HPA launcher making use of 200/300 bar handpumps?!??

[Crna Legija]

from the wiki some help withCO2 cartridges


i'm pretty sure you use up the hole 12grams every time with any descent sized chamber, not like a hammer valve where you only let out a tiny bit.

[Iceblade]

Yes, I know that.

My point was referring to what another member said earlier, that an increased chamber size will result in a lower pressure and therefore lower velocity. I think a bigger chamber will result in an unchanged pressure thus higher velocity, as a part of the liquid CO2 turns to gas, compenation and "filling" the bigger space up with the same pressure as before. Prerequisite is of course the assumption that "enough" (liquid) CO2 is there at all.

It's really hard for me to explain, hope you get what I mean here. ?

[Crna Legija]

ahhh yea i get ya, i too would say it be more powerful if you made the chamber just big enough to get all the co2 to turn into gas.

[Iceblade]

Yep, that's exactly my point!

Dunno if it's true or not. From the wiki article you get a different impression, but this may only be valid if there is no (more) liquid CO2 left or has never been there in the first place (= lower cartridge starting pressure).

[saefroch]

You told me that a bigger expansion chamber (using the same pressure source) would not increase the velocity of the mentioned launcher, but lower it instead. The reason would be that the amount of gas is limited due to the 12 gram CO2 capsule causing the pressure to be lower.

Normally you can achieve greater muzzle velocities by shrinking the chamber and proportionally increasing the pressure, but that may not necessarily be the case here, because getting more heat into the gas and thus raising the final expanded temperature, you may get a higher SOS in the gas and it may outweigh the losses from a lower pressure. I'm not sure that it would, but that's the only possible reason muzzle velocity would increase by making the expansion chamber larger.

I know that CO2 cartridges tend to preserve a fix maximum pressure, as long as the amount of gas in the capsule and thus the pressure surpasses a certain threshold. Above that threshold the CO2 liquifies, requiring less space, keeping the pressure constant.

Correct, the CO₂ condenses when its partial pressure inside the container is above its vapor pressure at that temperature.

Lalalalalalala gasification lalalalalaala

You mean boiling .

I thought this could be transferred to the above launcher. If the chamber is made bigger, the pressure will not fall, but stay the same, providing the CO2 capsule has enough pressure/liquid CO2. And with a bigger chamber, but the same pressure, the velocity should increase.

I highly doubt that's the case. I'm not about to throw numbers at it, but if you want to, go right ahead. The expansion chamber seems large enough to me to allow all the CO₂ to boil.

[Iceblade]

So then it would make sense to design such a launcher in a way that there is a pre-chamber before the "real chamber" with the burst disk and have pre-chamber and real chamber connected with another valve. that way it is possible to let all the CO2 flow into the pre-chamber, letting all CO2 boil, turn into gas and reach a warmer (surrounding) temperature. then, if there is only warm gas, the second valve can be opened releasing the CO2 into the real chamber with the burst disk.

on the other side, the gas and its pressure is split up between to chambers and thus a larger space, taking away a good amount of pressure again.

the question is, which effect is bigger?

making use of the entire pressure potential of the CO2 in the form of warmer gas or reducing the pressure by having two chambers...

the pre-chamber has to be as big as necessary and small as possible...


oh wait! is it possible to remedy this dilemma by simply using a burst disc with only breaks at the very maximum pressure possible? then it wouldn't be a problem if the CO2 takes its time to boil, heat up, turn into gas completely, until - and only then! - pressure is max and breaks the disc. best way to find out would be trial and error. using a burst disk which does not burst, and then go down to weaker discs gradually until one breaks. problem solved?

[jackssmirkingrevenge]

I say ditch the CO₂ altogether, make a small high pressure chamber charged with a shock pump. 12 gram capsules will get expensive very quickly anyway if you're using one per shot.

Air is less dense than CO₂ anyway so you have higher velocity potential - also, it's better for the environment

[Iceblade]

Is the silver bottom device the pressure chamber? Where can I get sth likes this, e.g. what is its original purpose?

I found a shock pump up to 21 bar... is that enough? Much less than CO2 cartridges...

That handle is from a high-pressure cleaner, right?

PS: Man, you are really quick in drawing those designs....

[jackssmirkingrevenge]

Is the silver bottom device the pressure chamber? Where can I get sth likes this, e.g. what is its original purpose?

Blessed with binary adhesives, any suitably strong piece of pipe can be the pressure chamber, regardless of its original purposes read up!

I found a shock pump up to 21 bar... is that enough? Much less than CO2 cartridges...

Ultimately, you're limited by the pressure the PVC pipe can take, so no matter what your pressure chamber pressure is, you don't want more than 200 psi or so behind the burst disk, assuming what you want is a large bore PVC launcher.

That handle is from a high-pressure cleaner, right?

An ordinary shop blowgun is good for anything under 400 psi,just five bucks posted.

PS: Man, you are really quick in drawing those designs....

Amazing what you can do with MS paint, google image search and a bit of imagination

[saefroch]

which effect is bigger?

My guess is the lower chamber pressure.

You could build what JSR suggests without any epoxy, you'd just have to be prepared to do a little shopping.

[Ragnarok]

If I got it right the 200 bar handpump would not provide for a more powerful source then 12 gramm CO2 cartridges then, despite these "only" have around 60 bar, like all CO2 tanks have?

A 200 bar source is a more powerful source than a 60 bar one, but if velocity is your only concern, then you're not going to get much more from it. You could fire heavier things with it, get the same velocities from a shorter barrel or some combination, but the upper velocities of a pneumatic design are not limited by lack of pressure, but by speed of sound.

(However, in this case, there is another factor - CO2 is a far denser gas, which means it has a lower speed of sound.)

More pressure can give more energy (but don't confuse energy with velocity), but given how hugely the risks increase between 20 bar and 200 bar, it's really not worth it.
With those kinds of energies, there's only two types of people: Those that haven't made a mistake yet and those that will never make another one.

[Iceblade]

JSR: So using CO2 cartridges is not a real option after all, if more than 200psi behind the disk of such a launcher is too much, is it? The cartridges have more psi....

saefroch: yeah, you're probably right about that. Wouldn't even know where to start shopping though, for example to connect that blowgun to the chamber :-/

Ragnarok: I understand the velocity vs. energy issue. When I was talking about more or less velocity I was assuming projectiles with similar weight and same lenth barrels. So what I actually meant was energy. Do you think the difference between CO2 and air due to their different SOS is really something I would notice from the effect/range of a projectile? Or is it a rather scientific issue which plays not a striking role for our purposes? The dangers of such high pressures are an important point. I decided to stick either to CO2 cartridges first, or to an HPA chamber and a shock pump like JSR suggested. First choice may be easier to build, while the second choice seems to have more energy...
hmmm....

[saefroch]

Do you think the difference between CO2 and air due to their different SOS is really something I would notice from the effect/range of a projectile? Or is it a rather scientific issue which plays not a striking role for our purposes?

*facepalm* If you're using such massive pressures, muzzle velocity will be determined by the SOS in the propellant gas. However...

If you're planning to build a launcher like the one we've been discussing, the pressures behind the projectile at any given time in the barrel are rather pitiful, so SOS doesn't come into play as much.

Also, if your muzzle velocity is approaching SOS in the propellant gas, you'll suddenly find yourself unable to retrieve the projectile if it's just pointed at the sky. Range becomes a non-concern.

[Iceblade]

So if it's not an issue, how come so many people note the fact that CO2 has a lower velocity potential than air? Unless I'm using ultra-light, neary weightless projectiles (or, like you say, use unreachable high pressures) the determining fact for velocity should be pressure alone and not gas-characteristics :-/

[jackssmirkingrevenge]

JSR: So using CO2 cartridges is not a real option after all, if more than 200psi behind the disk of such a launcher is too much, is it? The cartridges have more psi...

I assumed PVC fittings. If you go for steel fittings, heck, hook it up to a SCUBA tank and have a disk which bursts at 4500 psi

As expressed earlier in this thread:

Rather than focusing on high pressure, think of the following:

- what materials and techniques you have to work with
- what performance you want to achieve
- what budget you have to spend on the build
- what constraints in size or weight you might have

Before you define what you want your launcher to be able to do, and what resources you are willing to allocate to its construction and operation, any discussion is moot.

On the CO₂ vs Air discussion, here's an interesting comparison:

http://www.pyramydair.com/p/condor-air-rifle.shtml

http://www.pyramydair.com/s/m/AirForce_ ... coped/1669

Of course the air powered model is filled to much higher pressure but it gives a good idea of the performance limitations of CO₂.

Another good comparison: http://www.quackenbushairguns.com/xl.html