Which country designed the best engines for WWII?

[stona]

Napier is a strange case. After WW1 and until 1930 it was the aero engine company that received most money from the Air Ministry, and by a substantial margin, more than the 'other three' (Armstrong Siddeley, Bristol and Rolls Royce) combined in many of the years through the twenties.
The problem for Napier, and the company wasn't alone then and wouldn't be now, was a lack of investment in research and development. Between 1923 and 1930 the company received £5.4 million from its Air Ministry contracts, again more than twice that paid to any of the 'other three', annual profits averaged nearly £200,000 and gilt edged investments just shy of £1 million had been accumulated by 1927.
This was all on the back of the 'Lion' engine which dominated military sales until 1930. Unfortunately, whilst Rolls Royce and Bristol were already developing engines which would become, for example, the 'Kestrel' and 'Pegasus', Napier did absolutely nothing. Having declared higher profits than any other company within the aircraft industry in 1929 it made a loss in 1933.
It is impossible to over estimate the effect of this blase attitude to research and development within British industry. At the height of its prosperity, with money to be invested, Napier was approached by the Air Ministry to develop an engine along the lines of the American Curtiss D-12. The company refused, Rolls Royce took on the work, a D-12 was indeed made available to Rolls Royce at Derby in late 1925 at a time when the team there was under Hives and Rowledge. How much it influenced the Kestrel, the engine which established Rolls Royce as one of Britain's foremost engine manufacturers of the 1930s, can be debated elsewhere. Between 1930 and 1934 Rolls Royce produced 1,182 engines and profits rose from £177,000 to £597,000. Napier nearly went bust.
Had it not been for the looming war it is likely that 'Messrs. Napiers' (as the Air Ministry liked to call the company) would have been allowed to whither and die. By July 1935 the Permanent Under-Secretary, the brilliant Sir Christopher Bullock whose role in establishing the RAF as an independent service is woefully overlooked, was writing that it "was essential that Messrs. Napiers should be kept alive as a separate entity, in order to prevent the engine industry being constituted on too restricted a basis." This sums up government policy towards the firm throughout the immediate pre-war period. It was regarded as an essential cog in a rather larger machine. It was to this end that the initial order for six 'Sabres' was placed in 1937, and in the same year plans for a new Sabre engined fighter to replace the Spitfire and Hurricane were drawn up.
The trials and tribulations of the Sabre are well known, but we shouldn't forget that Rolls Royce and Bristol also struggled with their next generation engines. The Vulture failed consistently in 1939 and the Centaurus....
The decision to gamble on Napier and the Sabre engine was taken on two principle grounds, neither of them financial. The engine would provide a third string to the British bow, should the Vulture and Centaurus fail, and it kept Napier in the aero engine industry. Once the decision was made it started to need financing, the new factory at Liverpool etc.
An overlooked by product of the decision was the effect on Rolls Royce. It gave urgency to Merlin development and indirectly led to the abandonment of the Vulture and development of the Griffon.

Cheers

Steve

 

[Shortround6]

Thank you that informative post.

Part of the trouble England had with engines was that during the 20s and 30s the number of actual engine designers was rather small.
Arthur Rowledge Had worked at Napier from 1913 to 1921 during which time the Lion had been designed. In 1921 he went to work for Rolls Royce. Frank Halford, who designed the Sabre, had started doing engine work in WW I, post war he set up a consulting business nest door to Harry Ricardo. Halford designed the DH Gypsy engine before going to work for Napier and designing the Rapier and Dagger engines. Combining lots of small cylinders (high rpm) with sleeve valves gets the Sabre. Halford moved on to Jets and His design was taken on by de Havilland and became the Goblin. de Havilland absorbed Halford's consulting company in 1944.

Bristol had been dominated by Roy Fedden since it it acquired Cosmos engineering in 1920. Once Fedden got the sleeve valve bit between his teeth there was no stopping him until 1942 when the Bristol board of directors let him go. Had he been kept on the Centaurus Might have seen service sooner.

The problem with the Sleeve valve arrangement was that it took so long to sort out that most of the problems with the poppet valve engines that it was supposed to solve had already been solved by other means. Now maybe it served as a competition to other engine makers to spur them on but basically all the research and development came to nothing.

 

[swampyankee]

"Back on track here I think the Napier Sabre was the most advanced piston engine to see service in WW2, but I think that, on the whole, the US companies Pratt & Whitney Aircraft and Curtiss-Wright designed the best large engines."

The Napier Sabre may have been the most advanced, wither it was worth the time and money spent on it is another matter, since the answer, if it exists, has been buried since WW II. While at least some cost figures for P & W and Wright engines does surface on occasion both the Bristol and Napier sleeve valve R&D costs have remained unknown to the world at large. Somebody may be able to ferret out the cost of post war Bristol engines but since all Napier sales were to the government they may be much harder to find.
The Sabre, for all it's "advancement went exactly nowhere after the war. No commercial sales and darn little use in post war military aircraft except to complete a few existing contracts?
Now I will grant that the majority of post war P & W R-2800s difference from even the war time "C" series engines and that the post war Wright R-3350s also differeed from the majority of wartime R-3350s. Likewise post war Hercules and Centaurus engines differed form war times ones( Hercules went to main bearings with larger rollers for one thing).

AN awful lot of money and time was spent on the Sabre which wound up powering about 4500-4600 airplanes, almost all being single engine.

Finding out the prices for any big-ticket industrial item is not trivial; P&W, Bristol, and C-W didn't have public price lists. I suspect that the Centaurus and Hercules were more expensive than the corresponding engines from P&WA and C-W, but wouldn't bet more than a few cents (or pence) on it. I also suspect that the big radials weren't much cheaper than the gas turbines that were coming out, at least per horsepower. As a WAG, a post-ww2 production R-2800 or R-3350 would be around $20,000 to $30,000, although I'd not be shocked if that number was off by a factor of 2.

 

[swampyankee]

Napier is a strange case. After WW1 and until 1930 it was the aero engine company that received most money from the Air Ministry, and by a substantial margin, more than the 'other three' (Armstrong Siddeley, Bristol and Rolls Royce) combined in many of the years through the twenties.
The problem for Napier, and the company wasn't alone then and wouldn't be now, was a lack of investment in research and development. Between 1923 and 1930 the company received £5.4 million from its Air Ministry contracts, again more than twice that paid to any of the 'other three', annual profits averaged nearly £200,000 and gilt edged investments just shy of £1 million had been accumulated by 1927.
This was all on the back of the 'Lion' engine which dominated military sales until 1930. Unfortunately, whilst Rolls Royce and Bristol were already developing engines which would become, for example, the 'Kestrel' and 'Pegasus', Napier did absolutely nothing. Having declared higher profits than any other company within the aircraft industry in 1929 it made a loss in 1933.
It is impossible to over estimate the effect of this blase attitude to research and development within British industry. At the height of its prosperity, with money to be invested, Napier was approached by the Air Ministry to develop an engine along the lines of the American Curtiss D-12. The company refused, Rolls Royce took on the work, a D-12 was indeed made available to Rolls Royce at Derby in late 1925 at a time when the team there was under Hives and Rowledge. How much it influenced the Kestrel, the engine which established Rolls Royce as one of Britain's foremost engine manufacturers of the 1930s, can be debated elsewhere. Between 1930 and 1934 Rolls Royce produced 1,182 engines and profits rose from £177,000 to £597,000. Napier nearly went bust.
Had it not been for the looming war it is likely that 'Messrs. Napiers' (as the Air Ministry liked to call the company) would have been allowed to whither and die. By July 1935 the Permanent Under-Secretary, the brilliant Sir Christopher Bullock whose role in establishing the RAF as an independent service is woefully overlooked, was writing that it "was essential that Messrs. Napiers should be kept alive as a separate entity, in order to prevent the engine industry being constituted on too restricted a basis." This sums up government policy towards the firm throughout the immediate pre-war period. It was regarded as an essential cog in a rather larger machine. It was to this end that the initial order for six 'Sabres' was placed in 1937, and in the same year plans for a new Sabre engined fighter to replace the Spitfire and Hurricane were drawn up.
The trials and tribulations of the Sabre are well known, but we shouldn't forget that Rolls Royce and Bristol also struggled with their next generation engines. The Vulture failed consistently in 1939 and the Centaurus....
The decision to gamble on Napier and the Sabre engine was taken on two principle grounds, neither of them financial. The engine would provide a third string to the British bow, should the Vulture and Centaurus fail, and it kept Napier in the aero engine industry. Once the decision was made it started to need financing, the new factory at Liverpool etc.
An overlooked by product of the decision was the effect on Rolls Royce. It gave urgency to Merlin development and indirectly led to the abandonment of the Vulture and development of the Griffon.

Cheers

Steve

I remember reading -- and I don't know how true this is -- that Napier was also struggling with basic quality issues in their manufacturing process. They also seemed to have some very interesting, technically advanced, engines that must have been very expensive to produce and maintain: the Rapier and Dagger. Neither seemed to show any significant advantage, even on paper, over the contemporary engines of comparable power from the other British manufacturers. This brings up the question as to whether Napier had competent engineering and project management.

 

[Shortround6]

Finding out the prices for any big-ticket industrial item is not trivial; P&W, Bristol, and C-W didn't have public price lists. I suspect that the Centaurus and Hercules were more expensive than the corresponding engines from P&WA and C-W, but wouldn't bet more than a few cents (or pence) on it. I also suspect that the big radials weren't much cheaper than the gas turbines that were coming out, at least per horsepower. As a WAG, a post-ww2 production R-2800 or R-3350 would be around $20,000 to $30,000, although I'd not be shocked if that number was off by a factor of 2.

It is not just the cost of the engines, it is the cost of the R&D to bring the engine to production. Granted in the case of the R-2800 there were a number of sets of R&D costs. 1st set is the R&D for the 'A' series engines, 2nd set is the 'B' series, not a very big one. 3rd set is the 'C' series engines where everything was brand new. Continuing R&D on the 'C' series involved new pistons and connecting rods . The rods were lengthened 1 in and the wrist pins moved up in the pistons. Stroke and engine diameter remained the same.
Now the company will try to spread the R&D costs out over the production run of the engines but can't charge too much for the first few hundred (or even first few thousand) without making the engine uncompetitive price-wise.
One source says P& W spent 8 million dollars on the R-2800 but doesn't say what stage that brought them to. I would say either the A or B as the comparison was being made to the Wright R-2160. The P & W R-4360 took 25 million but it used a lot of parts/knowledge from the "C" Series R-2800. dividing 25 million by the 18,687 production run of the R-4360 means each engine built cost over $1300 in RD.
Once you have the design finalized (production ready) you can talk about actual production costs , materials and machine/man hours to make parts and assemble.

aside from very rough estimates the R & D costs and production costs of the sleeve valve engines are unknown to the public.

 

[stona]

The problem with Napier was that at the start of the major rearmament programmes the company had virtually no work. Almost all RAF first-line aircraft were powered by Rolls Royce and Bristol engines, companies which were developing the engines that would enter service in the late 1930s (Merlin, Perseus, Taurus). This was a concern to the Air Ministry and political pressure to keep the firm afloat.
In February 1937 Freeman was arguing that

"the loss of the experienced personnel making up the technical organisation [of Napier] would be a serious loss to the RAF."

In May 1939 he was expressing concerns that not only Napier, but also Armstrong Siddeley, were on the verge of leaving the aero engine business altogether. He echoed Bullocks sentiments, which I posted above, when he wrote that

"It was a most unhealthy position for the Air Ministry to be dependent upon two firms only."

It was a situation that the Air Ministry had largely brought upon itself. It had allowed Napier to get away with simply producing its Lion engine for too long and Armstrong Siddeley had maintained a market for its older engines by having other companies within the group design aircraft around them, like the Lynx powered Tutor trainer built by A.V.Roe. The Lynx was a design dating almost to WW1.
Now, it had to keep them in the business with the promise of lucrative contracts.

Cheers

Steve
.

 

[Shortround6]

Unfortunately the seeds to Napier's and Armstrong Siddeley's fall from prominence had been sown around 1930. No amount of high value contracts were going to produce much in the way modern, high power engines in 1938,39,or 40.

The Napier Rapier may have been a clever and somewhat innovative engine but an 8.8 liter engine , no matter how clever or innovative was going to be a replacement for the 24 liter Lion. The Rapier first ran in 1929, 2-3 years after the Kestrel, a 21.2 liter engine. Hlaford and Napier following up with the Dagger in 1934, a 16.8 liter engine that weighed as much as a Merlin,granted it had no liquid cooling system with it's attendant weight but it is doubtful that manufacturing costs could have been lower than a Merlin, 24 cylinders and two crankshafts? Air Ministry bought hundreds anyway for the Hector and Hereford. Given that track record the Sabre was pretty much of a long shot.

Armstrong Siddeley goes back to the early 20s and the inheritance of the RAF 8 14 cylinder radial. This slowly transformed into the Jaguar engine which was one of the BIG 3 of British engines during the 20s. Unfortunately Armstrong Siddeley kind of got stuck there and made 7 clinder single row versions, 5 cylinder versions, 10 cylinder two row versions and so on. It was periodically "updated" wiht at first a 0.25in bore job to become the Panther (1929) and then in 1932 a further 0.25 in bore increase and 0.5 increase in stroke introduced the Tiger. 7 cylinder versions (and others ?) of these larger cylinder engines filled the training market. Unfortunately, short of beating J. D. Siddeley about the head with a stout cricket bat there was no convincing him of the need for a center bearing in a 2 row 14 cylinder engine and this rather doomed the whole line to 2nd rank status. While the introduction of the first service 2 speed supercharger was innovative the leap to the 21 cylinder 3 row Deerhound certainly seems suspect in regards to cooling.

Both companies failing to come up with up to date, truly competitive products in the 1929-34 period to base further developments on put the AIr Ministry in the position of making leaps of faith when placing contracts in the late 30s to keep them in business.

 

[stona]

Both companies failing to come up with up to date, truly competitive products in the 1929-34 period to base further developments on put the AIr Ministry in the position of making leaps of faith when placing contracts in the late 30s to keep them in business.

This is precisely the point I'm making. The Air Ministry made that leap in the case of Napier in 1937. It wasn't just the engine contracts, an entire factory at Liverpool also had to be financed.
With the benefit of hindsight we can safely say that this was a waste of money. Britain would not have lost the war and the Air Ministry/RAF would not have suffered unduly if both Napier and the aero engine division of Armstrong Siddeley had been allowed to fall by the wayside. Whilst it seemed undesirable to concentrate British aero engine production in just two companies, especially before the role of the US in the war was clear, it would not have had the undesirable consequences feared.

Both companies were lazy. Napier was going along very nicely until the late 1920s, trading on ancient engines which a lack of foresight and planning allowed the company to believe would continue into the future. It didn't
Armstrong Siddeley is a slightly different case, the aero engine business being just one under that umbrella. It too was guilty of complacency and lack of foresight.
As a man who once owned several British motorcycles I could argue that this is a recurring theme in some British industries.

Cheers

Steve

 

[swampyankee]

It is not just the cost of the engines, it is the cost of the R&D to bring the engine to production. Granted in the case of the R-2800 there were a number of sets of R&D costs. 1st set is the R&D for the 'A' series engines, 2nd set is the 'B' series, not a very big one. 3rd set is the 'C' series engines where everything was brand new. Continuing R&D on the 'C' series involved new pistons and connecting rods . The rods were lengthened 1 in and the wrist pins moved up in the pistons. Stroke and engine diameter remained the same.
Now the company will try to spread the R&D costs out over the production run of the engines but can't charge too much for the first few hundred (or even first few thousand) without making the engine uncompetitive price-wise.
One source says P& W spent 8 million dollars on the R-2800 but doesn't say what stage that brought them to. I would say either the A or B as the comparison was being made to the Wright R-2160. The P & W R-4360 took 25 million but it used a lot of parts/knowledge from the "C" Series R-2800. dividing 25 million by the 18,687 production run of the R-4360 means each engine built cost over $1300 in RD.
Once you have the design finalized (production ready) you can talk about actual production costs , materials and machine/man hours to make parts and assemble.

aside from very rough estimates the R & D costs and production costs of the sleeve valve engines are unknown to the public.

I worked in the aircraft engine industry for a [too] short period of time; the actual costs of the engines my employer sold were a total mystery to me

I knew (within a few percent) the price at which one model was being sold to one company, but I have no idea what the exact same model would be priced at for a different customer.

"Back on track here I think the Napier Sabre was the most advanced piston engine to see service in WW2, but I think that, on the whole, the US companies Pratt & Whitney Aircraft and Curtiss-Wright designed the best large engines."

The Napier Sabre may have been the most advanced, wither it was worth the time and money spent on it is another matter, since the answer, if it exists, has been buried since WW II. While at least some cost figures for P & W and Wright engines does surface on occasion both the Bristol and Napier sleeve valve R&D costs have remained unknown to the world at large. Somebody may be able to ferret out the cost of post war Bristol engines but since all Napier sales were to the government they may be much harder to find.
The Sabre, for all it's "advancement went exactly nowhere after the war. No commercial sales and darn little use in post war military aircraft except to complete a few existing contracts?
Now I will grant that the majority of post war P & W R-2800s difference from even the war time "C" series engines and that the post war Wright R-3350s also differeed from the majority of wartime R-3350s. Likewise post war Hercules and Centaurus engines differed form war times ones( Hercules went to main bearings with larger rollers for one thing).

AN awful lot of money and time was spent on the Sabre which wound up powering about 4500-4600 airplanes, almost all being single engine.

The Sabre is probably the poster child for why "most advanced" does not necessarily equal "best."

 

[stona]

I found a list of engine prices for Bristol Mercury engines, originally produced to compare the cost of those produced by the parent and those produced by the 'Shadow Industry'.
The cost given is 'average price per engine'.

Bristol:
1st 500, £1,913. 2nd 500, £1,306. 3rd 500 £1,197. The price was reduced by almost 40% over the run of the first 1,500 engines.

Shadow Industry.
1st batch of 987, £1,496. 2nd batch of 887, £1,486. It seems here, where many production issues would already have been solved by Bristol, the price per engine was far more stable. Higher wages in the Midlands, where many of the shadow factories were located, was considered the principle reason that the engines produced there cost slightly more than those at the parent.

£1,500 per engine in 1939 money is equivalent to very approximately £97,000 in today's money.

Cheers

Steve

 

[wuzak]

I found a list of engine prices for Bristol Mercury engines, originally produced to compare the cost of those produced by the parent and those produced by the 'Shadow Industry'.
The cost given is 'average price per engine'.

Bristol:
1st 500, £1,913. 2nd 500, £1,306. 3rd 500 £1,197. The price was reduced by almost 40% over the run of the first 1,500 engines.

Shadow Industry.
1st batch of 987, £1,496. 2nd batch of 887, £1,486. It seems here, where many production issues would already have been solved by Bristol, the price per engine was far more stable. Higher wages in the Midlands, where many of the shadow factories were located, was considered the principle reason that the engines produced there cost slightly more than those at the parent.

£1,500 per engine in 1939 money is equivalent to very approximately £97,000 in today's money.

Cheers

Steve

Was there a royalty payment/licence fee for Bristol from the shadow factories?

 

[wuzak]

This is precisely the point I'm making. The Air Ministry made that leap in the case of Napier in 1937. It wasn't just the engine contracts, an entire factory at Liverpool also had to be financed.
With the benefit of hindsight we can safely say that this was a waste of money. Britain would not have lost the war and the Air Ministry/RAF would not have suffered unduly if both Napier and the aero engine division of Armstrong Siddeley had been allowed to fall by the wayside. Whilst it seemed undesirable to concentrate British aero engine production in just two companies, especially before the role of the US in the war was clear, it would not have had the undesirable consequences feared.

Both companies were lazy. Napier was going along very nicely until the late 1920s, trading on ancient engines which a lack of foresight and planning allowed the company to believe would continue into the future. It didn't
Armstrong Siddeley is a slightly different case, the aero engine business being just one under that umbrella. It too was guilty of complacency and lack of foresight.
As a man who once owned several British motorcycles I could argue that this is a recurring theme in some British industries.

Cheers

Steve

It does make one wonder what Rolls-Royce and Bristol could have made with the extra resources and engineering personnel.

Could Rolls-Royce have continued developing one or more of the projects cancelled in favour of the Merlin and Griffon - ie the Vulture, Peregrine and Exe. Or could they put it into work on teh Crecy, which was not allowed to be cancelled, but continued on low resources.

On Bristol's side, would the Hercules and Centaurus have been developed faster, the latter becoming a more important engine during the war?

Armstrong Siddeley was told to abandon their piston projects (Deerhound, Wolfhound, etc) around 1941 to concentrate on gas turbines. This new focus really only bore fruit when they took over the Metrovicks F.2 project after the war. So, perhaps, Armstrong Siddeley would have been better joining with Metrovicks in the beginning, rather than going it alone. Certainly the steam engineers at Metrovicks could have done with some aviation engineering help.

 

[Piper106]

I remember reading -- and I don't know how true this is -- that Napier was also struggling with basic quality issues in their manufacturing process.

I recall reading that some finished Sabres were being delivered with chips from machining operations inside. To me that indicates a breakdown not just in manufacturing but also inspection, and supervision at Napier. .

 

[Deleted member 68059]

This is fairly typical of the kind of things recorded in the British official papers of the period, I have a hundred pages more like it. You wont find things like this said about the Merlin (you can cherry pick one or two during some of the difficult times, but they are few and far between).

 

[stona]

Was there a royalty payment/licence fee for Bristol from the shadow factories?

No. Also shadow factory should be read to understand shadow group. The first shadow group comprised five firms from the motor industry (Austin, Standard, Rover, Humber and Daimler) all manufacturing various elements of the engines, initially 1500 then 2000 Mercury types, which were still assembled and tested by Bristol.
Cheers
Steve

 

[swampyankee]

I found a list of engine prices for Bristol Mercury engines, originally produced to compare the cost of those produced by the parent and those produced by the 'Shadow Industry'.
The cost given is 'average price per engine'.

Bristol:
1st 500, £1,913. 2nd 500, £1,306. 3rd 500 £1,197. The price was reduced by almost 40% over the run of the first 1,500 engines.

Shadow Industry.
1st batch of 987, £1,496. 2nd batch of 887, £1,486. It seems here, where many production issues would already have been solved by Bristol, the price per engine was far more stable. Higher wages in the Midlands, where many of the shadow factories were located, was considered the principle reason that the engines produced there cost slightly more than those at the parent.

£1,500 per engine in 1939 money is equivalent to very approximately £97,000 in today's money.

Cheers

Steve

That's not too far from my guesstimate for the price of one of the P&W or Curtiss radials. U.S. wages tended to be somewhat higher than British wages, but I think the US makers had more efficient production methods, so these probably balanced out.

As an aside, why did the UK's Air Ministry feel the need for a third first-tier aircraft engine maker? The US had C-W and Pratt, which split the airline market, Allison which really only sold to the USAAF, and a bunch of small engine companies for general aviation ( Continental, Lycoming, etc). Could part of the problem in the UK have been that Bristol's engine and airframe business were closely connected, as were those of Armstrong and Dehavilland, and the ministry felt this disadvantageous to its requirement for high performance aircraft? Even thogh Pratt and C-W were both owned by companies that also owned airframe makers (Vought-Sikorsky and Curtiss, respectively), most of their sales seemed to be to places like Boeing, Douglas, and Martin.

 

[fastmongrel]

As an aside, why did the UK's Air Ministry feel the need for a third first-tier aircraft engine maker?

Possibly due to fears one well aimed bombing raid could halve engine production.

 

[Shortround6]

What would be interesting is the price of a Bristol Perseus, The 9 cylinder sleeve valve engine to compare to the poppet valve Mercury.
Mercury's had also been being made for quite a few years so some of Bristols tooling was already paid for. Shadow factory tooling was paid for by the government?
First 500 Mercury engines would date back to the late 1920s??

 

[stona]

Very briefly, and from memory,I might get a chance later for a more detailed reply.
De Havilland had corners the civil market and really didn't feature in Air Ministry production plans. It did produce a basic trainer, based on a civilian model. When the Mosquito was finally ordered De Havilland were told not to allow it to interfere with work contracted from other companies (like Hawker and RR).

I am fairly sure that the engine price comparisons are under the first expansion scheme that led to the establishment of the shadow system, at least for Bristol. RR was opposed to the system, which is why it developed very differently for it.
I think the prices are from the late thirties and Scheme L, but I would need to check.

 

[tomo pauk]

...
As an aside, why did the UK's Air Ministry feel the need for a third first-tier aircraft engine maker?

Another 1st-tier engine maker is a stick against the two making a deal, that could've increased the prices while making their products less advanced. The Armstrong-Siddeley was to be the 4th, but the experiments with Deerhound went nowhere, while Tiger was a dog, not a tiger.
OTOH - looks like Fairey was not allowed to enter the race, posibly due the fears of the Air Ministry not be able to finnance five companies' designs?

Possibly due to fears one well aimed bombing raid could halve engine production.

The shadow factories were to provide redundancy, eg. RR producing their engines in 4 locations in the UK plus in the USA?