What's the purpose of the P-47's protruding barrel sleeves?

[DAVIDICUS]

This interesting question was posed by "spaters" on the Aircraft Requests forum. There have only been a few responders and the answers have been speculative. I thought that this question would receive more proper treatment and ultimate resolution here.

What was the purpose of the leading edge protruding barrel sleeves that housed the .50 cal. gun barrels?

 

[Matt308]

Purely a WAG...

A: To better facilitate quick changes of the BMGs without need to perform labor and time intensive boresighting procedures that result in aircraft downtime.

 

[Matt308]

It appears that I may be on to something. Check out

http://rwebs.net/avhistory/history/p-47.htm

"The four .50 cal machine guns in each wing of the P-47 are secured in the gun bays to Republic-designed mounts. Front mounts are conical-shaped and the guns are locked to these by rotating the locking ring of the gun bracket assembly; the rear-mounts are locked by simple levers which are part of the rear mount assemblies."

 

[DAVIDICUS]

Matt 308,

I don't see how the following is relevant:

"The four .50 cal machine guns in each wing of the P-47 are secured in the gun bays to Republic-designed mounts. Front mounts are conical-shaped and the guns are locked to these by rotating the locking ring of the gun bracket assembly; the rear-mounts are locked by simple levers which are part of the rear mount assemblies."

The front mounts are at the front of the receiver and operate without regard to the barrel's placement within the sleeve.

The only way that the barrel sleeves could aid in maintaining the secure and accurate placement of the guns after the sighting in process would be if the sleeves themselves were adjustable to fit necessary barrel adjustments made in the sighting in process and thereafter locked into position. I don't think that this was the case.

On the other thread, I suggested that they may just be a further barrel guide for the proper placement of the barrels that may have something to do with the guns being aligned parallel to the ground as opposed to centered in the leading edge of the wing.

 

[Matt308]

Hey it's a WAG. Seems to me that an aircraft that was designed with ground strafing as its primary mission would also focus upon means to streamline repetative and lengthy maintenance procedures. While I understand your point about the front mounts as being universal, it seems odd that these were noted as Republic's design. US military, being the fanatics for standards that they are, would likely spec a common locking ring for all .50 BMG unless perhaps Republic was on to something. The note about the rings being conical implies to me that perhaps the BMGs were being 'funneled' into a set alignment. Otherwise a simple locking ring with windage and elevation would have sufficed. In fact, if you designed a 4 gun carriage, you might be able to make a single adjustment to the carriage and simply insert the BMGs. Sounds reasonable. This might have been born of necessity since boresighting for windage and elevation for six BMGs was bad enough...but eight!?

Didn't the F9F panther have these sleeves too?

 

[Matt308]

Davidicus,

I searched my information on F9F and I can't dig up anything similar for its four 20mm. Must be my youthful imagination.

I would love to hear others thoughts, as I don't profess my response to be anything more than educated guess.

 

[DAVIDICUS]

Hey Matt308, can you pay a visit to the OFF-Topic / Misc. forum? I have a question about Washington.

 

[DAVIDICUS]

Does anyone have any information to add to this topic?

PBAILLON's spastic posting of topics unrelated to this forum is displacing current, active posts on the first page and relegating them to the second page where they don't benefit from exposure.

 

[Jank]

Did they have vents? Were they to help the cooling?

 

[DAVIDICUS]

Well, here's an update in this ongoing saga. I have learned that these barrel sleeves actually have a name. Blast tubes. I still haven't benn able to find out why they were employed or what the actual effect, if any, was as a result of their employment.

I believe they were constructed from stainless steel.

 

[DAVIDICUS]

Here's a picture of some M2's recovered from a downed P-47. Note that it appears that the blast tubes are attached to the guns themselves.

From: http://www.pacificwrecks.com/aircraft/p-47/42-75940.html

 

[Matt308]

Now that is interesting and appears to blow my theory all to hell. Unless...

No, I'll withold my comments and appear the fool instead of voicing them and removing all doubt.

What in the hell could those be used for? Might they have something to do with a design allowing extra cooling via the venturi effect? Can't see how though. Must be stainless steel. That's obvious. Stainless steel tubing? That's also VERY expensive.

Stainless steel is very common in modern rifle bores to maximize barrel life. These "blast tubes" don't appear to be much larger than a standard .50 BMG barrel with perforated heat sheild. Just random neuron firing to perhaps promote discussion.

 

[trackend]

Look at the sleeves Dave they appear to be attached to the gun breech mechanism and not the Barrel this would mean M2's recoil action would be inside the sleeve and in the pic of the P47's wing the sleeves appear to be mounted solidly to the leading edge of the wing with a stainless plate this to me points too some form of hot gas suppression within the wing/or a form of flash eliminator I shall keep looking however.

 

[Matt308]

Here's some more info. It appears that virtually every aircraft with fuselage/wing mounted machine guns and cannon made use of blast tubes. I found my most valuable information in a patent search of all the places!

The blast tubes apparently serve multiple purposes depending upon the mounting locations and the problems that are being solved. Blast tubes are used for any or a combination of the following:

1) To act as a blast suppressor by protecting the aircraft structure from the overpressure of firing the weapon. The overpressure for .50 BMG and up is rather acute and structurally fatiguing.

2) To act as a blast diffuser to minimize dispersion of rounds fired by multiple high caliber weapons in close proximity to each other.

3) To act as a blast diffuser to minimize muzzle flash.

4) To act as a means of venting hot gases from the aircraft structure.

5) To act as a structural means to minimize barrel "whipping" and thus minimize dispersion of rounds from point of aim.

6) To simplify weapon installation, removal and maintenance by aircraft armourers.

7) To facilitate a solid structural support for the weapons to minimize firing inaccuracies of the mounted gun due to wobbling of its firing axis relative to the aircraft. (seems similar to 5 above)

Here are a couple of pics from a Hurricane that I also found. Note that blast tubes appear to used for solving a different set of concerns than those of the P-47, as the mounting appears quite different.

 

[trackend]

Brilliant find there Matt I think thats solved Dave's query

 

[DAVIDICUS]

No, I don't think it does.

Yeah, blast tubes are quite common.

1) To act as a blast suppressor by protecting the aircraft structure from the overpressure of firing the weapon. The overpressure for .50 BMG and up is rather acute and structurally fatiguing.

Why then do we not find a similar design on all other U.S. aircraft with wing mounted .50's? We don't see blast tubes that extend beyond the leading edge of the wings on other "P" and "F" designated aircraft.

2) To act as a blast diffuser to minimize dispersion of rounds fired by multiple high caliber weapons in close proximity to each other.

Again, why then do we not see this feature employed with other aircraft?

3) To act as a blast diffuser to minimize muzzle flash.

The .50 BMG isn't a cartridge known for problematic muzzle flash. Why then do we not see this feature employed with other aircraft?

4) To act as a means of venting hot gases from the aircraft structure.

Why then do we not find a similar design on all other U.S. aircraft with wing mounted .50's? We don't see blast tubes that extend beyond the leading edge of the wings.

5) To act as a structural means to minimize barrel "whipping" and thus minimize dispersion of rounds from point of aim.

This is interesting. I find this valid. Barrels do indeed "whip" when fired.

6) To simplify weapon installation, removal and maintenance by aircraft armourers.

Matt308, can you explain this concept to me? I don't see how removal and installation of the guns would be facillitated by the barrels being located within these stainless steel blast tubes. The receiver is still locked into place at the forward and rear.

7) To facilitate a solid structural support for the weapons to minimize firing inaccuracies of the mounted gun due to wobbling of its firing axis relative to the aircraft. (seems similar to 5 above)

Agreed. it does appear related to #5.

It is possible that the P-47 was designed with these protruding blast tubes to solve problems that did not exist. Were the P-47's guns mounted far forward in the wing so that even in the absence of blast tubes, the barrels would protrude in a like manner? If so, why were they mounted so far forward that three of each battery of four protruded so far?

Were they merely a cosmetic addition designed to enhance the lethal appearance of the P-47?

 

[Matt308]

Aww c'mon David. Your busting my balls.

I noted the 7 findings as a compendium of rationale for blast tubes. Not as the final justification of the choice for the P-47. As a lawyer, you should know that putting forth a defense is not formulated upon a single thrust in logical argument, but rather from a collective set of arguments that overwhelmingly influence the conclusion as being truthful based upon the preponderance of the evidence.

Same in reverse engineering. These 7 features of blast tubes are the result of different installations, their resultant problems, and identification of a single solution. Engineering consideratios such as metallurgy, kinematics, environmental, thermodynamics, structural, lifecycle support of the equipment, and human machine interface all drive unique solutions that may be combined in a composite approach to maximize common design goals.

The basis for blast tubes is no different. The P-47 offered unigue design challenges; of which the blast tubes afforded an ability to address a "composite" of P-47 engineering concerns. I doubt that you will ever discover a comprehensive list of the all the top down derived requirements that forced the blast tube incorporation decision in the P-47. Like legal discourse, sometimes decisions are driven by politics, fate, historical legacy, and even pure ignorance (ie the not invented here syndrome).

In my search to discover the above 7 advantages of blast tubes, I did discover that virtually all...read that ALL...fighters contained some form of blast tube installation as part of their weapons package. Check out B-25s, PBYs with forward firing guns located in the keel, F3Fs, P-38s, P-39s, P-40s, Boeing P-26 Peashooters, etc. Hell even the Polikarpov I-16 used them.

We can all make suppositions about which of the 7 fit the P-47 design criteria. The primary wing spar on the P-47 is located right behing the gun bay and 4 guns must be staggered to accomodate the perpendicular feeding belts from the magazines. This is the primary reason for the staggered design of the .50 BMGs and thus perhaps one (1) rationale for the exposed blast tubes. However, to come full circle, I will bet you bottom dollar that ease of maintenance ranks right at the top.

Take the word of a lowly aerospace engineer. My advice is free and is worth exactly what you pay for it.

 

[DAVIDICUS]

I don't know why you think I'm busting your balls. And, "... putting forth a defense ..."? Let's leave our occupations out of this. After all, as a "lowly aerospace engineer" you should know that while many reasons may generally exist for a design feature on an aircraft, every aircraft will not necessarily benefit from said feature.

Most of what we do on this forum is to apply our common sense, education and life experience to issues that are unclear or subject to competing claims. I think it works out pretty good.

As you pointed out, all aircraft appear to have employed blast tubes so it isn't the existence but the protrusion that is the issue. You mentioned something interesting when you said that the main wing spar is right behind the gun bays. This would explain the forward placement of the guns since they were staggered.

Do you know if the Corsair, Hellcat, Wildcat and Warhawk have staggered mountings as well? I believe that the Mustang did not have staggered mountings.

BTW - Is the 308 after your name .308 Winchester aka 7.62 x 51 NATO?

 

[Matt308]

I think that you will find that most all wing mounted guns are forward of the main wing spar. Thus, you will find most wing mounted guns are staggered to minimize mechanical complications associated with more than two guns mounted side-by-side. A main wing spar mounted mounted forward of the gun bay in a fighter aircraft is highly unlikely due to the fact that it would seriously complicate the ability carry normal wing loads associated with maneuvers.

Don't take my "ball busting" comment too seriously. I'm just asserting myself in a forum other than in my wife's presence where I feel vindicated when I'm done.

Yes, Matt308 is taken from the same family of cartridges. I'm a fledgling, yet avid gun collector who needs another 5ft safe. I have carried my silly name across forums only to minimize my need to exercise the grey matter (which I find increasingly difficult as time and beer goes by)

No hard feelings?....

I'm off to read some Docter Seuss B Book to my youngest son who made a hat trick in soccer today. Talk with you later.

 

[DAVIDICUS]

I understand that guns are mounted ahead of ther main wing spar. I am assuming that the main wing spar of the P-47 was much further forward in the wing than in a Corsair or Hellcat (which I am further assuming had staggered guns). Do you agree?

Such an advanced main spar placement would explain why the blast tubes had to protrude so far.

How old is your son? I have one whom just turned 18 months. This morning I took him to the Skyport Cafe at the San Carlos Airport (a little airport near where I work) for pancakes and to watch the Cessna'a take off and land.