Supercharger Development & Aircraft Design Policy (USAAC)

[Zipper730]

I was starting to give "The Lockheed P-38 Lightning", by Warren M. Bodie a serious read, and there's a whole bunch of interesting stuff in here, one mention was in the prologue (page xi), which include two interesting mentions.

Using a devious approach to avoid the policy roadblocks, we even referred to it as an "Interceptor." That was actually a semantic dodge in working with the Military Requirements and the Type Specifications for the design competition in order to deviate widely from conventional design and operational philosophy dictated by Congress, the Executive Branch appointees and the War Department.

and

Thanks to the efforts of Gen. Malin Craig, absolutely no funds were available for years to develop two-stage, two-speed mechanical superchargers for any of our aero engines. Therefore, the Air Corps was committed to the turbo-supercharger that had been under development for well over a decade.

In regards to the first quote: I thought the requirements for aircraft were largely dictated solely by the US Army and US Army Air Corps. I didn't know Congress set any real regulations on this matter other than issues regarding to cost.

As for the second quote: Why were all funds for twin-stage superchargers done away with at General Craig's efforts? Was this solely because of an interest in turbochargers, or another reason?

 

[tomo pauk]

I was starting to give "The Lockheed P-38 Lightning", by Warren M. Bodie a serious read, and there's a whole bunch of interesting stuff in here, one mention was in the prologue (page xi), which include two interesting mentions.


and
In regards to the first quote: I thought the requirements for aircraft were largely dictated solely by the US Army and US Army Air Corps. I didn't know Congress set any real regulations on this matter other than issues regarding to cost.

As for the second quote: Why were all funds for twin-stage superchargers done away with at General Craig's efforts? Was this solely because of an interest in turbochargers, or another reason?

These are not words of Bodie, but of Benjamin Kelsey.
As for the 1st quote: it is sometimes easier if blame is shared by someone else?
For the 2nd quote: very cheesy statement - Criag was not holding a post in the US Army from 1st Aug 1939 to September 1941. Two years is a lot of time to develop a 2-stage supercharger in late 1930s/early 1940s if one really wants it, as seen by cooperation of US Navy with P&W; USA was not some poor backwater country after all.

 

[Shortround6]

I would note that the Navy and P & W were developing a single speed engine supercharger with a two speed plus neutral (unused at low altitude) auxiliary supercharger.
Contrast this to the Merlin two stage in which both impellers were turning at all times and the first and 2nd stage turned at the same speed, the two speed drive shifted both impellers at the same time.
Also note that the US was just introducing 100/100 fuel (not 100/130) in the very late 1930s and 1940.
Also note the P & W two stage engine that flew in th eUS 1939 fighter trials was good for 1050hp at 17,500ft in high gear on the auxiliary supercharger and a Merlin X engine using high gear on it's single stage engine was good for 1010hp at 17,750ft. and this is before Hooker got involved.

US superchargers simply weren't very good in general at this time. having one crappy supercharger blowing into the inlet of a 2nd crappy supercharger doesn't really get you that much.
The US had a dearth of supercharger designers at this time as P & W, Wright and Allison had all gotten fed up with the crappy superchargers General ELectric was selling (GE sold designs and licenses for mechanical drive superchargers in addition to the turbo chargers) and were trying to design their own. Some how many writers/critics want the US to magicly comeup with a bunch of supercharger designers that didn't exist in real life to design a supercharger that didn't really exist in either America or Europe ( It took RR how long to get their two stage engine into service?). Even the British had trouble, Bristol had set a world record for altitude using a two stage engine in 1938 but Bristol never got a two stage supercharger into service on any engine either during or after WW II. It was not as easy as some people think.
One critic claimed all a 2nd stage did was magnify the problems of the 1st stage.

 

[wuzak]

It took RR how long to get their two stage engine into service?

About 2 years, I'd guess.

Considering that existing production took precedent over the new engine for some of that time, as well as the production of existing airframes taking priority.

Also, the Merlin 2 stage was originally intended for the Wellington, but first saw service in the Spitfire.

Even the British had trouble, Bristol had set a world record for altitude using a two stage engine in 1938 but Bristol never got a two stage supercharger into service on any engine either during or after WW II

Bristol spent much of their time trying to make their engines work properly. Probably didn't have resources to spend on a 2 stage supercharger.

Napiers were working on a 2 stage Sabre before they had figured out production.

 

[Zipper730]

These are not words of Bodie, but of Benjamin Kelsey.
As for the 1st quote: it is sometimes easier if blame is shared by someone else?

Firstly, I didn't know that Kelsey made the quote -- I simply saw the statement, and figured it'd be from Bodie as it was his book.

For the 2nd quote: very cheesy statement - Criag was not holding a post in the US Army from 1st Aug 1939 to September 1941.

While true, the implication seemed to be that he had set-up a budget situation that had ensured there'd be little money for the time he was holding a post within the US Army.

 

[tomo pauk]

Firstly, I didn't know that Kelsey made the quote -- I simply saw the statement, and figured it'd be from Bodie as it was his book.

You can read at pg. xii that Kelsey wrote the intro containing the claims.

While true, the implication seemed to be that he had set-up a budget situation that had ensured there'd be little money for the time he was holding a post within the US Army.

And thus piling up all the blame to a General that was dead by 1945. How convenient.

 

[Shortround6]

Bristol spent much of their time trying to make their engines work properly. Probably didn't have resources to spend on a 2 stage supercharger.

Napiers were working on a 2 stage Sabre before they had figured out production.

After the war when Hooker left RR and went to work for Bristol on their jet engine he made a comment that he didn't think the Bristol engineers understood airflow very well.
Granted an axial flow jet is a LOT more complicated than a pair of centrifugal compressors but designing good compressors/superchargers was a lot harder than it looks.

Allison screwed up by trying to go cheap in initial work by using two identical superchargers (or at least two impellers the same diameter) while RR and Hooker realized that the airflow requirements at high altitude required a larger compressor and the vulture supercharger was a close match to the airflow they needed at the altitude they were interested in so they used parts from the Vulture in their first test rig (or rigs?) which allowed them to hone in on the proper size/features much quicker.

 

[drgondog]

Simply stated - Material Division under leadership of Oliver Echols, did not believe that a two speed-two stage supercharged engine was feasible. In-line engines weren't even considered with enough regard to experiment until Allison passed the 1000 Hp bench testing about the time the RFP for P-38 and P-39 emerged - driven by Kelsey - and as described adequately by Kelsey quotes in Bodie's P-38 work. The Ge Turbo was required at that time to push the P-38 performance envelope above 15,000 feet enough to qualify as a high altitude 'Interceptor'. The P-39 was described by Kelsey as 'the small solution' and never achieved promised capability until the P-63 limped out with the Allison auxiliary second stage - requiring a major redesign to accommodate the engine.

Secondly, the only promising in-line engine manufacturer that's actually developed a reliable supercharged engine was Allison. Allison was underfunded by GM, and starved for R&D funding, but there is no evidence that Allison believed they could develop such an integral 2s/2s engine if given the funding.

As to many references to Kelsey as a visionary, history doesn't support him as ever being an advocate of the P-51 until he was out from under Echols and in ETO operations where the P-38 was under-performing to the P-51B.

General Malin Craig was Army Chief of Staff and retired days before the Germans attacked Poland, not a staunch supporter of AAF vision but also not regarded as a meticulous bean counter looming over Arnold's shoulder regarding approved budgets for R&D. Simply stated again, Arnold drove R&D for AAF while under Malin and basically throughout WWII. Bomber development was number one priority until events in Europe cast doubts of unescorted bombers being able to carry out AWPD-1 to destroy enemy war making capability. Turbo supercharged engines, whether on interceptors or high altitude bombers were central to specifications for new development. The only other alternatives considered were to boost in-line power to continue to raise the ceiling of singles stage engines. The Continental, etc. were attempts that repeatedly failed to drive down weight and volume as well as achieve reliability requirements.

Materiel Command had a miserable record for developing high performance fighters capable of long range escort to support AWPD-1 and -2. Lack of in-line engine development in the US was central to the failures.

 

[fubar57]

For those like me who have no idea what AWPD-1 was....

 

[Zipper730]

I would note that the Navy and P & W were developing a single speed engine supercharger with a two speed plus neutral (unused at low altitude) auxiliary supercharger.

If most superchargers weren't that good -- why did the USN go for two-stage and the USAAC just go for turbos

 

[tomo pauk]

If most superchargers weren't that good -- why did the USN go for two-stage and the USAAC just go for turbos

A half-decent 2-stage supercharger is much better thing than a half-decent 1-stage supercharger. Adding a turbocharger is even better.

 

[GregP]

According to documents that Joe Yancey has, the USA owned the engine designs that were paid for by US funds. So, the Allison V-1710 design was owned by Congress and any changes to the design were approved by Congressional vote. That's why when the Army ordered, say, a V-1710-31, the components were to be delivered as a V-1710-31, not some improved version. On several occasions, Allison asked the government if they wanted a 2-stage V-1710 and they were told each time, "No."

Allison, being a relatively small company, and even a small division after GM bought them, didn't have the resources to develop advanced versions on their own, and the government got exactly what they ordered. The research that WAS done was paid for piecemeal by the government. Allison finally DID develop an auxiliary stage on their own, but that was after asking several times for funding for a more advanced development and being denied. The aux stage was a stand-alone addition, not part of the engine. People may wonder why, and it is because they were not allowed to modify the design without approval.

The Packard Merlin engine team was never saddled with government oversight like the Allison was because the US government didn't own the Merlin design. They were free to add improvements as they came up. All in all, Allison didn't do badly at all considering the bureaucratic red tape that any improvement necessitated.

Most people have no idea of amount of paperwork involved. When Ben Kelsey wanted to just increase allowable boost, the paperwork required almost a six-month study and recommendations from several levels of authority. Allison was a good company and still is. I doubt they ever want to allow ownership of one of their designs by anyone ever again!

 

[tomo pauk]

According to documents that Joe Yancey has, the USA owned the engine designs that were paid for by US funds. So, the Allison V-1710 design was owned by Congress and any changes to the design were approved by Congressional vote.

USA owned the actual engines they bought. V-1710 was designed for years without US Gov paying anything.

Most people have no idea of amount of paperwork involved. When Ben Kelsey wanted to just increase allowable boost, the paperwork required almost a six-month study and recommendations from several levels of authority. Allison was a good company and still is. I doubt they ever want to allow ownership of one of their designs by anyone ever again!

Willy-nily, whatever Allison nowadays designs is owned by Rolls Royce.

 

[Shortround6]

If most superchargers weren't that good -- why did the USN go for two-stage and the USAAC just go for turbos

You have several interrelated problems.
many of the superchargers weren't as efficient as they could be, this means they took more power to compress the same mass of air (volume and weight) to the desired pressure, say 6lbs of boost or roughly 42in MAP. Now the problem isn't so much that supercharger A took 10-20 more hp from the engine to reach the same goal (although that counted) as supercharger B but the extra power didn't just disappear. It was converted into heat in the intake charge and had two effects, the higher temperature charge actually had less mass/weight for the same volume and pressure which means less power and the higher temperature mixture was closer to the detonation limit if not over it which means max boost might have to be limited.

two stage superchargers and low octane fuel really needed good intercoolers to work well (or at all). trying to use two stage superchargers at high altitude called for a lot of compression of the air. A lot more than the compression needed than at low altitude to get the same manifold pressure. This means much more temperature rise in the supercharger.

A two stage supercharger will give the the same compression as a single stage supercharger (of the same basic efficiency) with less temperature rise. However two low efficiency compressors (superchargers) in series may give more temperature rise than a single high efficiency compressor/supercharger.

The US went to two stage superchargers (mechanical and turbo) in part to get around the fact that their single stage superchargers weren't that good.

 

[tomo pauk]

...
The US went to two stage superchargers (mechanical and turbo) in part to get around the fact that their single stage superchargers weren't that good.

The superchargers on radials were both small and inefficient.
For scale - BMW 801 and Hercules used impellers of diameter of 13 inches, engines of 2360 and 2560 cu in, respectively. The US R-2600 and R-2800 have had 11 in impeller on 1-stage S/Ced engines before late 1944. 10:1.
BTW - the superchagers on the BMW 801 and Hercules were also not that efficient, but the size of impeller helped. Neither of the two Europeans used curved/parabolic vanes until some time of 1944, depending on engine: BMW 801 with the E (prototype from 1943) and S (in series, late 1944), Hercules probably with 100 series. I'm not sure when the US radial engines received curved vanes on impeller*.
Some of radials (and pre-Hooker Merlins) also used air entries that messed with airflow before entering the supercharger, that again harms the altitude power - photo.

(photo of Centaurus internals, the curved vanes of the S/C are prominent)

*edit: the R-2600 have had curved vanes on impeller already in 1940

 

[GregP]

The V-1710 was designed originally for the US Navy as a dirigible engine, and was modified to become an aircraft engine. The engine was sort of of-again on-again, and didn't proceed very quickly in the years leading up to WWII, but it finally DID get into production. The first aircraft powered by an Allison V-1710 was the Consolidated Vultee XA-11A testbed aircraft. Sort of got better from there. That's when they got the idea to try the V-1710 in a P-36 airframe to produce the P-40. Some of the former P-36 pilots have said the radial P-36 was a better dogfighter than the inline P-40. The longitudinally distributed mass of V-1710 might lend some credence to that, but the P-40 WAS faster.

The US government not paying anything is exactly WHY the V-1710 took so long to develop, Tomo. But you know that.

Not saying it was right, wrong, or indifferent, just saying that is the way it happened.

 

[Shortround6]

According to "Vees for Victory" Allison had designed the engine and approached both the Army and Navy. The Army was short of funds and suggested the Navy might be interested. The Navy was interested in four 750hp airship engines but ordered the engine as it existed on paper in order to expedite the development of the version they were interested in. The Army was monitoring things. The Army started negations with Allison in Jan 1931 and it took until March of 1932 to order one engine which was based off the Navy engine. The Navy ordered 3 additional engines but these engines had no superchargers unlike the first two engines and were also reversible, being able to reverse direction of rotation and return to full power in 8 seconds. The Army engine was rated at 750hp at 2400rpm but had demonstrated 1070hp at 2800rpm.

The original Navy engine appears to have been rebuilt at least twice. The Navy Airship engines were "B" series engines while the first Navy engine and the first Army engine were both A series engines. Some of the charts/tables in Vees for victory do not agree with each other but one chart shows
1932 as the year the first navy engine was delivered and the engine at least started a 50 hr acceptance test at 750hp.
1933 has the Army getting their first engine (A series) and running their 50hr test at 1000hp, navy also gets their first reversible airship engine.
1934 has the Army getting their first long nose "C" series engine to try to pass the 150hr 1000hp test.
1935 sees the Navy get their other two airship engines and the Army get one engine (C) for initial fitting and ground testing in the A-11A aircraft.
1936 sees the Army get one engine for the 150hr 1000hp test and one engine for flight testing in the A-11A (both C series).

There were a lot of trials and tribulations and lack of funding but it doesn't seem that Allison and General Motors designed an airship engine. They designed a general purpose engine and adapted it to airship use in a period of very little government spending in order to keep the whole program alive. Allison had been given a tentative contract for a whopping 10 engines that was contingent on getting an engine to pass the 150hr 1000hp test and it was this promise that kept GM funding things. This test was successful passed in April of 1937 so Allison and GM had been hanging on for quite some time.

 

[Zipper730]

You have several interrelated problems.

So more horsepower was taken away to drive the shaft, and the temperatures brought the engine closer to pre-detonation and limited maximum pressure?

As for two-stage superchargers being dependent on intercoolers: I'm already aware of that. As a general rule, it would appear that the turbocharged engines that we used had the turbocharger being the auxiliary stage blower. I would figure that compression would be the same on both a twin-stage supercharger or a turbocharged engine except that less horsepower would be taken off the shaft, and exhaust thrust would be less.

 

[K5083]

Nobody has mentioned that the advantage of the Merlin 60 series for a fighter was compactness. Aux superchargers take more space. OK with a radial but problematic with a V12. Turbos are even harder to fit in. That's why there were only three operational turbo fighters in WW2. They were designed around the turbos. The turbo defined the shape. The turbo made the P-38 twin-boom, at the expense of versatility and made the P-47 so big.

 

[tomo pauk]

Nobody has mentioned that the advantage of the Merlin 60 series for a fighter was compactness. Aux superchargers take more space. OK with a radial but problematic with a V12.

Indeed, Merlin 60 series ticked most of the boxes - it was of modest size & weight, power at all altitudes was excellent, fuel consumption was okay, being a V12 meant that drag due to engine could be held as minimal as possible.
The main shortcomings of the 2-stage V-1710 (the only V12 with aux S/C worth talking about) is that it was a bit late, and then it was installed on an irellevant aircraft type. We know that P-40 accepted the 2-stage V-1710, and that it flew about as good as Fw 190D-9 by mid-1944.

Turbos are even harder to fit in. That's why there were only three operational turbo fighters in WW2. They were designed around the turbos. The turbo defined the shape. The turbo made the P-38 twin-boom, at the expense of versatility and made the P-47 so big.

P-38 was a very versatile aircraft.
A good part of P-47's size depended on it's big R-2800 engine, big fuel tankage, and it's battery of HMGs with plenty of ammo. Hi alt operation necesitates a big wing, too.
See F4U and F6F - big engine, big wing, wide fuselage, heavy gun firepower, heavy payload, no turbo (bar prototypes).