Alternate engines for the B-29

[wuzak]

The Boeing B-29 was designed around the Wright R-3350 Duplex Cyclone, which was rated at 2200hp for takeoff.

As we know the R-3350 ran into reliability problems in development, and issues continued after the B-29 entered service, notably with overheating problems.

What alternate engines could Boeing/AAF have looked at for the B-29 program.

American Engines

Allison V-3420
The obvious one is the engine that was the nominated backup plan - the Allison V-3420. The XB-19 had its R-3350s changed for V-3420s as part of the backup program. The V-3420 offered 2600hp for takeoff, but the turbos originally used in the XB-19 were experimental and unreliable. So when the V-3420 was put into a YB-29 to make the XB-39 it did without turbos. Even so, it was some 50mph faster than the regular B-29.

Pratt Whitney R-2800
Another alternative discussed in these pages is the R-2800. It had been proven reliable by a few years of service already, and had been successfully matched to turbos in other applications. Around the period that an engine change would have been contemplated the R-2800 was slightly less powerful than the R-3350 for takeoff. At other power settings it was significantly less powerful, however, such as in cruise mode.

Pratt Whitney R-4360
The R-4360 would prove to be the engine the B-29 was looking for when it was used for the XB-44 and B-50. Unfortunately the production of this engine would not start until 1944, and waiting for it would have delayed Superfortress production too much. Pratt Whitney had proposed the R-4360 for the B-29, a prototype being ordered in late 1943 and flying in May 1945. This was too late for the war.

Lycoming XH-2470
Another alternative which probably wasn't seriously considered was the Lycoming H-2470. The H-2470 was rated for 2300hp for takeoff and 2100hp normal when flown in the XP-54 in 1943. The engine was a little unreliable, and the AAF considered that it would not be ready by the end of the war.

Pratt Whitney X-1800 et al
Pratt Whitney had been developing a series of liquid cooled sleev valve H engine, the XH-2600 (X-1800) and the XH-3130/XH-3730. The XH-2600 was slightly less powerful than the R-3350, while the XH-3130/XH-3730 had the pwer but were massive constructions. These engines were cancelled so that P&W could concentrate on the R-4360.

Wright R-2160 Tornado
Another engine that had potential to make the power required was the Wright R-2160 Tornado. This was experiencing severe technical difficulties and never flew, and is unlikely to have made it to production in time. The resources, it was determined, were better spent sorting the R-3350.

That's about all the American engines I could think that would be even remotely suitable for the B-29.


British Engines

Bristol Centaurus
The Bristol Centaurus was almost directly analogous to the R-3350. Similar in size, weight, capacity and power, it entered production in 1942. Production was slow due to emphasis being placed on the smaller Hercules. There were some development difficulties, though by 1944 the Centaurus was rated at over 2500hp.

Napier Sabre
There is little doubt that the Napier Sabre had the grunt to power the B-29. By 1943 Napiers were still overcoming production and reliability issues. If Boeing ever seriously considered a British engine then this would have been it, except for its notorious reliability.

Rolls-Royce VultureThe VVulture also had a lot of reliability issues. These were mostly solved at the time of its cancellation in 1942. Vultures had run to 2500hp on the bench comfortably at that time also, but production had been stopped.

Rolls-Royce Griffon
Similar in capacity and weight to the Sabre, but slightly less powerful. Early single stage engines were rated at 1780hp, the two stage engines starting at 2000hp and finishing at about 2400hp by the war's end. Like the R-2800 probably lacked the normal power rating of the R-3350, though probably capable of matching it in takeoff and WER circumstances.

The only other British engines that may have been suitable were too late for the B-29 and the war. Those would be the RR Pennine (2740hp at takeoff) and the RR Eagle 22 (3200hp+).


So having looked through the options I think Boeing and the AAF were rather limited in their choice. The R-3350 was marginal for the B-29, especially for overloaded take-offs, but the only realistic alternative was the V-3420, and that was overlooked because of the fact the change would have had on production. Never mind that the V-3420 installation was designed as a QEC for a direct swap with the R-3350.

The only other realistic alternative would have been the Bristol Centaurus, but I'm sure the AAF were very reluctant to rely on an overseas engine.

 

[Readie]

The Boeing B-29 was designed around the Wright R-3350 Duplex Cyclone, which was rated at 2200hp for takeoff.

As we know the R-3350 ran into reliability problems in development, and issues continued after the B-29 entered service, notably with overheating problems.

What alternate engines could Boeing/AAF have looked at for the B-29 program.

American Engines

Allison V-3420
The obvious one is the engine that was the nominated backup plan - the Allison V-3420. The XB-19 had its R-3350s changed for V-3420s as part of the backup program. The V-3420 offered 2600hp for takeoff, but the turbos originally used in the XB-19 were experimental and unreliable. So when the V-3420 was put into a YB-29 to make the XB-39 it did without turbos. Even so, it was some 50mph faster than the regular B-29.

Pratt Whitney R-2800
Another alternative discussed in these pages is the R-2800. It had been proven reliable by a few years of service already, and had been successfully matched to turbos in other applications. Around the period that an engine change would have been contemplated the R-2800 was slightly less powerful than the R-3350 for takeoff. At other power settings it was significantly less powerful, however, such as in cruise mode.

Pratt Whitney R-4360
The R-4360 would prove to be the engine the B-29 was looking for when it was used for the XB-44 and B-50. Unfortunately the production of this engine would not start until 1944, and waiting for it would have delayed Superfortress production too much. Pratt Whitney had proposed the R-4360 for the B-29, a prototype being ordered in late 1943 and flying in May 1945. This was too late for the war.

Lycoming XH-2470
Another alternative which probably wasn't seriously considered was the Lycoming H-2470. The H-2470 was rated for 2300hp for takeoff and 2100hp normal when flown in the XP-54 in 1943. The engine was a little unreliable, and the AAF considered that it would not be ready by the end of the war.

Pratt Whitney X-1800 et al
Pratt Whitney had been developing a series of liquid cooled sleev valve H engine, the XH-2600 (X-1800) and the XH-3130/XH-3730. The XH-2600 was slightly less powerful than the R-3350, while the XH-3130/XH-3730 had the pwer but were massive constructions. These engines were cancelled so that P&W could concentrate on the R-4360.

Wright R-2160 Tornado
Another engine that had potential to make the power required was the Wright R-2160 Tornado. This was experiencing severe technical difficulties and never flew, and is unlikely to have made it to production in time. The resources, it was determined, were better spent sorting the R-3350.

That's about all the American engines I could think that would be even remotely suitable for the B-29.


British Engines

Bristol Centaurus
The Bristol Centaurus was almost directly analogous to the R-3350. Similar in size, weight, capacity and power, it entered production in 1942. Production was slow due to emphasis being placed on the smaller Hercules. There were some development difficulties, though by 1944 the Centaurus was rated at over 2500hp.

Napier Sabre
There is little doubt that the Napier Sabre had the grunt to power the B-29. By 1943 Napiers were still overcoming production and reliability issues. If Boeing ever seriously considered a British engine then this would have been it, except for its notorious reliability.

Rolls-Royce VultureThe VVulture also had a lot of reliability issues. These were mostly solved at the time of its cancellation in 1942. Vultures had run to 2500hp on the bench comfortably at that time also, but production had been stopped.

Rolls-Royce Griffon
Similar in capacity and weight to the Sabre, but slightly less powerful. Early single stage engines were rated at 1780hp, the two stage engines starting at 2000hp and finishing at about 2400hp by the war's end. Like the R-2800 probably lacked the normal power rating of the R-3350, though probably capable of matching it in takeoff and WER circumstances.

The only other British engines that may have been suitable were too late for the B-29 and the war. Those would be the RR Pennine (2740hp at takeoff) and the RR Eagle 22 (3200hp+).


So having looked through the options I think Boeing and the AAF were rather limited in their choice. The R-3350 was marginal for the B-29, especially for overloaded take-offs, but the only realistic alternative was the V-3420, and that was overlooked because of the fact the change would have had on production. Never mind that the V-3420 installation was designed as a QEC for a direct swap with the R-3350.

The only other realistic alternative would have been the Bristol Centaurus, but I'm sure the AAF were very reluctant to rely on an overseas engine.

The B29 would have been a better plane with a Griffon instead of the rather crude aircooled problematical radials.

Griffon 89
2,350 hp
Griffon 101
2,420 hp Two-stage, three-speed supercharger using Low Supercharger (L.S), Moderate Supercharger (M.S), or Full Supercharger (F.S); reduction gear ratio 4.45; Rolls-Royce fuel injection system.

Fabulous powerfull aeroengines.
Cheers
John

 

[Mustang nut]

I dont think using a British engine would be an option unless it was made under license, probably they would be best sticking with what they have.

Does wanyone know if the Russians reverse engineered the problems on B 29 engines?

 

[wuzak]

The B29 would have been a better plane with a Griffon instead of the rather crude aircooled problematical radials.

Griffon 89
2,350 hp
Griffon 101
2,420 hp Two-stage, three-speed supercharger using Low Supercharger (L.S), Moderate Supercharger (M.S), or Full Supercharger (F.S); reduction gear ratio 4.45; Rolls-Royce fuel injection system.

Fabulous powerfull aeroengines.
Cheers
John

The 89 and 101 were somewhat later than the B29 required. That said, I wonder if the single stage two speed unit could have been hooked up with a turbo (like the yanks liked for their big bombers) and boosted to similar levels.

If the B-29 adopted the Griffon maybe the contra props may have been pushed harder as well.

 

[wuzak]

I dont think using a British engine would be an option unless it was made under license, probably they would be best sticking with what they have.

Very true.

The shame about the Griffon was that it was made in relatively small numbers - 8000-9000 IIRC. I wonder how hard it would hav ebeen for Packard to change over to Griffon production, or set up another line for Griffons.

 

[Shortround6]

As has been stated the big problem with using the Griffon is not take-off power but cruising power. What was the Griffon rated for when running at maximum cruise lean condition? Put that together with the fact that hundreds if not thousands of B-29s had been built BEFORE the decision to bomb at low altitudes was made meant that a turbo-Griffon would have to have been made. Good as the R-R two stage superchargers were the turbo worked better at 30,000ft and above which is what the requirement was, mistaken though it might have been.

 

[Mustang nut]

Hey the B 29 is such a cheap plane lets change the engines to use up the budget

 

[fastmongrel]

Bristol had some very interesting big engine designs that might with a bit of a liberty have been ready for the B 29. Probably the most likely and suitable to have got flying by 1944 would have been the Bristol Orion but a very intriguing design sketch was for a 3500 hp 28 cylinder coupled Hercules engine. Though obviously Bristol didnt have the spare capacity or spare design staff to work on such projects in 1940/41. This link has some very interesting picture files Bristol Orion

 

[wuzak]

Would not the Centaurus be powerful enough for the first generation of B-29s?

 

[johnbr]

What the B-29 need was four 3500hp turboprop if they where ready sooner.

 

[FLYBOYJ]

Folks, be advised that had the war continued the B-29 "would have" been re-engined. The "B-29D" which later became the B-50 would have more than likely replaced many of the earlier B-29 that were delivered with the 3350s.

Even though I don't like Wiki, for the record...

Boeing XB-44 Superfortress - Wikipedia, the free encyclopedia

Boeing B-50 Superfortress - Wikipedia, the free encyclopedia

 

[fastmongrel]

What the B-29 need was four 3500hp turboprop if they where ready sooner.

The first succesful turboprop the Rolls Royce RB 50 Trent was flying by sept 1945 but wasnt anywhere near that figure. The Rolls Royce RB39 Clyde might have been the ideal engine at up to 3,500hp plus residual thrust but that wasnt flying till late 1946.

 

[Shortround6]

What the B-29 need was four 3500hp turboprop if they where ready sooner.

Please be aware that the turbo-prop is not a stepping stone to the jet engine. Rather you cannot build a successful turbo-prop until you can build a successful jet engine.

 

[wuzak]

Please be aware that the turbo-prop is not a stepping stone to the jet engine. Rather you cannot build a successful turbo-prop until you can build a successful jet engine.

Metrovicks were building the F1 to Griffith's design. That was a axial flow turboprop with separate power turbines (the Trent mentioned by fastmongrel did not). Griffith did not believe that jet power was sufficient at first, but hen Metrovicks stopped work on the F1 and commenced work with the F2 turbojet.

I wonder how far they could have got if not for the switch. F2s flew in the Meteor in 1942, IIRC, though they had a few issues that slowed their move into production.

F1 would not have been in the 3500hp class, though.

Basically, Metrovicks didn't have to develop a jet before they developed a turboprop.

 

[fastmongrel]

Please be aware that the turbo-prop is not a stepping stone to the jet engine. Rather you cannot build a successful turbo-prop until you can build a successful jet engine.

Not entirely true the Jendrassik_Cs-1 was the worlds first Turboprop and it came before Whittle or Heinkels turbojets got working. For a lot of people in the 30s the turboprop was the logical next step not the turbojet. Possibly this was because if your used to a big spinny thing at the front anything else was not logical.

 

[Shortround6]

Actually I think you both just proved my point.

"though they had a few issues that slowed their move into production"

"but problems with combustion stability limited the power to 400 hp, from the design goal of 1,000 hp" and"and continued work on the flame cans should have allowed it to develop to full power"

Mechanically a turbo-prop is nothing more and nothing less than turbo jet with a bigger turbine section that takes more power from the exhaust and uses it to turn a propeller via a reduction gearbox.

You need a compressor section in both ( and a number of jets and turbo-props have used identical compressors), you need a burner section for both ( again identical burner sections used by a number of jets and turbo props) and you need a turbine section. In the jet engine the turbine section just needs to pull enough power from the exhaust to power the compressor.

If you could develop a production ready turboprop you had a production ready jet engine, just leave a stage or two or three out of the turbine and ditch the propeller drive arrangements.

File:Cs-1 engraving.jpg - Wikipedia, the free encyclopedia

I will grant you that the Jendrassik engine would have been a bit on the small side (750-800lbs thrust?).

You have all the same problems with both engines.

Building a compressor with a pressure ratio close to 3 to 1, minimum.

Building a burner section that will have a stabilized flame front under a variety of conditions and not burn through.

Building a turbine section that will stand up to the heat of the exhaust gasses for the projected life of the engine. This may be harder for the Turboprop because you are trying to get more heat (energy) out of the exhaust gas stream.

Please be aware that experimental gas turbine engines for stationary power plants (or proposed ship propulsion) had been built before WW I. Their compressor sections did not have a high enough compression ratio to allow for much more than self sustaining running. In other words, while they ran they couldn't produce any real useful power to do anything else.

 

[FLYBOYJ]

Even if there was a functional turbo prop engine available in 1943 for the B-29, has any one thought about the availability of jet fuel? AV gas could have been used but would have caused many problems on early turbine engines (and it did on later aircraft that did use avgas). In the end sticking to a recip would be the most logical thing to do.

 

[Trilisser]

How about using diesel fuel, just making sure that it has as low as possible sulphur content...

 

[FLYBOYJ]

How about using diesel fuel, just making sure that it has as low as possible sulphur content...

you're now talking about making a special blend of fuel for one specific aircraft. Additionaly even low sulphur diesel fuel will play havoc on early FCUs, fuel nozzles and turbine blades. For the advantages that a turbo prop would have to offer, it would be more cost effective to stay with a recip. Consider the new technology, training mechanics and re-tooling engine shops that normally handeled recips, all this change happeneing right in the middle of a large campaign.

 

[fastmongrel]

Definitely no high power engines available that used diesel. Plus a high revving high power diesel light enough for flight is going to be a fabulously complicated engine. Dont know if any of the Jumo aerodiesels ever got more than about a 1,000hp. About the only diesel I can think of that would have been in the required power range was the Napier deltic and that was a complicated hefty lump designed for marine use.