Why Couldn't The Spitfire Be Modified To Be A Long Range Escort?

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Reading the book Target Berlin it talks of a Spitfire that flew from the UK to Berlin and back. Okay this was a PR Spitfire with no guns and more fuel. Why couldn't the Spitfire be modified with four guns and drop tanks to give it the range?
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My guess, and it is a guess, is that it is to do with the design of the wing creating too much drag compared to a single-engined fighter that could reach Berlin (a P-51 Mustang).
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It might be a fairly complicated answer depending on which mark of spitfire !!
The P-51D holds 184 gallons (wings). The military used drop tanks of a maximum capacity of 110 gallons each and had a 85 gallon rear fuselage tank
P51 D max fuel capacity = 485 (US) Gallons Mark 9 fuel system diagram http://www.spitfireperformance.com/spitfire9-fuelsystem-lr.jpg Mk 9 max fuel capacity = 212 (imp) galls with bubble canopy or 186 (imp) galls with standard (highback) fuselage. The P51 was designed from the outset to have a much higher fuel capacity than the spitfire ! The spitfire was sensitive with longitudinal stability/centre of gravity issues and with the larger rear fuselage tank on later marks (up to 75 galls) was very unstable until approx 50 galls burned off.The spitfire wing was so thin that I doubt very much it could have been modified to carry fuel and still carry guns. Spitfire handling test report... http://www.spitfireperformance.com/Spitfire_IX_ML-186_Handling.pdf

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One thing I've always wondered about the P-51 Mustang is why did it end-up with such an impressive range? Was long-range part of the design criteria? I doubt it, considering when and why it was originally ordered, so was this just a happy accident as part of the attempt to get high performance. I know the radiator design was particularly effective from a drag point-of-view. Or was the original (Allison-engined) range modest but the design proved capable of accommodating large amounts of additional (internal) fuel. Sorry for the thread-drift but it seems like this question will be answered as part of the original question.
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We did cover some of this stuff in an old thread and here is part of an old post of mine... The P51 was also longitudinally unstable with the 85 USG rear fuselage tank full and their normal operating procedure would be similar to ...
Just checked in the Robert Goebel autobio 'Mustang Ace',he confirmed that they did indeed completely fill the 85usg rear tank,He says that he would take off with main (wing) tanks selected,after take off he selected the rear tank and burned that down to approx 30usg at which point he emptied the drop tanks before reselecting the rear tank to empty that ! He also said that if too much fuel was left in the rear tank (C of G too far aft) the a/c would 'tighten up' in the turn and loss of control could follow.
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The P51 was designed from the outset with large wing tanks,the design of the wing meant it was easy to fit wing drop tanks as well !
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A table of P51 fuel capacities... Model XP-51 (170) P-51A (180) P-51B/C-1(180) P-51B/C(269) P-51D/K (269) All clean - no droppers

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Lots of reasons;- 1. It was designed from the outset as an air-superiority fighter mainly for defence, with no thought for or concession to any long-range capability. 2. As CD mentioned, it's wing, not being a Laminar-flow section, was less efficient at the required cruise speed. 3. It had very limited internal fuel capacity. 4. it had limited oil capacity. 5. It's wing wasn't designed with suitable hard-points as in the case of the Mustang. (Later, when bomb-racks and four cannons were added [Around the MkV..?] there were structural issues. Corrected on late marks.) 6. The Spitfire had a relatively limited CG range, and with such a small tailplane (..and fin.), could quite easily become very unstable if the CG crept too far back. None of these things were insuperable individually, but as a package they blocked any cost effective solution for a long time. In the late marks of Spitfire, many of these issues were at least partially addressed. Of course, the Spitfire was outfitted with quite a range of sizes of 'Slipper Tanks'. These were really only a glorified Ferry Tank, but could be dropped in extremis. They slowed the a/c and had to be dropped for any combat. Being a centreline tank, they also placed all of the load on the wings mainspar, as opposed to wing-mounted drop-tanks, such as on the P51, which tended to distribute the load more evenly. On the plus side, the Spitfire was generally a better pure fighter than the P51, but as the war progressed there were also many other types of fighter available to the RAF that were able to carry wing-tanks if required.

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The P51 was designed from the outset to have a much higher fuel capacity than the spitfire...
I wonder why; surely nobody was envisaging escorted daylight raids to Berlin in 1940?
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The USA is a big country ! Does one need a more complex answer than that ? ; )

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It wasn't designed for the USA; it was designed and ordered for the RAF! :dev2:

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Part of this question goes back to basic design principles, and what is nowadays called the Fuel Fraction - how much of the weight of an aircraft is devoted to fuel. In the early 1930s the RAF operated two different types of fighter - a long endurance patrol fighter (the Bristol Bulldog) and a short range interceptor (the Hawker Fury). The interceptor had a lower fuel fraction and thus a lighter airframe providing a superior climb rate and (by being a smaller design) higher top speed. Although the Fury was a very popular aircraft, in exercises it was found that the Interceptor principle didn't work as well as the patrol fighter - though in practice the Bulldog was approaching obsolescence and was too slow. The new requirements for replacement fighters therefore stressed a higher fuel fraction, but the studies showed that this came at an excessive penalty in climb rate. Climb rate was considered vital in view of the short warning times available, so the Spitfire and Hurricane were therefore designed with a lower fuel fraction. It isn't clear to me whether advance planning for the radar system influenced this. In the US, the larger internal distances to be flown and the lack of any nearby threat meant that US fighters were designed with a larger fuel fraction, carrying more internal fuel at the expense of a rapid climb rate which had little value in the US picture. Even so, this had to be increased by a wartime "crash" programme which resulted in the additional aft tanks on the P-51. As said above, the P-51 was unstable with the aft tanks full and this had to be used first. The Spitfire could (and eventually was) fitted with aft tanks, but the lower internal fuel of the Spitfire meant that it could be flown on an aft tank to a range where it could fight but then be left with insufficient fuel to get home. The same would obviously apply to larger external tanks, which were available but used for ferry. In practice the aft tank was of little more use than the belly tank, and these could not be used together. The solution would have been a larger tail to provide greater stability with fuel in the aft tanks, which did appear on the XX series but introducing this earlier would have meant a reduction in the number of aircraft coming off the line and this was not acceptable in the run-up to the invasion of Europe. I'd point out that the Mk.VIII Spitfire had a double boost of internal fuel over the Mk.IX, having a larger main fuselage tank and wing root tanks. This would have alleviated the problem considerably (at least) but the choice of equipping Fighter Command with Mk.VIIIs would have introduced even greater production problems.
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It wasn't designed for the USA; it was designed and ordered for the RAF!
I know mate... Presumably NA were forward thinking enough to realise that the USAAF could potentially be their biggest customer,flying in the US would benefit from large internal fuel capacity. I know much is made of the wing design and cooling system design but the major reason for the P51's great range was undoubtably its fuel capacity ! rgds baz

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RR worked out that a Mustang was some 50mph faster than a Spitfire, with the same engine. This came from a lower drag (mainly the radiator installation) and clearly this converts to a significant drag reduction in cruise too, and hence superior range for the same fuel as the Spitfire. Given that it is a significantly later design, this shouldn't be too surprising. There's no doubt that fuel is necessary to fly any distance, but the key question (as originally posed and hopefully answered above) is why more fuel couldn't have been put into the Spitfire.
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Aircraft performance is a complicated issue with many variables and I do agree that the P51 cooling system was very 'clean' - however if you half the fuel capacity then you at least approximately half the range given a similar a/c size and engine !! rgds baz

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Was the P-51 originally designed with wing-tanks only? From a practical point-of-view I suppose it is much easier to fit extra fuel into the fuselage of a fighter with wing-tanks (P-51) than it is to fit wing-tanks into a fighter that only has fuselage-tanks (Spitfire).
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Yes and Yes rgds baz

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What is the best way to increase the range of a Spitfire..? ...buy a Hawker Tempest!

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Re 14 The Mustang 50mph faster compared with which mark of Spitfire ?

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The only other long-range fighter, that I know anything about, from the time of the (early) Spitfire and the (early) P-51 is the Mitsubishi Zero... ...absolutely nothing advanced about it, plus extremely light construction (no armour or self-sealing fuel-tanks), but very long-range.....and carrier-capable! Presumable this was achieved by carrying a lot of fuel too.
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In my opinion, the OPs original question, was the Spit was a P.R.aircraft, no external tanks, no weapons equals the one thing it was designed for ...Speed, every pound of load means more fuel, it needed to be light, to possibly outrun the enemy fighters it may well have encountered, going to and from the target area. Just my opinion. Jim. Lincoln .7