High Altitude Heavy Bomber for RAF

[wuzak]

The added weight of a non-propulsive engine, plus the necessary ducting, intercoolers, and fuel load, would have eaten into payload, range, and overall efficiency. And all that complexity meant more maintenance headaches and more things to go wrong at 40,000 feet. In many ways, it was an elegant solution to a tough problem—but also one that created new trade-offs.

That was an option also explored by the Germans.

 

[wuzak]

Seems very over complicated to me. The Merlin was one of the most advanced engines of the war. Just equip the propulsion plants with turbochargers like was done in the R-3350 and there will be enough airflow into all crew compartments.

I don't see the need for 40,000 foot altitude design.

The B-29 Superfortress was designed for high-altitude strategic bombing and had a service ceiling of about 31,850 feet.

40,000ft was the target release altitude for the 10 ton deep penetration bomb that Barnes Wallis proposed.

As for cabin pressurisation, several Merlin variants had cabin blowers in their accessory drives. They were used on various PR Spitfires and the PR.XVI and B.XVI Mosquitoes.

 

[Clayton Magnet]

That was an option also explored by the Germans.

Prototype for the Pe-8 had provisions for a "Central Supercharging Unit" in the fuselage as well.

 

[Alan Stevens]

40,000ft was the target release altitude for the 10 ton deep penetration bomb that Barnes Wallis proposed.

As for cabin pressurisation, several Merlin variants had cabin blowers in their accessory drives. They were used on various PR Spitfires and the PR.XVI and B.XVI Mosquitoes.

Wallis's original plan also included designing a massive six-engine bomber called the Victory to carry the bomb to the required altitude—but wartime constraints made that impractical.

This concept eventually evolved into the Grand Slam bomb, a 22,000-pound (10,000 kg) behemoth that was the largest conventional bomb used in WWII. While the RAF never had an aircraft capable of reaching 40,000 feet with that payload, specially modified Avro Lancaster B Mk I (Special) bombers were able to drop the Grand Slam from around 20,000 to 25,000 feet

I once figured that the B-29 had a separate pressurization engine for the tail gun compartment -- wrongly -- the B-29's tail gunner compartment was pressurized via a dedicated duct from the aircraft's main pressurization system, which itself relied on compressed intake air bled from the engine turbo-superchargers.

 

[GrauGeist]

40,000ft was the target release altitude for the 10 ton deep penetration bomb that Barnes Wallis proposed.

The 40,000 ft. number was actually estimated to be above enemy anti-aircraft range.

The USAAF also used that altitude as a requirement for a high altitude, long range heavy bomber in 1941.

As it turns out, it wouldn't be until the B-36 came along, that the 40,000 ft. operating range was realized.

 

[wuzak]

Wallis's original plan also included designing a massive six-engine bomber called the Victory to carry the bomb to the required altitude—but wartime constraints made that impractical.

The premise of this thread is for the Victory Bomber, or a similar aircraft by another manufacturer, be developed.

 

[GrauGeist]

Question:
In 1938/39, which British aircraft manufacturer had the most experience (and success) with large aircraft?

Could one of these manufacturers have modified an existing airframe to an upscaled, six engine heavy bomber from that existing design?

 

[wuzak]

Question:
In 1938/39, which British aircraft manufacturer had the most experience (and success) with large aircraft?

Could one of these manufacturers have modified an existing airframe to an upscaled, six engine heavy bomber from that existing design?

Short?

 

[yulzari]

Seems very over complicated to me. The Merlin was one of the most advanced engines of the war. Just equip the propulsion plants with turbochargers like was done in the R-3350 and there will be enough airflow into all crew compartments.

I don't see the need for 40,000 foot altitude design.

The B-29 Superfortress was designed for high-altitude strategic bombing and had a service ceiling of about 31,850 feet.

I think that adding an extra common production engine plus a large familiar technology blower and ducting out would be quicker than changing to making four unfamiliar technology turbines and ducting in. But it is a matter of judgement not certainty and what suits the circumstances of national production. The extra engine is a better match to Britain's stretched full war economy whose sub machine gun production included a very large shed near my childhood home where Sten gun welding was done by part time housewives, school leavers and pensioners and also performing as trained Home Guard soldiers, Air Raid Wardens and Civil Defence Workers etc. Importing turbochargers would mean some being lost at sea and something not sent at all to make shipping room for them.

 

[don4331]

Question:
In 1938/39, which British aircraft manufacturer had the most experience (and success) with large aircraft?

Could one of these manufacturers have modified an existing airframe to an upscaled, six engine heavy bomber from that existing design?

If you're going back to the 38/39 timeframe, Short would be your manufacturer of choice.

It also means you would be looking at 6 - RR Vultures; they are the only engine that comes close to the required power on 87 octane fuel. At least the Vulture has 2 speed supercharging There isn't any Griffon, and the Merlin and Hercules aren't near powerful enough.

 

[wuzak]

Could they use a Vulture for an auxiliary blower engine? With Merlins or Griffons for the main power.

The Vulture was used to drive the experimental 2-stage superchargers for the Merlin, IIRC.

 

[Shortround6]

The French tried the extra engine in the fuselage trick just before the war (?) and the Germans tried later in the war with the Hs 130E.

A DB605T powering the central supercharger feeding the DB603 engines in each wing.
There were 3 experimental Do 217Ps with the same basic powerplant.

Basically recon machines as there was no room for bombs (or very many) once they stuffed the 3rd engine, central supercharger, intercool, 3rd engine radiator and needed fuel into the existing (?) fuselage.

A lot of countries issued specifications for bombers (and fighters) that were beyond the state of the art. These specifications helped push the state of the art but most (all?) of the extreme specifications failed. In 1939 the US Army was asking for a twin engine medium bomber that could carry a 2,000lb bomb load 2000 miles. What they got were the B-25 and B-26 which were lucky to fly 2000 miles with no bombs and a bomb bay full of fuel.

Anybody trying for a bomber that could fly at 40,000ft carrying a 20,000lb bomb in 1940 needed more years of development in aerodynamics, structures and propulsion to pull it off.
North American tried a medium bomber and flew it in April 1942.

Twin turbo R-2800s (P-47 engines) but the service ceiling was only 34,800ft and the normal bomb load was 2000lb.
You are not going to make 40,000ft using the airfoils and flaps of 1938/40 carrying the load you want.
The US post War B-50s were not just B-29s with new engines. They also used a new aluminum alloy for the wing skins and a restress structure.
The KB-50J tankers got to just under 40,000 without guns and with a J-47 5200lb thrust turbo jet under each wing but that was around 1950?

Now there is a huge gap between the Lancaster as modified to carry the big bombs and the what the thinking was in 1939/40. Where they might have wound up in 1944/45 could be quite a bit better than the Lancaster but it wasn't going to be what they were talking about in 1940.

 

[wuzak]

It also means you would be looking at 6 - RR Vultures; they are the only engine that comes close to the required power on 87 octane fuel. At least the Vulture has 2 speed supercharging There isn't any Griffon, and the Merlin and Hercules aren't near powerful enough.

If the Vulture was the choice, that would give it the stay of execution, at least for a time. It depends on whether they can sort the remaining issues.

If they can fix the reliability, use the better fuels and improve the supercharger like they did for the Merlin, a max output of 2,500hp or more could be feasible by the time this aircraft took to the air.

The Vulture II was rated at 1,780hp @ 4,000ft and 1,660hp @ 13,500ft with +6psi boost at 2,850rpm. That's the "international rating" and is also equivalent to the maximum climbing power (30 minute limit).

The 5 minute limit was also at +6psi, but with 3,000rpm was rated at 1,845hp @ 5,000ft and 1,710hp at 15,000ft.

Maximum cruise power was at 2,600rpm, +5psi boost. That gives 1,480hp at sea level.

This is with either D.T.D. 230 (87 octane) or R.D.E./F/100 (100 octane) fuel.

So, an aircraft fitted with 4 or 6 Vultures will struggle for altitude performance unless improved superchargers are available, or they can use turbochargers.

Turbochargers in the USA were still problematic at this time, and the USA was still officially neutral. So there may have to be some UK turbochager development along side their gas turbine developments.

 

[Shortround6]

Could they use a Vulture for an auxiliary blower engine? With Merlins or Griffons for the main power.

The Vulture was used to drive the experimental 2-stage superchargers for the Merlin, IIRC.

Are you talking about an engine used in the test house?
Or are you talking about Hooker using the impeller out of a Vulture as the first stage in the two stage Merlin prototype?
They had done a quick look at the airflow of the Vulture supercharger and found that it was close to what they estimated they needed for the 1st stage so they grabbed one Vulture impeller to shorten development time rather that design and machine one from scratch. How much they changed things I don't know but it certainly put them on the right track.
Now I think they were looking for a mass airflow (weigh of air) to support 1000hp at near 30,000ft.
The Vulture was good for about 1710hp at 15,000ft and by 30,000ft it was around 1015hp due to the change in air density.

 

[wuzak]

Are you talking about an engine used in the test house?

Yes, I was talking about it being used on a test bed.

But now I'm thinking it was driving a compressor for one of the gas turbine developments.

 

[Shortround6]

So, an aircraft fitted with 4 or 6 Vultures will struggle for altitude performance unless improved superchargers are available, or they can use turbochargers.

Turbochargers in the USA were still problematic at this time, and the USA was still officially neutral. So there may have to be some UK turbochager development along side their gas turbine developments.

Any engine is going to need improved superchargers.
0 ft.................................1.0 density ratio
10,000ft........................0.7884
15,000ft.......................0.6291
20,000ft.......................0.5326
25,000ft.......................0.4480
30,000ft.......................0.3740
35,000ft.......................0.3098
40,000ft.......................0.2447

rounding off you need over twice the cubic feet of air at 40,000ft than you need at 20,000ft or just over 2.5 times the amount of air at 40,000 to make the power the engine did at 15,000ft.
Now compressing the air that much means that it is going to get very hot, even with the temperature at those altitudes and you need an intercooler to get the temperature down to what you can use without running into detonation problems. High performance number fuel is your friend, Trying to use 87 octane with this level of supercharging doesn't work well.

The big problem with turbos in 1940 was how to manufacture them. GE's design wasn't all that good but not only did you need high temperature alloys, You needed to make thousands if not tens of thousands of blades for even a small number of turbos. Turns out that the alloy was very close to one used in dentures and there was a way precision cast them that required much less machining/polishing that trying to cut them out of solid stock.

It doesn't matter what you can design and build one or two of if you can't mass produce it.

 

[Elan Vital]

The French tried the extra engine in the fuselage trick just before the war (?)

SNCAC-150 first flew on May 11th, 1939 yes. In the end the bi-tri layout was deemed impractical and they were to go for one S-C per engine driven directly by each of the two engines.

Stratospheric or high alt French projects involved the following engines:

LeO 457
- GR 14R 04/05 with 2-speed cleaned up S-C, not ideal
- HS-12Z with the base Hispano S-C and Brown-Boveri turbos aka "Hisbo" (so akin to US turbo V-1710 setups), with ideas to use a 2-speed S-C instead for even better coverage, or Merlin X as the base engine + S-C+ turbo since that one already had 2-speed S-C.

SE 012
- HS-12Z "Hisbo" or with Szydlowski S-Cs

SNCAO LeO 455 "substratospheric"
- GR 14R 04/05

Amiot 357
- HS-12Z with turbos

Amiot 410
- 4 HS-12Y-50/51 with the Farman NC-C1 three-stage S-Cs (allegedly 12Y-54/55 with CR of 7)

MB 179:
- GR 14R 04/05

MB 140
- Quad GR 14N then 14R and 12Z

CAO-710
- Quad GR 14R

NC-150:
- Twin HS-12Y with first NC-C1 and then NC-C2 three-stage S-C ran by a single HS-12X, to be deleted in favor of running the S-Cs by the two engines
- NC-152 with twin HS-12Y/Z "Hisbo"

None of those were as ambitious as the aircraft requested here tho.

There were also NC-C3 S-Cs and "Hisra" HS-12 setups, this time with Rateau turbos.

 

[MikeMeech]

Hi
Any British Aviation Company that was going to build a high altitude bomber would have consulted Vickers, as their work with the Wellington Mk. V and Mk. VI in the early war years would have been very relevant. A brief good coverage of this project can be found in 'Vickers Aircraft since 1908' by Andrews and Morgan, pp. 346-353:

Also relevant would have been the work undertaken by Bristol Type 138A high altitude Monoplane, which exceeded 50,000 ft., which was powered by the Bristol Pegasus P.E.6S. An interesting comment on page 256 of 'Bristol Aircraft since 1910' by C H Barnes is "In the ten years since 1928 the height record has been broken nine times, once by a (Bristol) Jupiter, five times by a (Bristol) Pegasus and only three times by other engines;" This appears to show that pre-war Bristol engine manufacturer had some knowledge of operating engines at high altitude.

Mike

 

[Alan Stevens]

The 40,000 ft. number was actually estimated to be above enemy anti-aircraft range.

The USAAF also used that altitude as a requirement for a high altitude, long range heavy bomber in 1941.

As it turns out, it wouldn't be until the B-36 came along, that the 40,000 ft. operating range was realized.

Long after WW II at that.

The B-36 didn't fully achieve its high-altitude performance goals until the "Featherweight" modifications came into play, especially in the later B-36J variants.

The "Featherweight" modifications to the B-36 Peacemaker were approved in February 1954, with the first modified aircraft delivered in May 1954.

The "Featherweight" program stripped out most of the aircraft's defensive armament—removing up to six retractable gun turrets and their associated fire control systems. This significantly reduced weight and drag, allowing for:
=> Higher service ceilings—up to 44,500 feet in some configurations.
=> Increased fuel capacity—by saving weight, they could carry an extra 2,700 gallons, boosting range and endurance.
=> Smaller crew requirements—from 15 down to 13 in the Featherweight III models.
So yes, the B-36 finally met the original 1941 vision of a 40,000+ ft intercontinental bomber—but only after it shed a lot of its Cold War-era armor.

 

[GrauGeist]

Long after WW II at that.

The B-36 didn't fully achieve its high-altitude performance goals until the "Featherweight" modifications came into play, especially in the later B-36J variants.

The "Featherweight" modifications to the B-36 Peacemaker were approved in February 1954, with the first modified aircraft delivered in May 1954.

The "Featherweight" program stripped out most of the aircraft's defensive armament—removing up to six retractable gun turrets and their associated fire control systems. This significantly reduced weight and drag, allowing for:
=> Higher service ceilings—up to 44,500 feet in some configurations.
=> Increased fuel capacity—by saving weight, they could carry an extra 2,700 gallons, boosting range and endurance.
=> Smaller crew requirements—from 15 down to 13 in the Featherweight III models.
So yes, the B-36 finally met the original 1941 vision of a 40,000+ ft intercontinental bomber—but only after it shed a lot of its Cold War-era armor.

The standard B-36 in typical war dress could cruise at 40,000 feet.
It was actually the YB-36 that first reached the desired 40,000 foot mark, as it had several improvements over the XB-36 (which was limited to 30,000 feet).

The Featherweights could cruise at 50,000 feet, though their ceiling was about 55,000 feet. Plus they were faster, of course, achieving about 423 mph.