Blackburn Botha to Gloster Reaper

The Blackburn Botha was clearly underpowered for front line service and became a trainer. Despite it's inadequacy being evident from the first flights 580 of them were built, mostly using 930bhp Bristol Perseus.

At the same time the Gloster G39 (aka F9/37, Reaper) was proving excellent on 1,000bhp Bristol Taurus engines.
Could Blackburn's two factories at Brough and Dumbarton have made G39s using the Botha's Bristol Perseus which were nearly as powerful as the early Taurus and lighter? It would call on no other resources than the Botha did IOTL and produce an aeroplane equivalent to the early Bristol Beaufighter in 1940-42. With the Rolls Royce Peregrine at 880bhp the G39 could reach 330mph. One would imagine that it's operational use would mirror the Bristol Beaufighter IOTL.

Yes, interesting to where else it could be produced, I think it could have been produced earlier e.g. rather than the spec. you mention, become the 'back-up aircraft to the designs for the spec that became the Whirlwind. Gloster designed an aircraft to fulfil a turret-fighter requirement (Defiant won). A simplified version, no turret but two 20mm can in the nose & four 0.303" mgs, This gives an earlier prototype, and very few Blenheim 1Fs.

A big problem with both of your engine swap schemes is that the Perseus engine used the same cylinders as the contemporary Hercules. Larger Perseus production means fewer Hercules engines. Granted you are only proposing to use the engines used in the Botha. Perseus development practically stopped with the outbreak of WW II. While the Perseus was allowed to be run on 100 octane fuel (with suitable notations in the log books) I don't believe it was ever approved for increased boost/power ratings. Welcome corrections?
The second problem is the size of the Perseus. Sources disagree but the most favorable show a diameter of 52in and a frontal area of 14.7 sq ft. British radial cowlings of the time were not the most advanced. The increase in drag would be rather large compared to the Peregrine engined Version or the Taurus engines (46.25 in/11.7 sq ft).
The Gloster F9/37 carried a LOT less fuel (capacity 170imp gal?) than the Beaufighter which means it would have trouble performing the same missions as a Beaufighter. Not only would it be a Beaufighter lite, it would be a Beaufighter short range/short endurance. Much shorter nightfighter missions, much less range on shipping strikes or convoy protection missions. Room for early radar?

The claimed performance figures for the Gloster with Taurus engines were probably for the unarmed prototype (if those existed), a tad bigger Bf 110C day fighter was barely topping 520 km/h (~320 mph) and it have had V-12 engines with as much of exit thrust as possible back then. (no worries, I've believed myself the 580 km/h figure for the Gloster for quite some time )
So we'd probably have 300-310 mp/h for the Perseus-powered Gloster, and maybe 10 mph more with Taurus with, both with 'F' supercharger gearing.

Now - what could it do? My proposal is to cut the guns down to 2 cannons or 6 Brownings and install the torpedo rack. Use the generous central fuselage space, now devoid of slant-mounted cannons & ammo to install fuel tanks, so it can carry a torpedo to a meaningful distance. The wing is also generous, not thin, some space should be also there for fuel tanks. Low-level Perseus, of course.

The Gloster is a big aircraft, bigger than P-38, so it should be a good platform for Merlin. Having the Gloster as a night fighter frees the Mosquito production for bomber and fighter-bomber versions.

Bf 110 didn't get a lot of exhaust thrust.


Yes it is a model but shows the outer exhaust rather well. Inner exhausts were pointed downwards, why I don't know but one can guess the outer exhausts directed the exhaust over the top of the wing and kept the hot exhaust gases out of the radiator.

Perhaps downward inner pipes kept exhaust out of the rear cockpit????

Assuming you can get torpedoes for the RAF in 1939-40 and 41 in more than token numbers (more than needed for the Vildebeests and the token Beauforts) just stick medium supercharged Mercuries in a Blenheim, bolt the bomb bay doors shut and hang the torpedo under the fuselage. Much easier than redoing a short range (relatively) fighter into a long range torpedo bomber.

Shortround6 said:

A big problem with both of your engine swap schemes is that the Perseus engine used the same cylinders as the contemporary Hercules. Larger Perseus production means fewer Hercules engines. Granted you are only proposing to use the engines used in the Botha. Perseus development practically stopped with the outbreak of WW II. While the Perseus was allowed to be run on 100 octane fuel (with suitable notations in the log books) I don't believe it was ever approved for increased boost/power ratings. Welcome corrections?

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Well the numbers should not be a problem as we are using the ones IOTL used on the 580 Bothas. The benefits of higher boost and/or compression ratio that 100 octane could permit would be nice but twin Perseus gives you a solid 1,800bhp as a base.

The Perseus used much less fuel than the Hercules (after all twin Hercules are four Perseus) so the range would not be directly comparable to a Beaufighter's tankage.

If there is room for radar in a Defiant there is room in a G39. As to an operator?

The Blenheim was a gnat's slower than even a Beaufort. With an external torpedo (is there room for the tail on take off?) it must drop drastically to that of a Blackburn Roc on a bad day. Barely a third faster than a Vickers Vildebeest. Sidney Cotton tried putting lipstick on the pig but could not get even a stripped down smoothed out Blenheim up to a speed to evade anything. The torpedo is 167% of the Blenheim warload.

Myself I would like to mount twin Vickers 40mm S guns especially if they can be made belt feed. Dual purpose air and ground weapons with the HE rounds.

How big was the internal voulume we can roughly guess by checking out this picture:



There are 3 cannons behind pilot, plus there is some volume between pilot and cannon 'area'. Sure enough, the lower two cannons should be now pointing straight forward, rather than at angle. Two additional cannons must be added for (night)fighter version, whether in wing roots (similar to the Tigercat, but only half of the guns there; maybe the fuel tanks were there so scratch this in that case) or in a pod/tray under the fuselage.

The pom-pom HV should make a good airborne AT gun, it was belt fed.

Which version of the Perseus are you using? The one/s most quoted for the Bothas offered either 750hp for take-off or (with a cropped impeller) 880hp for take-off. the first engine could give 880hp at 15,500ft but cropping the impeller gave 950hp at 5,500ft. 100 oct, if used, gave 950hp for take-off in the cropped impeller engines, it was good for about an extra 70hp at continuous climb rating down low (like 2750ft). Power is no better than either the Peregrine or Taurus and the Taurus has more power and less drag.
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yulzari said:

The Perseus used much less fuel than the Hercules (after all twin Hercules are four Perseus) so the range would not be directly comparable to a Beaufighter's tankage.

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Gee, I always thought the Hercules was 14 cylinders, not an 18 cylinder. Fuel used is in proportion to the power used, not necessarily the engine size, few planes cruised around for hours at near full throttle. Granted the Beaufighter is bigger and has more drag but a pair of Hercules engines running at 60% power may not be that far off of a pair of Perseus engines running at 70-75% power on fuel per hour. The Gloster F9/37 carried for it's admittedly smaller engines roughly the same amount of fuel per engine as the Hurricane or Spitfire did for their single engines. Anybody really think the Gloster is going to stay in the air for hours longer than a Spitfire or Hurricane on 85 gals per engine?

Maybe you can stick radar in it. Tighter fit than anything except a Defiant though and the Defiant installation wasn't worked out until the fall of 1940. Claiming the F9/37 could be built as a nightfighter in the summer/fall of 1939 rather ignores the progress made with radar in the pre-war and first year/s of the war. Effective radar in a Defiant waited until the MK VI radar and near the end of 1941.

The Blenheim was a gnat's slower than even a Beaufort. With an external torpedo (is there room for the tail on take off?) it must drop drastically to that of a Blackburn Roc on a bad day. Barely a third faster than a Vickers Vildebeest. Sidney Cotton tried putting lipstick on the pig but could not get even a stripped down smoothed out Blenheim up to a speed to evade anything. The torpedo is 167% of the Blenheim warload.

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You have reasons for making those statements? BTW Cotton did get e Blenheim over 290mph but that isn't good enough for evading single seat interceptors. It is however a far cry from the under 200mph speed of a Blackburn Roc.

And the Torpedo is merely 126% of the max Blenheim warload. 1000lbs in the bomb bay and 320lbs (eight 40lb bombs) under under fuselage racks behind the bomb bay. Blenheim also carried a lot more fuel which can be traded off.[/QUOTE]

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Shortround6 said:

Which version of the Perseus are you using? The one/s most quoted for the Bothas offered either 750hp for take-off or (with a cropped impeller) 880hp for take-off. the first engine could give 880hp at 15,500ft but cropping the impeller gave 950hp at 5,500ft. 100 oct, if used, gave 950hp for take-off in the cropped impeller engines, it was good for about an extra 70hp at continuous climb rating down low (like 2750ft). Power is no better than either the Peregrine or Taurus and the Taurus has more power and less drag.

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The whole point is that it is the ones otherwise used IOTL for the Botha. I agree that the Peregrine, Perseus (and indeed Mercury) have power in much the same region and the Taurus, as a double Aquila, is smaller in diameter but the idea is to use engines as made IOTL but in the G39 rather than Botha. Not to choose a better engine for the G39. The factories existed and the power units etc. existed. Gloster had done the design work.

I believe that the Botha used low level rated Perseus as trainers as they needed no great altitude whilst the target tugs used higher altitude rated ones as they had to simulate enemy bombers.

Tomo, all five of the guns in your drawing are inclined deliberately to use 'no aim off' firing. I don't know what else was in the space but it looks like there is room for an AI set and operator. The same space had been intended for a gunner and his Boulton Paul type turret. Or a couple of S Guns firing below the pilot. Or indeed extra fuel. The S Gun was developed in time for this but in 15 round drum form. Belt would be better but maybe a larger drum as there is more room than in a Hurricane's wing. The Hispano installation is in 60 round drums. Unlike the Beaufighter there is no observer to reload the drums, but then that task needed straight and level flight if he wants to keep his fingers. Pom pom HV? Does this have a name?

A lot of bad information seems to be around about the Bristol sleeve valve engines, Some people can't seem to count as some web sites repeat the Taurus being a double Aquila. Just like the Hercules the Taurus was a 14 cylinder engine, two rows of 7 cylinders while the Aquila was a single row 9 cylinder. Neither the Hercules or the Taurus were doubled engines anymore than the R-2600 was a double Cyclone 9 (R-1820) Taurus also used a longer stroke than the Aquila.
P & W had the Wasp with 5.75 x 5.75 cylinders (nine of them) and the twin Wasp with 5.50 x 5.50 cylinders (two rows of seven) and the Double Wasp with 5.75 X 6.00 cylinders (18 of them, two rows of nine) but that doesn't mean a lot either as the P & W engines didn't really use any common parts even if they kept the same or similar bore and strokes.

The problem with re-engining the Gloster twin and trying to re-purpose it is really two fold. One is that no production facility exists at the time the the Botha is being built or you are trying to say use the Botha production facility for the Gloster F.9/37. The Botha was ordered pretty much off the drawing board and flew 4-5 months before the F.9/37 ( Botha flew in Dec 1938) , this was well after 442 Bothas had been ordered back in 1936. Production jigs and fixtures and long lead components had already been completed or were well along when the F.9/37 first flew. Trying to switch type of aircraft being produced is going to take months if not a year (maybe more). To be available in 1940 also requires somebody looking at drawings and calculations/estimates to not only decide the Botha is going to be the next thing to a Kiwi (flightless bird) but decide that the Perseus powered F.9/37 is going to be a workable aircraft around year before it first flies with the Taurus engines.
The 2nd part is trying to replace a torpedo plane/recon machine with an endurance of around 5 hours with a fighter?/recon/whatever with an endurance of around 2-3 hours.
The F.9/37 was a large plane physically (50 ft wing span, 37ft length and 386sq ft wing) but was actually a "light" airplane being about 90% of the weight of P-47 when empty and about 80-82% of the weight of the YP-38 (unarmed) empty. You have room (volume) to "add" all kinds of stuff. Do you have the strength? or do you need stronger spars, frames, landing gear etc if you add several thousand pounds to the aircraft.
Looking at the Blenheim would the F.9/37 using essential the same power engines be enough better performing due to smaller wing and fuselage (lower drag) to be worth making?

Indeed. Bristols mixed and matched a limited number of cylinder sizes and strokes to make their different engines. Crudely put, they just made a new crankcase for a given layout.

They kept on building the Bothas for two years despite the first flight findings. The idea is to get G39's produced instead. Not for the Botha role but in it's own right. What that role might be is another debate.

On the structure side. It was stressed to operate as fighter with a heavy turret. If that is not up to other tasks (not a torpedo carrier I would suggest) then the Welington was not strong enough to replace Pegasus with Hercules, the Spitfire certainly could not carry a Griffon much less a Bf109 a DB605 or a Fw190 the Jumo 213. Why would you add several thousand pounds to a G39? It comes off the drawing board with x5 cannon or hundreds of lb of turret/gunner. Hurricanes. Spitfires, P40s, P47, P51 etc. etc. managed to carry bomb loads of 500lb to 2,000lb without huge structural changes.

The Botha should have never been ordered in the first place. It was grossly underpowered fully service laden and could be out climbed by a man on a ladder. A recce machine that the pilot can't see out of? Even a Beaufort had trouble hold height fully laden on Taurus engines. To be fair to Blackburns (which I find difficult with the Roc, Botha and Firebrand) they recognised the problem and wanted Taurus but there was not the spare production (which is the real issue for the 'lets put the Taurus on Whirlwinds' school) being all allocated to Beauforts and Albacores.

Had the G39 proved valuable then future Perseus have the stretch to 1,200 bhp but replacing them with Hercules or Merlins would be better in engine production terms even though it would need work to accommodate the weight but then the Whirlwind was offered with Merlins so why not a G39 which was also stressed for Peregrines? But future development is not the point here.

Had the powers that be realised their error in ordering Bothas early enough then I see no reason why the G39 could not have been made instead. Bothas were not used for training and target towing because they were wanted for this. They were making the damned things so some use had to be found. The Beaufort was the 'modern' torpedo bomber alternative and the Hampden helped fill the Botha hole later on with both replaced by Beaufighters. I am open to suggestions as to what the Botha factories might have otherwise done and I sympathise with Vildebeest and Swordfish crews who had to make do with nothing better even into 1942.

Unles/until the Perseus gained cubic displacement, it won't go easily over 1000 HP, IMO.

yulzari said:

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Pom pom HV? Does this have a name?

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40 mm pom pom, firing the HV (high velocity) shell was standard AA gun of the RN in lead-up to the ww2 and during the war. The S gun used the same cartridge, but firing the AF ammo.

Shortround6 said:

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The 2nd part is trying to replace a torpedo plane/recon machine with an endurance of around 5 hours with a fighter?/recon/whatever with an endurance of around 2-3 hours.
The F.9/37 was a large plane physically (50 ft wing span, 37ft length and 386sq ft wing) but was actually a "light" airplane being about 90% of the weight of P-47 when empty and about 80-82% of the weight of the YP-38 (unarmed) empty. You have room (volume) to "add" all kinds of stuff. Do you have the strength? or do you need stronger spars, frames, landing gear etc if you add several thousand pounds to the aircraft.
Looking at the Blenheim would the F.9/37 using essential the same power engines be enough better performing due to smaller wing and fuselage (lower drag) to be worth making?

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Indeed, the main problem for the Gloster is that is a bit late in the game.
As for techincals - plenty of fighters were being fast to gain heavy underslung payload, waepons and bigger/heavier engines. The P-38 carried 2 heavy torpedoes rather early, the Spit received the 170 imp gal drop tank, Fw 190 got the 1800 kg bomb - weight of two US torpedoes! Fiat G.55S got the torpedo.
Gloster F.9/37, being a fighter, was certainily stressed for heavier G load than a contemporary bomber.

Performance - with Perseus, clean, we'd probably see 300+ mph, and with Taurus around 320 mph. A 50 mph gain over Blenheim/Beaufort is a good thing, IMO. Even a better thing is that Blackburn will produce an actually useful aircraft.
Merlin is needed for 350+ mph.

I would not make any comparisons with the German twin Me-110, however, there are many similarities in size etc. to the FW-187 - even more so when looking at the redesign that became the Reaper.
IMHO - I would not have considered the possibility of it carrying a torpedo - just because it could as an overload doesn't mean it's a good idea. I can see the potential in an 'fighter-bomber' role - in a 'can it carry bombs' scenario - even in 1940. After all if the Whirlwind could carry two 500lb bombs sure this one could as well. Moreover, I think I demonstrated a way - in an earlier post - it could have flown earlier - even if it was with Mercury engines.
Whilst, it would have been better than the Blenheim 1f as a nigh-fighter, the Mosquito would have much better range to do intruder missions.

The Fw 187 was a smaller aircraft than the Gloster, ie. wing area was some 20% greater on the Gloster. Fuselage of the Fw was so narrow that engine instruments were located on the inner side of the nacelles, while the Gloster's fuselage was wide enough to accomodate the 4 Browning turret. Just the fittment of second crew meber on the Fw was a tricky affair. Wing of the Gloster does not look thin either.
The wing of the Bf 110 was just some 6% of greater area than what was the case with the Gloster.

Maybe an interesting scenario would've been that Mossie does the bombing and really LR recon work (where the recce Spit can't reach), Gloster does the night fighting and fighter-bomber job, while the Beaufighter does the LR strike job?
Navalize the Gloster?

Myself I am at the toddling to walking stage here, let alone running before one can walk. For the G39 to be built I saw an opportunity to use the Bothas production and power plants. Minimum input and 580 G39s which has to be better than 580 Bothas. In 1939/40 they will be used as fighters .This frees up the Blenheim 1Fs which might fill some of the vacant OTL Botha training role. Remember in this period fighter squadrons in France are still changing to Hurricanes from Gladiators so a 300mph+ cannon armed fighter is not going to any other role at first. That is the base line.

Now one might postulate that they become used in France to attack German transport in the immediate rear of their advance. Prove themselves very able to disrupt the resupply of some German Divisions advancing at the speed of their horse drawn artillery and resupply with 40mm HE shells. However the lack of bombs to damage structures such as bridges or dug in positions, plus the expectation of invasion, leads to efforts to arm them with bombs too. Some bright transferred in FAA pilot points out that he has been dive bombing with his Swordfish with 500lb bombs on underwing racks and that Alabacores can carry 4 500lb bombs underwing. He borrows some and the G39 has a rush refit to take underwing bomb racks. RAF Army Cooperation instantly claim them for anti invasion tasking freeing up all the Tomahawks to be sent to Egypt, also with bomb racks. The success of the G39 in the army cooperation role leads to the decision to place further orders. Initially drawing on Perseus engines from Lysander contracts. Production of the Lysander is stopped forthwith and the Mercuries go to the Master production line whilst Perseus to the G39.

It is clear that the Perseus will not continue to be able to do the job against improving enemy aeroplanes. Bristol propose a 2 stage supercharged version with Centaurus cylinders predicting at least 1,200 bhp with power maintained at altitude. The Ministry is not confident that this will happen in a sensible timescale. Taurus production is spoken for. As is Hercules. To meet the need Rover sets up a renovation works to receive obsolete and worn Merlins from Battles, Hurricanes and Spitfires. They are broken down and a common rebuild standard applied and Blackburn modify the G39 for Merlins. As Merlin production improves then the G39 is ultimately built with new Merlins and the Rover works forms the Rover Meteor production and rebuild establishment for tank engines. The need to maintain Hurricane production is reduced and it ceases in late 1943.

By lat 1942 the Perseus G39 is going out of service in NW Europe and they are sent to India where they provide vital, accurate and effective close support whilst being able to cope with Japanese fighters in the theatre.

The Merlin G39 can do the same low level role as the new Typhoons and Hawkers attention is early drawn to bringing forward the Tempest and Typhoon production ceases by early 1944.

The G39 thus forms itself into a tactical strike role. The Beaufighter's longer range leads to it's concentration in use in maritime strike and in the Mediterranean theatre. The constant battle between bomber and fighter Mosquitos is reduced by the night fighter version of the G39 in early 1943. Mosquitos still are necessary for the night interdiction role with their greater range and speed but the G39 can do the night fighter task over the UK and Italy and the USAAF is very relieved to swap their worn out second hand Beaufighters for shiny new G39s.

For a real flight of fancy: the S Gun, even when loaded with AP ammunition, soon becomes unable to destroy the latest uparmoured German tanks except, with fortunate top or rear strikes. The 6 pounder is proposed and installed (not easily) in strengthened G39s. Early trials soon show that even the 6 pounder has trouble with the best German armour. However, as with it's tank peers, the barrels are replaced with 75mm ones and HE rounds are the normal load. Now the G39 can still kill German tanks other than from the front (the round receives the extra velocity for the aeroplanes airspeed in addition to the muzzle velocity of the gun) but is now a very potent pice of flying artillery. The weight of the installation does prevent also carrying the normal 2x500lb wing bombs or x8 60lb rocket projectiles but the accuracy is far better and the accuracy of the G39s 40mm cannon has allowed close support within yards of friendly troops. The use of canister rounds proves useful against V1s.

BTW I have used Gloster's nomenclature of G39 as the name Gloster Reaper has been applied to more than one airframe especially in the 'what if' world.

Some dimensions for comparisons:

Gloster 'twin' Span 50' 0", length 37' 0", wing area 386 sq. ft., Weigts - empty, 8,828 lb; loaded 11,615 lb, max speed (Peregrine) 2 x 885 hp = 330 mph.
Focke-Wulf FW 187 Span 50' 3", length 36' 5", wing area 327.222 sq. ft., weights empty 8,160 lb, loaded 11,000 lb., max speed 2 x 700hp 326 mph.

Seems pretty similar to me, size and weight, the extra speed per hp, is possibly accounted for by the smaller wing; whilst the Me-110 is in the region of 4,000 lbs heavier!

At ~5 km, where the Peregrine makes 885 HP (close to 900 metric HP, or CV, or PS), the Jumo 210D will provide barely above 500 PS. Max power in second gear was supposed to be 680 PS at 3,7 km (Soviet data; the German charts show much less power than Soviet ones); the Peregrine will do maybe 1000 HP there, admitedly on 100 oct fuel? Fw 187 never received the more powerful 210G.

Kinda shows how much more power was needed for the Gloster to be competitive - basically the Merlin or at least Taurus. We need to be clear what variants (crew? guns? ammo?) of the Gloster and Fw are compared in weight. The Bf 110C-1 was at ~13340 lbs, it carried less ammo and just 2 crew members vs. the C-2 and C-4.
It also shows how well the Fw 187 would've fared with just the HS 12Y engine, available in occupied Czechoslovakia, at 4 km perhaps some 200-250 HP more than Jumo 210.

The unloved Peregrine has the altitude power closer to the DB 601A or Jumo 211A-H., rather than to the most powerful Jumo 210 around. More power than HS 12Y.

I've done some measuring of the wing thickness (jpg from post #7 here to pdf, then use measuring tools - yes, I have that much time for now). Works out to 17.5% thickness at the root. For comparison sake, Whirlwind was supposed to be also at 17%, P-38 at 16%. Bf 110G at 18.5%.

A Taurus G39 would be nice but what will you do without to fit them? Production went to the Beaufort (leave Vildebests and Swordfish as torpedo bombers up to the end of 1943?) and to the Albacore (leaving the FAA to row bombs and torpedoes to the targets?). The Merlin was equally fully employed unless you want to do without Defiant night fighters in 1941/2 and there was no rework yet done to fit them. I contend that the Perseus was the only viable choice in 1938 to 1941. Afterwards then the world is the mollusc of your choice.

We do know that engines of up to 600kg are compatible. I suspect that much after that then we are talking of proper redesign to take the weight in fighter stresses and keep a sensible centre of gravity. A Merlin is in the region of 750kg+ wet and the Hercules more. Is that enough to trigger a full redesign?