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projectile testing - any volunteers?

A place to ask general spud cannon related questions.
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Unread postAuthor: Petitlu » Fri Oct 14, 2011 7:33 am

Here the ammunition which I fire with my PAC 3000psi cal 50 (12.7mm) and cal 16mm
The gun in 16mm not being striped I did not do that:

Ammunition very stable, skirt aluminum and point steel

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Unread postAuthor: saefroch » Fri Oct 14, 2011 9:15 am

Those look amazing. Are the ones with a longer tail section notably more stable?

And I notice that JSR's version two with the aluminium body/tail has a shorter nose/head than the Delrin version. That may have been what made the biggest difference, especially if the Al has a greater linear mass density than a Delrin cylinder of the same dimensions.
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Unread postAuthor: Petitlu » Fri Oct 14, 2011 9:24 am

In 16mm:
overall length: 80mm
Length skirt aluminum: 70mm
threaded rod steel: 30mm of which 20mm inside
Weight: 35g

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Unread postAuthor: Brian the brain » Fri Oct 14, 2011 9:45 am

The gun in 16mm not being striped I did not do that:


Striped = Rifled

That is where the name " rifle" comes from.


Seems like the quest is over...but again..those are shot at pretty high velocity.

What was the distance to the target?
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: Petitlu » Fri Oct 14, 2011 10:05 am

My 16mm is a tube out of copper and thus not striped
the target was with 30m
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 14, 2011 10:12 am

The holes it made in the plywood were perfectly round.


As Brian mentioned, high velocity does mean the projectile has less time to tumble before it hits the target, also worth nothing that plywood is a relatively hard target so other forces come into play, see the attached segment from one of my artillery books.

Although that really is a short arrow..not a " round" ...


The cross section is round :D

Next time post a picture of both at the same time.
That way we can observe the difference in the CG points.


Taking the maximum diameter of all projectiles being 6mm as a reference the dimensions can be worked out from the photos, I tried to take more vertical shots now.

What are the dimensions on that aluminium rod? How does the linear mass density compare to that of UHMWPE or Delrin? I think you'd want to continue trying to use plastics; I'm pretty sure aluminium rods (hollow or not) will get pricey...


6mm OD 5mm ID, I'm just using parts from my scrap box - remember this is just to play with projectile shapes, not meant for production.

Read the article - they state explicitly that it is drag stabilised


I was referring to the Gyrojet ;P

That will reduce base drag (which will shift the CP forward) and add weight to the back (which will shift CG to the back) so it can f### things up


That's pretty much what happened, read on. Duane postulated that the muzzle blast might be deflecting the conical tailed projectiles, I intend to try using a wad and see if it makes a difference.

Here the ammunition which I fire with my PAC 3000psi cal 50 (12.7mm) and cal 16mm


Very nice!

Seems like the quest is over...but again..those are shot at pretty high velocity.


Well we already know that a tube capped and weighted on one end makes a stable projectile - my contention is it's not very aerodynamic, but what I've found so far is something that maybe should have been obvious from the beginning - reducing drag also reduces the efficiency of drag stabilisation :roll:
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Attachments
CIMG3595.JPG
The mechanism of penetration *chuckle*
CIMG3592.JPG
balsa tail fitted, didn't change CG much but the projectile hit sideways at 10m
CIMG3608.JPG
lead weight in the nose and fins created by crimping the end of the tube in a 3 jaw chuck, flew straight
CIMG3606.JPG
lead weight in the nose, conical delrin tail, hit sideways at 10m
CIMG3604.JPG
original "special" projectile with shorter and hollowed out body, hit sideways at 10m
CIMG3599.JPG
lead weight in the nose, tail packed with balsa wood and filed to a conical section, hit sideways at 10m
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Unread postAuthor: Labtecpower » Fri Oct 14, 2011 10:18 am

As Brian mentioned, high velocity does mean the projectile has less time to tumble before it hits the target, also worth nothing that plywood is a relatively hard target so other forces come into play, see the attached segment from one of my artillery books.


The velocity was about 90-100 meters per second. Does that classify as high?

How would I be able to see if it tumbles? shooting it at a lower pressure into a sandbag?

Edit: Hehehe 666 posts :P
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 14, 2011 10:21 am

Labtecpower wrote:How would I be able to see if it tumbles? shooting it at a lower pressure into a sandbag?


Shoot it at a sheet of cardboard a bit further away, far enough for the muzzle blast not to influence the target.
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Unread postAuthor: Labtecpower » Fri Oct 14, 2011 10:23 am

Ok, I think i'm trying that in a while. Not going to do so in my backyard, too afraid I miss it and hit our wooden fence instead, pissing off my parents :wink:
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Unread postAuthor: Petitlu » Fri Oct 14, 2011 10:26 am

There I prepare balls style “brenneke” but without the wings which are useless for stability


But a rifled bore that changes all! :D
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Unread postAuthor: Labtecpower » Fri Oct 14, 2011 10:36 am

COG test:

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Unread postAuthor: al-xg » Fri Oct 14, 2011 10:39 am

Seems like the quest is over.


The quest for what though... These projectiles may be stable but probably have reasonably high drag too, not necessarily any good for long range.


The Delrin tail projectiles are probably upset by muzzle blast as they are only kept centred in the barrel near the tip at full bore diameter, also there is a more likely chance off pressure effects from muzzle blast acting orthogonally to the flight path.

Our estimated CPs are slightly optimistic, as the centroid method works for objects extruded straight out from the studied section generally having a higher drag than the real projectile. Although that empirical 1.5 calibers probably takes that into acount.

Edit: d'oh way too slow...
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Last edited by al-xg on Fri Oct 14, 2011 11:02 am, edited 1 time in total.
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Unread postAuthor: dewey-1 » Fri Oct 14, 2011 10:42 am

Petitlu wrote:In 16mm:
overall length: 80mm
Length skirt aluminum: 70mm
threaded rod steel: 30mm of which 20mm inside
Weight: 35g


Very nice ammo.

What is the thread size and pitch?
What is the wall thickness on aluminum tube?
Where is the center of gravity of this ammo?
I am guessing about 15 mm from nose tip.
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Unread postAuthor: saefroch » Fri Oct 14, 2011 11:28 am

Some math ahead of time would have been nice for this experiment :P, Could have saved JSR a log of machining.

Linear mass density of the Al rod: .0234g/mm
Linear mass density of the Delrin: .04g/mm
Linear mass density of UHMWPE: .027g/mm

The hollow aluminium rod clearly wins, especially with the crimped fins. I don't see too much use looking into other designs, unless you want to try a hollowed-out UHMWPE or balsa boattail plug in the Al to avoid fins.
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Last edited by saefroch on Fri Oct 14, 2011 11:30 am, edited 1 time in total.
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Unread postAuthor: al-xg » Fri Oct 14, 2011 11:30 am

reducing drag also reduces the efficiency of drag stabilisation


That's why fins are good, very little drag while flying straight but high drag if the projectile wobbles off axis.

A drag stabilised projectile doesn't have to be draggy the whole time.
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