SpudFiles

Here are some other methods with useful calculators for CP location of rockets/projectiles and other model rocket calculations.

http://my.execpc.com/~culp/rockets/Barrowman.html


http://www.mrhq.org/software/freeshar/freeshar.shtml


A CG of one caliber ahead of CP is nominal minimum but some times .5 to .75 "just" makes minimal stability.

my guess is that the muzzle brake/air stripper is not large enough

I would say that the paper cones are simply not strong enough to withstand high pressure shots. They deform in the barrel and tumble during flight. I have experienced that in the past as well.

Another possibility is that the cones haven't got the perfect shape or are not properly aligned to the nail. With higher pressure the speed of the projectile increases and also the air which acts onto it.

It is like a car that speeds up and with increasing speed the slightest oversteering can make it tumble.


A good test for cone darts is to spin them between your fingers and you will see if they are aligned properly.

For you AutoCAD guys here, another useful of finding CP/CG is by comparing the COG of a 3D rendering of projectile (CG) with a COG of an outline drawing (CP). Finding COG of a drawn object in AutoCAD use the massprop function.

One thing that gets overlooked in projectile design is overall length...

Normally with most recreational spudguns you're dealing with sub-transonic flows, but for those of us who pllay with highpressures/high velocities, the airflow around the projectile will change, and when you are dealing with 900+ fps projectiles the pressure wave can envelope the fins/tailcone/tail in a "vacuum" in essence negating any drag stabilization designed into the round... It doesn't take mach+ speeds to create these "vacuum pockets", depending on the shape of the nose it can actually occur at even well below transonic speeds...

But basically, at transonic speeds and higher you need the projectile to be shaped in such a way that the fins/cones/etc are either wide enough or long enough to actually reach outside the envelope...

jeepkahn wrote:...when you are dealing with 900+ fps projectiles the pressure wave can envelope the fins/tailcone/tail in a "vacuum" in essence negating any drag stabilization designed into the round...

Although it can cause some issues, with the right projectile design, you can actually use this to your advantage - although, more at longer ranges.

In a similar method to supercavitating torpedoes, if the majority of your drag is skin friction (and in a lot of darts, it will be), if this "vacuum pocket" shields the majority of the side of the dart, you can save quite heavily on drag.

Of course, in air, if the vacuum pocket covers the tail of the projectile, you'll get extra drag as a result of a pressure difference between the nose and the tail of the projectile.

The trick is to balance out nose drag against skin drag, but still keep the fins doing their job - at whatever your chosen velocity is.
This is obviously quite a lot of number crunching, but you can end up with velocity coefficients that look like 99.99% at the end of it.

You can then have further complication between trying to match drag efficiency against penetrative ability.
For longer ranges, it may be worth having a projectile with a destructible aerodynamic nose, which on impact breaks apart to reveal a nose better shaped for penetration.

Anyway, I've got to go shopping, so I'll ramble more later.

Ragnarok wrote:You can then have further complication between trying to match drag efficiency against penetrative ability.
For longer ranges, it may be worth having a projectile with a destructible aerodynamic nose, which on impact breaks apart to reveal a nose better shaped for penetration.

Bindun

Anyway, I've got to go shopping, so I'll ramble more later.

I hope it's either stout or copper fittings

Hmm.. The idea of a breakable aerodynamic nose is intriguing

jackssmirkingrevenge wrote:Bindun

I didn't say it hadn't. Any real projectile idea that you can propose for spudgunning has already been thought of in ballistics before.

The fact that it has been done doesn't however stop it being worth trying here.

The question remains of whether such a nose could offer enough improvement that enough energy is saved from improved aerodynamic or penetration efficiency to still win out after you subtract the energy needed to break the first nose apart.
As I hope to be losing less than 10% of the muzzle energy over 400 metres even with a dual-purpose nose anyway, my guess is any improvement will come more from a more optimally shaped penetrator than from better aerodynamics.

A further issue arises depending on whether I'm happy to completely mono-task the projectile design with the dual nose set-up. Improving it versus the steel target I'm planning on will negatively affect its performance versus other targets.

The design brief doesn't say it has to work well against other targets, but it's already shaping up as being capable of trouncing what the original design brief said anyway. Originally, the plan was for 6mm steel at 400 metres. If you could hit at 800m (the sight arrangement wouldn't actually allow it though), it should be capable of going through 12mm.

Hmm. Perhaps I've got too into this project.

I hope it's either stout or copper fittings

Afraid not. Assorted foodstuffs before the shops are cleared out (the current weather conditions mean that deliveries to the stores will be affected).

This dart has been tested to 300 psi.

It takes about 30 seconds to make.

Close cell foam 3/8" cut with sharpened bicycle frame tubing.

Push point of 10 nail through a disk and slide it up to the head.

Place some hot glue on the nail head and stick on another disk.

I like the way it breech loads and seals.

I'm merging all your dart threads together. Keep up the good work stu.

boyntonstu wrote:This dart has been tested to 300 psi.

It takes about 30 seconds to make.

Close cell foam 3/8" cut with sharpened bicycle frame tubing.

Push point of 10 nail through a disk and slide it up to the head.

Place some hot glue on the nail head and stick on another disk.

I like the way it breech loads and seals.

it would be nice if you could cover the entire nail with that foam and then shape it into a bullet using some kind of technique... could you melt the foam to shape it?
btw. very nice idea with the foam I never thought of that but my paper nail dart cones also take 30 sec to build a piece and they handle the 150psi I throw at em every day with ease

FighterAce wrote:

boyntonstu wrote:This dart has been tested to 300 psi.

It takes about 30 seconds to make.

Close cell foam 3/8" cut with sharpened bicycle frame tubing.

Push point of 10 nail through a disk and slide it up to the head.

Place some hot glue on the nail head and stick on another disk.

I like the way it breech loads and seals.

it would be nice if you could cover the entire nail with that foam and then shape it into a bullet using some kind of technique... could you melt the foam to shape it?
btw. very nice idea with the foam I never thought of that but my paper nail dart cones also take 30 sec to build a piece and they handle the 150psi I throw at em every day with ease

I copied the concept from Theairgunman who uses plastic 12 Gauge shotshell wads.

Unfortunately, 12 Gauge wads are too small for 3/4" type L copper tubing.

These foam wads compress to fit and yet they slide easily.

A few other ideas:

Use 14 Gauge shotshell hulls.

Place a small paper nose cone in front of the foam disks for a more aero shape.

Use a stepped set of increasing diameter foam disks to approximate a cone.

My goal is 550 psi.

What was the max pressure that you shot your beautiful nail darts?


Hint: I tried your fold and roll technique with heavier than index card stock.

It would break and crack when tightly folded.

Solution: Spray on a little water on the first fold to soften it for folding.


Works like a charm!

Keep up the good work!

BoyntonStu

boyntonstu wrote:

FighterAce wrote:

boyntonstu wrote:This dart has been tested to 300 psi.

It takes about 30 seconds to make.

Close cell foam 3/8" cut with sharpened bicycle frame tubing.

Push point of 10 nail through a disk and slide it up to the head.

Place some hot glue on the nail head and stick on another disk.

I like the way it breech loads and seals.

it would be nice if you could cover the entire nail with that foam and then shape it into a bullet using some kind of technique... could you melt the foam to shape it?
btw. very nice idea with the foam I never thought of that but my paper nail dart cones also take 30 sec to build a piece and they handle the 150psi I throw at em every day with ease

I copied the concept from Theairgunman who uses plastic 12 Gauge shotshell wads.

Unfortunately, 12 Gauge wads are too small for 3/4" type L copper tubing.

These foam wads compress to fit and yet they slide easily.

A few other ideas:

Use 14 Gauge shotshell hulls.

Place a small paper nose cone in front of the foam disks for a more aero shape.

Use a stepped set of increasing diameter foam disks to approximate a cone.

My goal is 550 psi.

What was the max pressure that you shot your beautiful nail darts?


Hint: I tried your fold and roll technique with heavier than index card stock.

It would break and crack when tightly folded.

Solution: Spray on a little water on the first fold to soften it for folding.


Works like a charm!

Keep up the good work!

BoyntonStu

Unfortunately my pump only goes up to 150 psi and I have no way of testing it at higher pressure. But I can tell you I'm very impressed how they handle it. Recently I gave the templates for it to my friend who also has an air gun. His cones fell apart in mid air so I guess there's a lot to do with your technique of making them. Its not just fold, glue, done.

btw. good idea with the water but how does the glue hold wet paper?

FighterAce wrote:Unfortunately my pump only goes up to 150 psi and I have no way of testing it at higher pressure. But I can tell you I'm very impressed how they handle it. Recently I gave the templates for it to my friend who also has an air gun. His cones fell apart in mid air so I guess there's a lot to do with your technique of making them. Its not just fold, glue, done.

btw. good idea with the water but how does the glue hold wet paper?

I use hot melt glue many layers away from the thin damp area without difficulty.

Just spray on a fine mist an wipe off. Mask areas to remain dry.

Damp is good, wet is too much.