B-36 - Why a Pusher??

[XBe02Drvr]

I doubt the electronics would have been negotiable, but what kind of changes could have been made to the intake duct?

Actually, even back in electron tube days, the second generation of a system was often lighter and more compact than the original, as long as designers could resist the urge to cram in new features.
I'm no expert on intake ducting, but my understanding is that the Dog Sabre's intake was not the straight shot down the throat of the compressor that the "open mouth" Sabres had, due to the bend around the radome. Early jet engines tended to be a little fussy about smooth airflow into the face of the compressor, and would sometimes compressor stall or flame out just from the wakes of bullets or cannon shells leaving the nose guns. The early axial flow versions were especially susceptible to this. Sometimes a sudden yaw or the wake turbulence from another aircraft would cause a flameout. My instructor said he got a lot of practice at windmilling relights in the dog.

 

[Zipper730]

Actually, even back in electron tube days, the second generation of a system was often lighter and more compact than the original, as long as designers could resist the urge to cram in new features.

If I recall the F-86's radar had some new features built into it to reduce workload.

I'm no expert on intake ducting, but my understanding is that the Dog Sabre's intake was not the straight shot down the throat of the compressor that the "open mouth" Sabres had, due to the bend around the radome.

It was an S-duct.

 

[XBe02Drvr]

It was an S-duct

For a jet that's going to be expected to maneuver at near-sonic speeds and has an engine fussy about intake flow, that's got to be a problem. The fine points of airflow turbulence and compressibility in ductwork were still being worked out on sliderules in those days, and before the advent of variable stators, the range of airflow conditions compressor blades could handle without stalling was relatively narrow.

 

[SaparotRob]

I love aviation gobbledygook!

 

[Joe Broady]

With one jet pod (or any two piston engines) inoperative, the weight necessary to bring service ceiling down to sea level is more than 550,000 lb.

That's a purely theoretical figure, well beyond any possible weight. If fuel tanks and bomb bays are loaded to capacity (184,000 + 84,000 lb) the B-36H weighs 449,000 lb. Max allowable weight by the book is 357,000 lb, which gives a load factor of 2.0, "the minimum generally considered acceptable." Nevertheless it does say, "gross weight limitation is 455,000 pounds for 8,000 foot ground run. (Rate of climb with normal rated power 675 FPM.)"

 

[11bwmech]

I just saw this thread, and had to go all the way through it. I am probably the only person on the forum who has actually flown in the B-36. I was stationed at Wheelus AFB, in Tripoli, Libya 1954-1955. I made two flights on the B-36 as an observer (non-crew). Both times we went from Wheelus to somewhere over Europe, then turned around and came back by a different route. I thought it was an enjoyable plane to fly in. On the down side, on both flights we had to shut down the #3 engine. First time for overheating and the second time for a pitch control failure of some kind. The plane flew just as well without it !

As for the pros and cons of the plane, as in the discussion of the thread, I honestly don't know that much about the technical aspects of the plane.

Charles

You aren't the only one. I was a B-36 prop mechanic and flew in it a lot. The props had their own oil supply which had to be serviced after every flight. So when they had an RON, a prop man had to go along.

 

[11bwmech]

And how about trim?

That's what I was talking about. The aircraft was usually trimmed with a positive angle. I'm not an engineer, but when I was flying in a B-36, that's the way it was.

 

[rinkol]

I haven't ever seen quantitative papers or books about this design decision, but qualitatively speaking it's more or less as you said. The B-36 specification heavily emphasized long range and thus cruise performance. As a rule a pusher configuration will be more efficient in cruise, subject to considerations like rotation on take off, which might force the pusher prop to be smaller diameter to clear the ground on rotation. However in B-36 case they found that pusher was definitely more efficient in cruise. Some early concepts leading to the B-36, and some wind tunnel models even, had push-pull nacelles or conventional tractor nacelles. but the designers believed pusher would win, and at least at the state of the art in wind tunnel testing at the time the tests proved them right.

The disadvantage of pusher is not so much that wing downwash will actually make it less efficient net, but the vibration considerations of uneven flow, not only wing downwash in cruise, but also the engine exhaust stream flowing through the prop. These were issues with the B-36 which had flight restrictions related to prop vibration. Also, besides the cruise configuration, the wing interaction issues become more serious with flaps down. Note in photo's of B-36's even fully extended flaps are continued right in front of the props. And also in low speed flight, the tractor prop helps generate lift which the pusher doens't to the same degree. And then there's more potential for FOD from stuff thrown into the props by the main gear; again see B-36 photo's: the main gear legs are within the radius of the inboard props.

Re: B-47, can't see how it could be compared to the B-36 in practically any way. The whole concept of its use was different, relying on friendly bases close to the USSR (many B-47's were homebased in CONUS, but practically speaking would need staging bases in Allied countries, even with aerial refueling). And it was an example of much more advanced state of art. The planes shared the J47 engine, but those were just an add on to the B-36, rather than the engine techology around which the B-47 was designed. And the B-47 was close enough in speed to enough of the Soviet interceptor force to make large scale B-47 nuclear raids essentially unstoppable (and there were lots of B-47's), even if groups of subsonic MiG's could intercept a single B-47 as they proved in certain RB-47 intrusions into Soviet territory or nearby. Although, even the B-36's capabilities were adequate to make the US nuclear deterrent credible v the USSR into the late 1950's. It's one thing to shoot down a bomber or cause a few % losses per sortie and make a conventional bombing campaign too costly. It's quite another to shoot down enough planes to make a nuclear attack's results tolerable: that was extremely difficult to do, requiring almost perfection for the defense and giving every advantage to the offense.

Joe

In the case of the B36, the pusher arrangement lent itself to burying the engines in the wings. In the 1940s, this was seen as a good way to reduce drag - special engines, such as the Wright Tornado, were built to facilitate such schemes. There was also the idea of trying to preserve laminar air flow over the wing. though I am not sure to what extent this was achieved.

The pusher installation had the problem of requiring drive shafts at the cost of extra mechanical complexity and weight. The B-35 was supposed to use a pusher engine configuration with contra-rotating propellers, but this failed to mature.

 

[11bwmech]

The B-36 Featherweight program, classified for many years, did not use the J-47 pods as I remember. Now I will have to look it up.

Featherweights used jets. They got rid of gun turrets, insulation and "crew comfort" items. I'm not sure what else.

 

[Builder 2010]

I had to page through the entire thread to make sure that no one posted this. It's the thing that the B-36 was designed to haul. It's the Mark 17 Thermonuclear weapon that weighs 45,000 lbs. They built the Peacemaker around it, just like they built the A-10 around the 30mm cannon. The B-36's bomb bay is like a banquet hall. It is vast. They duplicated it in the B-52.

 

[XBe02Drvr]

It's the Mark 17 Thermonuclear weapon that weighs 45,000 lbs.

"Holy fireball, Batman, that's max gross for a Fokker F27!"

 

[Simon Thomas]

With a bomb that powerful, does the plane that dropped it have any chance of surviving the kaboom?

 

[swampyankee]

I'm barely old enough to have seen B-36s in flight. When I was a mere lad, barely higher than a grasshopper's knees, I would see (or hear) them on the last leg of their flight from wherever the hell they were based to the Arctic and back, although I wanted to fly B-47's.

 

[GrauGeist]

I had to page through the entire thread to make sure that no one posted this. It's the thing that the B-36 was designed to haul. It's the Mark 17 Thermonuclear weapon that weighs 45,000 lbs. They built the Peacemaker around it, just like they built the A-10 around the 30mm cannon. The B-36's bomb bay is like a banquet hall. It is vast. They duplicated it in the B-52.

View attachment 609717

With all due respect, the B-36's design and specs dated to '41 with it's original completion date by '45 (it was delayed and finally first flew in '46).
The Mark 17 was developed in the early 50's - ten years after the B-36 was being prototyped.

There was a postwar modification called the "Grand Slam Installation", initiated in 1950, to allow for larger nuclear weapons.

 

[swampyankee]

With a bomb that powerful, does the plane that dropped it have any chance of surviving the kaboom?

One of the reasons the Soviets didn't replicate the Tsar Bomba was because it was too risky for the bomber. I don't know how powerful was the Mk17, but it is, surprisingly, possible for a bomb to be too powerful. The Soviets decided that line was crossed with their Tsar Bomba. I think the USAF (perhaps surprisingly) figured it out before building one.

 

[nuuumannn]

Early jet engines tended to be a little fussy about smooth airflow into the face of the compressor, and would sometimes compressor stall or flame out just from the wakes of bullets or cannon shells leaving the nose guns. The early axial flow versions were especially susceptible to this.

I'm looking at YOU, first gen RR Avon in the Hunter!

 

[GreenKnight121]

One of the reasons the Soviets didn't replicate the Tsar Bomba was because it was too risky for the bomber. I don't know how powerful was the Mk17, but it is, surprisingly, possible for a bomb to be too powerful. The Soviets decided that line was crossed with their Tsar Bomba. I think the USAF (perhaps surprisingly) figured it out before building one.

Mk 17 yield was 15 megatonnes.

Tsar Bomba had a yield of 50 Mt (210 PJ). It was to be the basis of a larger bomb, but used inert material in what was to be the 3rd stage of the larger bomb.
In theory, the bomb would have had a yield in excess of 100 Mt (420 PJ) if it had included a uranium-238 fusion tamper but, because only one bomb was built to completion, that capability has never been demonstrated.
The test of a complete, three-stage, 100 Mt bomb was rejected due to the extremely high level of radioactive contamination that would be caused by the fission reaction of large quantities of third stage uranium.

 

[chipieal]

One of the historical mysteries surrounding the 36, why the B-60 was never produced. It would seem in the cost conscious atmosphere of post WWII, a proven airplane which had swept back wings and all jet engines would be far more desirable than a clean sheet bomber that essentially did the same thing. The few sources I have found, talked about "inferior performance" to the 52. Could it be that Boeing had better lobbyists than Convair? I think in this configuration, the true advantage of The 36 air frame would finally be utilized. I hate to what if, but the 36 was and is my favorite bomber ever since the first time a viewed Strategic Air Command. (at 8 years old)

 

[special ed]

For anyone genuinely interested why the B-35 & B-49 bombers were cancelled, there still exists on you tube a half hour video I first saw on TV about 30 years ago. It is a story of greed, politics and power. Spend the time to watch: Flying Wings-John K. Northrop's final interview-1979
Afterwards one might check Symington's interference with the NFL (Wikipedia).

 

[XBe02Drvr]

One of the historical mysteries surrounding the 36, why the B-60 was never produced.

the 36 was and is my favorite bomber ever since the first time a viewed Strategic Air Command. (at 8 years old)

To anybody who wasn't already a B36 true believer, the answer should be pretty obvious.
Large scale multi engine swept wing technology was a whole new arena with no body of experience to rely on, and Boeing was acquiring that experience with the B47 before the B36 was even in service. By the time Convair had any spare brain cells available to look beyond the B36 and all of its other projects, the B52 was already well underway and on track to be a more advanced and capable aircraft.