high altitude bomber

[Civettone]

I am fascinated by the concept of the high altitude bomber. At first it seems a clear concept: a bomber so high it is difficult to shoot down by interceptors or anti-aircraft guns. The most famous one is probably the Junkers Ju 86 which was used in small numbers as a recon and as a bomber. But the paradox seems to be that the added weight of the bombs is detrimental to high altitude performance. Indeed, the Ju 86 bomber used as a mere nuisance bomber, carrying a single 250 kg bomb.

The American B-17 and B-24 were equipped with turbo superchargers and this allowed them to operate at 8-9 km (30k feet). I could be wrong but I believe the old story of the Mosquito carrying a bigger bombload than the B-17 comes from a B-17 flying at its maximum altitude. In fact, the B-17 had a payload similar to the Lancaster but no way it could carry that at high altitude.

So, what determines how much a bomber can carry at various altitudes? Let's say for instance the Heinkel He 274. This was a planned very high altitude bomber, powered by four turbocompressed DB 603s or BMW 801s, with a ceiling of 14 km (46k ft) but how much of its maximum payload of 4 ton could it carry that high? Also, would it have made a difference if it was an external payload (Fritz guided bomb) as ultimately an aircraft can fly at any altitude as long as it has sufficient speed?

 

[tomo pauk]

Totally off-topic : my, my, look who's back

 

[Civettone]

Totally off-topic : my, my, look who's back

Hi buddy, I was talking about you just last week. Forever grateful for your hospitality many years ago.
I have been following this forum and others, and have been reading many of your excellent posts over the years.

 

[tomo pauk]

Hi buddy, I was talking about you just last week. Forever grateful for your hospitality many years ago.
I have been following this forum and others, and have been reading many of your excellent posts over the years.

Don't mention it
Seems like you have avoided a lot of my ... not-so-excellent posts

 

[pbehn]

I am fascinated by the concept of the high altitude bomber. At first it seems a clear concept: a bomber so high it is difficult to shoot down by interceptors or anti-aircraft guns. The most famous one is probably the Junkers Ju 86 which was used in small numbers as a recon and as a bomber. But the paradox seems to be that the added weight of the bombs is detrimental to high altitude performance. Indeed, the Ju 86 bomber used as a mere nuisance bomber, carrying a single 250 kg bomb.

The American B-17 and B-24 were equipped with turbo superchargers and this allowed them to operate at 8-9 km (30k feet). I could be wrong but I believe the old story of the Mosquito carrying a bigger bombload than the B-17 comes from a B-17 flying at its maximum altitude. In fact, the B-17 had a payload similar to the Lancaster but no way it could carry that at high altitude.

So, what determines how much a bomber can carry at various altitudes? Let's say for instance the Heinkel He 274. This was a planned very high altitude bomber, powered by four turbocompressed DB 603s or BMW 801s, with a ceiling of 14 km (46k ft) but how much of its maximum payload of 4 ton could it carry that high? Also, would it have made a difference if it was an external payload (Fritz guided bomb) as ultimately an aircraft can fly at any altitude as long as it has sufficient speed?

View attachment 747014

The higher you go the harder it is to generate both power and lift, also the higher you go the more difficult it is to hit anything other than a city. The Spitfires that intercepted Ju 86s were also modified with various things removed (guns armour radios) to lose weight. At the heights involved the Spitfire was so "marginal" on performance that one gun not firing caused it to stall. As nuisance raids raids with no losses they were worth it, when they started to be shot down or even hit, it wasnt worth the risk for the few they had in service. No matter what the bomb site dropping dumb bombs from 40,000+ ft means you cant hit anything that a small bomb could damage.

 

[Shortround6]

means you cant hit anything that a small bomb could damage.

Well, you can hit something you could damage.
Just not what you intended/aimed for.

 

[tomo pauk]

So, what determines how much a bomber can carry at various altitudes?

Hi-alt bomber needs to have favorable wing loading and power loading in order to be a viable hi-alt bomber.
B-17 was very good in this, and it's bomb load was far heavier than 4000 lbs. Eg. see here - with 10000 lb of internal bombs, it was still supposed to have the range of 1600 miles when flying at 25000 ft.

I could be wrong but I believe the old story of the Mosquito carrying a bigger bombload than the B-17 comes from a B-17 flying at its maximum altitude. In fact, the B-17 had a payload similar to the Lancaster but no way it could carry that at high altitude.

You know what they say about the old stories
There were also the Mosquitoes that were carrying 5000 lbs and still capable of service ceiling of 36000 ft on max weight, these being powered by very hi-alt Merlin 70 series engines. See here.

 

[pbehn]

Well, you can hit something you could damage.
Just not what you intended/aimed for.

There were few raids, dropping small loads, so air raid sirens were not used to warn civilians. As is almost always the case with such things they got lucky, one bomb hit a bus station, killed 48 and wounded 48.

 

[yulzari]

The higher you go the more difficult it is to hit the target, in period.

 

[MIflyer]

Well, the Ju86P and Ju86R were highly successful in one respect. They inspired the British to create a high altitude Wellington to emulate them. And that in turn led to a request to Stanley Hooker of RR to create a back-up engine for the original turbosupercharged radial design, which he did by combining a Merlin with the supercharger off the failed Vulture engine. And that in turn led to Herman Goring looking at the sky above Berlin in March 1944 and saying, "We have just lost the war."

 

[Civettone]

Hi-alt bomber needs to have favorable wing loading and power loading in order to be a viable hi-alt bomber.
B-17 was very good in this, and it's bomb load was far heavier than 4000 lbs. Eg. see here - with 10000 lb of internal bombs, it was still supposed to have the range of 1600 miles when flying at 25000 ft.



You know what they say about the old stories
There were also the Mosquitoes that were carrying 5000 lbs and still capable of service ceiling of 36000 ft on max weight, these being powered by very hi-alt Merlin 70 series engines. See here.

Hi Tomo

The Mosquito is such an impressive aircraft. In my opinion, the best aircraft of WW2. It's interesting that the B.35 was such a great performer at high altitude despite having normal wings. I am curious what the impact is of a larger high ratio wing compared to a multi-stage engine.

The B-17 is quite interesting in that regard. Although equipped with turbo superchargers it struggled to reach high altitudes. Only with reduced bomb load could it reach 31.5k ft. A B-17G at maximum weight wouldn't get above 26.5k ft. That brings me back to the Heinkel He 274. A service ceiling of 46k feet but would it have been able to carry its maximum load of 4 ton? Or the Ju 388 its 3 ton at 40k ft?

By the way, and OT, what's a Tomo gun?

 

[nuuumannn]

Well, the Ju86P and Ju86R were highly successful in one respect. They inspired the British to create a high altitude Wellington to emulate them. And that in turn led to a request to Stanley Hooker of RR to create a back-up engine for the original turbosupercharged radial design, which he did by combining a Merlin with the supercharger off the failed Vulture engine.

That's not true. The concept of the high-altitude pressurised Wellington dates back to an Air Ministry requirement in 1938. The first was the Wellington V powered by the turbosupercharged Hercules VIII, the exact date of its first flight is not known but it was airworthy in September 1940. Rolls had already begun development of the two-speed, two-staged Merlin for the high-altitude Wellington by this time, although it was not installed in the Wellington Mk.VI until 1941.

I've not seen any reference to the 60-Series Merlin using the Vulture's supercharger before. According to what you've said, what did they have in common? The Vulture had a two-speed single-stage supercharger with a down-draught carburetor inlet, whereas the Merlins were all up-draught carbs. The 60-Series Merlins were fitted with two-speed, two-stage superchargers separated by an intercooler. Both the Vulture and the first Merlin 60 had SU carbs, but the units were different, particularly in the ducting casing.

 

[nuuumannn]

I could be wrong but I believe the old story of the Mosquito carrying a bigger bombload than the B-17 comes from a B-17 flying at its maximum altitude. In fact, the B-17 had a payload similar to the Lancaster but no way it could carry that at high altitude.

Yeah, this old thing gets a lot of traction, but the maximum internal only load of the B-17 was 8,000 lbs. There's no way a Mosquito could carry that. Granted, the B-17's range was shortened, it still could reach an altitude in excess of 25,000 ft. The B-17 had excellent altitude performance for its time, and I once had a discussion with someone about this theory and it stems from a presumption of range versus payload figures, which are not necessarily recounted accurately with every telling.

For example, according to books I have, the B-17G could carry a bomb load of 6,000 lbs over 2,000 miles, whereas the Mossie B.XVI could manage 1,500 miles with 4,000 lbs internally. Both aircraft had the same maximum altitude of 37,000 ft, although the books from don't specify warload at that height. The Mosquito Mk.IV had a maximum range of 2,040 miles but only had a 2,000 lb internal load, its maximum altitude was 34,000 ft. Impressive for 1942. By comparison the same year the B-17E/F could manage 2,000 miles carrying 4,000 lbs internally, and both had higher maximum altitudes than the Mosquito B.IV.

 

[Shortround6]

I've not seen any reference to the 60-Series Merlin using the Vulture's supercharger before.

I have seen reference to Hooker using the Vulture impeller (not sure about other parts) in a test rig.
They had calculated the airflow needed at the desired height and found that the Vulture supercharger would give the airflow they wanted (or close enough) to match the input of the Merlin supercharger to give a good result. They don't say what they changed, only that it saved time using an off the shelf part rather than custom making a new impeller while they did testing. More proof of concept?
How much fining tuning they did (size/shape of the diffuser and shape/angle of the vanes) I have no idea when they went into production.

The first stage had to flow enough pounds of air per minute/hour at the desired altitude to give the power desired. If you tried to just add a second Merlin supercharger in series it would choke the 2nd stage. They needed both volume and pressure.


Allison was trying to use two identical size impellers in their early tests. Failed miserably. Going cheap did not work.

 

[nuuumannn]

I have seen reference to Hooker using the Vulture impeller (not sure about other parts) in a test rig.

That certainly makes sense, but the supercharger units between the two engines were definitely not the same. From what I have read, the 60-Series Merlin was based on the Merlin 46, although again the superchargers were different.

 

[Shortround6]

B-17 was rarely (if ever) used as designed. It was also operating well over 10,000lbs higher that it was designed which seriously affected altitude.

From Joe Baugher's web site.

B-17E...Weights: 32,350 pounds empty, 40,260 pounds gross, 53,000 pounds maximum.
B-17G... 32,720 pounds empty, 55,000 pounds normal loaded, 72,000 pounds maximum.

Expecting late model B-17s to fly in formation at over 30,000ft over Germany is like expecting the tooth fairy to shower hundred dollar bills for one tooth.

 

[nuuumannn]

B-17 was rarely (if ever) used as designed. It was also operating well over 10,000lbs higher that it was designed which seriously affected altitude.

From Joe Baugher's web site.

B-17E...Weights: 32,350 pounds empty, 40,260 pounds gross, 53,000 pounds maximum.
B-17G... 32,720 pounds empty, 55,000 pounds normal loaded, 72,000 pounds maximum.

Expecting late model B-17s to fly in formation at over 30,000ft over Germany is like expecting the tooth fairy to shower hundred dollar bills for one tooth.

This is true, but the theory behind the Mossie carrying a bigger bomb load across a greater distance than the B-17 simply doesn't add up. The unfortunate off-shoot of this theory is that people use it to reinforce their ideas that heavy bombers should have been replaced with Mosquitoes.

Issues with the B-17's altitude not being what the designers had claimed arose with British use of the aircraft in 1941. Granted, these were early model B-17Cs, but they struggled at altitudes above 25,000 ft, suffering severe icing. One was taken to 30,000 feet before its controls froze and it spiral-dived into the ground, killing all aboard.

 

[Shortround6]

That certainly makes sense, but the supercharger units between the two engines were definitely not the same. From what I have read, the 60-Series Merlin was based on the Merlin 46, although again the superchargers were different.

Just going by "The Merlin in perspective: The combat years (Historical series)" as I sure don't have any Merlin supercharger impellers laying around

Standard Merlin used a 10.25in impeller.
Merlin 46/47 used a 10.85in impeller.
Most (all?) two stage Merlins used a 10.1in impeller on the 2nd stage and a 11.5in and 12in impellers on the 1st stage.

Again that is just the size (dia) of the impeller and no mention of anything done to the blades, inlet vanes, diffuser size, diffuser vanes and so on.

 

[Shortround6]

This is true, but the theory behind the Mossie carrying a bigger bomb load across a greater distance than the B-17 simply doesn't add up. The unfortunate off-shoot of this theory is that people use it to reinforce their ideas that heavy bombers should have been replaced with Mosquitoes.

Issues with the B-17's altitude not being what the designers had claimed arose with British use of the aircraft in 1941. Granted, these were early model B-17Cs, but they struggled at altitudes above 25,000 ft, suffering severe icing. One was taken to 30,000 feet before its controls froze and it spiral-dived into the ground, killing all aboard.

Some (but not all?) later B-17s had de-icing boots installed. Some versions flying today may have had them removed to ease the maintenance load and since they are not flying at high altitudes (or even in really bad weather) it makes a lot of sense.

The whole Mosquito carrying a the same bombload as B-17 was a throwaway line to a war correspondent that made a good headline. It was never a solid fact.

 

[Shortround6]

At times B-17s were carrying an average of around 4000lbs to certain targets in Germany. However this average was comprised of planes carrying 5000lbs of HE, (five 1000lb bombs or ten 500lb bombs) and planes carrying in the low 3,000lbs worth if incendiaries. The incendiaries being much bulkier than HE bombs they simply couldn't fit more in the bomb bay.

As opposition (or escorts went deeper) changed perhaps the bombers flew somewhat more direct routes, carried less 'extra' .50 cal ammo or other factors changed which allowed for the B-17s to carry 6000lb of HE bombs, the racks were not full with the 5000lb load. There was, I think (?) and intermediated stage on the B-17 between the 53,000lb max and 72,000lb max weight (B-17F=56,500 pounds maximum.)and that may have been both weather and field dependent. Some of the older planes were field modified with upgraded landing gear.

That is as close as I am willing to go (not going to search dozens of raid reports, I have looked up a few a while back) and I have not seen anybody actually break down that 4000lb claim.