Recoil effect on path of projectile.

[Blaster]

When firing a cannon, how much effect does recoil have on the strike of the projectile??  This has probably been discussed on this forum many times but I just have not located it.  So, has the projectile actually exited the bore prior to the recoil, which could determine a hit or miss of the aimed point.  I believe someone had indicated that normally the projectile is already well down range before the cannon tube actually moves with the recoil.  I'm very interested since although I have a pretty high hill on my property, that I use as a backstop, there is a neighbor about a 1/4 to 1/2 mile behind my hill and I'd rather not have a projectile enter his space (I don't want to cause any injury/damage and especially since he is my Dentist and sure don't want any retaliation when I'm in "the chair" for my next visit). Any help will sure be appreciated.  I recently fired a little 20 m/m deck gun and the gun made a complete backward flip and actually wound up pointing right back at the original target.  That started some real concern.  I had taken a movie of that gymnastic gyration .  The projectile plowed through the ground, just above the top of the target, leaving a trench about eight feet in length,  six inches wide and a few inches deep.  Never did find the projectile.  Any suggestions will be appreciated.  I have no concern about my mortars as I know where those projectiles are going!! :?
Blaster (Bob in CO)

[Cat Whisperer]

The laws of physics are just that - laws.  The energy imparted to the ball is the same as the engergy to the cannon.  (Allow a little difference to account for the mass of the powder/burning gasses.)

The ball moves in a very restricted direction - down the tube and beyond.  It is moved to a high speed because it's mass is small (ft-lbs of energy = mass x acceleration).  In the same respect, the cannon moves in the opposite direction, allbeit slowly, because the mass is very great (same ft-lbs of energy = larger mass x smaller acceleration).  

Both move at the same time because it's the same force.  BUT the cannon moves MUCH more slowly, against much more resistance.

The resistance the cannon faces can influence it's direction of movement; BUT that is of importance ONLY for a VERY short period of time (when the ball is still in the barrel).

So from a perspective of AIM (for ONE shot) you only have to restrict the movement (or direction of the barrel) for a very brief period of time after the fuse is lit.

Observe match rifles - the line of the axis of the barrel is aligned with the stock - so the recoil is straight back.  Some rifles have a lot of 'drop' at the heal of the stock - and the muzzle lifts up on recoil.

[Blaster]

The resistance the cannon faces can influence it's direction of movement; BUT that is of importance ONLY for a VERY short period of time (when the ball is still in the barrel).

 OK Tim.  So now it sounds to me like the shorter the barrel, the less influence the recoil will have on elevation segment during the flight of the projectile.  Right??  Blaster (Bob in CO)

[Cat Whisperer]

I think we're reading the THEORY the same way.

Of importance is (if the cannon is flipping) the height of the bore's axis above the majority of the mass of the mount.  Might want to make it heavier or bolt it to something a bit longer.  Look also at repeatability.  

Another comparison - revolvers.  Measure sometime the distance from the center of the bore to the height of the front and rear sights.

[Blaster]

Ya know, I did neglect to mention one important thing in my post regarding the little 20 m/m deck gun making a complete back flip.  Although this little thing weighs around 35 lbs and is mounted on wheels (on the front only as the back is plain with a little spade type shelf to dig in upon recoil) I should have allowed it to move to the rear on it's own recoil.  Instead, I though I be tricky and have the rear end butted against something solid such as a board well anchored into the ground.  If I had not done that, it would have had the freedom to move back with the recoil, which would most likely had eliminated that fearful backward flip.  The fellow who sold me the barrel for this gun had told me that he had fired his on numerous occasions without the benefit of a carriage - would merely place the barrel in a wooden box/cradle and fire it like that.  Of course, the barrel would wind up way behind the original firing point.  Just as information and for your or any one elses thoughts.  Thanks, Blaster (Bob in CO)

[Blaster]

Cat Whisperer, many thanks for your input on this recoil issue.  I had though that this subject would get a lot of attention, opinions and comments from other concerned shooters.  Guess I was sure wrong on this one.  I'll put this recoil matter to sleep.  Blaster

[Double D]

Actually, Bob it is an issue. Some of us are just slow starters.

When I fire my Boat howitzer with cylinderical slugs I have to tie it with a block and tackle system much like that used on a ship..



That block and tackle system wasn't just to pull the gun back into battery after firing , it also was used to help aim the gun and served as a rudimentary recoil arrester.

Iyou look closely at the postion of the rope these two pictures you cansee how  much the gun is recoiling.





When i shoot the round ball load I get virtuall no recoil.

As far as the effect of recoil on the flight path, it has to have some effect as soon as the gun is fired. What effet, i'm not sure.  

One of the things I want to do is put sights on my cannon and shoot it.  Once I do that I will have to what effect weight of slug and heavier recoil has on point of impact

[Blaster]

OK DD, now that was really interesting.  The response from CW and yourself is the type information that I was hoping generate when I started this deal about recoil.  I still have a whole lot to learn about many things having to do with the firing of cannon/mortars. You have really got me guessing as to just why the cylinderical projectile would generate so much more recoil over the ball assuming the weights are fairly similar.  Would you care to furnish the approximate weight of each of those projectiles?  Also, do you allow approximately the same amount of windage for both of them??  Finally,  great pix!  Thanks.
Blaster (Bob in CO)

[Cat Whisperer]

How recoil - or the mechanism used to disapate it - affects accuracy is very important topic - escpecially if you're in the business of killing people (as in the armed forces that is) because it's important to kill the right ones.

A major step in handling recoil was the institution of a recoil absorbing device - actually to spread the force out over time by allowing the barrel to move back while the carriage remained (for the moment) steady.  My guess is that it improved accuracy because it allowed the barrel to move in a straight line and reduced the variability of the effect while the round was still in the tube.

Building a strong base for the huge mortars is commonly observed, as in the pictures of the Dictator and others.  Some of the rail guns of WWI and II used the movement of the carriage down the track to advantage as well.  Further, the large naval guns would generally fire only broadside - as the ship would roll, absorbing the recoil, but if fired forward or back the shock to the ship was intense.

[Double D]

The Cylinderical slug  weighs 7 ozs.  The round ball is 1 inch diameter  and i don't know what i weights but it is a lot less.

The ball has windage, the slug was built to slip fit when I thought windage was something you did after eating a bowl of beans.

[guardsgunner]

When we first fired our mt. how. size gun at 400 yds. , we fired several rounds and could not figure out why it kept going to the left. It was noticed that the gun was recoiling to the left and not straight back. we found a large clump of weeds slightly left of the trail that was forcing the trail right at the beginning of the recoil and throwing the shot before it could leave the barrel. This gun only move about 18" with a full charge. Didn't take much to move it.Recoil has to be straight back.

[Blaster]

OK guardsgunner, sounds logical now that the carriage must be able to move straight back to eliminate influence on the left/right placement of the shot.  Shows that the projectile is still deep enough in the bore to make a major difference....And DD,  on that windage/bean thing.  A couple hardboiled eggs and a couple beers will have the same effect however it'll seem like you were just witnessing a blackpowder cannon being fired (with similar sound and air quality).  Now back on the subject,  I now wonder if making a wooden Naval carriage completely stationery, would there be enough recoil to really damage that carriage.  Seems like something has got to give.  I'll find out soon as the brand new mud dries up.  We just had three inches of fresh snow by noon today and by 2PM it was completely melted making a whole lot of fresh mud.  Blaster (Bob in CO)

[Cat Whisperer]

 ....  The ball has windage, the slug was built to slip fit when I thought windage was something you did after eating a bowl of beans.

We'll have to add that to the glossary Sticky.


.

[John N]

One would think that mortars are a lot less susceptible to recoil effects than their longer barreled cousins. A mortar’s projectile doesn’t spend much time under the influence of “interior ballistics”, given a mortar's very short barrel.

Here are some pictures of my sleigh carriage beer can mortar. The first shot is taken the moment before firing. Note the position of the carriageÂ’s base relative to the ground.



The next shot shows the moment of firing. The can is still somewhere in the fireball having just emerged from the barrel. Note how the rear of the carriage has squatted down into the dirt, appearing to cant the barrelÂ’s elevation slightly upwards.



Here is the next frame, 0.067 seconds latter. The can is now visible towards the top of the frame, already tumbling:



I hope to repeat this shot sequence with my new digital camera. It will record 640x320 fine resolution movies at 30 frames a second, twice as fast as my current camera. I might get lucky and capture the can just as it is exiting the muzzle and see how much the carriage base has moved.

As a side note, my heavier coehern mortar simply slides straight back on its flat base and doesnÂ’t seem to significantly alter itÂ’s elevation at the moment of firing like the sleigh carriage mortar.

[CAV Trooper]

Here's a web site about recoil that you guys might find interesting:

http://www.bsharp.org/physics/stuff/recoil.html

It appears that there's two parts to the total recoil of a gun. Primary recoil is from the projectile moving down the barrel just after firing. Secondary recoil comes from the escaping gas after the projectile leaves the muzzle. It's actually the secondary recoil that is felt the most by a shooter.

This is a link to a calculator for determining how much recoil is produced by a gun. It's designed for rifles and pistols but it works for cannons and mortars. The only problem is that you need to know the muzzle velocity of the projectile.

http://www.huntamerica.com/recoil_calculator/

[Cat Whisperer]

John N -

Great sequence of picutres.

THanks especially for the one showing the mortar tipping into the dirt.  It's something you'd never see otherwise.

Cav T -

Good links!  Information is where you find it.  You dug out some good ones!

[GGaskill]

Recoil begins at the same time projectile motion begins, as the same force causes both.  Once the projectile is out of the barrel, recoil can have no effect on the projectile or its direction.  So the question is what effect recoil will have on the barrel during the short period of time the projectile is still in the barrel.

Anything causing nonuniform motion during the recoil while the shot is still in the bore will effect aim; more resistance from one wheel than the other, change in elevation of the wheels due to bumps or dips in the path of the wheel, a flexible carriage that allows the breech to dip when the charge is fired.  If the muzzle dips when the charge is fired, the trunnions are above center which will effect the aim since the breech doesn't maintain constant contact with the carriage.  If the trunnions are much below center, there will be a bending force applied to the carriage which will cause the gun to shoot high, although that should be fairly uniform.  Lots of things can cause problems if the design is not made to prevent them.

[John N]

Here are some follow up pictures of my sleigh carriage mortar, this time firing off of a piece of plywood instead of directly off the ground. Because of its design, the rear of this carriage tends to dig into the dirt under recoil. This resistance causes the front of the mortar to rise up and in some cases flip over as seen in this shot:



I fired six beer cans down range to see how the mortar would behave under recoil when placed on a sheet of plywood. It seemed to work beautifully. This 5 shot sequence was taken of one of the firings. In the first picture, you can see a small puff of smoke exiting the muzzle past the can. (If the shot had been taken a hundredth of a second later, I might have caught the can exiting the barrel. Oh well, thereÂ’s always the next time).











As you can see the mortar is still leaving the ground for a tenth of a second or two, but the plywood shooting platform does reduce the upward motion quite a bit and most of the recoil energy is expended in a “more controlled” rearward motion.

[CAV Trooper]

John,

Great pictures. While I can't disagree with GGaskill, it's fairly clear that the majority of the recoil effect takes place AFTER the can has left the tube. That's what happens with modern guns.

I was always under the impression that the biggest accuracy problem caused by recoil was getting the gun back into battery and lining up the NEXT shot so it would hit about the same place as the previous one.

[Cat Whisperer]

....  I was always under the impression that the biggest accuracy problem caused by recoil was getting the gun back into battery and lining up the NEXT shot so it would hit about the same place as the previous one.

And the REPEATABILITY word comes up again.  Consistancy in everything we do will gain us the best accuracy.  Then comes controlling the variables that cause the inaccuracy one at a time.  Wind, I'm sure, will be a big one.

[GGaskill]

No question that the vast majority of the motion of recoil occurs after the shot is gone but that motion has no effect on the direction of flight of the shot.

[CAV Trooper]

No question that the vast majority of the motion of recoil occurs after the shot is gone but that motion has no effect on the direction of flight of the shot.

Absolutely correct.

However, there IS something else that effects the accuracy of the shot that hasn't been mentioned yet. That is; due to windage, when the gun is fired, the projectile bounces off of the walls of the tube as it travels it's length. The last point where it hits the tube before it exits the muzzle determines the shots final direction.

I'm sure you all know what I mean but just in case, here's a really crude and exaggerated  picture of the process:



I don't know how pronounced this would be in a beer can mortar but in a cannon, it does have some small effect on where the shot goes. The bad thing is that there's no way to predict, or repeat, the final direction of the shot.

You know, the more you think about it, the more you realize how complex the overall process of shooting these things really is.

[Cat Whisperer]

Add to the loosness (and the location of the sphere 'bouncing' down the bore) is the huge pressure of  the gasses of combustion pushing -  which would push to one side or the other if the sphere was not centered at the moment of exit.

I have wondered, having shot a few golf balls in my time, what causes them to hook or slice.

Going further, looking at the causes of vibration in machine tools, one finds that one of the causes of movement in the cutting tools is a "soft-foot" or one of the 4 supports of the lathe or mill is not supported adequately.  Hence, we may not recognize the importance of the effect of only having three of four wheels of the cannon supported.  With a two-wheel cannon the 'trails' have one point of contact (except on modern split-trail cannon) which gives excellent 3 points of contact with the ground.

Perhaps not the most significant causes of variation, but worth considering.

[GGaskill]

I have wondered about the "shot bouncing down the barrel" question and it is probably not a question that can be answered by strictly inductive methods by us amateurs.  But it is my opinion that the windage allows a cushion of gas around the shot that would tend to keep it centered in the bore much like the old apprentice trick of spinning a ball bearing on the end of an air hose.  The actual problem may be that we can't control the spin axis of the shot so it comes out spinning one way one time and another the next.

It may have been an actual problem in the old days of 1/25 windage and erratically performing gunpowder but today's powder is very good stuff, the bores are usually pretty round, and the shot pretty uniform.

[guardsgunner]

GG,
    The old idea on how to shot a smoothbore musket is just that. Say a .674-.678 ball in a .690 barrel. It was thought that the gas would act evenly around the ball going down the cushion.  The tighter fit the gas to go to one side and bounce in down the causing a wider shot group.
    It could be a least partially responsible for the slice.

[Cat Whisperer]

OK - here's some evidence.

What it means is open to interpretation, but  I sumit these two pictures of a golf ball fired recently from a smooth bore 2 caliber long mortar.

The golf ball is marked 440 Nitro 1.  Fitting.  

It went about 20 yards out from a very mild charge of FFFg.

It was blackened on most of one side, (flash brightened it up a bit) in a pattern that is almost round - any variation  from roundness could be explained by the variation of the pattern of dimples.

It, like most cast cannon balls shot by mortars, flew true or maybe a little turning whilst in the air.

There were NO visible marks from scraping the bore where one would expect to find them.

FWIW

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[guardsgunner]

And how bad did it slice?

[CAV Trooper]

There were NO visible marks from scraping the bore where one would expect to find them.

CW,

From the picture it looks like the ball didn't spin at all. At least not when there was flame from burning powder in contact with it.

As to not finding scrape marks from the barrel...I'm not sure that you would in your situation or with any mortar for that matter. With a tube that's only 2 calibers long the ball probably wouldn't touch more than once or maybe twice, if at all.

Since you built it, I gather the bore has a nice smooth finish and as you know, golf balls have a pretty tough hide on them. I don't think it would get scuffed by contact with the steel when fired. I've hacked a bunch of them around a golf course and they usually don't get scuffed by the clubs. Not that I've noticed anyway.

I'd love to try shooting one in a long barrel cannon that has a rough bore and see what happens.