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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: jimmy101 » Wed Apr 23, 2008 6:35 pm

I'll refrain from getting into too much of a pissing contest with a git but this should be fun ...
THUNDERLORD wrote: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.

This assumption is based on the condition of the gun when the ammo exits the barrel. At ammo exit, the average velocity of the gases is basically 1/2 the muzzle velocity. Why? Because the gases directly behind the ammo can not be moving any faster than the ammo. The gases at the breech end of the gun are not moving at all. Neglecting things like friction, temperature gradients etc., the velocity of the gases is linear with the distance from the breech (for a gun of constant chamber/barrel diameter like a real rifle). Since velocity is linear with distance then the average velocity of the gases at ammo exiting the barrel is half the muzzle velocity.

For a momentum calculation all you need is the average velocity of the object. Very basic physics and probably more than accurate enough for most spudguns.

1) Einstien plagarized things he saw in a patent applications for his various theories? Care to find the patents the he copied from? Having an opinion on something is one thing, pulling stuff like "he plagarized" out'a your ass is just a sign of ignorance.

2) Einstien was not an average student by any stretch of the imagination. He published his first paper before he entered college. How many scientific paper did you publish while in High School?
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Unread postAuthor: VH_man » Wed Apr 23, 2008 7:05 pm

Hmm... Rag i have an intersting concept for you.......

say i have a 15lb launcher (use www.convertit.com if you dont feel like converting it) and i want to have enough kick for me to feel it when firing a marble...... what pvelocity do i have to get it up to (if you need the pressure too i can try to figure out that value once i have the velocity)? (a marble is about 4-5g)

if you cant do it thats fine.... im just kind of wanting a cannon that goes BLAM and gives your shoulder a nice jolt, and doing things scientificaly first is always fun.
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Unread postAuthor: Ragnarok » Wed Apr 23, 2008 7:49 pm

jimmy101 wrote:This assumption is based on the condition of the gun when the ammo exits the barrel.

Yes, but the recoil of the gasses will occur mostly after the projectile has left the barrel.
What you've said is good for if you were trying to get an idea of how any recoil would affect the projectile's velocity and vector - given that the muzzle blast won't have yet contributed to the recoil energy while the projectile remains in the barrel.

But for an idea of the actual FRE, then a different approach is needed.

In fact, here's a thought - with an under-over, the recoil of the gases while a projectile is still in the barrel, although not completely nullified, will partly be negated by the opposite flow of gas within the chamber.

Presumably then, this might make an under-over intrinsically more accurate than an inline launcher - not to mention the possibility of connecting the barrel to the chamber to hold it steady.

@VHman: I can answer that one quite easily. HEAL will kick quite firmly when firing a 7 gram ~11/16" marble (but mostly due to the mass of the gasses), coming back rather harder than I remember the L98 Cadet rifle (essentially a manually cycled SA80, so in NATO 5.56) ever doing.

That's at around 900 fps and 300 psi, with a similar weight to the launcher you're proposing - it weighs around 6 kilos (13 lbs).

So I guess that a launcher of a moderate size at around 300 psi would create a decent kick.
If that's infeasibly high, no worries, cause HEAL is still a something of a kicker at ~150 psi blank fires.

Really, with a decent valve, and enough pressure, as long as the launcher isn't over-heavy, you should get the results you want.
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Unread postAuthor: jackssmirkingrevenge » Wed Apr 23, 2008 11:28 pm

From experience, blank shots recoil far, far less than those with a projectile thrown in. Your theory holds true of course (otherwise, these wouldn't work at all) but I can't say that I've felt double the recoil with a projectile compared to a blank shot. It's subjective, of course, but that's where we get the notion of "felt recoil" from.
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Unread postAuthor: dongfang » Thu Apr 24, 2008 8:10 am

EDIT: Hmm, someone else (Jack) already suggested this.
EDIT: Just ignore this, then...


Forgive me, but is this not rather simple:

The momentum of the gun and that of the projectile are equal in magnitude and opposite in direction.

That is:

m_gun * v_gun = m_proj * v_proj

Knowing the masses of your gun and projectile, and the muzzle velocity, you can calculate:

v_gun = (m_proj * v_proj) / m_gun

Your can then calculate the kinetic energy of the gun - if you really care. I'd say, knowing the velocity is good enough really.

(or - make the same calculations for a #12 shotgun, compare).

If you want an upper-bound estimate of the influence (on recoil) of the air propelled forward, find the mass of the compressed air in the gun and add that to the projectile mass.

About momentum:

I use "conservation of linear momentum", considering gun and projectile a closed system.

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