Radial engines more favored in Germany, 1935-45?

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tomo pauk

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Apr 3, 2008
Unlike, say, USA or Japan, there was only one German radial engine in-service that is worth talking about - BMW 801 - with two 9 cyl radials whose genealogy can be traced to 1920s. BMW 800 project never materialized, while really big radials also meant nothing at the end of the day.

Premise of the thread is that engine companies are pushed earlier towards the own development, so there is more to choose from, so to say. Small radials (9 or even 7 cyl) can be a made instead of the air-cooled V12s, while being lighter, cheaper and making a bit better power. Small to medium sized 14 cyl can be alternative engine for bombers and even fighters (German fighters were small and light, to help out wrt. performance even if the engine power is not over the top). 18 cyl engines can be alternative to the complicated and troublesome 24 cyl liquid engines like the Jumo 222, DB 606/610 etc.

For the needs of this thread, Jumo and DB remain in the liquid cooled engines' business. Merge the other companies as you see fit, and probably as early as possible, and kill of designs you see fit. No licence deals with foreign companies.
 
The first problem I see is that a smaller radial (in terms of power) would be viewed as not meeting the requirements anytime after about 1941? By then the Germans had recognized the need for more power than an engine like the R-1820/-1830 could put out.

I am not familiar with the early-war efforts by the different potential engine suppliers. Maybe if they embraced the higher rpm air-cooled X/H engine concept and applied the technical side to a radial?
 
The first problem I see is that a smaller radial (in terms of power) would be viewed as not meeting the requirements anytime after about 1941? By then the Germans had recognized the need for more power than an engine like the R-1820/-1830 could put out.

Note that timeline starts in 1935 - plenty of time to cook something before the shooting starts :)
 
Note that timeline starts in 1935 - plenty of time to cook something before the shooting starts :)
Cook what exactly?
Between 1935 and 1943 Wright built 4 completely different R-1820 engines, with no interchangeable parts.
Four different crankcases, One aluminum and 3 different steel crankcases. Between the 1st and 2nd steel crankcases there was change of about 50lbs (?).
Not as bad (barely) was the R-2600 series. To go from 1600hp to 1700hp required a new engine. Aluminum to steel crankcase and everything that went into the crankcase and on it (both cylinders, gear case and supercharger, accessories section.

Building 7 & 9 cylinder 350-800hp trainer/small airline engines does not really translate into design experience for 1000-1600hp 9 & 14 cylinder engines.

One of the major problems with building high powered radials was figuring out how to make them in quantity. Doesn't matter if the hand built prototype can make the desired power and pass a 100 hour test if you cannot make the engines in quantity. Forging the cylinder barrels and machining the necessary finning (or a useable cooling cuff) and making the needed cylinder heads. Can you cast the needed heads or do you have to forge them? How many forging operations? how many machine hours to finish each head after casting/forging?
Trainer engines that use low grade fuel and little or no supercharging put a lot less strain on things and generate less heat. Smaller cylinders have a better wall to volume ratio and cool better, as do lower rpm engines.

Do you have the necessary machinery and in many cases for true mass production you need special custom made machinery.

If you want high powered engines start designing high powered engines. You will probably get a few (or more than few) failures, but you are going to get failures trying to scale something up or trying to get a crankset to run 20% faster let alone trying to add a 2nd cylinder bank.
 
If you want high powered engines start designing high powered engines. You will probably get a few (or more than few) failures, but you are going to get failures trying to scale something up or trying to get a crankset to run 20% faster let alone trying to add a 2nd cylinder bank.

Roger that.
Some particular scenario wrt. high power radials you have in mind?
 
P&W was fooling around with two row engines in 1929, they were not the first, but they may have been one of the first to use the center bearing. It took 3 years to actually introduce it for sale. A two bearing two row is shorter, it is lighter, but you are going to hit limits soon with crankshaft flex. BMW used 3 bearing pretty much from the start I believe?
Many engines failed because they tried to use new ideas. They need to use sound principles. Easy to say now, in the early 30s they were dealing with a lot of unknowns.
But with air cooled engines one of the basic problems was known before WW I, how do you keep the engine cool? This was so basic that the whole rotary engine thing was done to keep the cylinders cool. Crappy fins and poor cooling at 80mph? spin the whole engine at 1000-1400rp to get the needed air flow.
Once you stop spinning the engine you need to figure out how to make the fins the right size and pitch to get the cooling you need. If you can't do that it doesn't matter what kind of valves you use.
 
P&W was fooling around with two row engines in 1929, they were not the first, but they may have been one of the first to use the center bearing. It took 3 years to actually introduce it for sale. A two bearing two row is shorter, it is lighter, but you are going to hit limits soon with crankshaft flex. BMW used 3 bearing pretty much from the start I believe?

On the BMW 139, they tried to be smart-asses, and all 3 bearings were between the crank throws - yes, no bearings on the ends. Once that backfired in their faces, they redesigned it into a 'normal' 2-row radial, now named BMW 801, with central bearing + one at each end. 801 gained ~200 kg vs. the 139, and it was also longer (necessitating relocation of the cockpit on the Fw 190 back for CoG reasons; that also left more space between the firewall and cockpit, so now the cowl guns & their ammo were the option - both blessing and curse; greater weight also necessitated greater wing to keep wing loading ... manageable).

A way to improve/speed up BMW radial engine fortunes might've been earlier dictum by RLM that BMW focuses on radial engines, say 1936 instead of 1938? No BMW 116, no 117, have them design radials instad.
 
BMW options:
- Start designing a 2-row radial instead of the 116 and 117 - they might opt for something sized like the R-2180/Ha-5/Ha-41 to be ready for flight tests by 1937? Best-case scenario for service use in 1939 is perhaps 1400 HP low gear, 1200-1250 HP high gear on 87 oct fuel, at least by looking what R-2180A and Ha-41 were capable for; 700-750 kg bare engine weight? Platforms for the engine might include the He 111, Ju 87 and 88, Bf 110, prototype of the Fw 190 and Do 217. By late 1941, improve to 1500/1300 HP, like the Ha 109. For 1942, C3 fuel version.
- For service use in 1943 and on, move on to 18 cyl sibling. Japanese equivalent of that was the rare and temperamental Ha 219/Ha 44, perhaps BMW can do a better job here? The 14 cyl engine should be getting the 2-stage S/C.
 
Can we put a radial in Germany's tanks? No cooling system, though there is the Sherman-like height issue.

They were working on air cooled diesels, Porsche was working on the X-16 SLA 16, and Tatra had prototypes for air-cooled V-8 and V-12 diesels. AFAIU the motivation behind air cooling wasn't power/weight, but ruggedness, better tolerance for battle damage, and no issues with the coolant freezing in the Russian winter or boiling off in the African desert.


 
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For the needs of this thread, Jumo and DB remain in the liquid cooled engines' business. Merge the other companies as you see fit, and probably as early as possible, and kill of designs you see fit. No licence deals with foreign companies.

With DB and Jumo remaining committed to liquid cooling, what latitude is there to do something dramatically different than OTL? In OTL BMW buys Bramo, and then has its hands full with the 801 in the run-up and for pretty much the duration of the war. Ok, so maybe Argus makes a small radial instead of the air-cooled V-12 used for trainers and a few low-volume aircraft. Maybe(?) a slight improvement, but won't really affect the course of the war one way or another.

I suspect that if you want to see some major changes, either DB or Jumo would also need to pivot to radials in order to bring enough R&D muscle to be able to produce another powerful radial in addition to the 801.

One potential problem with the radial strategy is that with radials running hotter than liquid cooled engines, this might exacerbate the valve issues that were already a huge problem for them in the OTL. Also requiring higher octane gas than equivalent liquid cooled engines again due to higher CHT, which was a problem for Germany.

That being said, without really pushing the envelope in radial engine performance, they could have used something like a simple cheap robust radial capable of running on B4 instead of the Jumo 211 as their standard bomber and transport engine.
 
With DB and Jumo remaining committed to liquid cooling, what latitude is there to do something dramatically different than OTL? In OTL BMW buys Bramo, and then has its hands full with the 801 in the run-up and for pretty much the duration of the war. Ok, so maybe Argus makes a small radial instead of the air-cooled V-12 used for trainers and a few low-volume aircraft. Maybe(?) a slight improvement, but won't really affect the course of the war one way or another.

I suspect that if you want to see some major changes, either DB or Jumo would also need to pivot to radials in order to bring enough R&D muscle to be able to produce another powerful radial in addition to the 801.
I certainly don't expect that 'my' scenario will affect the outcome of the ww2. What it might change is the technical back and forth within Germany, and slightly in the Allied camp.
As for the small radial - making 30000+ of 7 cyl radials of 20L and 500-600 HP should've been easier on the German budget, manhours and raw material supply than making the same number of V12 air cooled engines.

One potential problem with the radial strategy is that with radials running hotter than liquid cooled engines, this might exacerbate the valve issues that were already a huge problem for them in the OTL. Also requiring higher octane gas than equivalent liquid cooled engines again due to higher CHT, which was a problem for Germany.

That being said, without really pushing the envelope in radial engine performance, they could have used something like a simple cheap robust radial capable of running on B4 instead of the Jumo 211 as their standard bomber and transport engine.

Radial engines were running well enough with 87 oct fuel. Granted, the 100 oct fuel is a boon if it can be had.
A simple, cheap robust radial engine for LW might've looked like the Ha 41/109 series by Nakajima, or as Kasei by Mitsubishi (perhaps even short-stroke Kasei so it is more compact) - each of these engines were very light when compared with BMW 801. No-nonsense engines, and were able to run on Japanese fuel that was in it's best day of 91-92 octane. Useful for fighters, bombers, transports. Fuel injected versions for the fighters, and pressure-carburated versions with smaller valve overlap (= a bit less power and worse consumption) for transports; bombers can use either of the two.
Introduce the MW 50 system earlier, talk latest in mid-1943.
 
I certainly don't expect that 'my' scenario will affect the outcome of the ww2.

Sure, wasn't saying that. But just replacing the Argus 400 series engines with a radial of roughly equal power is, in the end, a rather minute change.

As for the small radial - making 30000+ of 7 cyl radials of 20L and 500-600 HP should've been easier on the German budget, manhours and raw material supply than making the same number of V12 air cooled engines.

Maybe? Where does the cost difference come from? Are you assuming the cost is proportional to the number of cylinders (and thus proportional to the number of pistons, valves etc.)?

But if we look at a couple of close competitors in this size class, both the P&W R-985 and Wright R-975 were 9 cylinder engines, in fact relatively few 7 cylinder radials were actually made? Which would slightly narrow the gap to a V-12.

Radial engines were running well enough with 87 oct fuel. Granted, the 100 oct fuel is a boon if it can be had.

Yes.. I was thinking of the "conventional" wisdom saying radials need higher octane than an equivalent inline. But that's a bit of a non-sensical comparison, as you can also give the inline higher octane. Yes, radials ran hotter, but they compensated by making less power/displacement and lighter construction. I guess the question is if you give a radial 100 octane instead of 87, how many % more power can you in principle get out of it vs. the same thing with an inline (assuming the engine otherwise is sturdy enough, S/C can cram enough air into it etc etc). Is there any reason to assume the relative performance increase would be any different for an air cooled vs. a liquid cooled engine? (I fear this might lead down a rabbit hole of knock tolerance of fuel as a function of CHT.)

pressure-carburated versions with smaller valve overlap (= a bit less power and worse consumption) for transports; bombers can use either of the two.
Introduce the MW 50 system earlier, talk latest in mid-1943.

Yeah, I don't think you can usefully run particularly high valve overlap without direct injection, if you don't want to see half your fuel go out the tailpipe unburned.

MW50 for TO/WEP might to an extent alleviate the need for higher octane gasoline.
 
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Probably not feasible as the Panther was already taller than the M4A3 Sherman anyway.
True. Those torsion bars really raise the floor. What the Germans needed was externally mounted bogie supension. And if the engine is at the rear, put the drive sprocket at the rear, like the British Cromwell, Centurion, US Pershing, and most modern MBTs. These two changes will lower the tank and allow for a radial engine.
 
Sure, wasn't saying that. But just replacing the Argus 400 series engines with a radial of roughly equal power is, in the end, a rather minute change.
Maybe? Where does the cost difference come from? Are you assuming the cost is proportional to the number of cylinders (and thus proportional to the number of pistons, valves etc.)?
But if we look at a couple of close competitors in this size class, both the P&W R-985 and Wright R-975 were 9 cylinder engines, so you might want 9 in order to comfortably reach the power levels required? Which would slightly narrow the gap to a V-12.

Smaller parts count should account for something on an engine that is bound to be made in many thousands in a country that has problems with securing a good deal of raw materials. Granted, a 9 cyl can work - perhaps short-stroke the BMW 132 or Bramo 323 (sorta "German Mercury"), down-rate it so it can use lower octane fuel and be done with that? A bespoke 7 or 9 cyl might be even better, or copy the Americans with the R-985 or 975? Or a bigger A-S Cheetah?

Yes.. I was thinking of the "conventional" wisdom saying radials need higher octane than an equivalent inline. But that's a bit of a non-sensical comparison, as you can also give the inline higher octane. Yes, radials ran hotter, but they compensated by making less power/displacement and lighter construction. I guess the question is if you give a radial 100 octane instead of 87, how many % more power can you in principle get out of it vs. the same thing with an inline (assuming the engine otherwise is sturdy enough, S/C can cram enough air into it etc etc). Is there any reason to assume the relative performance increase would be any different for an air cooled vs. a liquid cooled engine? (I fear this might lead down a rabbit hole of knock tolerance of fuel as a function of CHT.)

Liquid cooled engines were in theory better able to take advantage of the high octane fuel. For the Germans, however it seems like the BMW 801 was no worse in this regard than the DB engines (German engines were not taking all of the advantage of the very high rich rating of the C3 fuel anyway, per British reports; C3 rich rating being noted as better than on the Allied 130 grade fuel as war progressed). Eg. the BMW 801C gained ~300 HP vs. 1943 801D, despite the increased CR of the 801D (compare with the small V-1710 making 700-800 HP more on 130 grade vs. 91 octane, but also against the DB 605 gaining ~200 HP, and the 601 being worse still). Engine is supposed to actually withstand the increased stress that over-boosting brings along.
 
True. Those torsion bars really raise the floor. What the Germans needed was externally mounted bogie supension. And if the engine is at the rear, put the drive sprocket at the rear, like the British Cromwell, Centurion, US Pershing, and most modern MBTs. These two changes will lower the tank and allow for a radial engine.

Nah, I think torsion bars were the correct choice, at least in the sense that most tanks, both Western and USSR, have used torsion bars since WWII, presumably for good reasons. As for putting the transmission in the rear, that's probably the solution. That might lead to a very front-heavy tank however, as the front hull armor and the heavy turret are packed at the front. It sort-of worked for the T-34, but that was a relatively light tank and the same approach might not have worked well for a heavier tank like the Panther. Then again, it worked fine for the Centurion. Many other post-war tanks mounted the engine transversely, which also helped keep the rear end reasonably short and thus the tank overall better balanced.
 
Nah, I think torsion bars were the correct choice, at least in the sense that most tanks, both Western and USSR, have used torsion bars since WWII, presumably for good reasons. As for putting the transmission in the rear, that's probably the solution. That might lead to a very front-heavy tank however, as the front hull armor and the heavy turret are packed at the front. It sort-of worked for the T-34, but that was a relatively light tank and the same approach might not have worked well for a heavier tank like the Panther. Then again, it worked fine for the Centurion. Many other post-war tanks mounted the engine transversely, which also helped keep the rear end reasonably short and thus the tank overall better balanced.
I've always liked the Merkava. Put the engine in the front as added protection for the crew. Plus you can carry four infantrymen in the back, or more ammo/stuff.

What the Germans really need is a fully tracked APC. Height is less important, so we can use the aero radial above to power our fast moving infantry carrier.
 
The US used radial engines in tanks for a rather strange reason for the US.
They simply did not have a suitable size/power engine in production in the late 30s or 1940, despite having literally hundreds of car, truck, bus, industrial and marine engines in production.
They had radial engines in production in a variety of sizes and they could buy them cheap (relatively) in small numbers for experimental/low production series tanks.
War broke out a little quicker than expected and while the radial engine tanks had helped with the development of transmissions, steering gear and suspensions while not worrying about experimental engines breaking down at the same time they were not what was wanted long term.
The US used the radials as they geared up tank production but they needed the radials for aircraft production, 99.9% of the tank engines were trainer engines in aircraft.
The US did use a few weird and wonderful engine concoctions to power their tanks while they sorted things out.
 

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