Pressure, Recoil, and Newton

[1911Tuner]

Mud

misANTHrope said:


So, in summary: the key difference between the obstructed/non-obstructed cases are the ability of the bullet to transfer its forward force to the barrel and balance the rearward force on the slide. With an obstruction, all forces are balanced. With only friction to transfer the force, rearward force on the slide exceeds forward force on the barrel, and the pistol cycles.
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Precisely!
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And:

Clear as mud?

It is to me...

Refer to Isometric Tension versus pushing a medicine ball.

Consider pushing that medicine ball while freestanding, and again pushing it against a wall while you are chained TO the wall. Force is acting in opposite directions in both cases, and that force vector is your arms.
The force vector is going to apply force to the ball and your body. If you are freestanding, both you and the ball will be propelled away from ground zero. When the ball is solidly blocked by a wall, and you are chained to the wall, nothing will happen except the application of force that isn't sufficient
to move either.

Mr Keenan's experiment can be duplicated without firing the gun. Silver solder a rod to the barrel. Make the rod long enough to pass through a
one-ton steel plate. Thread the rod and attach a nut. Pull on the grip of the pistol with your right hand and try to pull the slide to the rear with the other. The principle is the same. The barrel is being forcefully pulled against the locking lugs in the slide. The slide is being pulled in opposite directions.

When the gun fires, the force is then BETWEEN the two objects, pushing them in opposite directions equally while the lugs resist. The higher the force, the more tightly they lock, and the harder they resist.

Want to see one fire without the slide moving? Attach a five-pound weight to the slide. If it moves at all it won't be very much.

 

[cracked butt]

Think of a gun not as a powder burning device ejecting a bullet, but as a spring loaded device ejecting a bullet. In essence a spring and charge of powder do the same thing, but in different forms- they both store energy- one as mechanical energy and the other as chemical energy.

So you have your gun with a supercoiled 50,000lb spring held back by your trigger mechanism. You pull the triger, the spring is released, pushing both at the breachface and the ball at the same time. To make the comparison as similar to powder as possible, the spring both weighs 5 gr. and is not attached to the breachface, and the ball weighs 230 gr.

What is going to happen is that the spring is going to push back on the breechface and forward on the ball. the ball will ejct from the barrel followed by the spring- as its the path of least resistance- versus pushing rearward against a much heavier object.

I'm a little rusty on my physics- its been a few years for me too, but conservation of momentum is (correct me if I'm wrong) Mass1xvelocity1=mass2xvelocity2

lets say the gun weighs 3 lbs, the ball and spring weigh a total of 235 gr. and the ball+spring are launched at 800 fps.

Mass1= 235 gr
velocity1=800 fps
Mass2=21000 grains
velocity2=?

235grx800fps=21000grx?
so..

(235gr x 800fps)/21000= Velocity2

Velocity2 therefore =8.9 fps

In order to launch a bullet at 800 fps your gun would have to travel backwards at 8.9 fps. some of this would be mitigated by the recoil spring which will cause the gun to move backwards slower over a longer period of time, and the force not being aimed straight back into the hand as the axis of the barrel is above the hand.

 

[1911Tuner]

Spring Analogy

Bingo, cracked butt.

The force vector...be it a spring, or an explosion...or a sudden burst of compressed air is applying force in both directions equally. Initially,
nothing moves...except I have seen slow-motion videos of the bun moving sharply FORWARD for a very short distance just prior to the recoil cycle.

 

[benEzra]

(firearms_instructor)
In a firearm with a rifled barrel, does the gun recoil much, if at all, before the bullet leaves the muzzle?

Theoretically, the gun is almost in full recoil by the time the bullet leaves the barrel. It gains some added recoil from the gases still being accelerated down the barrel, but I ignored that in my example. The key to understanding this situation is that the rifle accelerates backwards at the same time the bullet accelerates forwards; just as the forward push on the bullet ceases when the bullet leaves the barrel, the backward push on the rifle also ceases (ignoring residual gas effects).

What is special about the bullet, though? Would you agree that it could just as easily be the slide that was being "fired"?

Yes. I meant by "intrinsically tied together" that the acceleration of both the bullet and the rifle begin at the same time.

BTW, anyone remember the Hedgehog antisubmarine weapon from WW2? That system had a stationary "bullet" (solid rod attached to the weapon mount) and fired the "barrel" (hollow, finned cylinder with a bomb on the end).

(Jim Keenan)
Your initial take is correct; mass x velocity in one direction equals mass x velocity in the other. But your second conclusion, that "Recoil is transmitted to the firearm by the unopposed gas pressure upon the breechface. No surprise there", is not correct. In fact, recoil began before the gas pressure was unopposed; it began when the bullet first moved. The bullet exit causes pressure to drop almost instantly to the ambient level. The slight remaining pressure (called "residual pressure") can have some effects, but it is not enough to cause recoil.

Entire correct. I meant that the backward force on the gun was no longer unopposed by the forward force on the gun imposed by the imaginary locking mechanism, but my wording was misleading.

 

[cracked butt]

Here's the $64000 question- can you make a short recoil operated pistol function in the weightlessness of space?

 

[1911Tuner]

64k Question

can you make a short recoil operated pistol function in the weightlessness of space?

Sure. Mass and weight aren't the same thing. An object has mass, no matter where it is...and it still takes a measure of force to move it.

 

[RJ357]

When a repair crew had to go up and work on the Hubbel telescope, they trained underwater with large heavy objects. Even though the parts would be weightless in orbit, they still had mass. The crew had to be careful with the massive parts in orbit, as they would be hard to get moving, and once moving, they would be hard to stop.

 

[cracked butt]

Sure. Mass and weight aren't the same thing. An object has mass, no matter where it is...and it still takes a measure of force to move it.

Alright my bad, I asked the wrong question...



On Earth, when you fire a pistol, you are firing from a fairly solid platform- meaning the friction of the ground keeps you from sliding backward uncontrollably when you fire. If you do something as innocuous as limp wrist the gun, it can have functional failures. I'm wondering if in space if you are free floating on a space walk, would this lack of friction cause a situation similar to limpwristing? or would the amount of one's mass be enough to compensate?
If I were selected by NASA to go on the next shuttle flight as a test subject on the effects of living in space on individuals who like to lay on the couch, put down a few Leinenkugels and watch womens' beach volleyball, and I wanted to bring my 1911 to ward off space goblins on an occasional space walk, what poundage of recoil spring should I switch to assuming my mass is 112kg and I'll be wearing another 25 kg of space gear?

I used to love problems like this, but unfortunately, the gerbil wheels in that part of my brain have rusted up and need oil, some have stopped altogether, and perhaps alot of the rodents have died

 

[RJ357]

cracked butt -

You can create a similar situation on earth by sitting (or standing) on a swing hanging from very long ropes. The purpose of the long ropes is to keep all movement horizontal. That way gravity will have little effect on your movement. All resistance to sideways motion is virtually due to mass.
You can also do a similar thing by standing on the floor and leaning backward as far as possible (without falling). The friction of your feet on the floor would then have little affect in holding you.

Now if you can imagine this, you can see, I think, that your mass would probably be sufficient to make the gun cycle reliably. You would, however, go floating off backwards. And would also be rotating backwards. This is in space, of course.

 

[cracked butt]

I thought of using a pendulum to the same effect, but as you stated, it would have to be a very long rope. I guess maybe the best way to test it on earth is to skydive from an airplane and then shoot while freefalling.

 

[RJ357]

That would work. In fact, if it weren't for the air, it would be identical to being in deep space or orbit. You would be completely weightless.

 

[Jim K]

Cracked butt asked:

"On Earth, when you fire a pistol, you are firing from a fairly solid platform- meaning the friction of the ground keeps you from sliding backward uncontrollably when you fire. If you do something as innocuous as limp wrist the gun, it can have functional failures. I'm wondering if in space if you are free floating on a space walk, would this lack of friction cause a situation similar to limpwristing? or would the amount of one's mass be enough to compensate?"

Your mass here on earth is enough to resist the recoil and let the pistol operate, and the same would be true in space. The difference would be that since there is no gravity to keep you tied to the earth, firing the pistol would move you, at least a short distance. You could, in theory, do a space walk and get around by firing a pistol in the direction opposite that you want to move. But watch those bullets; they have mass too, and will do to a human target just what they would do on terra firma.

This thread has been a lot of fun.

Just for the record, Hatcher tried the blocked barrel experiment with a Model 1903 rifle and service ammunition. The gun was suspended on strings and fired remotely. There was no recoil. The pressure stayed up quite well and there was a loud bang when the bolt was opened.

I have done the same thing with a 1911A1 pistol and GI ball, except that the gun was hand held. The result was the same. The gun did not recoil, the slide did not move (I had a strain gauge on it). The case evidently did not obturate as well as the rifle case, because the pressure leaked out over a couple of minutes. But please don't try this one at home; you have to ruin a barrel and there are a couple of things you need to do to keep the pistol from being damaged.

Jim

 

[John Ross]

RJ357 said, replying to my statement that the "Pressure on the breechface" stuff is irrelevant:

"If that is true, then the "pressure on the bullet stuff" is also irrelavent. There is no fundamental difference between the bullet and the breechface (and whatever is rigidly attached to the breechface). Both are masses. Both are free to move."

The key factor is not "pressure on the breechface (or anything else)," it's MOVEMENT. The breechface and the sides of the barrel and the base of the bullet all have pressure on them, but the gun only recoils when something moves, and it recoils in the opposite direction of the something that moved.

If you have a plugged barrel and have ground the locking lugs off your bolt, the gun will recoil FORWARD, away from your shoulder, because the bolt will be going backwards through your head.

If your chamber has a flaw on the right side and ruptures, blowing out chunks of metal to the right, the gun will recoil to the left.



And to answer the guy that asked if the gun started recoiling before the bullet got out the muzzle:

YES. If you want to know how much, do some calculations based on the center of gravity staying the same (it does.)

Chamber a round.
Find the balance point of the loaded rifle on a knife edge.
Mark that spot.
Chamber an empty case.
Dump the powder in the barrel and distribute it evenly along its length.
Jam a bullet in the muzzle so it doesn't fall out.
Find the new balance point.

The distance between the two knife marks is how far the gun will have moved in free recoil at the moment the bullet exits the muzzle.

Let's not make this more complicated than necessary.

JR