A 'simple' V-1710: possible/plausible limits on ww2 tech?

[tomo pauk]

From quite number of threads, it looks like the installation of a 2-stage supercharged V-1710 on existing aircraft was not a trivial task, let alone trying to shove in the turbo somewhere. So let's say Allison tries to keep V-1710 in a compact form, while trying to install as good and as capable 1-stage supercharger as possible. On technology of ww2, what might be the limits to capability growth, both with regard to low-altitude power, and with regard to hi-altitude power, all while retaining reasonable reliability? Possible timeline?

(yes, I know that mating the S/C from 2-stage Merlin would've provided better results, but that is not within the scope of this thread)

 

[overbeck]

For the Derwent turbojet Sir Frank Whittle made a blower with a 4:1 pressure ratio at 78% efficiency, or 73% after Sir Stanley deleted the inlet guide vanes due to their structural weakness. The chance of Allison finding and hiring a Whittle was practically zero, but the Germans at DB managed 4:1 at about 70% before the end of the war, so if Allison pulled out all the stops in 1940, hired some serious talent, and built a blower research lab with a good test rig, maybe they could've had a V-1710 with a single-stage, two-speed 4:1 supercharger in production for the Mustang in 1943.

Assuming a 1" drop through the carburetor, 4:1 gets us 44.5" manifold pressure at 23,000 feet, so maybe about 1100 hp. ADI would boost that a bit, and it would raise allowable WEP boost lower down, which charge temperature would severely limit otherwise. In contrast, the V-1650-7 made 1390 hp at 21,000 ft from 60.5" manifold pressure. At that height our what-if V-1710 would blow 48.8" and make about 1200 hp. A bit weak but at least good enough for a stop-gap escort fighter while Packard sorted out their production problems, plus a bit lighter and with less cooling drag than the P-51B.

FWIW I see significant parallels between Allison's actions up to 1940 and the failure of Westinghouse to keep up with the gas turbine market in the 1950s. From the thesis at enginehistory.org entitled "The Westinghouse Aviation Gas Turbine Division 1950-1960: A Case Study in the Role of Failure in Technology and Business":

"The main rivals of the Westinghouse AGT Division, General Electric and Pratt & Whitney Aircraft, succeeded not just in developing significant R&D facilities and resources but in using those facilities and resources to produce products which were one step ahead of the requirements of its customers. This action permitted the military services to go forward with the development of a wider range of airframe applications for these new engines; it also helped spur the development of nonmilitary gas turbine engine-powered aircraft, thus broadening the customer base of the engine manufacturers. The Westinghouse AGT Division displayed little initiative in developing its own engine designs, and missed the resulting opportunities to broaden its market coverage."

The author concluded that Westinghouse always did what the Navy asked for, never more, and that was a major factor in their undoing. Allison's problem was they only ever did what the Army asked for, and didn't realize the Army didn't always know what it needed soon enough for Allison to design it in time, or, in the case of the two-stage Merlin with air/liquid aftercooler, the Army didn't know what it needed until it was presented to them. Making a 4:1 blower or something even better would take a serious shift in culture at Allison toward a far more entrepreneurial attitude, probably driven by a new Chief Engineer or CEO.

 

[Shortround6]

Making a 4:1 blower or something even better would take a serious shift in culture at Allison toward a far more entrepreneurial attitude, probably driven by a new Chief Engineer or CEO

Part of that attitude was the fact that in the Spring of 1939 when the Army placed the big order for P-40s the Army owed Allison 900,000 dollars for work already done at the armies request. GM had already given 500,000 dollars to Allison for the V-12 engine project. The engine 'shop' was attached to a rather profitable bearing manufacturing plant that could supply any more money (or was paying the profits to GM).
The Army never paid up. Allison had to forget about the debt in order to get permission to export the V-12 engine to France and Great Britain in 1939.
Army also wanted the pusher engines for the YP-37, the remote gear box engines for the P-39 and the turbo engines for the P-38s. At one point Allison did ask the army which projects they wanted Allison to work on as the y didn't have the staff to work on all of the armies ideas.

It is hard to be entrepreneurial for very long when the customer won't pay his bills. A bankrupt company doesn't build much of anything.

Army contracting during the was such that they only paid for a completed successful test (and timely payment even then was a rarity). In other words if an engine (or part) failed part way through a test session the contractor (Allison or Continental) had to tear down the engine, make new part/s, rebuild the engine and restart or resume the test all at their own cost. Army was not going to pay any additional money for repairs or delays.

 

[overbeck]

Part of that attitude was the fact that in the Spring of 1939 when the Army placed the big order for P-40s the Army owed Allison 900,000 dollars for work already done at the armies request. GM had already given 500,000 dollars to Allison for the V-12 engine project.

I'm thinking more of taking a big financial risk to receive large war orders for a product that was better than anything else available at the time. It would take more staff and work space. They could have done it if the will existed. However, they were receiving large orders anyway...until they were cancelled. With the benefit of 20/20 hindsight, if your vast engine plant is running at half capacity near the peak of the largest conflict in world history because the product is obsolete, you know you should have taken more financial risks. Given the culture at Allison and possibly GM as well, it would have taken a miracle for them to place such a bet, even in the midst of an arms race. Even RR didn't develop a two-stage until asked, but they did redevelop their single-stage blower on their own initiative per SH's analysis.

 

[swampyankee]

Both P&WA and Curtiss-Wright had a revenue stream outside of government contracts and financial reasons to develop technology outside of government requests; Allison had neither.

 

[wuzak]

Army also wanted the pusher engines for the YP-37

?

Do you mean YFM-1?

 

[tomo pauk]

For the Derwent turbojet Sir Frank Whittle made a blower with a 4:1 pressure ratio at 78% efficiency, or 73% after Sir Stanley deleted the inlet guide vanes due to their structural weakness. The chance of Allison finding and hiring a Whittle was practically zero, but the Germans at DB managed 4:1 at about 70% before the end of the war, so if Allison pulled out all the stops in 1940, hired some serious talent, and built a blower research lab with a good test rig, maybe they could've had a V-1710 with a single-stage, two-speed 4:1 supercharger in production for the Mustang in 1943.

Assuming a 1" drop through the carburetor, 4:1 gets us 44.5" manifold pressure at 23,000 feet, so maybe about 1100 hp. ADI would boost that a bit, and it would raise allowable WEP boost lower down, which charge temperature would severely limit otherwise. In contrast, the V-1650-7 made 1390 hp at 21,000 ft from 60.5" manifold pressure. At that height our what-if V-1710 would blow 48.8" and make about 1200 hp. A bit weak but at least good enough for a stop-gap escort fighter while Packard sorted out their production problems, plus a bit lighter and with less cooling drag than the P-51B.
...

Thank you, that looks very convincing.

(we can also recall that Allison developed the 2-stage supercharged V-1710 quite a bit earlier than DB installed the big SC from the DB 603 on the 605, and a lot earlier than the 2-stage supercharged versions of DB engines saw the light of day; they also worked on improving other bits and pieces - crankshaft, crankcase, valve springs, piston rings - so it not like people at Allison were just twiddling their thumbs in 1942-43)

Before things slip off topic - we're probably looking at altitude power comparable to the Merlin 46/47 or the fully rated DB 605A? The impeller is the key, diameter of 11 in, give or take, although 12 in is even more convincing.

(Germans went to 260mm for the impellers of DB 601A to E, and 280mm for the DB 603A - roughly 10.25 and 11 in respectively, 77% efficiency was provided for the S/C of DB 603A; I don't have good data for the DB 605A; RR went to 10.85 in for the Merlin 46/47; Nakajima did 11.4 in for early Sakaes, 12 in for later versions, even the humble Zuiseis from Mitsubishi gotten 10.25 in and then 12 in impeller; granted, not all superchargers were created equal)

Such a what-if V-1710 will obviously need more than one speed for it's supercharger, so it's either a 2-speed gearbox, or the hydraulic drive (as known from historical 2-stage V-1710s for the auxiliary S/C stage, or on the DB engines). The water/alcohol injection, as it was historically so for 2-stage engines. So all in all, it should've provided power at all altitudes similar to the DB 605A/AM?

 

[overbeck]

Both P&WA and Curtiss-Wright had a revenue stream outside of government contracts and financial reasons to develop technology outside of government requests; Allison had neither.

It would require another investment from GM, which would be a hard sell if those making the decision were not aviation people.

Before things slip off topic - we're probably looking at altitude power comparable to the Merlin 46/47 or the fully rated DB 605A? The impeller is the key, diameter of 11 in, give or take, although 12 in is even more convincing.

I'm not sure if 1100 hp at 22,000 ft for the Merlin 46 was with or without ram effect, but probably with as the maximum speed for the Spitfire V with a fuel injected 46 occurred at 22,500 ft. In that case a V-1710 with 4:1 blower would critical a few thousand feet higher than the 46. The Allison had the advantage of 7-8% higher power at the same boost. For the same reason, with a similar blower the DB 605 would make 1100 hp at higher altitude than the V-1710.

Such a what-if V-1710 will obviously need more than one speed for it's supercharger, so it's either a 2-speed gearbox, or the hydraulic drive (as known from historical 2-stage V-1710s for the auxiliary S/C stage, or on the DB engines). The water/alcohol injection, as it was historically so for 2-stage engines. So all in all, it should've provided power at all altitudes similar to the DB 605A/AM?

It would take a much beefier hydraulic drive than the one on the auxiliary stage, with the usual costs/benefits of that type of drive; heating the oil being one of the costs. A two-speed would cause less trouble but have the usual drop in performance around gear-shift altitude.

Space constraints may have forced them to curve the diffuser around, with the vanes bending around the curve too, i.e. turn the exit flow from centrifugal to axial. The NACA tested such designs after the war for gas turbines, and they worked.

It may be possible to raise the pressure ratio of the original 9.5 inch blower and make it variable at the same time, without changing the accessory case at all. Extend the shaft and attach two axial stages, with a swirl throttle to provide the initial swirl for the first stage, and variable stator vanes using the same mechanism. Even the last set of stators should be variable, because the swirl allows reduction of the centrifugal blower's pressure ratio. The diameter of the axial stages must be larger than the centrifugal inlet to get a high enough tip speed, which means necking down the flow after the axial blower. That may cause flow separation. Extending the shaft may cause bearing problems, and the 8.8 gears must be able to take the extra load. Of course the chances of anyone circa 1940 not only dreaming up such a device but also getting it built were practically zero. It's fun to imagine though.

 

[tomo pauk]

I'm not sure if 1100 hp at 22,000 ft for the Merlin 46 was with or without ram effect, but probably with as the maximum speed for the Spitfire V with a fuel injected 46 occurred at 22,500 ft. In that case a V-1710 with 4:1 blower would critical a few thousand feet higher than the 46. The Allison had the advantage of 7-8% higher power at the same boost. For the same reason, with a similar blower the DB 605 would make 1100 hp at higher altitude than the V-1710.

The 1-stage engines on most of Spitfires were outfitted with lousy carburetors, and, after Spit II, were also with ice guard - all of that meant the increase of rated altitude due to the ram effect was barely noticeable. The provisional power chart for Merlin 46/47 claims 1100 HP at 22000 ft: chart.
The fully rated DB 605A was supposed to do 1200 PS @ 2800 rpm at 6700 m (20000 ft; 1120 on 2600 rpm), at least when looking at power chart.
The V-1710 by winter of 1943/44 was being modified and bench tested for 3200 rpm operation, so that's another way to gain some performance at all altitudes.

But at any rate, even 1100 HP at 21000 ft will mean a far better 'base engine' than what the plain vanilla V-1710 was doing. With 'WER wet', this would've meant propelling the P-40 close to 400 mph, P-39 beyond 400 mph, and P-51 in-between historical P-51A and D (~ 430 mph?).

It would take a much beefier hydraulic drive than the one on the auxiliary stage, with the usual costs/benefits of that type of drive; heating the oil being one of the costs. A two-speed would cause less trouble but have the usual drop in performance around gear-shift altitude.

Space constraints may have forced them to curve the diffuser around, with the vanes bending around the curve too, i.e. turn the exit flow from centrifugal to axial. The NACA tested such designs after the war for gas turbines, and they worked.

The 'no free lunch' rule applies as ever.

It may be possible to raise the pressure ratio of the original 9.5 inch blower and make it variable at the same time, without changing the accessory case at all. Extend the shaft and attach two axial stages, with a swirl throttle to provide the initial swirl for the first stage, and variable stator vanes using the same mechanism. Even the last set of stators should be variable, because the swirl allows reduction of the centrifugal blower's pressure ratio. The diameter of the axial stages must be larger than the centrifugal inlet to get a high enough tip speed, which means necking down the flow after the axial blower. That may cause flow separation. Extending the shaft may cause bearing problems, and the 8.8 gears must be able to take the extra load. Of course the chances of anyone circa 1940 not only dreaming up such a device but also getting it built were practically zero. It's fun to imagine though.

Adding stages = not 1 stage anymore
OTOH, applications of the swirl throttle, like it was done on Mikulin's engines and later copied for Jumo 213 series would've improved power under the critical altitude a good deal, even if the S/C gearing has more than 1 S/C speed, while lessening need for water/alcohol injection.

 

[NevadaK]

Part of that attitude was the fact that in the Spring of 1939 when the Army placed the big order for P-40s the Army owed Allison 900,000 dollars for work already done at the armies request. GM had already given 500,000 dollars to Allison for the V-12 engine project. The engine 'shop' was attached to a rather profitable bearing manufacturing plant that could supply any more money (or was paying the profits to GM).
The Army never paid up. Allison had to forget about the debt in order to get permission to export the V-12 engine to France and Great Britain in 1939.
Army also wanted the pusher engines for the YP-37, the remote gear box engines for the P-39 and the turbo engines for the P-38s. At one point Allison did ask the army which projects they wanted Allison to work on as the y didn't have the staff to work on all of the armies ideas.

It is hard to be entrepreneurial for very long when the customer won't pay his bills. A bankrupt company doesn't build much of anything.

Army contracting during the was such that they only paid for a completed successful test (and timely payment even then was a rarity). In other words if an engine (or part) failed part way through a test session the contractor (Allison or Continental) had to tear down the engine, make new part/s, rebuild the engine and restart or resume the test all at their own cost. Army was not going to pay any additional money for repairs or delays.

This says so much about the development of the V-1710. In 1938/1939 there was a significant lack of investment capital in the US and projects like the V-1710 would have suffered accordingly. The US recovery from the Depression had stagnated and there were serious concerns that the country would slide back into economic failure. Even with GM's support, there would have been little development funding provided for the effort and with indecisiveness on the Army's part it's easy to see how there would be missed opportunities.

This explains a lot. Thanks.

 

[wuzak]

The Allison had the advantage of 7-8% higher power at the same boost.

Really?

Actually boost is one part of the equation. The rated altitude of that boost is also important.

If you look at a 2 speed supercharged engine, such as the Merlin XX, the power is higher in low gear than it is in high gear, even with the same boost.

The Merlin 24 had about 9% more power in low blower than high blower at +18psi boost.

The supercharger needs a higher pressure ratio to get the same boost at altitude, and it uses more power to do so.

Typically the Allison single speed engine was rated at a lower altitude than the Merlin single stage engine.

Also, late in the war Allison had the opportunity to test the 2 stage Merlin supercharger with the V-1710, and found near identical performance.

 

[overbeck]

Really?

Actually boost is one part of the equation. The rated altitude of that boost is also important.

My point is that more power at the same boost makes it easier to get any given power output at a particular high altitude level, because a lower pressure ratio is required. The blower is easier to design and may be less complex. So if I was given the task of making 1000 hp at 30,000 ft, and could choose a Merlin or a V-1710, the latter is an obvious choice. It's a bit lighter too.

 

[Shortround6]

The Allison used a bit higher compression and it's pent roof head might have helped a bit with breathing, lots of other stuff going on so attributing it to one thing is a bit hard.

On the other hand the higher compression does put a bit of limit on the allowable boost with a given type of fuel. So the Merlin will actually make more power using a given type fuel because it can use more boost before detonation sets it.

You want to make 1000hp at 30,000ft you need a lot compression going on in the superchargers, one will not do it. If you want 42in of MAP at 30,000ft you have to compress the outside air about 4.7 times which is beyond any single stage supercharger/compressor of the time. You are also going to need either intercooling, or ADI or both.
The higher compression in the cylinder means it is likely to hit the detonation limit sooner using the same or slightly less boost.

 

[overbeck]

The pent roof should have raised the detonation limit too, to offset the higher compression. At 120 degrees F carb temperature (i.e. a manifold temp at least as high as the Merlin 61 under most conditions, probably higher) the Allison -39 had a significant safety margin at 60" MAP, and it needed 62" or 63" to match the Merlin at 67". Someone (I don't remember who/when) once posted MAP limits for 87 octane in P-40 variants, and IIRC it was about 4" higher for the Allison.

There's no problem using charge cooling. Allison had problems with it in reality, but we're talking what might have been done without those constraints.

 

[Shortround6]

Make sure you are comparing 87 octane to 87 octane, A number of manuals give the limits for Allison running on 91 octane, a common fuel used in the US but unknown in England (OK they knew about it, they just didn't use it or stock it.) The difference is greater than it appears.
On the PN scale 87 octane is 68.29
On the PN scale 91 octane is 75.68

There is a P-40 manual that gives manifold pressure limits (and RPM) for both the Allison and Merlin engines used in the P-40 on both 100 octane and 91 octane fuel.

In only one instance is the Allison allowed to use higher Manifold pressure than the Allison, Max take-off on 91 octane fuel, it all other flight conditions the Merlin is either the same or several inches higher. The Manual gives specific details about fuel systems and electrical switches for the P-40M and N.

In no case using 91 octane fuel was the pilot supposed to use the full rich mixture setting except in extreme emergency.

 

[wuzak]

My point is that more power at the same boost makes it easier to get any given power output at a particular high altitude level, because a lower pressure ratio is required. The blower is easier to design and may be less complex. So if I was given the task of making 1000 hp at 30,000 ft, and could choose a Merlin or a V-1710, the latter is an obvious choice. It's a bit lighter too.

I would like to know which particular variants of each engine you are comparing, and at what altitude? And in which installation?

The V-1710 had 3.7% greater capacity (and piston area) which would have helped.

 

[overbeck]

I would like to know which particular variants of each engine you are comparing, and at what altitude? And in which installation?

I calculated from the displacement and compression ratio, and a comparison of the P-40F with later variants powered by the V-1710-81 says the calculation is about right. I've never seen any power charts which disagree with there being a significant difference between the two engines, FWIW.