WI: earlier/more/better Mustangs

Key to more Mustangs is USAAC ordering hundreds more A-36 Apaches. Once they recognize how much faster the A-36 airframe is, USAAF orders thousands of P-51B, etc.

Yes, P-51Ds had a large, L-shaped fuel tank behind and below the cockpit. Yes, that fuel tank pushed the centre-of-gravity too far aft, reducing stability and reducing maneuverability. But, on long-range escort missions, most of that (un-balancing) fuel had been burned off before they encountered significant numbers of German interceptors.

Trivia, most the surviving P-51s that are still flying (in 2016) have lost their long-range fuel-tanks and have a second seat installed in its place.

Speaking of German interceptors ...... Yes, Messerschmitt 109s were "twitchy" during landings and take-offs, but I suspect that many were damaged during landing after being shoot-to-pieces in battle and collapsed on landing.
Given the Me-109's modular construction, mechanics could (comparatively) easily canabilize 3 damaged 109s to re-assemble one airworthy airframe.

Early in the war, it was not obvious that Merlins were significantly better than Allisons, ergo little incentive to shut-down Allison production lines to re-tool them for Merlins. Alison's were still competitive at lower altitudes.
At a minimum, all Allison needed was an engineer as bright as Ricardo or Hooker to refine their super-chargers so that they competed directly with Merlins at higher altitudes.
 
Key to more Mustangs is USAAC ordering hundreds more A-36 Apaches. Once they recognize how much faster the A-36 airframe is, USAAF orders thousands of P-51B, etc.

They ordered the A-36 in the time they should've ordered the P-51/P51A. The 1st P-51B was ordered 8 (eight) months before the A-36 flew combat sorties - Aug 1942 vs. April 1943.
Further: no 2-stage (Packard) Merlins = no P-51Bs, no matter how many P51Bs are oredered by Americans.

Yes, P-51Ds had a large, L-shaped fuel tank behind and below the cockpit. Yes, that fuel tank pushed the centre-of-gravity too far aft, reducing stability and reducing maneuverability. But, on long-range escort missions, most of that (un-balancing) fuel had been burned off before they encountered significant numbers of German interceptors.

The fuselage tank was rectangular, not L-shaped. Also installed in vast majority of P-51Bs, all P-51Cs, and retrofitted in the 1st P-51Bs that were produced without it.
Under the pilot were the 'normal' tanks, each spreading between centreline and armament, between the spars, hence they were called wing tanks.

Early in the war, it was not obvious that Merlins were significantly better than Allisons, ergo little incentive to shut-down Allison production lines to re-tool them for Merlins. Alison's were still competitive at lower altitudes.
At a minimum, all Allison needed was an engineer as bright as Ricardo or Hooker to refine their super-chargers so that they competed directly with Merlins at higher altitudes.

It was obvoius. Eg. the US technical delegation noted before the war that Merlin has bigger induction system (ie. the supercharger was bigger, 10.25 in vs. 9.50 in), that gave better hi-alt performance, worth almost 3000 ft for same power. Also, the Merlin III was allowed for increased boost on 100 oct fuel even before ww2 started, a three years advantage vs. V-1710. Once Merlin XX is introduced in mid 1940, featuring Hooker's S/C and general clean-up of intake part of engine, the bar was further upped by 5000 ft, low level performance is again increased, and V-1710 has barely introduced in production.

Allison does not need Ricardo or Hooker (it was Hives at the helm when Merlin emerged, and Farman's ideas and patents were in pipeline before Hooker arrived), it needs green light from the Army when they suggested 2-stage supercharger, plus that Army funnels the funds to them, rather than to money holes of engines in Continental and Lycoming. There is no need to retool the Allison factory.
 
A bit about Ricardo - the US Army liked his hi-performance engines' concepts so much they started designing the engines following it's lead. The projects were pushed down the throat of Lycoming and Continenta, and never gave any service-ready engine, so Ricardo might be better left out of picture.

On topic:
With 2 production sources that either produce Mustangs or are about to do that, it is time to improve design. The OTL decision to install 2-stage supercharged Merlin on Mustang was a really excellent example of Anglo-American cooperation, so this happen also here. It will add another 10-40 mph on the already high speed figure, depending on altitude. Once Allison learns about the contract for the XP-78 (25th July 1942 OTL), later renamed to XP-51B, they burn the midning oil in order to get Mustang outfitted with their 2-stage enigne. That should've happen some time in Autumn 1942, the engine is not very satisfying during the tests although the speeds of 430 mph are obtained. People at Bell are flabergasted, their XP-39E cannot do 390 mph with similar engine. Even worse, the new XP-63A was barely touches 400 mph.
 
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