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projectile testing - any volunteers?

A place to ask general spud cannon related questions.
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Unread postAuthor: dewey-1 » Thu Oct 13, 2011 5:26 pm

JSR

Where is the actual balancing point?
Measured from nosecone tip.

Maybe due to the shape of the tail end, there will not be enough drag for stabilization. Adding fins would help.

Like others stated, hollow out the body and try again.
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Unread postAuthor: saefroch » Thu Oct 13, 2011 6:28 pm

If you still want to go finless, you'll probably need to change the tail, since you've already pushed the density of the nose as high as is reasonable, though you could shorten the nose a bit, the cylindrical section could have been removed.

How are you attaching the two? Does it have something like a tang? That could be screwing with the balance point a little, replacing such a thing with epoxy might help.

Your Delrin tail (I think) was poorly chosen. I'd go for a UHMWPE, lower friction (.25 vs. .12) and only two-thirds of the density. McMaster sells UHMWPE rods, but I think all their stock is on a roll, so it'll be sightly curved. Heat-treatment while chucked into lathe to straighten it?

If that doesn't work, I suggest a tungsten head.
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Unread postAuthor: POLAND_SPUD » Thu Oct 13, 2011 7:48 pm

I suggest a tungsten head
Why not DU?

he's already FUBAR I don't think that some radiation can hurt him
:D


Personally I'd choose lead... not sure if can be lathed but he can always make a form for it
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Unread postAuthor: saefroch » Thu Oct 13, 2011 7:56 pm

POLAND_SPUD wrote:
I suggest a tungsten head
Why not DU?

he's already FUBAR I don't think that some radiation can hurt him
:D


Personally I'd choose lead... not sure if can be lathed but he can always make a form for it


Because Tungsten's more dense than both :D
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Unread postAuthor: POLAND_SPUD » Thu Oct 13, 2011 8:03 pm

Because Tungsten's more dense than both
Not sure if you noticed but this whole purpose of this thread is to design a cheap and simple low drag ammo

I don't think tungsten is cheap and probably it isn't easily machinable
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Unread postAuthor: velocity3x » Thu Oct 13, 2011 8:10 pm

jackssmirkingrevenge wrote:The inset shows the impact hole in a sheet of cardboard 10 metres away from the muzzle, I shot this out a 300 psi 6mm piston launcher. It was clearly travelling sideways when it hit, this is not looking like it will work.


Could it be that 10 meters doesn't allow enough distance / time for the projectile to stabilize in flight?
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Unread postAuthor: saefroch » Thu Oct 13, 2011 8:38 pm

POLAND_SPUD wrote:
Because Tungsten's more dense than both
Not sure if you noticed but this whole purpose of this thread is to design a cheap and simple low drag ammo

I don't think tungsten is cheap and probably it isn't easily machinable
Prices should be right here Tungsten appears cheaper than uranium... or you could look here and here.
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Unread postAuthor: Moonbogg » Thu Oct 13, 2011 9:11 pm

Make a sureshot round and add a pointy nose?
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 14, 2011 12:00 am

POLAND_SPUD wrote:isn't it what we what to achieve ??

Image


Pretty much, why such rounds haven't replaced buckshot is beyond me!

and this (a 16mm drag stabilised rocket projectile)


Looks like a modern take on the Gyrojet, though really the latter was spin stabilised by angling the thrust from the rocket to rotate the projectile about its axis of flight like a rifled bullet.

Where is the actual balancing point?
Measured from nosecone tip.


Found the nose cone but not the tail :-/ should have measured it before firing.

How are you attaching the two? Does it have something like a tang? That could be screwing with the balance point a little, replacing such a thing with epoxy might help.


Friction fit but the "tang" is very short anyway.

Your Delrin tail (I think) was poorly chosen. I'd go for a UHMWPE


Sadly I have been unable to find a local source for UHMW and getting it in reasonable quantities from elsewhere is prohibitive, though I might be on the verge of getting a little help with that :) in any case, a hollow tail is a better proposition.

I don't think tungsten is cheap and probably it isn't easily machinable


"probably"? Definitely! Not too keen on machining lead either, can't be healthy. One option could be to drill out the brass nose cone and hammer in *cough* cold swage some old airgun pellets.

Could it be that 10 meters doesn't allow enough distance / time for the projectile to stabilize in flight?


It was clearly travelling sideways, if it's fishtailing so much I doubt it would have recovered after a longer distance.

Make a sureshot round and add a pointy nose?


They work on the same principle, what I'm trying to do is make it more aerodynamic, so the tail shape is of more significance than a pointy or round nose. Clearly a weighted tube works, but the abrupt shoulder at the end isn't good for drag - on the other hand, reducing this drag might remove the stabilisiation benefit as dewey alluded to earlier.

edit: found it, the CG is well forward of the CP, but clearly not enough.
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 14, 2011 1:48 am

Update: made a quick brass nosecone mounted in a length of aluminium tube and the result is dramatically different, this time the projectile was clearly traveling dead straight.

Next step is to make a very light (hollowed out balsa wood to change the shape without changing the CG too much) conical tail section and see if I can get the same results.
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Unread postAuthor: Labtecpower » Fri Oct 14, 2011 2:04 am

Strange enough this thing flew pretty straight;

[youtube]http://www.youtube.com/watch?v=yxpdd9s-wAw[/youtube]

The holes it made in the plywood were perfectly round.
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Unread postAuthor: Brian the brain » Fri Oct 14, 2011 4:12 am

Had the same result with a can of energydrink.
If it travels fast enough it will not have the time to tumble over that kind of a distance.

This does not mean the projectile is stabilised during flight.

It does add to the discussion though.
Speed itself is an important factor in accuracy.

Great result JSR.
Although that really is a short arrow..not a " round" ...
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Unread postAuthor: dewey-1 » Fri Oct 14, 2011 4:13 am

JSR;

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

If you take the picture overhead such that it appears like a drawing, I can measure the CG point if I know the lengths of each.

The results of the hollow tube (assuming it is the same length as the Delrin one), appears that more drag is induced therefore stabilization.

From the pictures, the CG points look to be very similar.
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Unread postAuthor: saefroch » Fri Oct 14, 2011 6:54 am

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...

Can Tungsten be machined using ceramic machining tools? I'm thinking high-alumina and silica composite or the like.

Wait... you can't purchase from McMaster?
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Unread postAuthor: POLAND_SPUD » Fri Oct 14, 2011 7:09 am

Looks like a modern take on the Gyrojet, though really the latter was spin stabilised by angling the thrust from the rocket to rotate the projectile about its axis of flight like a rifled bullet
Nahh
Read the article - they state explicitly that it is drag stabilised


Oh a success ! :D
nice work

conical tail section and see if I can get the same results
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
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