The Great Thrust Vector Debate

[sm]

Great Thread !

Does the barrel push the slide through the locking lugs and hood...or
does the thrust exerted on the breechface move the slide, causing the slide to pull the barrel along with it?

- Tuner

My .02
Thrust exerted on Breechface due to

... function of the expanded case in the chamber exerting the force against the breechface and

- Tuner

In my thinking , the opposite and equal travel of gases along the same plane. It would seem to me with gases going down bbl - the opposite being gases against breechface - path of least resistance if you will.

 

[P95Carry]

In firing, pressure drives the bullet forward so the same pressure pushes the slide rearward to balance the momentum. Cause and effect. Can't separate them.

Just as Jim says. Agreed.

Think also of a sprinter on starting blocks (the analogy to a breech face) .. at ''the gun'' he applies thrust against the blocks ... in order to accelerate forward and get into running mode. His foot pressure is totally comparable with rearward gas pressure vector during the time bullet is accelerated down barrel.

It matters not that we are talking about a gas under pressure .. this is a force, held in control briefly by the resistive inertia of the barrel/slide system... which takes time to react ... time enough for bullet to exit.

Equal and opposites .. no case to be answered!

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Oh just to add ... of course, gas pressure is applying force in all directions ... but if chamber/ barrel/case head can withstand pressured without rupturing then we have ''confinement'' .. and thus all forces act and are contained within the barrel's long axis... the bolt face remaining as the ''starting block).

As buller leaves muzzle ... gases now have an 'escape route'' ... the easy option .. and so rapidly cease to have any rearward thrust remaining. The obstruction (bullet) ... is gorn!

 

[RJ357]

It matters not that we are talking about a gas under pressure .. this is a force

Force on breechface is equal to area of breechface (square inches) multiplied by pressure of gas (lbs / square inch).

 

[Tamara]

'Nother thought experiment:

Does a .45 blank at least partially cycle the slide despite the total absence of a bullet? Will a 9mm blank cycle a 9mm 1911 with a light enough recoil spring, again, despite the lack of a projo engaging the rifling?

(Reverse the thought experiment: Would a caseless delayed blowback gun work?)

 

[R.H. Lee]

Does a .45 blank at least partially cycle the slide despite the total absence of a bullet?

I'll stick my neck out and say I don't think so. No bullet means no mass going down the bore, so there is no opposite reaction.

But then I'm no physics professor, either.

 

[RJ357]

1911Tuner -

Are you asking if the action of the 1911 in particular is responsible for the lack of recoil with a frozen bullet? In other words, it's the fact that the barrel and slide are locked together that's responsible?

(We were supposed to wait for Jim Keenan to reply, weren't we?)
(sorry)

 

[RJ357]

Does a .45 blank at least partially cycle the slide despite the total absence of a bullet?

Movie guns require a partial obstruction in the barrel to cycle with blanks. The absense of the bullet results in a very low pressure impulse.

Would a caseless delayed blowback gun work?)

I would say yes. Almost all of the slides movement is from momentum. The cause of the movement is very short, and almost gone by the time the slide unlocks.

 

[sm]

Tamara -

Would you explain your post a bit more in detail please.

I have used the .45 shotshells and they did in fact cycle 1911s. I have used the primer only cases with Wax bullets and have had these cycle . The Speer Rubber did not feed at first very well , I cured that ...still these did not cycle . Must have been the "crimp" affecting the gases?

Tamara is this similar to what you refer to?
Anyone else?

Am in the ballpark ?

 

[baker]

RJ357 said:

If you look even more closely, at the molecular level, what you will see are gas molecules transferring energy (momentum) to the molecules of the breechface by collision. And it is this alone that produces the force.

I respectfully disagree, of course the gas molecules are transferring lots of energy to the breechface and forcing it back, BUT, if there were no mass leaving the gun, the effect would be reduced.

If 230 grains leaves the gun at 850 fps, there IS an equal, opposite force upon the gun. If you stood on rollerskates and threw an object, you would move backwards with the same energy exerted upon your mass, minus friction.

Expanding gas alone is not the whole equation.

 

[P95Carry]

The only mass a blank round has to work against - is the density of the atmosphere .... not enough to create much of a vector rearward.

Contain those expanding gasses by obstructing it's discharge route - even partially, and this simulates to a degree the obstructive effect of a bullet.

Steve ..... if those .45's were shotshells then the BB count (load) will still act as an ''obstruction'' - resistance mass.... or if you like a mass that needs accelerated ... even if in a wad. Still equal and opposite will apply.

 

[RJ357]

I respectfully disagree, of course the gas molecules are transferring lots of energy to the breechface and forcing it back, BUT, if there were no mass leaving the gun, the effect would be reduced.

The effect on the breechface would not be reduced. If the pressure remains, so will the force. What will change is the movement of the barrel (and slide). If there were no mass leaving the gun, it would have to be because the bullet was frozen in the barrel. There would then be an equal force against the base of the bullet (and hence the barrel) in the opposite direction. The pressure and force against the breechface remains.

In fact, a release of gas would result in a decrease in breechface pressure.

 

[middy]

Keenen is right.

Kuhnhausen says it is pressure against the breechface that causes the slide to move. Wrong.

It's all in the barrel.

When the powder ignites, the pressure is in all directions, essentially fusing the casing and the barrel as the soft brass is pushed into the steel of the chamber. It can't slide out of the chamber and exert pressure against the breechface. If the bullet is attached to the barrel by rod and set screw, the cartridge and barrel are now basically just a pipe bomb without enough powder to blow. (Incidentally, I'm quite suprised this experiment doesn't blow the case every time.)

The slide moves because it is locked to the barrel initially, not because of pressure against the breechface.

The barrel moves because if the bullet is free to move out of the barrel the same force is applied against the barrel/casing in the opposite direction.

OK, actually, Keenen is only mostly right, it's not just the mass of the bullet, it's also the "rocket effect" of the burning powder (I think). Now it's like a pipe bomb with a loose plug in one end. Set it off and the pipe and the plug will go flying in opposite directions (assuming you don't have an overcharge which would fragment the whole thing before the plug has time to get out of the way). The lighter plug (the bullet) will, of course, fly faster.

Edited to add:

But even without a plug, imagine a pipe bomb with one end open... it's still going to go flying. Like a rocket. (Hey that is a rocket!)

 

[sm]

Tamara

Chris

I can understand the shotshells being mass or obstruction. I don't know the exact "weight" of that payload . I'm trying to keep the weight - for sake of this discussion at 230 gr .

The Speer Rubber bullets we just stuck in. The Wax ones were from a block of paraffin , so I would think more resistance becasue of more "crimp" is the rason these cycled. ???

Hey - I'm learning. Some stuff I kinda understand but don't know why I do. It is like I pass up some steps. I jsut kinda see it in my mind - I never took physics ....

 

[P95Carry]

middy -

Sure cases expand but - not IMO with enough frictional ''grip'' to cling onto the chamber walls! So I beg to differ.... the case WILL move back to impact the bolt. A ''component'' might be chamber ''grip'' but not much to matter.

Consider the revo --- the reason the recoil shield is there .... it prevents further rearward movement of a case ... remember too - the case ''sets back'' such that a smallish firing pin ''dimple'' becomes a deeper depression .. due to set back against a firing pin that has not had time to drop back! The primer tries to escape the pocket but is whacked by the pin .. again.... and will rarely if ever by other than flush with case head .. so proving set back.

Sorry - the force (yeah ... pressure over area) ... IS exerted on bolt ... has to be. Pressures in these realms will overcome any ''case grip'' in a chamber.!

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Steve ..... a 230 grn ''equivalent'' will be fine ... leaving aside bullet/barrel friction .. this'd equate well OK to a bullet mass.

Wax bullets? ..... well, if crimp good then certainly some useful opposite reaction but - if mass not there then it'll be a diminished rearward force ... has to be.

 

[RJ357]

The law of equal and opposite reaction is important in that it describes the relationship between the two actions.
1) that they are equal
2) that they are opposite

But it does not explain why.
Just as the law of gravity describes how objects fall and in what direction, but does not explain why.

Kinetic forces cannot act over distance. The interacting objects must be in physical contact. It is not a case of radiant energy like light, or forces involving fields like gravity or magnetism. The only thing that matters at the breechface is the pressure, as it is the only thing in contact with it. The actions some distance away may alter the pressure, but it is only the pressure that determines the force.
Whether the breechface (and anything attached to it) moves or not depends on what other forces are acting on the system. If the breechface force is the greater, then it will move.

I hate to make it sound complicated, but it isn't, really. If you look at everything discussed with nothing more in mind than the pressure in just the two directions, it explains everything.

 

[Tamara]

middy,

Kuhnhausen says it is pressure against the breechface that causes the slide to move. Wrong.

It's all in the barrel.

When the powder ignites, the pressure is in all directions, essentially fusing the casing and the barrel as the soft brass is pushed into the steel of the chamber.

So, does a PPK cycle this way, too? Or does .380 brass have a lower coefficient of friction than .45 brass?

How does the .45 casing know whether it's in a 1911 (and supposed to drag on the chamber walls) or in a Grease Gun (and supposed to shove on the bolt face)? That's some durned smart brass...

 

[P95Carry]

thing that matters at the breechface is the pressure, as it is the only thing in contact with it

RJ, that pressure is being exerted within a now empty case .... which makes that case want to travel out of the chamber rearward. It cannot because of the presence of the bolt face - it is constrained.

So the pressure is ''transmitted'' - via said case....... which desperately wants to exit its confining chamber but cannot.

Just as the law of gravity describes how objects fall and in what direction, but does not explain why.

Gravity is an acceleration .. it acts on objects with mass. It attempts to accelerate them at 9.81 m/s toward the earth's center .. only something resisting that acceleration will prevent this becoming fact. We stand on ground .. and that applies a counter force to the acceleration such that equilibrium exists.

All good fun eh!

 

[Tamara]

In clarification, however, what pushes the breechface back is the case. Why is the case heading back? Because the gasses (and bullet, if any) are moving forward. If there's nothing moving forward, nothing will move back. (There needs to be an action for there to be an equal and opposite reaction...)

I'd still need to watch the blocked bore test with my own eyes (and somebody else's gun... )

 

[RJ357]

RJ, that pressure is being exerted within a now empty case .... which makes that case want to travel out of the chamber rearward. It cannot because of the presence of the bolt face - it is constrained.

So the pressure is ''transmitted'' - via said case....... which desperately wants to exit its confining chamber but cannot.

You are correct. I ignored the case to simplify, since it is the same amount of force in the same direction.

Gravity is an acceleration .. it acts on objects with mass. It attempts to accelerate them at 9.81 m/s toward the earth's center .. only something resisting that acceleration will prevent this becoming fact. We stand on ground .. and that applies a counter force to the acceleration such that equilibrium exists.

Also correct. However, it is not attempting to accelerate objects, it is accelerating objects. The force that you feel standing on the ground is the result. If allowed to move freely, you will feel no force, zero G.
Part of Einstein's general theory of relativity concerned this equivalence of gravity and acceleration.
Still doesn't explain why, though.

All good fun eh!

Excellent fun!

 

[RJ357]

If there's nothing moving forward, nothing will move back. (There needs to be an action for there to be an equal and opposite reaction...)

Aha, but what if the bullet is attached to wall with a good strong rod? Will the gun move back now? What is the action?

 

[P95Carry]

''Hey Doc .... my brain hurts!!''

 

[JohnKSa]

Pressure does not cause recoil.

If nothing escapes the system then there can be tremendous pressure and there will still be zero recoil--zero movement for that matter.

Recoil is proportional to the mass velocity product of whatever escapes the system and in the opposite direction of the vector of the escaping material.

If no mass escapes then the product (mass times velocity) is zero. That means that recoil is zero as well.

Rockets move because they eject a lot of mass at a tremendous velocity out the back of the rocket motor--not because of the pressure inside the motor pushing against the rocket. Ion engines generate virtually no pressure and yet can attain tremendous velocities because the ions are ejected at very high speeds.

Blanks cause recoil although it is much reduced. They cause recoil because mass (in the form of gas and unburnt powder) escape through the muzzle. Restricting the muzzle increases the velocity of the escaping gas and therefore increases the recoil since it is proportional to the mass velocity product of the escaping gas. Just like reducing the size of the hole in the end of a hose with your thumb increases the velocity of the escaping water.

If you lock the bullet in the barrel by fastening it to the barrel then there will be no recoil because there is no motion. If gas vents then there will be recoil in the opposite direction that the gas is vents.

If you lock the bullet in the barrel by putting a rod down the barrel and fastening the rod to a wall, then there will be recoil-like motion because you have turned the gun into the projectile. The wall is now the breech and the gun is the "bullet". It's not recoil though, the recoil force now goes into the rod and will damage the rod or the wall if sufficient force is applied.

Locking the bullet in a straight blowback gun is similar to putting a rod down the barrel and bracing it against a wall. You make the barrel the breech and the slide the projectile. There's movement of the slide in the rearward direction, but that's not recoil in this case. The recoil will be in the FORWARD direction as the barrel recoils forward in proportion to the velocity mass product of the slide's rearward motion.

 

[Tamara]

JohnKSa,

All true.

But what causes the slide to move is pressure exerted by the recoiling brass against the breechfrace.

The brass is recoiling in that direction because a projectile and a bunch of gas just went off in the other direction. Newton tells the brass to get moving, so it does, shoving the breechface out of its way and hopping politely out of the ejection port (after banging its little head on the ejector.)

Er, not to put it too anthropomorphically.

(In a straight blowback pistol, of course, gas pressure plays a much bigger part in the rearward movement and ejection of the case, since there's no mechanism holding the breech shut until most of the gasses get out of the bore...)

 

[JohnKSa]

But what causes the slide to move is pressure exerted by the recoiling brass against the breechfrace.

Not pressure. The brass gets the recoil boot and transfers it to the breech--just like those little desk games--the balls on the strings. When one ball hits the stationary balls, the force is transferred through the motionless balls to the one on the end which then jumps.

Recoil is motion--that has to result from force--which is a function of motion (acceleration). Pressure, in and of itself, does not imply motion and therefore can not, in and of itself cause recoil. Until something moves, there is no recoil.

In fact, the pressure inside the brass case actually slows the recoil velocity by increasing the friction between the chamber wall and the outside of the swelled case.

 

[Tamara]

Pressure, in and of itself, does not imply motion and therefore can not, in and of itself cause recoil. Until something moves, there is no recoil.

If one exerts pressure on a movable object, it tends to move. We're wandering into semantics now.

The breechface has inertia imparted to it and is set in motion by a cartridge case that is recoiling from newtonian forces, as well as having the pressure of expanding gasses acting on it. Since it is not fixed in space, it is trying to move, too, just like the bullet. (Set off the cartridge outside of the gun and watch the brass case depart the premises due to the rapidly expanding gasses inside, right?)