went for the bombers, and it was the better machine of the two. It was not what won the battle — radar, the control rooms, a hundred-octane fuel and the pilots did that — but it was the symbol, and it stayed in front-line service to the end because its airframe could absorb an engine that doubled in power.

657 km/hTop speed (Mk IX)20,351Built

· WHY IT MATTERED

A wing that could take twice the engine

Mitchell’s elliptical wing was thin enough to be fast and strong enough to be doubled in load. In 1939 it flew behind 1,030 hp; by 1944 the same shape flew behind 2,000, and the Spitfire IX outclimbed and outran the Mk V it replaced by 110 km/h at 30,000 ft. Almost no other 1936 design survived the whole war in front-line service.

GO DEEPERThe Spitfire in the Aviation timeline →How the Merlin doubled its power →

· WHY IT MATTERED

Eight guns, then cannon

The first Spitfires carried eight rifle-calibre Brownings, four in each wing, firing about 1.7 kg of bullets a second for eighteen seconds. Against armoured bombers that was too little, and from 1941 two 20 mm Hispano cannon took the middle of the wing. The cannon’s heavier shell mattered more than its rate of fire: one hit did what a dozen bullets could not.

· WHY IT MATTERED

The battle was a system

Chain Home radar saw the raids forming over France; the sector control rooms plotted them and sent squadrons up only when they were needed; hundred-octane petrol from America let the Merlin run twelve pounds of boost for five minutes at a time. The Spitfire and Hurricane were the sharp end of a system, and it was the system that made the Luftwaffe’s losses unsustainable.

GO DEEPERThe Merlin in the Engines timeline →

· THE ENGINE

The Merlin: 1,030 to 2,000 hp on the same 27 litres

The Merlin never grew: bore and stroke stayed at 137 by 152 mm from 1936 to 1945. Its power more than doubled because of what was bolted to its back. A single-stage supercharger gave the Merlin III its 1,030 hp; hundred-octane fuel let it take more boost for a few minutes; and in 1942 Stanley Hooker’s two-stage, intercooled blower on the Merlin 61 cost about 200 hp to drive and gave back 300 hp at 30,000 ft. The Spitfire IX that carried it was the answer to the Fw 190 — and the same engine, built by Packard, made the Mustang.

Rolls-Royce Merlin III · V-12 · 27.0 L · single-stage supercharger · 1,030 hp; Merlin 61 (Mk IX) two-stage, intercooled · 1,565 hp at 3,730 m

GO DEEPEROpen the full demo →

· WEAPONS AND SPECIALITY

Convergence: eight guns aimed at one point

Wing guns do not fire straight ahead: they are angled inwards to meet at a chosen range, 230 metres by 1940, so the eight streams of bullets arrived together on one patch of enemy aircraft. Too far or too close and they arrived apart. With the Hispano the arithmetic changed: two cannon and four machine guns fired about 3.5 kg a second and a 20 mm shell exploded inside what it hit.

· TRY IT

Single stage, two speeds, two stages: the Merlin against altitude

The lab plots power against height for the three Merlins the Spitfire flew with. Watch the single-stage engine run out of breath above 5,000 m, the two-speed engine get a second hump, and the two-stage Merlin 61 hold its power to 8,000 m and beyond — the difference between a Mk V and a Mk IX in one curve.

Readout

Power1,396 hp

Per engine1,396 hp

True airspeed657 km/h

Service ceiling13,100 m

A simplified model calibrated to the published rating and top speed — the shape of the curve is the point, not the third decimal.