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Best advice is to download GGDT and have a play with the parameters, you will gain a much better understanding of what affects performance and to what degree.
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
With a little help from the forum you can then come up with a design that is suited to your needs.
Some basics that I'm sure many of you are aware of:
F also = m*a
You can shoot LIGHT projectiles REALLY fast with low pressure....if you've enough barrel length and chamber volume matched with sufficient flow rate to maintain the pressure (and thus force and acceleration) at the projectile.
Taken to extremes of barrel length, you will see frictional losses from the flow interacting with the barrel, as well as compression of the barrel volume ahead of the projectile.
Can you shoot "comparatively" heavy projectiles "fast" with low pressure? Again, if you've a long enough barrel, matched up with enough chamber volume and sufficient flow rate...sure.
Question is, can you live with 10+ feet of barrel in order to get a respectable projectile up to sufficient velocity? Kind of depends on your idea of "respectable" and "sufficient" velocity.
If not...you increase the pressure... which increases the force applied to the projectile....which increases the acceleration... yielding the velocity you're after over a shorter distance (barrel length).
Pumping sucks, and setting up to use N2 isn't free. Most spudders are either cheap buggers, or younger folks with small budgets. Not everyone can lay out the $$$ for a proper 4000+psi N2 setup. (you won't get even one 3000psi fill from a 3000psi bottle)
I suspect that's more the reason that people go with hybrids.
Comparatively easy (not to mention less costly) to get some propane, and a fridgy or stirrup pump to charge a mid to small sized chamber to past 20x mixes for VERY impressive power.
Construction of a "basic" burst disc hybrid isn't really difficult either. Most of the simpler ones are just threaded together...only remotely challenging aspects are the ignition and fueling systems.
As Jack suggests...get ggdt and play around with it some.
"It could be that the purpose of your life is to serve as a warning to others" – unknown
Liberalism is a mental disorder, reality is it's cure.
I see, you all have really helped me to understand what this is about. I will download that program and play around.
One last thing. You all know this version I suppose:
Since a 12 gram CO2 cartridge is used, I was surprised that the velocity seems rather low because I was thinking about pressure only. Then from what I learned here, I THINK the velocity is the "low" NOT because of the pressure (which is actually quite high), but due to the fact that this launcher lacks a proper chamber. With a 1000ml chamber between the cartridge and the ball valve, the velocity would increase, wouldn't it (neglecting the rather inefficient ball valve for a moment)?
If you still use the same 12g cart to fill a 1000cc chamber the performance would be lower, you will get more power from a small high pressure chamber then a large low pressure on, because you dump the air quicker with a small high pressure one.
That cannon linked needs a bigger valve, looks like he used a 1/2'' ball valve where he should have at lest a 1.5'' one.
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Add me on ps3: wannafuk, 8/11/11 cant wait
Again, download GGDT.
You can model your launcher, and get X performance.
Then you start playing with parameters - barrel length, chamber size, pressure, valve flow, opening time, projectile weight etc - and you will see how it affects the performance, and decide if its worth it.
For example, imagine you're getting 200 ft/lbs with a 1000mL chamber.
You increase the chamber volume to 2000mL, and power increases to 205 ft/lbs.
This means you increased power slightly, but doubled your pumping effort.
You can then decide if this is worthwhile before commiting to building your launcher, it's much easier to play around virtually than through trial and error.
Almost everything about that launcher is wrong. Crappy C:B ratio, very slow valve, and the chamber itself prevents good flow into the barrel. It's actually a great demonstration of how it's very easy to take a high pressure source and turn it into an overall low-performance build.
The velocity would not increase by increasing the chamber volume, because there would be no more gas in the chamber, since it is limited by the 12 grams in the cartridge.
Then why does the guy who constructed this launcher state:
"If distance is really a concern making the expansion chamber tube longer will add some distance but lowers the portability."
Trying to understand how this launcher could be improved (it doesn't need to be that portable for my purposes):
- Faster and bigger valve of course. QEV?
- Relocate the chamber directly behind the barrel, so that air doesn't have to flow around the corner
- use two 12 gramms cartridges at the same time?
- what is C:B ratio?
- still unsure about the chamer size yet... :-/
I think to gain first experience building this simple launcher is fine for me. I also like the 12 gramm adapter choice, it's easy and has a high pressure. But when building this I'd like to avoid obvious design mistakes as far as possible (obvious for you guys, not for me... yet )
Making the expansion chamber larger will allow the gas to pick up more heat from the environment upon decompression into the expansion chamber, which will increase the final temperature of the gases in the barrel. The greater temperature will give a higher speed of sound, more volume, and more pressure behind the projectile. Probably the reason I didn't think of that earlier is that I usually compress the propellant gas just before shooting, so the gases in the chamber are very hot and after expansion in the barrel are not so far below room temperature as the expanded CO<sub>2</sub>.
C:B ratio is the ratio of chamber volume to barrel volume.
You could get MUCH better performance using a burst disk. I'll draw up a design shortly.
edit: this will tear the head off and defecate down the throat of that launcher you posted
Wow, again most helpful replies folks Thanks esspecially for the design suggestion - nice naming of the .png file also I think I will build just that. Is it drawn in the right dimensions/relations?
Furthermore, apart from the valve really no steel is needed?
When using the burst disk the negative properties of the ball valve become irrelevant, right?
it's not really drawn to scale, you can make the barrel longer. The only steel needed is the parts that hold the co2 besides the burst disk part.
Sometimes words just pop into my head.
No - because you have to download GGDT and work it out for yourself!
(subtle hint )
Assuming a disk burst pressure lower than the PVC rating
Here's a GGDT model to start you off, that's not something you want to shoot at people!
In your model you selected "air", which I guess should be changed to "CO2" for a start. But I'll go and figure out the functions of the program myself.
Thanks also for your subtle hint (the one telling me to look down the barrel of a ready-to-shoot launcher). I'll make sure to follow it, but only with other ammo than nerv balls
Here's a better modelled golf ball launcher, under 3 feet in length and with more muzzle energy that a 0.22" rimfire rifle.
Cool, this one looks better than the first. Thank you!
Something else came to my head.
To 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.
Are you really sure about this? I'm asking because of the nature of CO2. 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 contant.
At CO2 rifles for example (this is where I come from originally) this is the main reason why CO2 is preferred: it provides for a constant pressure until, towards the end, the pressure falls rapidly. It works the following: cartridge is inserted, let's assume it has 60 bar. The CO2 is mostly liquified. As the capsule is attached, a small amount of CO2 flows out of the capsule into a first chamber. The pressure in the capsule therefore is lowered, which causes a part of the liquified CO2 to turn into gas, which in return - as it requires more space - increases the pressure again. When the pressure has reached the original 60 bar again, the "gasification" (is that a word?) process stops and the pressure in the capsule and in the chamber is the same again. This process repeats after each shoot: pressure falls, liquid turns into gas and pressure rises until pressure is the same. Only towards the end, when no liquid CO2 is present anymore in the capsule/chamber is it that the pressure falls below 60 bar permanently, decreasing continually with each shoot. The exact bar when CO2 liquifies/gasifies is of course strongly dependet of the temperature.
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.
Where is my error of thinking here?
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