Why the USAAF gave up on the A-36 in favour of the P-47.

[tcolvin]

I believe this is the answer to the question asked by Nick Beale on June 20, 2010 (page 4 of the thread 'The RAF and dive-bombing').

Nick asked, “Can I change the question? Why did the USAAF give up on the A-36 in favour of the P-47?”


The short answer is that the order for 500 (only) A-36s was a USAAF subterfuge (or scam?) for keeping the Mustang/Apache (Allison V-1710 engine) in production after Lend-lease funds ran out and the 1942 USAAC budget for Pursuit fighters had been used up.

The USAAC budget for Attack aircraft, however, still had funds, which could be accessed by turning the P-51 into the A-36 (fitting air brakes and cannon).

By the time the 500 A-36s had been delivered, the USAAF had decided that the P-51B (Packard Merlin) was a long-range bomber escort with potential to achieve air superiority over the Reich alongside the P-47 Thunderbolt.

In this environment an act of Congress would have been required to build additional A-36s instead of P-51Bs.

As the A-36s vanished through attrition, the units operating them switched to old P-40s and then to P-47s or P-51s that could not perform dive-bombing.

None of these aircraft had the accuracy of the A-36, but no one in authority in the USAAF gave a monkey so long as the only complaints came from brown jobs.




The long answer:

1. Feb 1940; RAF (British Purchasing Commission) commissioned the North American NA73 fighter ('pursuit') plane concept with high-speed laminar flow wing, with ventral air scoop to eliminate drag through the 'Meredith Effect', and with 4 x 0.3-inch and 4 x 0.5-inch machine-guns. (BPC actually specified 4 x 0.303 mgs with first production a/c to be received by January 1941). The initial request was for NAA to produce the P-40 Tomahawk under licence, but Kindleberger said he could produce a better aircraft in the same time frame.
2. March 1940 – MAP ordered 320 NA-73 Mustangs.
3. May 5, 1940, design drafting began
4. Sept 9, 1940, completion of airframe, less engine
5. Sept 1941, 150 NA91/P-51 Apache/Mustang Mk 1A (4 x 20-mm cannon) ordered under Lend-Lease
6. Oct 26, 1940, first flight of NA73 with Allison V-1710 fitted with single-speed single-stage supercharger (Allison turbocharger wouldn't fit within the narrow Mustang airframe)
7. Sept 1941, first production delivery. MAP ordered another 300 a/c (cumulative 620).
8. Nov 1941, Mustang Mk 1A entered RAF service and 20 were sent to RAF Army Cooperation Squadrons as a low-altitude recce and ground support aircraft. It outperformed the Spitfire VC by 30mph at 5,000-ft, and by 35mph at 15,000-ft despite the Spitfire's more powerful engine. But above 15,000-ft, performance fell off. The RAF asked to see how the Mustang Mk 1A would perform when fitted with the Merlin engine (two-stage supercharger).
9. Of the 150 Mustang 1s built; 93 went to the RAF who fitted one with a Merlin at Loughborough, 55 were diverted to the USAAF for Photo Recce purposes, and 2 had trial installations of the Packard Merlin (XP78)
10. The USAAC wanted to order P-51s, but their 1942 Pursuit plane budget was used up
11. So instead, on April 16, 1942 the USAAC ordered 500 A-36s with funds from the 1942 Attack plane budget. This necessitated redesigned wings to contain A-31 Vengeance dive brakes and 500-lb bomb shackles
12. May 29, 1942, maiden flight of P-51
13. September 1942, first A-36 produced. One was delivered to the RAF as Mustang Mk1 Dive Bomber
14. November 30, 1942, first flight of XP-51B (Packard V-1650-3)
15. Early 1943, production of P-51B began as B-17s and B-24s were being shot out of the European skies. P-51Bs got absolute priority, and no more A-36s could conceivably be built. This was the death sentence for the only USAAC dive-bomber ever used in combat in WWII, and with great success.
16. Without new A-36 production, units operating them had to switch to P-40 Tomahawks, and then to P-47 Thunderbolts or P-51 Mustang fighter-bombers.

Tony

[SteveB]

Tony

I think there are few bits to add to your analysis.

It has been stated that the first time consideration was given to putting a Merlin in the P-51 was when Ronald Harker flew the Mustang I at AFDU on 30/4/42. As you say the first Mustang had flown in October 1940 and I believe the XP-40F had flown with a Merlin in June 1941 so it would be a surprise to find that consideration had not been given to putting a Merlin in the P-51 airframe before April 1942. Ray Wagner writes that the USAAF authorized the NAA Merlin project on 12/6/42.

The peak number of RAF Allison Mustang sqns was 16 and that number started to decline from the end of 1943 and by June 1944 there were just five Allison Mustang sqns in 2TAF. It seems that this decline in availability of Allison Mustangs was anticipated because the RAF experimented (unsuccessfully) with Typhoons for the TacR role and then after D-day introduced the Spitfire IX and finally the XIV to the TacR role. The RAF wanted either more Allison Mustangs or they wanted more Allison engines but it seems the Americans would supply neither. Yes it is correct that the A-36 and the P-51A/Mustang II kept the line going at Inglewood but it also seems to be the case that the US authorities wanted to focus all production planning on the Merlin Mustang and wanted supplies of Allison engines for other types especially the P-38.

Steve

[tcolvin]

Interesting, Steve, but could you explain, please, why the RAF wanted more Allison Mustangs/Allison engines rather than Merlin Mustangs.
Did the Allison Mustang have an advantage?

I understood the RAF insisted their aircraft performed at altitude, and this was the reason they rejected the P-39 Aircobra. (I have heard an opinion that the VVS had no such requirement, and therefore accepted the P-39, because most aerial actions in Russia took place below 15,000-ft).

Packard undertook in September 1940 to build Merlins for both the US and British governments. The first Packard-built Merlin was fired up in Detroit on a test-bed on August 2, 1941.

So the XP-40F must therefore have been fitted with a RR-built Merlin in June 1941.

And the MAP could have specified Merlin Mustangs from the get-go had the RAF wanted them. Which comes back to the first question above, as to why the Allison Mustang was preferred.

Tony

[Graham Boak]

The Allison Mustang was not preferred to the Merlin-engined one: it was preferred to the other types that were available for low-level fighter/recce, because of its longer range. The Merlin Mustangs were kept for use as fighters. If the RAF could have had all the Merlin Mustangs it wanted, some would have been free for FR roles.

[SteveB]

Tony

I, for one, am not certain whether the RAF’s policy on engine performance remained the same in 1943-44. Clearly in 1940-41 when the P-40 and the P-39 were being delivered and trialled in the UK, RAF planning for home-based units was focussed on the continuing expectation of an invasion and especially concerned about intercepting high-flying day bombers and their escorts. I agree that this was apparently a significant reason why both aircraft types were rejected for RAF Fighter Command although there were quite a few other important technical factors for both types.

I don’t think it was the case that in 1940-41 the RAF had clearly identified the potential of the low-level/TacR/fighter reconnaissance role even if ACC had. I don’t think it would have been possible at that time to allocate Hurricanes or Spitfires to ACC to develop that role but it did become possible to develop that role through the introduction of the P-40 and then the Mustang in ACC. By 1943-44 the Allison Mustang had proved itself in this role and, as I understand it, was competitive with the Merlin Mustang for speed at low altitude and, as Graham has said, with all other types available to the RAF as far as range was concerned. Perhaps the decisions were based largely on a “horses for courses” basis but it was certainly the case that in 1943-44 the RAF had a lot of unused Mk I airframes into which I assume it was not a simple task to drop a Merlin engine and apparently there were no replacement Allison engines.

My understanding is that in the planning for Merlin Mustang production it became clear at an early stage that there was never going to be enough airframes or engines to meet all the expectations the allied air forces had for the aircraft. As you say, MAP may have been able to specify Merlin Mustangs “from the get go had the RAF wanted them” but the BAC files do not confirm that they would have been allocated to the RAF. The BAC files show that, in the process of agreeing the bi-annual planning protocols that under-pinned the lend-lease allocations, the BAC went into negotiations with expectations (from RAF/MAP?) that they were not able to agree with the Americans and the reviews of the six month protocols show that the final allocations of Merlin Mustangs to the British were usually less than had been previously agreed in those protocols.

I think the engine in the XP-40F was a Merlin XX?

Steve

[tcolvin]

According to 'Jane's 1945/46'; "Owing to poor performance at height the Mustang I (P-51) was re-mustered as a low-altitude reconnaissance fighter and posted to the RAF Army Co-operation Command ....... an oblique camera for tactical photographic reconnaissance was installed ..... first operational with ACC on July 27, 1942".

In other words, the RAF realised it had ordered an air-superiority Mustang that did not perform, so got rid of them to ACC and asked for the Mustang III (P-51B) with the Merlin.

Jane's states; "The original conversion was made in GB by RR by the installation of the Merlin 61 in the Mustang II. The success of the conversion was such that steps were immediately taken by NAA to re-design the P-51 to take the 1,520 hp Packard V-1650-3 (Packard-built Merlin 68 with two-speed two stage supercharger and aftercooler). The airframe was strengthened ........".

The RAF had begun its trick of getting rid of under-performing air-superiority fighters (Allison Mustangs,Typhoons and Spitfires) by over-selling them to the army as ground support Tac-R and fighter-bombers.
It got rid of accurate dive-bombers such as the Henley and Vengeance to target-towing duties. The RAF could not foresee the time when the GAF had lost command of the sky over the Reich and with it the means of shooting down conventional dive-bombers.

Also the RAF convinced itself that high-speed fighters could perform neither vertical dive-bombing nor long-range bomber escorting, even when faced with the contrary evidence. The RAF managed to suppress news of the success of the A-36 and Vengeance, but had to accept the success of the P-51B (and the earlier P-47) when it started in January 1944 to operate as a fighter over Germany on bomber escort duties and to destroy the GAF.

The A-36 was a weapon system that could have had a strategic impact and ended the war in 1943. It was cheap enough to be produced in volume, and accurate enough and with sufficient range to destroy the 100 or so electricity generating stations on which German war production relied. The vulnerability of the German electricity system to such an attack was highlighted in the postwar USAAF Strategic Bombing Survey.

Tony

[Graham Boak]

It would be interesting to learn the mechanism whereby the RAF "suppressed" information regarding aircraft operated by other air forces.

I suspect the RAF could foresee the Luftwaffe losing command of the air. What it couldn't do is rely upon that assumption. What it could rely on was the success of light flak in suppressing the operation of slow large targets over the battlefield. It had learnt that hard lesson over Masstricht in 1940, if nowhere else. The real enemy of the Allied dive bomber was not the RAF but the Quad 20mm.

As for a long range fighter: Portal was looking for such an aircraft from a very early date. Just why do you think the RAF wanted the Merlin Mustang, and why it was not available for FR duties in 1944/45? To claim a lack of interest inside the RAF for such an aircraft is provably wrong. What it could not do was immediately alter the previous design emphasis on high rate of climb.

Given air superiority, as you yourself argue, aircraft of lesser performance can operate successfully. It would make little sense to swap priorities and place the very highest performing aircraft into ground attack and let your inferior fighters fail to gain that very necessary superiority. However, you completely overlook the matter of differential altitude performance. The Allison Mustang was a much better aircraft at low level than higher, that is why it makes sense to operate it in a role that demands just that. Particularly as the numbers available and the numbers required were close. The same argument applies to the Typhoon. There was little or nothing inferior about it at low-level. It makes sense to employ it there.

If, as you argue, the RAF was only interested in dumping inferior aircraft into the tactical role, there were an awful lot of Spitfire Mk.Vs that could have been so employed - but weren't. The RAF employed the right horses for courses, when it could.

[tcolvin]

Graham;

I would say the mechanism was 'cold water', which the RAF liberally poured over all reports of dive-bombing accuracy and long-range high-performance fighters.
The RAF was for some time in a position to lecture the USAAF from a position of hard-won experience - Battle of Heligoland Bight, Meuse bridges and all that.
They could and did say "told you so" when the B17s and B24s were slaughtered in daylight.
The RAF poured water, I would suspect, by giving credence to the 'wired shut' story of the dive brakes on the A-36, which resulted after a couple of French pilots entered the dive before deploying the brakes and tore the wings off their A-36s. The absence of an anti-foaming agent in the hydraulic oil led to examples of A-36 dive-brake failure that the RAF also would have played up. I could go on (and on), but almost any book by Peter C. Smith will give you chapter and verse on the RAF's absolute refusal to accept dive-bombers, and the lengths they went to rubbish them.
The RAF air marshals were opinionated and bullying, and notorious for spin and political correctness. They brooked no dive-bombers or real cooperation with the army.

Which brings us to Portal's bitterly held opposition to the long-range fighter. If Portal was looking for one, as you say, then he had a very curious way of expressing himself.

I would refer you to Appendix G of Terraine's 'The Right Of The Line', titled 'Sir Charles Portal and the long-range fighter question', which begins; 'My repeated assertion of the direct involvement of Sir Charles Portal, while CAS, in the question of long-range fighters for the RAF, and his personal opposition to such a weapon has been questioned. Yet it is strongly documented in the Official History".
Terraine gives quotes;
May 27, 1941 Portal told the PM; "The long range fighter, whether built specifically as such, or whether given increased range by fitting extra tanks, will be at a disadvantage compared with the short-range high performance fighter". (NB the Mustang was flying by then).
June 3, 1941, Portal replied to Churchill's urging to increase the fighter's range, by repeating "that long-range fighters could never hold their own against short-range fighters".
In 1942, Webster & Frankland say Portal stated as a fact that the production of an aircraft with the range of a bomber and the performance of an interceptor fighter was a 'technical impossibility'.
Portal had arguments with General Arnold about the unacceptable (to Arnold but not to Portal) spectacle of 1,461 fighters inactive in Britain while USAAF bombers were being shot out of the skies. ('Told you bombers couldn't operate in daylight' was what the RAF brass thought, and probably said).
Portal remained unmoved even when Arnold "caused some Spitfires to be specially equipped to fly the Atlantic".
Terraine concludes his Appendix; "The advent of the P-51B in December 1943 settled the matter; it is clear that it constituted a blind spot in Portal's war direction".
I would add this was certainly not the only one.

The right horse for the course of tactical and strategic accuracy was the dive-bomber. Pace Portal, the A-36 could do both successfully, and even, it would appear, in the absence of aerial supremacy. German electricity generation could have been destroyed in 1943.
For Portal, however, the only imperative was the heavy bomber force and the mission to burn out the hundred biggest German cities.

Tony

[Nick Beale]

"The RAF had begun its trick of getting rid of under-performing air-superiority fighters … Spitfires …"

The Spitfire was an " under-performing air-superiority fighter"? Granted the Mk. V was overtaken by the Fw 190 but the Mk. IX was a viable air-to-air combat aircraft from its introduction until the end of the war, was it not?

"The RAF could not foresee the time when the GAF had lost command of the sky over the Reich and with it the means of shooting down conventional dive-bombers."

But as it turned out, the only mean of bringing about that German loss of command was to build fighters in huge numbers.

[Graham Boak]

There were not "some" Spitfires modified to fly across the Atlantic. There was one - it involved removing all the armament (which really would have been particularly useful for a long range fighter), converting the wing to an unsealed fuel tank and the fitting of a fuselage tank that rendered the aircraft only capable of very limited manoeuvres. The result was a useless freak.

That the Mustang was built with a large internal fuel capacity to RAF requirements might, I'd have thought, make it rather difficult to argue that the RAF was totally opposed to long range fighters.

There were three developments that lead to the successful development of the long range escort fighter.

The first is the availability of a fighter with a high fuel fraction. Given that the RAF's prime responsibility was the defence of the United Kingdom with short warning times of attack, this led prewar to the development of aircraft where light weight required a low fuel fraction. You might find it interesting to compare prewar use of the high fuel fraction fighter such as the Bulldog as opposed to the low FF fighter such as the Fury. This approach is not something that could be rapidly turned around: it takes a number of years from the issue of a specification to the appearance of the resulting type in significant squadron numbers. The Tempest is perhaps an example, which reached RAF service not far behind the Merlin Mustang.

The second was the widespread adoption of drop tanks, something that did not begin until late in 1940 and was fairly rapidly developed for the Spitfire and other RAF fighters. However, there is a limit to the value of drop tanks for a fighter with low internal fuel loads - they are not carried in combat and there is no point in flying out further than you can return after combat.

The third was the addition of extra fuel tanks behind the pilot on the P-47 and P-51. Until then the operating radius of the P-47 was little further than that of the Spitfire. These tanks required a sizable amount of space and a sufficient cg range: even then the fuel in these tanks had to be used up immediately after takeoff and the aircraft had to be very carefully handled, with a high workload on the pilot. The Spitfire had a smaller fuselage, with less space behind and a more limited cg range. A rear fuselage tank was developed, but was not acceptable for service until the heavier Griffon variants, which being thirstier than the Merlin added little to the practical range.

The combination of these factors, not all of which could be successfully predicted in 1941, are what lay behind the eventual success of the long range fighter in late 1943/early 1944. The early RAF interest in the Merlin Mustang, and the development of fighters such as the Tempest and Hornet, give the lie to ill-informed claims that the RAF were not interested in long range fighters. They were when they could get them.

It is worth remembering that the RAF was not monolithic: in addition to the bomber barons there were fighter-biased marshals, Tedder with his development of successful ground-air co-operation that showed the way for all other nations, and those pressing for increased maritime emphasis. All were working towards eventual victory, all were developing their own particular areas. That at some stages a priority was given to the bomber offensive does not rule out all development elsewhere.

Repeating that the divebomber is the best solution for air-to-ground in all circumstances does not make it true.