Missing AAF Bomber Generation

[Vincenzo]

This is obviously going nowhere.

full agree on this

 

[jimh]

The B-17 was designed in the mid 30's with no practicle experience in heavy bomber technology. The B-17, while a great design has alot of shortcomings as a bomber...primarily being the bombload. There is ALOT of wasted space in the B-17 airframe. When Consolidated was approached about building B-17's Rueben Fleet said we can do better. The B-24 was bred around the bombay and the wing. Consolidated also "modernized" with the introduction of the nosewheel, hydraulic systems, and bombay doors that did not create drag when opened. The 24 really is second generation technology. The B-29 can almost be considered 3rd generation with the introduction of pressurization, amongst other innovations. The USAAF ordered a feasiblity study of a jet engine bomber as early as 1943...and from this Boeing achieved the B-47 in 1947. The writing was on the wall with the advent of the jet engine.

jim

 

[Shortround6]

No but you can factor in ALL of th engines abilities. Once the American bombers had turbo charged engines they had 1000hp available continuous at 25,000ft. Not WER or 5 minute rating. This was more than a non- turbo R-2600 could offer or even a single stage Merlin regardless of take off power.
With design work starting on the B-29 in 1940 (and contract for 2 prototypes and a static test airframe placed on August 24, 1940 and signed Sept 2, 1940) there is precious little time to squeeze in a 2nd generation bomber without holding up the B-29/B-32.
The American bombers had the ability to cruise much faster than they did, but only at the penalty of bomb load and/or range. Swapping R-2600s for their existing engines wouldn't have changed things much without a large change in allowable gross weight. With a 3-4000lb increase in empty weight and more fuel needed the bomb load would have shrunk to next to nothing. These planes were already running on the ragged edge of overload having had their gross weight increased several times and at times operating at 20-40% higher than their original Gross weight. Trying to operate at higher weights would mean structural strengthening, new landing gear, relocation of equipment to handle CG changes. You might as well design a whole new airplane which is what they did.
Please look at the B-23 as the one example of an existing bomber re-engined with the R-2600. And it was done before the power turrets were available. unfortunately that was the window of opportunity for the R-2600. By the time the power turrets were available the R-2800 and R-3350 were the engines of choice for the next generation of bombers. Another example the redesign of the Martin Maryland into the Martin Baltimore.

 

[Vincenzo]

just a terminology note
we can't use the word Generation for the means of word Class so for me
B-17 and B-24 are of the same class but different generation

 

[davparlr]

The B-17 was designed in the mid 30's with no practicle experience in heavy bomber technology. The B-17, while a great design has alot of shortcomings as a bomber...primarily being the bombload. There is ALOT of wasted space in the B-17 airframe. When Consolidated was approached about building B-17's Rueben Fleet said we can do better. The B-24 was bred around the bombay and the wing. Consolidated also "modernized" with the introduction of the nosewheel, hydraulic systems, and bombay doors that did not create drag when opened. The 24 really is second generation technology.

jim

In my mind a generational change requires a significant performance upgrade. Here is a comparison between the B-17G to B-24H

Empty weight (lb)
B-17 36,135
B-24 36,500

Gross weight (lb)
B-17 55,000
B-24 56,000

Max loaded weight (lb)
B-17 65,500
B-24 65,000

Fuel (gal)
B-17 2810-3630
B-24 2814-3614

Max speed (mph)
B-17 287
B-24 290

Cruising speed
B-17 182 mph
B-24 215 mph

Range
B-17 2000 miles/6000lb bombload
B-24 2100miles/5000lb bombload

Service Ceiling (ft)
B-17 35,000
B-24 28,000

Notes:
1) The B-17 is considered more rugged than the B-24 but, in my mind, I don't think the case has been made.
2) It is, however, pretty well accepted that the B-17 was a much better flying aircraft
3) The B-24 does cruise faster, which I consider important, however, the B-17 has a higher ceiling, which I also consider important. Awash here.
4) The B-24 is considered a longer range aircraft, but I don't see it here. If so, I don't think it is significant.

I do not see anything significant in performance here, like, say, the difference between the P-51A and P-51B, which I would consider generational, all just because of an engine change.

The B-24 may have had some goodies like tricycle landing gear (is there any report on whether the B-24 was safer to taxi than the B-17?) but I don't care if they put on white wall tires I would not consider any a improvement generational unless there is some significant performance increases, which there is not.

No but you can factor in ALL of th engines abilities. Once the American bombers had turbo charged engines they had 1000hp available continuous at 25,000ft. Not WER or 5 minute rating. This was more than a non- turbo R-2600 could offer or even a single stage Merlin regardless of take off power.
With design work starting on the B-29 in 1940 (and contract for 2 prototypes and a static test airframe placed on August 24, 1940 and signed Sept 2, 1940) there is precious little time to squeeze in a 2nd generation bomber without holding up the B-29/B-32.

I agree. Once they had their eye on the B-29/32 they decided to lay all their bets on them.

The American bombers had the ability to cruise much faster than they did, but only at the penalty of bomb load and/or range. Swapping R-2600s for their existing engines wouldn't have changed things much without a large change in allowable gross weight. With a 3-4000lb increase in empty weight and more fuel needed the bomb load would have shrunk to next to nothing. These planes were already running on the ragged edge of overload having had their gross weight increased several times and at times operating at 20-40% higher than their original Gross weight. Trying to operate at higher weights would mean structural strengthening, new landing gear, relocation of equipment to handle CG changes. You might as well design a whole new airplane which is what they did.

I am not sure that I agree with this. You are assuming the aircraft structural strength was the limiting factor in loading and not the poor engine performance. The B-17 certainly had a reputation for being robust and, in fact, the XB-38 engine modifications show that, with only a 2500 lb increase of empty weight, max gross weight was increased 11,000 lbs over the original B-17E airframe, an increase of 8500 lbs. I suspect the aircraft was capable of handling heavier loading than the baseline B-17. I also suspect the B-24 would be similar. It seems like there is still growth for even more powerful engines.

Please look at the B-23 as the one example of an existing bomber re-engined with the R-2600. And it was done before the power turrets were available. unfortunately that was the window of opportunity for the R-2600. By the time the power turrets were available the R-2800 and R-3350 were the engines of choice for the next generation of bombers. Another example the redesign of the Martin Maryland into the Martin Baltimore.

It did require substantial upgrades to the base aircraft, but the performance gains were also substantial. The B-23, which also included aero clean-up, performance was significantly better than the B-18 in speed and an increase of 7000 ft in ceiling. The max airspeed of the DB7 went from 314 mph to 347 mph for the DB7A, cruise airspeed went from 270 mph to 295 mph and the aircraft went from a good aircraft to a great one in its A-20 designation.

The B-18 and the B-17 were contemporary aircraft. If the B-17 had updated when the B-18 was in order to become the B-23, there is little doubt this configuration could be available by mid 1943. It is somewhat of a surprise that the B-24 was not upgraded in its original proposal. Of course Consolidated was originally requested to build the B-17. This certainly would have been the time to do so. Obviously the AAF had a different plan.

 

[Denniss]

The lower ceiling made the B-24 an interesting target for german aircraft - they would not loose as much engine power as if they were chasing B-17s.
I remember to have read comments from german pilots that they preferred the B-24 because of this and because they considered it not as damage-resistant as the B-17.

 

[davparlr]

The lower ceiling made the B-24 an interesting target for german aircraft - they would not loose as much engine power as if they were chasing B-17s.
I remember to have read comments from german pilots that they preferred the B-24 because of this and because they considered it not as damage-resistant as the B-17.

The loss of climbing ability from 20k ft to 25k ft of the Bf-109G is about 27%, the loss of the weaker Fw-190 is about 20%. In addition, area of coverage of a Flak gun can be significantly less with another mile altitude to get to.

 

[spicmart]

Yes! That would have helped a lot. While upgrading the B-17 was experimented with using the XB-38, I could not find any effort to up-engine the B-24, which could probably use it to good advantage in that it had an advanced wing.

It was. Found that on B24.net:

"As far back as 1942 it had been clear that a single fin would be better, and on 6 March 1943 a converted B-24D flew with the fin and rudder of a Douglas B-23. After refinement, the whole tail end of this machine was grafted onto another aircraft (42-40234, originally a B-24D but with a nose turret) to become the XB-24K. Ford also fitted 1,350-hp (1007-kW) Dash-75 engines, and the result was a bomber that was considerably faster, had more than double the full-load rate of climb and much better power of manoeuvre. Convair was busy with further major improvements including longer nacelles housing larger oil tanks, an Emerson ball nose turret and lightweight ball turret in the tail, a completely new cockpit window arrangement giving better pilot view, and a further refined tail, and this became the next standard model after the B-24J, the B-24N. Thousands were ordered, the XB-24N flying in November 1944, but only seven YB-24Ns had flown when production stopped on 31 May 1945, 5,168 being cancelled."