Part One: The British breakthrough that didn’t

There are a lot of ways to end a sentence that begins “If it weren’t for the Rolls-Royce Merlin…”

In fact, it would be a fun parlour game next time the avgeeks got together.

If it weren’t for the Rolls-Royce Merlin… the Cromwell tank would have been a tin pillbox could be one slightly off-piste option. 

But I digress.

The version I actually want to focus on is “If it weren’t for the Rolls-Royce Merlin… the P-51 Mustang would have been a fairly forgettable (i.e. forgotten) strike-recon workhorse.”

 Of myth and legend

The story of the P-51 is its own land of myth and legend. Of how it was designed and built in 102 days (it wasn’t); that the key to its performance was its laminar flow airfoil (unlikely); that it was superior to the German fighters (debatable); and that it could escort bombers all the way to Berlin, fight the Luftwaffe, and then escort the bombers home again (that’s not how it worked).

What is undisputed is that the Mustang emerged as a game-changing escort fighter with excellent performance and incredible range. And every telling of the Mustang story has, as its pivot point, the switch from the smooth, beloved but un-supercharged Allison V-1710 to the superb, fuel-sipping, twin blower Merlin 65/Packard V-1650-3.

And the rest, as they say, is history…

Surprise, surprise

History, however, deserves better.

Given how that change made the Mustang, the process deserves more than a line or two in some YouTuber’s P-51 explainer. Even authors who do expand on this critical evolution will usually settle for half the story. The truth – surprise, surprise – is a multi-layered saga of foresight, innovation and native talent that is in danger of being segué-ed right out of history.  

And bear with me if you already know the Rolls-Royce half of this story. I still might surprise you when we head back to NAA in California.

A 12,000 foot advantage

Developed through the blessings of Sir Henry Self’s British Purchasing Commission, The Mustang I came to England with the Allison V1710 engine. The British soon discovered that their new fighter was incredibly fast at low level, but more of a target at altitudes above 15,000 feet. There is no official record of their disappointment, but the aircraft was quietly assigned to tactical reconnaissance duties. Bomber escort is somewhat futile when the defending fighters start with a 12,000 foot advantage.

Enter the man who, conventionally at least, gave the Mustang wings.

A somewhat oxymoronic experiment

In May 1942, literally days after RAF Mustang Is made their first foray over enemy territory with No. 26 Squadron, Rolls-Royce senior service liaison test pilot Ronald Harker suggested to his superiors that the new fighter  could be a lot more if it were harnessed to their Merlin 61 engine, which was being perfected for the new Spitfire Mk.IX.  

Harker had been inspired by taking a Mustang I for a flight at the invitation of his friend Wing Commander Ian Campbell-Orde, who was in charge of the RAF Air Fighting Development Unit (AFDU) at Duxford. Campbell-Orde had Mustang AG422 on strength for high altitude testing – a somewhat oxymoronic experiment – but Harker was more than impressed by its performance lower down. 

He noted that the Mustang was considerably faster than Curtiss P-40s with the same engine at all altitudes, and faster than the frontline Spitfire Vc below 15,000 feet. Immediately after flying the Mustang, he wrote to his superiors that “…with a powerful and good engine like the Merlin 61, its performance should be outstanding…”

Taken up quickly

Harker’s recommendation was taken up quickly and enthusiastically by Roll-Royce senior management, who went over some reluctant heads at the Air Ministry and used their political connections to push the proposal through.

Interestingly, they enjoyed the personal backing of Air Marshall Sir Wilfrid Freeman, who had famously championed the De Havilland Mosquito along with several other pivotal RAF aircraft, and must surely rank as one of the most consequential figures in the whole 1939-1945 air war.

As part of this campaign, Rolls-Royce’s Chief Aerodynamic Engineer Witold Challier calculated that a Merlin XX Mustang could easily achieve a level speed of 400 mph and that the Merlin 61 would increase this to 441 mph at 25,600 ft (7,800 m).

The engineers at Allison must have felt ill.

Optimised for middle altitudes

The Merlin 61 had been developed for use in new high altitude Spitfires (Mk.VII and Mk.VIII) from the Merlin 60, which was itself developed for a new high altitude version of the venerable Wellington bomber. To that end, the new variants featured an innovative two-stage supercharger, which gave unrivalled performance all the way up to 38,000 feet (and beyond).

As a rushed stop gap to counter the arrival of Focke-Wulf Fw-190s in theatre, the new engine was married to the Mk.Vc Spitfire airframe and became the ’interim’ Mk.IX. 

As it happened, the Merlin 61 would never see the inside of a Mustang. For reasons unknown, the Mustang experiments proceeded with the Merlin 65 (a further subvariant of the -61) which had its twin blowers tweaked to optimise their performance at middle altitudes. 

And for the record, variants of the Spitfire IX would be powered by the Merlin 61, 63, 63A, 66 and 70. Mechanics nicknamed the Merlin “the watchmaker’s nightmare” – but all those variants can’t have been much fun for RAF Maintenance Command either!

Mustang X

As a result of Harker’s initiative, Rolls-Royce received six RAF Mustangs Is (NA-73s) to convert and test under the special designation ‘Mustang X.’ 

Work began on four of them (AL975, AM203, AM208 and AL963) on 2 June 1942. The remaining two would not fly with Merlin engines: AG518 was only used for engine installations studies, while AM121 was held in reserve. 

AL975 was first out of the gate on 13 October 1942, with a Rotol four-bladed propeller to harness the extra power and Rolls-Royce Chief Test Pilot Ronald Sheperd at the controls. By November, the small test fleet was delivering everything Harker had expected – including a new top speed of 433 mph at 22,000 feet and a new absolute ceiling of 40,600 feet.

Critically, every great aircraft’s friend Wilfrid Freeman again played a role. To ensure there would be no ifs, buts or maybes about the new fighter, he arranged for two of the Mustang X aircraft to be delivered to US Eighth Air Force commander Major General Carl Spaatz for American evaluation.

This tactical gift virtually guaranteed there would be strong demand for the conversion from the forces that mattered most. And the rest, as they say, is history…

Except that it’s not. There is a lot more to this tale – and the best (as they say) is yet to come.

See you in Part Two!