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Air Source Question

This area is for the discussion of BB or airsoft sub machine guns, and or other small arms such as pellets.
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Air Source Question

Unread postAuthor: vng21092 » Wed Dec 04, 2013 9:44 pm

I don't have an air compressor nor do I want to buy one, the bike pump I have at the moment only charges my air chamber slightly over 100psi. Does anyone have a better recommendation, a pump that would allow me to achieve higher pressures? Would this work? http://www.amazon.com/RockShox-High-Pressure-Shock-Pump-300/dp/B005543YUE/ref=sr_1_1?ie=UTF8&qid=1386211175&sr=8-1&keywords=bicycle+shock+pump, if so how high do you think it can go? Can you recommend something?
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Re: Air Source Question

Unread postAuthor: Ragnarok » Wed Dec 04, 2013 11:00 pm

Assuming this is related to your question about vortex blocks, it's not suitable.

While shock pumps are capable of relatively high pressures, they are very low volume and are not feasible for filling more than the smallest of air chambers - I think you'd have to be a bit of a masochist to try filling a chamber of more than a (very) few cubic inches.
Those things just don't move enough air to be in anyway feasible for powering a BBMG. It'd take hours (if not days!) to pump up the air tank and seconds to empty it.

There's a distinct limit to what manual pumps are really suitable for.I use this home-made contraption to get a 600 cc (~36 ci) chamber up to about 25 bar (~360 psi); Despite a distinct volume advantage over a shock pump, it's rather a work-out to use, particularly doing it more than a few times in a row.
Ultimately, I'm looking to replace it with a two stage design, where a high volume first stage feeds pre-compressed air to a higher pressure second stage. In theory, at the outside, something like that might just be plausible for filling a moderately air tank to high pressures. But it'd still be a lot of work.

In short, BBMGs aren't really suited for manual pumps. But if you're really set on doing it that way, your best shot is a typical track pump (most will do somewhere like 120-150 psi) and as large an air tank as you can stomach pumping up (repeatedly). You need volume more than you need pressure.

Although, if you're asking about a single shot pellet gun with a very small chamber... sure, a shock pump would probably be fine.
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Re: Air Source Question

Unread postAuthor: vng21092 » Thu Dec 05, 2013 4:13 am

wow that thing looks like a workout, but it wasn't related to the vortex, I just had a semi-auto build going on and I wanted to know. Thanks anyway, Btw, for a slide valve-qev setup, what do you think is a good firing chamber size? I'm aiming for 4.5mm-8mm bbs, and currently the firing chamber is a half inch pipe, probably 3 or 4 inches in length, should I make it bigger?
(Just in case it's not called firing chamber)
http://imgur.com/cI6ibAO
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Re: Air Source Question

Unread postAuthor: Ragnarok » Thu Dec 05, 2013 6:07 am

If that's the launcher in mind, I'd guess your "stock" tank is probably too big to easily fill with a shock pump.

With regard to the chamber... that size should be fine. If anything, I'd be more likely to recommend smaller over larger - it wouldn't make all that much difference to the muzzle velocity, but would make a bigger one to the number of shots you could get out of one charge.
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Re: Air Source Question

Unread postAuthor: vng21092 » Thu Dec 05, 2013 3:24 pm

oh, in that case what do you think is the smallest size I can make it where it would still have decent velocity? if I wanna go for say... .177 cal with a 4.5 mm x 18 inch barrel. also, if you dont mind, can you explain why a smaller chamber wouldn't really affect muzzle velocity? I just thought a bigger firing chamber meant more air through the barrel thus carrying the projectile further.
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Re: Air Source Question

Unread postAuthor: Ragnarok » Thu Dec 05, 2013 4:45 pm

vng21092 wrote:also, if you dont mind, can you explain why a smaller chamber wouldn't really affect muzzle velocity?

It will have some effect, but you'll be firing projectiles from a barrel many dozens of calibres long with moderately high pressures. You'll be right into the transonic region with that, so a lot of the energy will be being wasted on the heavily diminishing returns that pneumatics get into as they approach the sound barrier.

Chamber size is a diminishing return itself, even without modelling the losses in flow. A C:B ratio of 0.5:1 is a lot more efficient than 1:1 is. Here were some results I cooked up a while ago for one of my launchers:
Image
The X axis is the ratio of the chamber volume to barrel volume, and the Y axis is the fraction of the theoretical maximum energy achievable with an infinitely sized chamber.

The blue line would be the energy of the projectile without any losses in flow. You can see it's levelling off though.
The red line is with the losses in flow - you can see this levels off faster, and is practically flat from about 5:1 upwards.
The green line is with flow losses, as if the valve were half the diameter. This is practically flat from about 2:1 upwards.

You've got quite a good valve. But still, a 1:1 chamber will still be using twice as much air as a 0.5:1 chamber for only a potential third increase in energy (even without bearing in mind that a huge portion of that potential will be lost because of the transonic inefficiency).

For a 4.5mm barrel, 45cm long... well, I had anticipated a slightly longer barrel than this with my suggestion. Still, the point is still valid.
For a steel BB, a ballpark at 300 psi would be 650 fps. Halve the size of the chamber and you'd be in the region of 580 fps - a bit of a drop, but you'd get twice as many shots!
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Re: Air Source Question

Unread postAuthor: vng21092 » Thu Dec 05, 2013 4:57 pm

wow thank you so much for that explanation. Would you mind sharing where you buy your barrels from? The closest thing I've found to a 4.5mm barrel is a 6mm brake line in which the inner diameter is about 4.8mm, but brake lines are not durable, they bend way too easily. I work in a home depot and they've got nothing that small so any suggestions? I don't want to buy an actual airsoft barrel as they are pretty expensive. This is what I've got so far, muzzle loaded until I can make a better method, do you think a longer barrel would produce higher velocity?http://imgur.com/ThvPmY8
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Re: Air Source Question

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Re: Air Source Question

Unread postAuthor: Ragnarok » Fri Dec 06, 2013 10:47 pm

vng21092 wrote:Would you mind sharing where you buy your barrels from?

I'm afraid I'm not of much use there.

Firstly, I don't buy a lot of barrels in that calibre (what I have used in that size is mostly brass modelling tubing, but that's only normally useful for less than 12" lengths).
Secondly, the way I spell "calibre" means my shops of choice aren't exactly particularly close to you!

Really, you probably need someone else to answer this question for you. What suggestions I can make are that you could either try sleeving the brake-line inside something else to protect it or try looking around for small diameter stainless steel tubing on the internet.

This is what I've got so far, muzzle loaded until I can make a better method, do you think a longer barrel would produce higher velocity?http://imgur.com/ThvPmY8

Under most circumstances, a greater barrel length will improve muzzle velocity.
It's another of those diminishing returns, but doubling barrel length will typically have a greater effect than doubling chamber length. (With the plus side that it doesn't use up twice as much air!)
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