A two-stage Merlin in the cleanest airframe of the war: it flew to Berlin and back, and broke the Luftwaffe in a spring.
1944·710 km/h at 7,600 m·2,660 km·Packard Merlin V-1650-7, 1,490 hp·12,800 m·~15,500
1· 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
2· 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.
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.
4· 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.
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.
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.
7· 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.
North American P-51 Mustang, 8 of 10
Deep dive · How the Mustang reached Berlin· opened from North American P-51 Mustang
Timelines · WW2 aircraft · go deeper · North American P-51 Mustang
How the Mustang reached Berlin1944
Low drag, a tank behind the pilot and two under the wings: a fighter with a bomber’s range.
1,020 L·2 × 284 L·2,660 km·~930 km
01The idea
Fuel plan for an escort: climb, cruise out, fifteen minutes of fighting, cruise home, reserve
Escorting a bomber to Berlin is not a matter of how far a fighter can fly: it must get there with the bombers, fight for a quarter of an hour at full power, and come home with a margin for weather. That is a fuel plan, and in 1943 no single-engined fighter could make it. The Mustang could because of two things: the lowest drag of any fighter of the war, and a decision to fill every space in it with fuel.
02Why it matters
Drag is range
The Mustang’s laminar-flow wing and ducted radiator gave it a zero-lift drag coefficient of 0.0163, and range is inversely proportional to drag. A Spitfire on the same fuel could not have gone half as far.
03Why it matters
The tank behind the pilot
In July 1943 an 85-gallon tank went behind the seat. Full, it moved the centre of gravity so far back that the aircraft was unstable — pilots were told to burn it off first and not to dogfight until it was — and with two drop tanks it gave 1,020 litres inside and 570 more outside.
04Why it matters
Doolittle changes the orders
Escorts had been told to stay with the bombers. In January 1944 General Doolittle told them to go after the German fighters wherever they found them. Within two months the Luftwaffe’s day-fighter force had lost the pilots it could not replace, and the bombers went where they liked.
05Try it
The fuel plan
Build the escort’s fuel plan and see whether Berlin falls inside the radius.
Readout
Escort radius941 km
Fuel for cruising827 kg
Berlininside the radius
The planning arithmetic the Eighth Air Force did, in round numbers: rich mixture for the climb and the fight, L/D 14.6 for the cruise, still air. Drop tanks are burned first and go the moment the fight starts.