F.6/42-type fighter, but with Merlin power

Well, how about this: you take a new fighter design designed around the two stage Merlin family (60 series engines, which were basically brand new in 1942, with the possibility to run later versions if/when they get developed). It's designed primarily as an interceptor armed with 4x20mm cannons, is intended to be about the same size as a Spitfire 8 or 9 (or slightly larger), and has a range of maybe 700-800 miles on internal fuel (not as short ranged as a Spitfire). And is intended for the ground attack role to be capable of carrying similar ordinance.

I'm aware that such a fighter doesn't meet F.6/42 as written or envisioned at time, but this is for if a smaller aircraft that can fulfill some of the F.6/42 specs that was Merlin powered was envisioned.

Hence, what do you think such a plane would've looked like, what features would it have had, and so on? You're in the driver's seat as far as designing such an aircraft.

Some things to maybe consider. The lightweight P-51 Mustangs were built to British engineering standards (so would this aircraft), and the result was that the XP-51F/G were about as light at combat weight as the Spitfire IX, had more range (though not as much with normal internal fuel as the earlier Merlin Mustangs), and could/was intended carry most of the ordinance of the P-51B/C/D/K models (6 HVARs, 2000 lbs of bombs). Notably, air to air armament was envisioned to be the 4 .50 MGs of the B and C models (though with more ammo, 400-440 rpg vs 380 inboard/270 outboard) instead of the 6 of the D, K an H models. However, it does seem that the F/G was judged to be too radical for mass production at the time, hence the H model.

For the Merlin 100 engines, a bone-stock P-51B was fitted with Merlin 60 series engines that were 100 series test beds and ultimately production spec 100 engines, and it gained approx 15 mph in top speed at most altitudes, and gained nearly 900 fpm in climb rate according to documents from Rolls-Royce at World War II Aircraft Performance.

And at least one Merlin engined fighter was being developed at around this time as a lightweight, high altitude interceptor. There was two Miles M23 projects. One, from 1940-41, was a lightweight, high speed interceptor that broadly resembled a scaled down Spitfire. There was also the M23A (which is of focus here) was intended to be a dedicated high altitude interceptor powered by a two-stage Merlin and was envisioned to supplement aircraft like the Spitfire 7 (a dedicated high altitude interceptor version of the Spitfire 8) and the ultimately largely stillborn Westland Welkin.

As far as I know, the only reason a Merlin powered fighter couldn't meet this specification was that it explicitly called for the Sabre or Centaurus (and maybe Griffon, I'm not sure) which I imagine is largely because it was simply assumed that only a larger engine would be sufficient, as it wasn't fully appreciated just how much potential the Merlin still had. So while a Merlin fighter couldn't have met the specification itself, it may well have met the basic need behind the specification, which in principle I would define as essentially "improve upon the Tempest V." On a technical level I think this could absolutely have been done with the Merlin, and the benefits of doing so are clear — the Merlin's already in production, it's much cheaper and easier to maintain than the Centaurus or Sabre, and offers much better fuel economy.

The drawback is that most of those Merlins are already spoken for, and it's unclear whether they could have increase production enough to meet existing demands plus the ~1500 or so Merlin F.6/42s you'd be getting instead of Tempest. You could potentially get around this by adapting an existing type (most likely the Spitfire, since the lightweight Mustang hasn't been envisioned yet), but to do that you'll need to interrupt production, which the Air Ministry doesn't want — moving to a ventral radiator on the Spitfire, which would go some way to helping it meet this spec, was supposedly vetoed for exactly that reason. There's also the question of what Hawker is meant to do in the meantime, just keep producing the Typhoons (and eventually Tempests) this new fighter is intended to make obsolete? Hawker could of course have developed their own Merlin-powered fighter, had they the foresight to do so, and it's fun to imagine what that might look like (a mini-Tempest I? Could be quite a looker, nevermind performer), but then you've potentially got the allocation problem again, and even if that doesn't end up being an issue for Rolls-Royce/Packard, what of Napier and Bristol? They've sunk a lot of money into developing these sleeve valve engines, and have set a lot of productive capacity aside to produce them. Retooling to make Merlin's isn't going to be feasible, it'll take too long and cost too much, so it's better to make use of that existing capacity even if you could make something at least as good on a cheaper per-unit basis making Merlins instead. Bristol has other engines in production, and developing the Centaurus was basically just a matter of scaling those up a bit, they would probably survive its cancellation just fine, but Napier could be in a tough spot once Typhoon production winds down in favour of this new Merlin fighter.

So on a technical level, it could absolutely be done, and there are half a dozen different ways to envision how (Supermarine, North American, Hawker, de Havilland, even Miles). But for strategic industrial reasons, it would probably have to be Hawker, and I suspect the specification was written with them in mind for pretty much this reason; similar concerns may also have played into the specification's requirement of the Sabre of Centaurus engine. I also think the only way it makes sense is with the presumption that the Merlin will eventually be capable of ~2000hp for takeoff, which I don't think it would have been impossible to foresee at the time, but certainly wouldn't have been possible in 1942, so you're probably not looking at this fighter getting into production much before the Tempest — which is to say just in time for D-Day (incidentally also around when Merlins started using 150 octane I believe, someone can correct me if I'm wrong).

I wonder if I should create a thread separate from this one (or maybe put it in here even) of asking about maybe a "super Spitfire" powered by a Merlin, or Hawker making a "baby" Tempest or Fury with a Merlin? I already did one for if de Havilland did a "what if" single seat single engine fighter.

I know that I posted above about seeing if this would be better served from here if this was broken off into a "super Spitfire" thread or a "baby Tempest" thread, or (new idea) a "British or USAAF equal to the Ki-61/inline-engine powered Ki-100 of improved performance".

However, I was thinking about armament. I think I posted about having 6 .50 MGs with 300 (min/normal) to 400 (max/overload) rpg, or 4 20mm cannons with 150-200 rpgs (same min/max and normal/overload conditions apply). But what about 2 .50s with 300-400 rpg and 2 20mm cannons with 150-200 rpg?

Not 100% sure if I brought it up here previously (armament), but I was thinking about this last night actually.

BarnOwlLover said:

I know that I posted above about seeing if this would be better served from here if this was broken off into a "super Spitfire" thread or a "baby Tempest" thread, or (new idea) a "British or USAAF equal to the Ki-61/inline-engine powered Ki-100 of improved performance".

However, I was thinking about armament. I think I posted about having 6 .50 MGs with 300 (min/normal) to 400 (max/overload) rpg, or 4 20mm cannons with 150-200 rpgs (same min/max and normal/overload conditions apply). But what about 2 .50s with 300-400 rpg and 2 20mm cannons with 150-200 rpg?

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One choice that US/UK designers skipped is the 'gun shooting through the spinner'. I can understand their choice of avoiding the weight, complexity, and lower RoF of synchronized guns, but the motor gun avoids that while being in the optimal centerline position, warm and cozy behind the engine. So in a hypothetical 'super Spitfire', or 'Spitfire done right' scenario, I'd like the RAE/RAF to have required inline engines to be designed with a tube between the cylinder banks exiting through the reduction gear for a motor gun, just like the German inverted V-12's (biggest change compared to the historical Merlin is probably how to place the supercharger behind the engine?).
With this arrangement an early war 'super Spitfire' could have been armed with a 20mm Hispano as the motor gun, and, say, 4x.303 in the wings. Then later on as the belt-fed Hispano arrives on the scene, get rid of the MG's entirely and go for a 3x20mm armament which should be sufficient until the end of the war.

Hispano-Suiza 12X/Y/Z engines were capable of using cannons mounted behind the engine, as well as the Russian Klimov derivatives of the 12Y, and all had their superchargers mounted behind the engine like a Merlin, Griffon or Allison.

IMO, aside from gun harmonization/convergence issues, the wing guns were the way to go in the end. You could carry heavier armament, often with more rounds per gun, and heavier weapons.

The reason I'd suggest say 1 .50 and 1 20mm per wing (like the Spitfire E-wing) is that it does weigh less than say 3 .50s per wing, similar salvo fire power, you still get cannons, and could be upgraded to 4 20mm cannons without an issue (though for some weight increase).

I do wonder aside from the Hurricane why there was never a mass produced four cannon Merlin powered fighter that was single seat/single engine? We do know that Spitfire Vs, VII, VIII and IXs were capable of it, but it was rarely fitted. And inerestingly, one of the XP-51Bs started out as a four cannon armed Mustang IA.

BarnOwlLover said:

I do wonder aside from the Hurricane why there was never a mass produced four cannon Merlin powered fighter that was single seat/single engine?

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What is the cut-off number for a ww2 fighter to be considered as 'mass produced'?

Actually saw production and use as a standard configuration. And, no, the Spitfire IV with 4-6 20mm cannons (two of which were mock ups for the 6 cannon version) and the few Spitfire V's and IX's that flew in North Africa with 4 20mm cannons don't count (again, I don't know why it wasn't standardized for the northern ETO until the Spitfire 21, even with the provision for it as early as the Spitfire III prototype).

BarnOwlLover said:

Actually saw production and use as a standard configuration. And, no, the Spitfire IV with 4-6 20mm cannons (two of which were mock ups for the 6 cannon version) and the few Spitfire V's and IX's that flew in North Africa with 4 20mm cannons don't count (again, I don't know why it wasn't standardized for the northern ETO until the Spitfire 21, even with the provision for it as early as the Spitfire III prototype).

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Who decides what counts?

FLYBOYJ

How's that for sucking up?

BarnOwlLover said:

... with 4 20mm cannons don't count (again, I don't know why it wasn't standardized for the northern ETO until the Spitfire 21, even with the provision for it as early as the Spitfire III prototype).

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Certainly the RAF, by that point, was in love with autocannons and would much have preferred the punch of them instead of MG's. I guess the simple reason is that they thought 4 wing mounted cannons with ammo was too heavy and affected maneuverability too much? Not only the weight itself, but also the weight being out on the wings would affect roll rate. (Let me take the opportunity to reiterate my love for motor cannons, no issues with weight being placed far outside the centerline here.. )

Additionally, when they introduced the new stiffer version of the E wing to deal with the aileron reversal issue, that was also the time when they introduced 4x20mm as the standard armament. Perhaps not a coincidence? That is, the heavy armament would have further exacerbated the aileron reversal issue with the old wing?

BarnOwlLover said:

IMO, aside from gun harmonization/convergence issues, the wing guns were the way to go in the end. You could carry heavier armament, often with more rounds per gun, and heavier weapons.

The reason I'd suggest say 1 .50 and 1 20mm per wing (like the Spitfire E-wing) is that it does weigh less than say 3 .50s per wing, similar salvo fire power, you still get cannons, and could be upgraded to 4 20mm cannons without an issue (though for some weight increase).

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They don't need to be exclusive of each other, you can have both a motor cannon and wing mounted weapons. And unless we're thinking of post-WWII weapons like gatling guns or revolver cannons, a single motor gun isn't gonna cut it, so yes, you'd need something else in addition to the motor gun.

The 3x20mm arrangement I suggested above would have more punch as well as less weight than the 2x20mm + 2x.50 arrangement.

z42 said:

One choice that US/UK designers skipped is the 'gun shooting through the spinner'. I can understand their choice of avoiding the weight, complexity, and lower RoF of synchronized guns, but the motor gun avoids that while being in the optimal centerline position, warm and cozy behind the engine. So in a hypothetical 'super Spitfire', or 'Spitfire done right' scenario, I'd like the RAE/RAF to have required inline engines to be designed with a tube between the cylinder banks exiting through the reduction gear for a motor gun, just like the German inverted V-12's (biggest change compared to the historical Merlin is probably how to place the supercharger behind the engine?).

Click to expand...

BarnOwlLover said:

Hispano-Suiza 12X/Y/Z engines were capable of using cannons mounted behind the engine, as well as the Russian Klimov derivatives of the 12Y, and all had their superchargers mounted behind the engine like a Merlin, Griffon or Allison.

Click to expand...

If you look at the Hispano V12s you can see the supercharger is smaller than the Merlin's (or the V-1710's), and the intake ports are on the outside of the engine, the same side as the exhausts.

The Merlin and V-1710 had very substantial intake systems in the vee, where the gun would go.

And would the gun protrude into the fuel tank area of the Spitfire and Hurricane?

wuzak said:

If you look at the Hispano V12s you can see the supercharger is smaller than the Merlin's (or the V-1710's), and the intake ports are on the outside of the engine, the same side as the exhausts.

The Merlin and V-1710 had very substantial intake systems in the vee, where the gun would go.

And would the gun protrude into the fuel tank area of the Spitfire and Hurricane?

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Yes, you'd have to move the induction piping above the gun tube. But just looking at a Merlin, if it had been designed like that from the start I don't think it would have bee impossible. DB and Junkers managed to do it, after all.
And yes, the motor gun would be right where the fuel tank would be in the Spitfire and Hurricane. But with less weight of guns in the wings, maybe that fuel volume could be moved to wing tanks instead (and if we're on the topic of 'Spitfire done right', move the radiator to under the fuselage as well, further freeing up weight in the wings). Which might otherwise be a good idea too, considering the horrible burns experienced by pilots who had a fire in that tank right ahead of them. Then again, a hit in that area causing a detonation of the motor cannon magazine wouldn't have been pretty either.

The less we mess with engines - unless it is about the power increase - the better. Or, in other words, great_engine > great_gun_setup, when it is about the ww2 fighters.

tomo pauk said:

The less we mess with engines - unless it is about the power increase - the better. Or, in other words, great_engine > great_gun_setup, when it is about the ww2 fighters.

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I'm not saying the Merlin should have been redesigned in the middle of the war. Just saying that had the RAE/RAF(?) specified back in the early 1930'ies(?) that any future inline engines be designed with a gun tube, it could have been accommodated without undue impact on the engine design.

z42 said:

I'm not saying the Merlin should have been redesigned in the middle of the war. Just saying that had the RAE/RAF(?) specified back in the early 1930'ies(?) that any future inline engines be designed with a gun tube, it could have been accommodated without undue impact on the engine design.

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How much of a difference that would've made for, let's say, 1940?

z42 said:

And yes, the motor gun would be right where the fuel tank would be in the Spitfire and Hurricane. But with less weight of guns in the wings, maybe that fuel volume could be moved to wing tanks instead (and if we're on the topic of 'Spitfire done right', move the radiator to under the fuselage as well, further freeing up weight in the wings). Which might otherwise be a good idea too, considering the horrible burns experienced by pilots who had a fire in that tank right ahead of them. Then again, a hit in that area causing a detonation of the motor cannon magazine wouldn't have been pretty either.

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People have a fascination with the radiator under the fuselage, I guess because of the P-51.

But not all under fuselage radiator set-ups were great.

The Hawker Hurricane's worked OK. The Hawker Tornado's didn't work at all well, and it was moved to the chin.

The XP-40 had an under fuselage radiator when it first flew, but not long after it had a chin radiator.

The under fuselage radiator would be in the area that later Spitfires would have their rear fuselage tanks.

wuzak said:

People have a fascination with the radiator under the fuselage, I guess because of the P-51.

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Best part of the radiators was within the fuselage of the P-51.
On the Hurricane (and Defiant), the whole bulk of both both radiators was under the fuselage, meaning it generated a greater drag.

wuzak said:

The under fuselage radiator would be in the area that later Spitfires would have their rear fuselage tanks.

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Hopefully the Spitfire will never get the under-fuselage radiator in the 1st place - burry it as much as possible within the fuselage, just like the best part of the Spitfire's original radiator was burried in tthe wing.
With wing now free of radiators, install the fuel tanks in these places - even better from the CoG view. The top rear radiator still can fit on it's old location.