Just point out (for the dozenth time) that were differences between the Spitfire and P-51 that affected how they were used and what you could do with them.
Maybe if you took yes for an answer you would not need to repeat. No one here is trying to turn the Spitfire into the Mustang. They are trying to keep the Spitfire range comparable to the 1943/44 P-47, in a similar time frame.
The MK VIII was supposed to be the longer range version.
So it did not have the longest range on internal fuel of any Merlin version until the 1944 upgrades?
Probably not a good idea to put rear fuselage tanks in the early MK IXs.
Even with the extra forward weight you note? You are now in 1942, when no one else is thinking of or proposing rear fuselage tanks.
There isn't a lot of opportunity to build long range MK IXs (and by long range I mean 150 Imp gallons internal).
You could build a plane with more range than the MK IX but how much is it really going to get you and are you willing to screw up production of the MK IX in the fall/winter of 1942 to get it?
That is the mark VIII plus 33 gallon rear fuselage tank. Why require it in 1942 to early 1943 after noting even the USAAF did not need that sort of range escorts given its operations at the time?
What version of the Spitfire do you want to stop in order to free up engineering staff (and workers for prototypes ) for the "somewhat" longer ranged MK IX Spitfires.
Why the somewhat and supposed ranges? How about two things, fit the larger internal fuel tanks into the Castle Bromwich Spitfires as it switches to mark IX, then add the 33 gallon rear fuselage tank in say October 1943, the 12th month of mark VIII production and the 14th month of mark VII. Given the exchanges about fighter ranges and drop tanks between the USAAF and RAF in mid 1943?
"It might have been quite possible to make a long (or longer) ranged version of the Spitfire, but it might have required more engineering change"
" however you may very well need, ... You may want ... you may want ... You may want ..."
Have you noticed your Spitfires tend to turn up later, thirstier, more fragile, harder to fly and modify than the ones the British built?
The time line is clear enough, the mark VIII with 90 gallon external tank as built has a longer range than the P-47 to July 1943, matches the P-47 range in August 1943, becomes less in September and much less in early 1944. Add a 33 gallon rear fuselage tank and it matches the early 1944 P-47. Pushing the Spitfire to the edge of its stability envelope and weights would take a lot of resources and time, to come up with something with less range than the off the shelf P-51B and later could supply. Adding a 33 gallon rear fuselage tank to remain competitive with the P-47 range in 1943/44 was clearly possible. Historically none of the 1943 Spitfires came with rear fuselage tanks, cut down rear fuselages or E wings, they were all 1944 changes, the XIV was (just) a 1943 production aircraft, the F.21 was a 1943 design one.
At 66 gallons per hour, 320 mph, translates to 4.85 miles per gallon, the Spitfire VIII had 660-434 = 226 miles more range than the Spitfire IX by having 124-85 = 39 gallons more fuel, 226/39 = 5.8 miles per gallon at economic cruise. So to reach 320 miles at fast cruise cost 66 gallons, at economic cruise 55.2 gallons. Looking at the map on page 8 here and knowing where the front line was in September 1944 it was possible for aircraft in Britain heading for the Ruhr to avoid hostile airspace until the German border, including flying direct courses, no need for high speed cruise until quite near the target, anyway the Germans were not contesting Netherlands airspace with fighters very often, if at all. That was the profile for the Spitfire longer range escort missions in 1944/45 and there were open continental airfields if something went wrong. Spitfire IX 85 gallons, add the 28 gallons of wing tanks and 66 gallons in the rear fuselage, total 179, deduct 103.5 for 300 miles of range, 23 for take off and climb, 36 gallons for combat leaves 16.5 gallons, you could almost do the trip on internal fuel with adequate reserves, alternatively have 113 gallons internal less 52 for return and 36 for combat leaves 27 gallons, the 90 gallon external tank handling the 52 for outward and 23 for climb. The Big Ben anti V-2 operations consisted of Spitfire fighter bombers from Britain making an attack, landing in Belgium, rearming and refuelling and doing another attack on the return journey.
As far as anyone can find the RAF did not run a single mission with a Spitfire VIII/IX/XVI formation using 66/75 gallons of rear fuselage plus wing tanks and a 90 gallon external tank. So there is no evidence for how well the combination went in service use, only the test pilot's reports. As noted that sort of fuel load is about 300 pounds heavier than the standard Spitfire VIII carrying 1,000 pounds of bombs, so at the least that needs to be engineered. To bring it back to the weights actually flown means a reduction of around 40 gallons. So say the 33 gallon rear fuselage tank and 90 external or the 75 internal and 50 external. While the usual rule is better to carry things internally the effects on handling by that much rear fuselage fuel means I suspect many pilots would have preferred the larger external tank. The test pilot's reports make it clear 33 gallons in the rear fuselage was completely acceptable, going beyond that caused increasingly difficult handling problems. Also having two smaller tanks in the rear fuselage was better than one big one because of the way the fuel would move during things like turns when the tank was part filled.
A Spitfire VIII with a 33 gallon rear fuselage tank and 90 gallon overload tank by end 1943 is not an engineering problem, it is whether it was asked for. Similar for taking the hit to production and switching Castle Bromwich to using the bigger fuselage and the wing tanks in 1943. Going beyond 33 gallons in the rear fuselage is an engineering issue, both in terms of handling and weight, it would not be a 1943 aircraft without a lot of work starting in 1942 and probably luck in coming up with the right answers early, if there actually were answers that could be implemented within the constraints of the time (Spiteful tail cure for F.21 series problems for example). Post war rules meant the RAF only used the rear fuselage tanks in the Merlin Spitfires at least with special permission, similarly post war Mosquitoes were flown at maximum weights of at least a couple of tons less than routine wartime weights and even during the war the RAF was wary about filling the rear fuselage tank of its Mustangs.
The various changes made to the Spitfire in 1944 are not well documented, even things like the cut down rear fuselage and E wing which are visible in photographs still have uncertainties. The internal fuel arrangements more so. Add the ongoing monthly quota for Spitfire IX to Russia, which would not want most of the longer range and which prolonged the mark IX production.
The Portal comments about long range were made in 1941, by then or at latest the end of 1941 the PR Spitfires had proved the range could be extended and the pilots could handle the flying hours and navigation loads. The early 1942 P-51 showed the combination of long range and high performance was quite possible. The Spitfire mark VII and VIII were the considered redesign starting in 1941 to make the best use of the Merlin 60 series engines and included adding another 37 gallons of fuel which combined with the 90 gallon external tank meant being able to reach out to around 300 miles at fast cruise and towards 400 at economic. In any case with its world wide commitments adding a long range fighter force in Britain plus associated long range day bombers would require reallocation of resources on a major scale to be effective. As we know the US supplied the bomber force but that was all plans in 1942 and into 1943, taking much longer than the timetables said and not asking for fighter escorts.
The Spitfire XIV as introduced was considered weight limited, not surprising given the engine change and the need for a larger tail, it reverted to the mark IX forward fuselage tanks, 87 versus 96 in the VII and VIII, but kept the wing tanks, the FR version did add rear fuselage cameras. The XIVE added a 33 gallon rear fuselage tank, the FR XIVE added cameras as well. The XIV was considered an interim version with the XVIII the definite one, it had more strengthening and a pair of 33 gallon rear fuselage tanks plus things like a wing that eliminated the residual support for the outboard 0.303 inch machine guns.
On top of the evolution of the original airframe and wing came the redesign for the F.21 and later series, the F.21 began production at Castle Bromwich in April 1944, with the probable aim of shifting all production to the new model, but handling problems kept the F.21 out of the war until just near the end, in fact until after the F.22 started production began, the F.21 came with 124.5 gallons of internal fuel, forward fuselage and wings. The F.22 introduced the rear fuselage tanks during production, the much later F.24 came with a pair of 33 gallon rear fuselage tanks.
Packard Merlin production was was quite consistent July to December 1942, 801, 800, 800, 800, 796, 849, not so consistent January to June 1944, 1,564, 1,490, 2,000, 1,905, 1,275, 2,239
Supermarine Spitfire output July 1940 to April 1941, 137, 127, 100, 61, 74, 42, 49, 66, 67, 74. The mark V began series production in March, mark I ended in April. Overlaying the September damage there was usually a reduction in output during winter, harder to make test flights, plus there was the chance for the staff to recover from the major efforts during the Battle of Britain. January to June 1940 production was 37, 51, 40, 60, 77, 93
Weights, Merlin III 1,375 pounds, Merlin 46 1,425 pounds, Merlin 61 1,640 pounds, Griffon 61 1,980 pounds.
As a final note the British economy was tightly stretched in 1943, even simple things like putting drop tanks into series production involved time lags and trade offs where production of other items was cut and this continued into 1944. The US histories note the required resources for Operation Overlord had world wide effects, they also note some of the effects on the British economy, like cutting steel production. The invasion force was taking over ports for holding its shipping and then for moving to France, reducing the port's ability to handle normal economic traffic, while generating large scale moves to handle the supplies and equipment arriving, then departing from different ports. In the first half of 1944 the British added 28,300 more people or 13.1% more labour to the railway system, including Italians. Despite this in May restrictions were placed on civil rail traffic.
Overlord needed plenty of coasters but the US had been very successfully building largely ocean going tonnage. The rather busy British coaster fleet would need to be used, which meant withdrawal from service for modifications then withdrawal as part of the invasion. Initially some 396 coasters totaling 623,000 DWT were considered suitable and each used underwent an on average 3 month refit (extra armament, accommodation, strengthened masts and smoke making equipment) the refit program began in October 1943 just before the railways began having their usual winter problems. As it turned out the reduction in war damage to the coaster fleet meant the tonnage available in the winter of 1943-4 was about the same as the previous year. Despite agreements to hand back ships after the initial invasion it turned out that between December 1943 and the end of 1944 another 64 coasters of 100,000 DWT were added to the invasion fleet. The US helped by allowing use in Britain of locomotives and rolling stock meant for France. This resulted in post invasion exchanges of we want our Coasters/Trains back, you first.
The real world restrictions on what ifs.