SpudFiles

I have been around here for awhile now so its about time I posted something that isn't off topic..
This is my first attempt at making an air gun, many years ago I made a fairly large spray and pray cannon out of DWV. It was pretty powerful and fun, but its unpredictability and concerns for my well being were what made me decide to go with compressed air, at least for now...

The overall goal was for it to be 'semi' semi automatic, aka didn't need to be refilled between every shot but loading was still done manually. This was going to be achieved with a sprung piston, controlling airflow exiting pilot valve and minimizing airflow from a reservoir to the guns chamber. So the piston would close before pressure could build enough to keep it forced open. Perhaps a cross section could help explain this: I choose 6mm for two main reasons:
- I had a broken arrow lying around which is the perfect diameter for 6mm bb pellets
- I didn't have money for anything larger

So as to serve as a trial run before building larger and more possibly more complex creations when funding allows, I made this.

The design was inspired by one of wyz2285s guns, found here


Where I live, galvanized fittings are cheaper than PVC (plus as far as I am concerned metal is better than plastic,) so that is why metal was used for a low cost build. Other cost saving measures involve using cheese wax or electrical tape for sealing the threads (I find cheese wax works very well).

The piston is a small section of television antenna or similar aluminium tube with epoxy filling about 1/3 of it. Initially a piece of gumboot was used for the sealing face, but this wore out after awhile. I am currently using some tyre inner tube, which is holding out very well.

The piston (shown below) is lightly sprung which stops leaks forming when initially filling the chamber with air (it still leaks below 20 psi). The piston slides inside the casing of an old AA battery, this along with some electrical tape and oil makes for a near air tight seal.

The screw was an experiment to stop unnecessary air passing the piston rather than exiting the barrel, when piston is fully depressed the intent was for the screws body to fill the main exit hole in the back of the battery. It doesn't work well.The AA battery has a strip of paper and more electrical tape wrapped around it so it forms a tight fit into the galv fitting. This also helps decrease pilot volume. One downside of the design is that barrels cant be interchanged, easily. The arrow is epoxied into the reducer fitting which also holds it at the right length to interface with the piston. At one point I sleeved the barrel with a smaller diameter aluminium tube, perfect for .177 pellets or 4.5mm bbs. I found that this had a large effect on projectile speed and as such have reverted back just the arrow.
Similarly I also have a larger aluminum tube, initially to protect the carbon arrow. As it is overly long, it has a silencing effect when firing. Again I prefer not to use it as the added length makes it too unwieldy and I cant be bothered to cut it down.

The stock is made from 30mm diameter steel tubing. On my quest to save money and not having anything like JB weld on hand, I let a friend who had just got a welder have some practice at welding thin metal. It doesn't bother me, its better than what I could do for the price.
Holes were then cut and filed until the gun was able to slot in place, its an interference fit with the galvanized elbow, and a 'securing' screw holds the hook on the blowgun firm. Its a surprisingly secure fit, yet removal is simple.The finish on the stock was achieved by scrubbing it with scouring pad and then clear coating it. Its a nice finish that photos don't do justice.

Filling is done with a fridge compressor, it was the cheapest option for me, just had to jump into a few skips to salvage some stuff Now the interesting part.

Throughout the build and tuning process many items have been fired and with different methods. The most effective I have found to date is a 4.5mm steel bb with a 6mm lightweight bb as a sabot. This has been measured at 700fps when charged with 130psi, I feel sometimes it fires faster than this however.
The lightweight bb by itself has been recorded at just over 1000 fps with maybe 150psi (it was past the end of the pressure gauge).To stop the bb rolling out of the barrel, there is a broken piece of hard drive magnet stuck to the inside of the chamber.

Other random damage pictures: Damage tends to just be holes in stuff, which isn't too exciting..

Excellent stuff! Just two points of contention:

Firstly;

ToasT wrote:Damage tends to just be holes in stuff, which isn't too exciting..

Making holes in stuff is the best part of the hobby for me

Secondly, no videos?

150 psi, and you're using a fridge compressor?? Shame! Crank it up to at least 400.

no videos?

I will get around to making some videos of putting some holes in stuff.

150 psi

Yea, it is a shame. I have been meaning to get some better quality quick connect fittings, my current one leaks and about 120psi is the steady state of air in to air out, with effort I estimate I can get this to somewhere around 150.

The whole compressor set up needs to be improved, currently the hose is held onto the compressor outlet with tape, not sure what it can hold. Also need to get a pressure gauge that reads past 120.

The lightweight bb by itself has been recorded at just over 1000 fps with maybe 150psi (it was past the end of the pressure ga

aha, i had a little dispute with someone about i can achieve the speed of sound. i see you got 1000 fps with 150 psi. I shoot over 350 psi. i downloaded a chrono software for my laptop and i got at every shoot 1225 fps( the maximum for it ). but i think it is wrong. I need something to compare it with.
try your gun with higher pressures if you want more power and a nice recoil.

In the past I have marked out a distance, say 10 metres, placed some audio recording device at the mid point, and shot at a target over this distance. Then by using audacity (a computer program) measuring the time between audio spikes you can calculate the average speed. Having the audio device in the middle factors out the speed of sound.

Theoretically I think you can achieve the speed of sound with a pneumatic, but not exceed it. At least with constant area pipe. Something about compression and fluid flow.

To restate the dispute I had with bravotome:

Sound is the propagation of pressure, so the speed of sound in a gas (or any other state of matter) is also how fast pressure can propagate through it. And if the propellant's pressure can only propagate at its speed of sound, it can't accelerate something faster than that.

So, achieving supersonic velocities requires the propellant's SoS to exceed the ambient SoS - requiring either hot or lighter than air gases. However, the inherent problem with pneumatics is that rapid decompression of a gas reduces its temperature and hence its speed of sound.

There are mechanisms by which the temperature in a pneumatic can potentially rise above ambient, but these aren't achieved casually. Hence why I default to scepticism when it comes to claims of supersonic pneumatics that don't have reliable chronograph data backing them up.

Ragnarok wrote:To restate the dispute I had with bravotome:

Sound is the propagation of pressure, so the speed of sound in a gas (or any other state of matter) is also how fast pressure can propagate through it. And if the propellant's pressure can only propagate at its speed of sound, it can't accelerate something faster than that.

So, achieving supersonic velocities requires the propellant's SoS to exceed the ambient SoS - requiring either hot or lighter than air gases. However, the inherent problem with pneumatics is that rapid decompression of a gas reduces its temperature and hence its speed of sound.

There are mechanisms by which the temperature in a pneumatic can potentially rise above ambient, but these aren't achieved casually. Hence why I default to scepticism when it comes to claims of supersonic pneumatics that don't have reliable chronograph data backing them up.

Interesting, I thought certain PCP rifles had archived supersonic muzzle velocities, but physically I agree with you.
However, I finally bought a decent chronograph similar to a Chrony, so I can finally test my hybrids and HPA launchers.

wyz2285 wrote:Interesting, I thought certain PCP rifles had archived supersonic muzzle velocities

With enough brute force, it's possible to compress the gas between the valve and projectile, producing hotter than normal gas in the barrel.

Actually, I've just been outside trying to test that theory with HEAL. I'm still looking at the results, but expect a report later on.

If I remember correctly, (actually I'm pretty certain ) that JSR broke the sound barrier with a 6mm burst disc pneumatic.

a spring gun gamo is able to shut a Pb pelet at 1000 fps and do same penetration test on a book as my gun with a bb.. I am able to shoot at the end of the barrel @ 28-30bar pressure a plastic bb with the speed of sound or more. How else i can hun. by 15-20 m a pest with a plastic bb.
I tried the softchrono software but it seems not to work, i only got 1225 fps witch is the maximum that it can read.. If anybody else tested it it in comparation with a chronograph please let me know how to calibrate.

I mean if 1000 Fps is achieved with @ 150-200 Psi, what happens to 400 psi. ? We are not talking about the speed of light !
lets stop it here and continue when i'll get myself a chronograph.
ok, than take a look at this man that shoots 60 bar with metal bb. It is way over speed of sound [youtube][/youtube] ....your presumtions are incorect. there are custom pneumatics guns that shoots over 2000 fps.


'''You also increase the density of air in proportion with the pressure"""--- yes and the efect is that the exausted air expands even faster and more power as well.

bravootome wrote:I mean if 1000 Fps is achieved with @ 150-200 Psi, what happens to 400 psi.

You also increase the density of air in proportion with the pressure, so your gains in force are offset by also increasing the accelerating mass.
(Contrary to first instincts, the mass of air gets quite significant when pressurised to this extent. Frequently, the air in my launchers weighs more than the projectile).

As I've said, I'm not saying you're definitely not going super-sonic, but I still need to see decent evidence to be convinced.

Use Helium......

While it initially sounds appealing, Ragnarok's assumption about the maximum attainable speed in an "ideal" gun (one with a massive propellant gas, but assuming an ideal gas EoS, 1D behaviour, no viscous effects, preburned (or pneumatic) propellant, and whatever else I forgot) is incorrect: the maximum attainable velocity is not c_0 (the initial sound speed), but rather 2c_0/(γ-1). The full derivation can be found in "The Theory of High Speed Guns", section 10. The basis of the derivation is Riemann's solution for a 1D travelling wave in a semi-infinite tube, which shows up in Landau's "Fluid Mechanics" (and no doubt several other places).

Simple inspection will show that the maximum attainable speed (called the "escape velocity" in this situation) is different from the initial (and in the ideal case, maximum) sound speed - the gas close to the projectile base is accelerated by the pressure gradient along with the projectile. Imagine, if it helps, a series of thin, weightless pistons spaced through the tube - pressure drops the further one gets toward the projectile, and each space between pistons behaves like a little chamber, accelerating the gas mass in front of it to some speed above the base speed provided by the mass before it. The escape velocity occurs when the projectile base pressure is equal to zero. If γ=3, escape velocity = c_0, but this is an impossibly high adiabatic index, so the escape velocity is always higher than the initial sound speed.

The sound speed limitation certainly has an effect - in extreme cases, it will cause the breech to still be at the pre-shot pressure after the projectile has departed. It prevents instantaneous "communication" between distant parts of the system, but it does not prevent the projectile from exceeding the initial sound speed. In fact, with air at room temperature, the escape velocity would be a monstrous 1700m/s.

I'm surprised to see that this conflation of sound speed with escape velocity has persisted so long around here. Excellent treatments of the subject have been available since at least the 1960s.

wyz2285 wrote:If I remember correctly, (actually I'm pretty certain ) that JSR broke the sound barrier with a 6mm burst disc pneumatic.

He did, and someone [who?][citation needed] linked to it on the Wikipedia page on spudguns!

DYI wrote:I'm surprised to see that this conflation of sound speed with escape velocity has persisted so long around here. Excellent treatments of the subject have been available since at least the 1960s.

Yep, I don't know why we are still having this discussion, high performance PCPs regularly go well past the speed of sound running only on compressed air.

Warning, kiwi accents:

[youtube][/youtube]

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