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"In the world of spuds today"

A place to ask general spud cannon related questions.
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Unread postAuthor: elitesniper » Thu Mar 13, 2008 9:46 pm

edit: daberno thank you so much for that idea i will be sure to try that this weekend.
And yes GGDT does help a lot I have discovered that with a 50ft 6mm barrel on the PG I can break 2500fps, no typo.

nice~! i wonder what that will do on my big blue :) but where will we find break line that long? hehe :D
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Unread postAuthor: bigbob12345 » Thu Mar 13, 2008 10:03 pm

never mind its actually 1900fps I accedently made the chamber volume more than it is but if you used helium you would reach 2800fps :twisted:
This is all with a .12 gram aisoft bb in a 50ft 6mm barrel at 100psi.
but you could probably get the 2500fps with your big blue as it has more chamber volume.
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Unread postAuthor: Ragnarok » Fri Mar 14, 2008 8:18 am

POLAND_SPUD wrote:rag I think that it would be really great if you could add a simple Cd calculator to this.... Cd is just way too important for ballistics... it's possibly the least known variable for all of us

Well, a Cd calculator is quite a difficult thing. However, there is going to be a table of Drag coefficients for common shapes, and given that similar shapes tend to have sufficently similar drag coefficients for our purposes, that should easily cover most situations.

iknowmy3tables wrote:cool launch program, only if there were a chamber pressure variable.

This is an external ballistics calculator, not an internal ballistics one.
For now, it's designed to work hand in hand with GGDT and EVBEC, with a muzzle velocity calculated in another program - or rather better, actually measured in real life - and calculate a range from that information, rather than have me bite off more than I can chew.

Not to say I'll never make an internal ballistics tool, but for now, GGDT does a very good job of that under most circumstances - the obvious exception being discussed below.

@bigbob12345: Not a typo, but another mistake I'm afraid.
GGDT is known to give rather... generous, shall we say... results for projectiles that reach or exceed the sound barrier.

It's hard to break the speed of sound in a gas, and impossible to break the particle speed. In air, the speed of sound is around 1120 fps, depending on temperature, humidity, and other factors.

You need a lot of brute force for the job, and therefore, air powered cannons, even one using pressures of 300 or 400 psi, have severe trouble breaking 1000 fps.
Getting to around 1100 is near impossible (although JSR did once manage it), and going past that is going to get even harder.

As the average particle speed in air is somewhere between 1600 and 1700 fps depending on the temperature, and it's not actually possible for a gas, regardless of it's pressure, to accelerate something past it's particle speed... which makes the 1900 fps figure you have completely erroneous.

As the particle speed is approached, you lose effective pressure, and thus acceleration, which makes it hard to even get to 75% of the particle speed (and that will only happen after you've dealt with the first opponent of the sound barrier)
1200 fps might just be achievable with high pressures, a light projectile, a low friction barrel and a REALLY powerful valve.
But given the trouble with the sound barrier, seldom will I believe a velocity claim of more than about 90% of the SOS from GGDT, and even then I normally just round back down to around 90%.

In your case, I think you should be happy with achieving over 900 fps with 100 psi, regardless of barrel length. Getting over 1000 fps is the region of brute force cannons using 3 or 4 times that pressure.
However, with helium, 2800 fps is potentially achievable, as that's not much over 90% of the SOS in helium (around 3000 fps) - but you'll still need more than 100 psi.

You should also bear in mind, if you're using CO2 in your KAMG, that's going to blunt performance even more than with air. The speed of sound in CO2 (at room temperature, let alone when cooled) is only 900 fps, and based on the gas dynamics of CO2, you're not going be be capable of getting half as close to the sound barrier in air as an air powered launcher is capable of.
CO2 is not a gas that's suitable for breaking the sound barrier.
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Unread postAuthor: bigbob12345 » Fri Mar 14, 2008 8:30 am

Oh, Okay.
but if it says 2500fps with air in a launcher I plan to build is it possible that it would make it over the sound barrier because it is so high over the sound barrier. Or is it so generous that it would be below the SOS?

And for the CO2 in the KAMG ill just be using that most of the time for all the shots were Im really going for something(SOS, penetrating 1/4in steel) I will take 20min to pump it up with my high pressure pump I am working on.
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Unread postAuthor: Jared Haehnel » Fri Mar 14, 2008 8:34 am

As the average particle speed in air is somewhere between 1600 and 1700 fps depending on the temperature, and it's not actually possible for a gas, regardless of it's pressure, to accelerate something past it's particle speed... which makes the 1900 fps figure you have completely erroneous.


To quote Rag, probably not because the air doesn't expand that fast. You would have to use a very light object to match the speed of the expanding air.
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Unread postAuthor: Ragnarok » Fri Mar 14, 2008 9:15 am

bigbob12345 wrote:Oh, Okay.

It's a mistake that arised when D_Hall tried to improve the algorithm to cope with supersonic PCP airguns. Unfortunately, it didn't help regular launchers get a good result.
Earlier versions of GGDT can give better results around that region.

but if it says 2500fps with air in a launcher I plan to build is it possible that it would make it over the sound barrier because it is so high over the sound barrier. Or is it so generous that it would be below the SOS?

There are so many variables which affect it.
However, I have to be honest, you're more likely to be looking at a figure in the 900-1000 fps range, which is a little way south of the sound barrier.
GGDT predicted a 2100 fps figure for a super light projectile I had, and when I tried it in the real world, that wasn't quite capable of breaking the sound barrier either (but it was within a walking pace of it) - and I was using 300 psi, not 100, so there was a lot more brute force around to try to defeat the sound barrier with, and it still wasn't enough.

Often longer barrels can actually harm the chances of a supersonic muzzle velocity. Pressure effects ahead of the barrel are quite significant at those speeds, and as the pressure behind the projectile falls (which with a moderately low C:B ratio is quite significant) you can actually see more deceleration in the barrel than friction alone might suggest - even more than the projectile might suffer from drag outside that barrel in some cases - because the air has to be pushed forwards and out of the barrel, rather than just sideways around the projectile, which requires a lot more force.

It's not totally impossible to break the sound barrier with room temperature air, but it is at the least difficult.
You shouldn't get hung up on breaking the sound barrier. It's not that important in the long run, as you'll have to use a projectile so light it has nearly no kinetic energy, and which will also slow down so fast it has very little range.
Concentrate more on projectiles that go quite fast but have enough weight to do some damage.

To add a metaphor (or whatever it's called) - Think of it a bit like a girlfriend.
You could choose an attractive airheaded bimbo who hasn't ever had to develop a personality or a sense of humour, just relying on her looks - or a slightly less attractive girl who's actually got some brains, a sense of humour and a great personality.
You'd soon get bored with the airhead (or at least, I would), but the other girl might well be one you'd be happy to live with for the rest of your life.

Same here. A supersonic BB will soon grow boring, but a slower projectile with a bit more weight behind it and which can do some damage - that could remain fun for ever.

And for the CO2 in the KAMG ill just be using that most of the time for all the shots were Im really going for something(SOS, penetrating 1/4in steel) I will take 20min to pump it up with my high pressure pump I am working on.

20 minutes? Ouch. That's going to leave your arms sore.
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Unread postAuthor: Jared Haehnel » Fri Mar 14, 2008 9:31 am

Often longer barrels can actually harm the chances of a supersonic muzzle velocity. Pressure effects ahead of the barrel are quite significant at those speeds,


Perhaps venting ahead of the projectile would help? Or creating a vacuum in front of the object...just a thought..
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Unread postAuthor: Ragnarok » Fri Mar 14, 2008 9:43 am

@Jared: Yes, evacuating the barrel (creating a vacuum ahead of the projectile) is quite a common suggestion for helping with the problem, although seldom actually put into practise.
Using the practise, supersonic velocities would be a much more viable achievement - with enough vacuum, then even moderately low pressures could (probably) be used to exceed the sound barrier.

There is the slight problem with what is used as a burst disk across the end of the barrel to maintain the vacuum, but aluminium foil should probably suffice in most cases.

Venting/porting the end of the barrel won't really help the situation however.
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Unread postAuthor: Jared Haehnel » Fri Mar 14, 2008 9:50 am

Hey, some people will do anything in the name of velocity...

I wouldn't think it would be that hard. Place a tee in the barrel and hook up a shop vac....
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Unread postAuthor: Ragnarok » Fri Mar 14, 2008 10:30 am

Jared Haehnel wrote:I wouldn't think it would be that hard. Place a tee in the barrel and hook up a shop vac....

It's very much dependent on the level of vacuum attained. You really want at least 75 or 80% vacuum, and ideally 90-95% or more.

For that kind of thing, you really need a dedicated vacuum pump.
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Unread postAuthor: Jared Haehnel » Fri Mar 14, 2008 10:41 am

Your right that's probably out of the range of most shop vacs on the market...
It could make enough of a difference if you were with in a stones throw of breaking the SOS
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Unread postAuthor: jimmy101 » Fri Mar 14, 2008 12:10 pm

Ragnarok wrote:Already working on an auto fill tool. It's got all the basic things I can think of: Golf balls, baseballs, ping pong balls, tennis balls, 12g CO2 cartridges, batteries of different sizes, approximations for both .177 and .22 airgun pellets... and anything else that comes to mind.

As for BBs, there are so many varieties it's hard to know where to start - plastic in nearly infinite numbers of densities (from 0.12g up to 0.43g last I knew), copper, lead, steel, all of those in both .177 and 6mm sizes. Then add on 1/4", 3/8", 1/2", 3/4" and 1" steel ball bearings, and all the various sizes of buckshot and birdshot...
Suddenly the data table is an utter mess - I can think of at least 40 different spherical BBs and shot.

I don't know whether it would be better instead to put in a calculator for working out the mass of a standard sphere or cylinder based on it's dimensions and density (of course, there would be a table of densities for lots of different materials, and theoretically a table of shot sizes).

Spuds are also tricky. They come in lots of lengths and densities, with odd shapes that make it hard to know a drag co-efficent. My best guess remains at 0.5-0.6 for that, but it very much depends on how they're cut, and for all I know it could be more again.
Any thoughts on that matter?


Standard 0.177 steel/copper BBs would be real nice to have since they are by far the most common ammo of the "BB class" (except perhaps whatever the standard Airsoft ammo is).

Spuds are tricky. The best you can do is an educated guess. Probably not worth trying to get an extremely accurate Cd or dimensions or mass since those values will vary a fair amount from spud to spud. Perhaps just values for full spuds in 1", 1.5 and 2" barrels and half spuds in the same barrels? The user can always tweak the values a bit if they think they have a better estimate of one of the parameters. Heck, even if the values aren't all that good at least it gives folks a reference point.
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Unread postAuthor: Ragnarok » Fri Mar 14, 2008 3:50 pm

jimmy101 wrote:Standard 0.177 steel/copper BBs would be real nice to have since they are by far the most common ammo of the "BB class" (except perhaps whatever the standard Airsoft ammo is).

6mm plastic normally, 0.12g is generally considered lightweight, 0.2g normal weight, and 0.25g and above heavyweight - although there are heavier 8mm versions, but I don't hear much about spudders using those.

Well, I'm never sure with my educated guesses about spuds. Comparing the shapes to the drag of other shapes, I can't guess a better Cd than 0.5 to 0.6.
As I suppose they tumble a lot, 0.6 is probably the better guess.

It's tricky - there are so many variables. For example, I tend to cut the ends of my spuds square after they come out of the spud cutter. A cylinder is a very poor drag shape I know, but they do fly a little more predictably than if they have a slightly angled end - they tend to take slightly longer to start tumbling, and in spite of the worse shape, they tend to fly further anyway.

It's a tough one.
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Unread postAuthor: Fnord » Fri Mar 14, 2008 4:21 pm

The problem with potatoes is the cd changes rapidly during flight. I've also observed them spinning like a rifle bullet out of an unrifled barrel. It's just too random to pin solid numbers on.

For an output I would give a min/max range rather than a solid number; using the best and worst case numbers to give the extremes of the range.
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Unread postAuthor: bigbob12345 » Fri Mar 14, 2008 5:57 pm

Ragnarok wrote:
And for the CO2 in the KAMG ill just be using that most of the time for all the shots were Im really going for something(SOS, penetrating 1/4in steel) I will take 20min to pump it up with my high pressure pump I am working on.

20 minutes? Ouch. That's going to leave your arms sore.


I probably overestimated a little there Ill revise that to 15min which will still put my arms through hell but its all for a good cause :)
Im also making the chamber a decent amount bigger(is 65ci, will be 95ci)
and the pump time is estamated with the extended chamber.
With the current 65ci chamber I would say about 10min.
The pump is designed for small JSR like projects as I am getting into things that fire BBs so it will take a while to pump up larger guns.

OK, in the world of spuds today I attached the popoff to a sprinkler to actuate my autopiston and it worked :D . I dont have the autopiston complete and still need to get a loading system but in a couple weeks I should have full auto.

apart from that more burst disk testing with the KAMG as i am having sealing problems at higher pressures with my old fav burst disk material, brown tape.
I have come back to aluminum foil as it just seals really well.

I started work on a small pvc piston valved pnuematic(4ft by 1in chamber, 1in piston valve 1/2in porting, 1/2in ball valve pilot, 6ft by 1/2in sdr21 paintball barrel) Ill GGDT it in a couple minutes.
thats it
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