Bullet RPM ?

PineCone

PineCone

New member
If one shoots a bullet straight up into the air and it leaves the muzzle, 3000ft/sec., 8 twist, spinning at, lets say, 270,000 RPM. At some point it reaches zero velocity and starts coming back down.
How fast does the RPM slow down ?
When does it stop spinning ?
Is it still spinning when it reaches max altitude ?
Is it still spinning when it reaches the ground ?
Just wondering.
Thanks.

PC
 
Myth busters did somthing like this but could not get bullets to go STRAIT up...assuming that you could the bullet would essentially stall out at the end of its climb and even though it should in theory still be spinning it would not fall strait backwards and would attempt to flip over. The bullet would not do a perfect 180 degree turn and so would be tumbling when it hit the ground.
Again this is all theory to me and I have like zero scientific facts for you, but that seems right to me
DaFunk
 
Yup, once it reaches it max height it tumbles back to earth, penetration was about 6-9" or so when it hit dirt with the 9mm and rifles they used.
 
Originally Posted By: Boeydafunk......it should in theory still be spinning it would not fall strait backwards and would attempt to flip over. The bullet would not do a perfect 180 degree turn and so would be tumbling when it hit the ground.


But if it is still spinning fast enough to stabilize it, even a little bit, who knows how it would "tumble".
So my question is still unanswered.
Does it stop spinning before max altitude?
Does it stop spinning before groud impact?
Does it stop spinning at groud impact?
Of course the spin slows down when it leaves the muzzle, but by how much ?

Originally Posted By: 5spd....tumbles back to earth, penetration was about 6-9" or so when it hit dirt with the 9mm and rifles they used.

6-9" is pretty good penetration considering the bullet starts at zero velocity.
Makes me think it was pretty stable at impact, even if it didn't have much downward velocity.
Thanks.

PC
 
Originally Posted By: 5spdYup, once it reaches it max height it tumbles back to earth, penetration was about 6-9" or so when it hit dirt with the 9mm and rifles they used.

Wasn't it more like 2" too 3" or less penetration in the ground?

DAB
 
Originally Posted By: DAB
Wasn't it more like 2" too 3" or less penetration in the ground?


9" does seem like alot.
It's hard to penetrate that much gravel my range backstop.

PC
 
Im prolly waaay off, i usually am, but wouldnt it be kind of like a football spinning up and then come back down?..


hmmm.... as i typed that i realized that if it was still spinning when gravity got hold of it and started bringing it back down it would stableize untill it lost its spin... but falling like that would drag stop its spinning? if the bullet was heavyer in the tail woulnt it just fall like a rain drop? did i answer any questions? or did i figure myself back to the start? or just make more? i think i have lost myself....

OK OK OK...

Sooooo.... following the football logic.... when you fire a bullet parallel to the ground when would it stop spinning... waaaay after impact i would guess. soo will gravity stop and start the bullet back down if you shot straight in the air. i guess to answer that we would need to know at what rate the bullet loses spin vs. the rate it loses upward ennergy and if the spin wins it will prolly spin on the way back down... like the football as it drops back down...


I DONT KNOW!!!!!!!!!!!
 
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The bullet won't stop spinning till it hits the ground. There isn't enough drag to absorb the rotational energy in the limited time available. The rotation isn't affected by anything but time and circumfrential drag. The bullet can be completely stopped in vertical motion at the moment it reaches the top of it's vertical trajectory and it will still be spinning because there is nothing to stop it.

The bullet will not fly point first coming back down, at least I wouldn't think it would because there isn't enough force or velocity to turn it around with the rigid gyroscopic axis. The bullet only flies point first leaving the muzzle because it is pointed that way and the spin tends to keep it moving with the nose pointed into the oncoming air (more or less, it isn't moving exactly nose first for several reasons but this isn't the place to explain all that).

It may also tumble because it may not have enough spin to be stable when it is subsonic, especially if it is a long nose rifle bullet. Some long nose rifle bullets are not stable when they go subsonic even though they are still spinning as Brian Litz points out in his book "Applied Ballistics For Long Range Shooting".

It probably won't come straight back down either unless it is perfectly windless which almost never happens over that much vertical height.

BTW: Brian Litz's book is outstanding. The easiest to understand and most useful book I've read on the subject.

I didn't see the program mentioned.

Fitch
 
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Originally Posted By: Fitch
....bullet can be completely stopped in vertical motion at the moment it reaches the top of it's vertical trajectory and it will still be spinning......

....The bullet will not fly point first coming back down, at least I wouldn't think it would because there isn't enough force or velocity to turn it around with the rigid gyroscopic axis
Fitch

That's what I was thinking....It starts "backing down"...still spinning.
Can't flip too easily because of the gyro effect.
But my question remains: How fast is it still spinning ?
I suppose a bullet with a larger diameter would have more momentum and stay spinning longer if it started at the same RPM.
I'm thinking about a top spinning on the floor....spins for a long time.

PC
 
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