P-51 Mustang (1944): the Merlin escort to Berlin

1944·710 km/h at 7,600 m·2,660 km·Packard Merlin V-1650-7, 1,490 hp·12,800 m·~15,500

· THE STORY

The escort that reached Berlin

The Mustang was designed in 1940 in 102 days for the British, with an Allison engine that ran out of breath above 15,000 ft, and for two years it was a good low-level aircraft nobody was excited about. Then a Rolls-Royce test pilot flew one and asked for a Merlin in it. With the two-stage Merlin the same airframe did 700 km/h at 25,000 ft, and with a tank behind the pilot and two under the wings it could escort the bombers to Berlin and fight when it got there. In the spring of 1944 the Luftwaffe’s fighter force met it and did not recover.

710 km/h at 7,600 mTop speed2,660 kmRange with tanks

· WHY IT MATTERED

The transformation

The Allison Mustang made 1,150 hp at 3,600 m and almost nothing higher. Ronald Harker flew one in May 1942 and wrote a memo; the first Merlin conversion flew in October. The Merlin 61 gave 1,390 hp at 7,200 m, where the Allison had nothing, and the predicted speed went from 630 km/h at 4,500 m to 710 km/h at 8,500 m. Nothing about the airframe changed. It had been waiting.

GO DEEPERHow the Mustang reached Berlin →

· WHY IT MATTERED

The cleanest fighter of the war

The Mustang’s wing was the first on a production fighter designed for laminar flow, keeping the air attached smoothly further back along the section; its belly radiator was ducted so that the heat it dumped into the airflow gave a little thrust back. Its zero-lift drag coefficient of 0.0163 was the lowest of any fighter of the war, and low drag is range.

· WHY IT MATTERED

Escort to Berlin, and orders to hunt

In July 1943 the Air Force ordered maximum internal fuel: an 85-gallon tank behind the pilot that made the aircraft tail-heavy until it was burned off, plus two drop tanks. In January 1944 General Doolittle changed the escorts’ orders from staying with the bombers to going after the German fighters wherever they were. Mustangs were over Berlin on 4 March.

GO DEEPERThe Merlin in the Engines timeline →

· THE ENGINE

A Merlin built in Detroit

Packard built the Merlin under licence with American tooling and tolerances, and the V-1650-7 was the Merlin 66 with the two-stage blower geared for the heights the escorts flew at. Its pressure carburettor injected fuel into the eye of the supercharger, so unlike the Spitfire’s early Merlins it never cut out under negative g. On 150-grade fuel it could hold 1,720 hp for five minutes, and it still had 1,270 hp above 30,000 ft — the altitude where the Allison had given up two years before.

Packard V-1650-7 (Merlin 66) · V-12 · 27.0 L · two-stage two-speed supercharger, aftercooled · pressure carburettor · 1,490 hp, 1,720 hp war-emergency

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· WEAPONS AND SPECIALITY

Six heavy machine guns and 1,840 rounds

The Mustang D carried no cannon. Six Browning heavy machine guns, three in each wing, fired about 3.4 kg of bullets a second for some twenty-five seconds — less punch per hit than a cannon but far more time on the trigger, and against fighters, where a few hits in the engine or the pilot were enough, it was the right choice. The earlier B and C models had four guns that jammed in hard turns until the mountings were redesigned.

· TRY IT

Escort radius: the fuel plan that reached Berlin

The lab’s range tab is set to a P-51D: 1,020 litres inside and two drop tanks, a climb, a cruise out, fifteen minutes at war-emergency power, the cruise home and a reserve. What is left is a radius from East Anglia. Take the fuselage tank away and watch Berlin fall outside it.

Readout

Still-air range2,660 km

Radius, out and back1,330 km

Fuel on board1,143 kg

Fuel fraction25 %

North American P-51 Mustang: Breguet with cruise L/D 14.6, scaled so that the published 2,660 km comes out with the drop tank — as the speed tab is scaled to the published speed. Still air, no reserve.