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Calculating recoil

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: Ragnarok » Wed Apr 23, 2008 12:58 pm

@hyldgaard: A 45% co-efficent for a piston valve is perfectly reasonable.

jackssmirkingrevenge wrote:Granted, though in the case of spudguns the recoil of the air alone is going to be negligible compared to what would happen if you had a projectile in the barrel.

You'd be surprised.
Let's take an example. In the case of HEAL, the chamber contains about 18 grams of air, a huge mass compared to firearms, which typically use no more than a few grams.

With a 25g projectile leaving the barrel at 180 m/s, those 18 grams of air will leave the barrel at around 225 m/s (on average).
Believe it or not, that makes the air responsible for a majority of the recoil.

@BTB: When dealing with recoil, it is typical to express it as free recoil energy, rather than velocity or momentum.

For the FRE, you do need to know the cannon mass, as heavier cannons will have a lower FRE. Assuming all else is equal, a cannon with twice the weight will have half the FRE.

In practise, it's more complex than that, but the general rule of thumb still applies.

@Thunderlord: The thread where I discuss the maths can be seen here: http://www.spudfiles.com/forums/topic-t11431.html

While a coilgun certainly has recoil (the camera flash one I have actually has a surprisingly noticeable jerk on firing), my point was that it doesn't have a propellant gas to complicate the recoil calculation, so the momentum of the coilgun will be equal to that of the projectile.

You do however seem to be a little mixed up about magnetism and plasma.

Magnetism is a force, formed either by electric current, or by a majority alignment of magnetic cells in a ferromagnetic material. Plasma is an ionised gas.
Magnetism does affect plasma, but they are separate from each other.

Now, magnetism does have a limited mass. As Einstein's famous E=mc<sup>2</sup> says, energy has mass (although absolutely minuscule amounts). Magnetic fields store energy, so they therefore have some mass.

However, in a coilgun, the mass in these magnetic fields is not accelerated. After a coil has been fired, the energy is converted back into electrical current, which becomes back E.M.F across the coil. Depending on geometry, this energy can then be returned to the capacitors via a half-bridge circuit, or can be discarded in the form of coil heat.

So, for those reasons, the recoil momentum of a coilgun is always equal to the momentum of the projectile.
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Unread postAuthor: jimmy101 » Wed Apr 23, 2008 1:19 pm

THUNDERLORD wrote:BTW a coil gun has recoil just not rearward if it works by pulling the projectile in. As I'm sure you know magnetism is a form of mass(plasma).

I don't think so. A coil gun (electromagnetic solenoid with a ferromagnetic shell) recoils exactly the same way any other type of gun does. Ignoring friction, mass of air in barrel etc (ie., using high school physics);
V<sub>gun</sub>M<sub>gun</sub>=-V<sub>projectile</sub>M<sub>projectile</sub>

Magnetisim has nothing to do with plasma. A magnetic field has zero mass (neglecting the incredibaly small quantum mechanical affects). Plasma is a form of matter and has mass. Coil guns do not use plasma. Combustion spudguns contain small amounts of plasma for very brief periods. Electrical sparks in air are a form of plasma.

EDIT: Ooops, Raga already corrected it.
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Last edited by jimmy101 on Wed Apr 23, 2008 1:21 pm, edited 1 time in total.
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Unread postAuthor: THUNDERLORD » Wed Apr 23, 2008 1:21 pm

@Ragnarok, Thankyou very much for the mathematics link, I got to get ready for work so I'll check it out when I get back tonight.

I need to be more critical about what I write. I meant that magnetism is the same catagory of state of matter as plasma.

It actually goes back to occultist/alchemy (primitive science) the pentagram symbol for example the five points were said to be "Earth, Air, Fire, Water and Spirit.

They weren't as stupid as revisionist historians like to claim. for example, earth is the solid state of matter, water -liquid state, air--gas state fire would be plasma or magnetism for example and spirit well that's a seperate discussion all together.

Just look at a magnet floating above a magnet the actual magnet has mass so the only way scientificaly it can be floating is that the (invisible to the eye)mass of the magnetism of each magnet is equal to half the mass of each solid (earth state matter) magnet. so, Yes, magnetism is mass. But I need to start explaining stuff more exact. 8)
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Unread postAuthor: jimmy101 » Wed Apr 23, 2008 1:26 pm

THUNDERLORD wrote:I need to be more critical about what I write. I meant that magnetism is the same catagory of state of matter as plasma

Nope, wrong again. Plasma and magnetism are not the "same catagory". Magnetism is an electromagnetic field. Plasma is not. Plasma responds to an electromagnetic field but that does not mean that the two are related to each other. Like I said before, plasma has mass, an electromagnetic field does not. That alone says that a magnetic field is not a state of matter since all matter has mass. Einstein's E=mc<sup>2</sup> does not mean that energy of the field has mass, it tells you how much mass you would get if the energy in the field were converted into mass (or vice versa).
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Unread postAuthor: Ragnarok » Wed Apr 23, 2008 1:35 pm

OK, I went off and did the maths, and the estimate for the recoil energy is approximately 105 Joules (77 ft-lbs).

That's roughly equal to the free recoil of a 0.50" cal rifle, and there are some rifles with more FRE than that, so I would suggest that the recoil is not uncontrollable, but it will certainly have a very significant kick.

If it's to be shouldered, you should be looking at a good stock, recoil pad and a decent shooting position.
Roughly 40% of the recoil momentum is due to the air, so a good muzzle brake could be used to turn that to your advantage.
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Unread postAuthor: jimmy101 » Wed Apr 23, 2008 1:36 pm

Ragnarok et al:

Can't you do a reasonably accurate job of calculating recoil energy by just including the mass of air and assuming the air is moving at 1/2 the muzzle velocity? (The average velocity of the gases in the gun is the muzzle velocity over 2 assuming the chamber and barrel have the same diameter.) Add in the frictional force between the ammo and barrel (M is mass, V is velocity)...

-M<sub>gun</sub>V<sub>gun</sub> = M<sub>projectile</sub>V<sub>projectile</sub> + (1/2)M<sub>gas</sub>V<sub>projectile</sub> + F<sub>friction</sub>
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Unread postAuthor: Ragnarok » Wed Apr 23, 2008 1:53 pm

jimmy101 wrote:Can't you do a reasonably accurate job of calculating recoil energy by just including the mass of air and assuming the air is moving at 1/2 the muzzle velocity?

I wouldn't have thought so.

As the eventual vector of the propellant gases is out of the barrel, and in most cases, significantly faster than the muzzle velocity (presumably near the speed of sound at whichever temperature, as there is little else to restrict the gas' speed other than friction), I would have guessed that would have severely underestimated the momentum of the gasses.

The spreadsheet I use takes the total energy available and works out how it should be divided it up between the different sources - friction, gas cooling, projectile energy, gas kinetic energy, and launcher FRE.

It assumes sound, heat and other things are a later form of parts of those energy outputs.
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Unread postAuthor: POLAND_SPUD » Wed Apr 23, 2008 2:49 pm

@hyldgaard check BTB's vids - his gun is more less similar to your design so it would give you a rough idea
http://www.spudfiles.com/forums/overkil ... t8855.html
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Unread postAuthor: THUNDERLORD » Wed Apr 23, 2008 3:05 pm

@jimmy101, how do you explain magnetic propulsion of satelites in space? or the fact that when enough magnetism is applied to any material it can be propelled?
Also you state to "assume the air is moving half velocity of projectiles" the gas propelling the projectile is moving faster in some cases and actualy overtakes the projectiles in some slow speed photos so you are the one who's wrong again.

You can continue to catagorize things how you want and I will continue to catagorize plasma, magnetism and fire together as states of mass.

As for the Einstein stuff I tend to believe he was an average student who paid the smarter students for thier work so he could get through college, go on to work in the PATENT office where he saw some information he plaigarized as well to go on become one of the most famous people in modern history and science.

True genius!!! How's that for revisionalist history?
BTW Thanks for the many electronics references you have given which are very useful.

EDIT: I understand THUNDERLORD may be too long for some of you so you may refer to me as LORDTHUNDER.
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Unread postAuthor: clide » Wed Apr 23, 2008 4:02 pm

Ragnarok wrote:If it's to be shouldered, you should be looking at a good stock, recoil pad and a decent shooting position.


I'm not sure how much recoil it had, but I have seriously injured my shoulder with the kick from one of my guns, so if you do have a powerful pneumatic gun then it is certainly something you should be concerned about. The problem was mostly because the gun had a stock that was never designed for a strong recoil. With a properly padded stock you will probably get knocked on your ass before reaching a power level that will injure your shoulder. If your gun almost knocks you over when you fire it then you should probably consider mounting it on a stand or using less pressure.
As for the Einstein stuff I tend to believe he was an average student who paid the smarter students for thier work so he could get through college, go on to work in the PATENT office where he saw some information he plaigarized as well to go on become one of the most famous people in modern history and science.

If someone tried to patent general relativity then I would almost say they deserve to have the idea stolen :roll:
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Unread postAuthor: Ragnarok » Wed Apr 23, 2008 4:51 pm

@THUNDERLORD: Although there are some things on which Jimmy is not entirely correct, he is certainly one of the forum's more knowledgeable users.

Having read what you've said, I think I can see where your problem lies. I believe you are mixing force and mass.
Mass is not "propulsion". Force is propulsion. Mass and force are distinct - despite the unfortunate nature of the imperial system having pounds as both mass and force.

Now, in what you said earlier, with the two magnets, one floating over the other - for our purposes there is no mass in the magnetic field, but there must be enough FORCE generated by that field to cancel the gravitational force acting on the upper magnet.

It's not particularly easy to explain the difference in words, but in short, Force is the amount of push or pull, Mass is the amount of stuff.

Now - Time to link to Wikipedia.

http://en.wikipedia.org/wiki/Mass
http://en.wikipedia.org/wiki/Force

In order to clear up your misunderstanding over magnetism being mass, try this:

For an experiment, take an iron bar. Weigh it (although this strictly measures gravitational force, in Earth's gravitational field Mass and Force are proportional, so you can presume the measurement is of the mass).
Note the weight. Now find a way to magnetise it - stroke it with another magnet, direct it at the North pole and hit it with a hammer, whatever.
Weigh it again. It will not have gained any mass from being magnetised.

Fire and Plasma are physical stuff, and are thus mass (you can hold them, assuming you don't like your hands) - Magnetism isn't physical stuff, and therefore cannot be considered mass.
I mean, go and try picking up magnetism - not a magnet, but the actual field around it. You simply can't.

I know you want to categorise things your way. I don't know where you are in life, but making the assumption you are still at school (if not, I apologise), the confusion of mass and force will not get you many marks at all in physics tests.
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Unread postAuthor: DYI » Wed Apr 23, 2008 5:46 pm

how do you explain magnetic propulsion of satelites in space? or the fact that when enough magnetism is applied to any material it can be propelled?


Well for one, there's no such thing as a magnetically propelled satelite, unless someone managed to alter the laws of physics without telling us about it. There is, however, such a thing as an ion drive, which propels plasma out the back by accelerating it with a magnetic field. The plasma has mass, not the magnetism.
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Unread postAuthor: Ragnarok » Wed Apr 23, 2008 5:53 pm

DYI wrote:Well for one, there's no such thing as a magnetically propelled satelite, unless someone managed to alter the laws of physics without telling us about it.

Damn those blasted agents, constantly changing the Matrix on the slightest of whims. I mean, I don't mess around with the laws of physics...

*Bends spoon with mind*
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Unread postAuthor: Blitz » Wed Apr 23, 2008 6:05 pm

I knew I felt that deja-vu for a reason. :shock:
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Unread postAuthor: jimmy101 » Wed Apr 23, 2008 6:14 pm

Ragnarok wrote:
jimmy101 wrote:Can't you do a reasonably accurate job of calculating recoil energy by just including the mass of air and assuming the air is moving at 1/2 the muzzle velocity?

I wouldn't have thought so.

As the eventual vector of the propellant gases is out of the barrel, and in most cases, significantly faster than the muzzle velocity (presumably near the speed of sound at whichever temperature, as there is little else to restrict the gas' speed other than friction), I would have guessed that would have severely underestimated the momentum of the gasses.

But isn't only part of the gases ejected from the gun? The gun doesn't end up at zero pressure.

And, doesn't turbulance keep the gas flow significantly subsonic, even after the ammo has left the barrel? I seem to recall Reynolds numbers in the vicinity of 100,000 for a combustion gun. For a pneumatic at say 120 PSIG the Reynolds number would be about 8x higher? In other words, the gas can't get out of it's own way and gas flow would be significantly sub-sonic, not only in terms of wahtever the speed of sound is for the given gun conditions but also in terms of the ~1100 FPS for air outside the gun.

I wonder, what is the pressure and temperature of the gases at exit for a 120 PSIG pneumatic with say a 2:1 C:B. How much do the gases cool off and how much does the speed of sound in the gases drop?
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