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Physics Project...

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: sade » Wed Mar 12, 2008 8:33 pm

Ok. To begin with, I am not a total spudgun noob. I have a decent amount of experience with combustion cannons, but that's it.

I am in an advanced physics class and we will be building different launchers for chicken eggs. I want to know whether or not it would be plausible to build a pneumatic air cannon that will launch an egg (in a small spherical container) upwards of 100 ft straight up into the air. Safety is a very large concern, and I will need to be able to convince the instructor that the cannon is safe when used correctly. Also, portability and storage is a pretty big concern, but if that is asking too much then both can be sacraficed.

If it is plausible, could anyone help point me in the right direction as far as materials, design, and parts? I've looked a good bit through the wiki, but I would like some real feedback on this idea.

100 ft straight up is the minimum I am looking for, if possible. Of course, I want to get the most performance and distance as possible.
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Unread postAuthor: OuchProgramme » Wed Mar 12, 2008 8:35 pm

100ft up should be easy. A sabot with ALOT of insulation should be the trick.

Insulating the bottom and top since firing the egg will be a concern
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Unread postAuthor: DYI » Wed Mar 12, 2008 8:44 pm

These criteria can be achieved quite easily - all you need is low pressure, and a long barrel for greater acceleration time. You may also want to include some dead space and choke the flow a bit to reduce the initial shock on the projectile.

I don't have GGDT on the computer I'm using right now, but I think that 200 yards straight up is quite feasible with an aerodynamic sabot and a 10' or longer barrel.
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Unread postAuthor: BC Pneumatics » Wed Mar 12, 2008 9:20 pm

Something like this might fit your needs:
http://www.bcarms.com/product_info.php?products_id=93

Your biggest concern will probably be cushioning the egg against the Gs experienced at take off, but it shouldn't be too bad with some foam padding.
(Eggs are surprisingly durable.)
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Unread postAuthor: bigbob12345 » Wed Mar 12, 2008 9:34 pm

i actually think you should use a ball valve as it has a slower opening rate which would be less likely to damage the egg.
Even with a ball valve 100ft in the air should be easially acheivanle even with a small chamber.
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Unread postAuthor: BC Pneumatics » Wed Mar 12, 2008 10:41 pm

I have to admit, big bob is right. May want to go with a ball valve to reduce shock, just in case.

To hit 100+ft at 90* you will need about a 2" chamber 2ft long, with a 2" ball valve and a 3ft 2" barrel. Should have about a 15% safety marine if 100ft is a requirement. Estimated with 130psi of air. Probably about 160psi with CO2, if you wanted easy portability and a higher rate of fire.
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Unread postAuthor: The_Kangsta » Wed Mar 12, 2008 11:47 pm

BC Pneumatics wrote:Something like this might fit your needs:
http://www.bcarms.com/product_info.php?products_id=93


Sorry for going off topic, but would an inflater like this have enough flow to be useful for a portable bbmg?
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Unread postAuthor: psycix » Thu Mar 13, 2008 7:28 am

bigbob12345 wrote:i actually think you should use a ball valve as it has a slower opening rate which would be less likely to damage the egg.
Even with a ball valve 100ft in the air should be easially acheivanle even with a small chamber.


Im not sure if I can agree with that.
It is the acceleration wich may break the egg, and not the "speed of acceleratio increment".
For consistancy, you could just take an unmodded sprinkler (mod if more power is needed)

Just take some long barrel, fairly low pressure and a sprinkler.
If you use pressure rated pipe its 100% safe and with this all you will achieve your goal.
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Unread postAuthor: jackssmirkingrevenge » Thu Mar 13, 2008 7:43 am

DYI wrote:These criteria can be achieved quite easily - all you need is low pressure, and a long barrel for greater acceleration time.


Agreed, a low pressure high volume launcher is ideal in this case in order to preserve the egg's integrity. Casting a foam sabot around a particular egg will reduce stresses.
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Unread postAuthor: Ragnarok » Thu Mar 13, 2008 9:03 am

BC Pneumatics wrote:I have to admit, big bob is right. May want to go with a ball valve to reduce shock, just in case.

Myself, like psycix, I wouldn't agree - why not just use a sprinkler valve and a lower pressure in the first place? If there is a real problem with the rate of loading being too fast, then it could be left electrically operated.
Lots of wadding, and some prior experimentation to determine a suitable pressure would be fine.

Eggs are actually tougher than people give them credit for - especially when the load is applied moderately gently and evenly to one of the ends. They only break when they're subjected to pretty high local forces, like happens when you drop them - you get a large impulse over a short time, acting on a small area.
Pressure is of course evenly distributed, which means that they should survive reasonably impressive acceleration from an air cannon.
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Unread postAuthor: BC Pneumatics » Thu Mar 13, 2008 10:05 am

I know that eggs are very strong, I have been the class demo for supporting my entire weight spread over 4 vertical eggs. As I mentioned before, a ball valve would only be a precaution, and you may as well err on the side of caution, as I doubt Sade would be very happy about having to do it over.

Psycix, I am not sure what you are trying to say, but I do want to clarify that we are worried about the g-forces experienced by the egg at launch destroying the egg. Since g-force is in fact acceleration (with one G ~ 9.8m/s^2) the slower we accelerate the egg to its final velocity, the less Gs it will experience.

It is still a bit difficult for me to imagine the sloppy internals of an egg doing any damage to the shell during a high g-force acceleration, so it would probably come down to your padding, which would just have to be soft, but retain it's shape. As long as there is nothing hard for the egg to collide with, then I do not think you would haver a problem regardless of what type of launcher you used.

You mentioned that portability and storage were concerns, that is why I opted for some of the smaller methods available. If you are worried about shock to the egg at launch then a ball valve will keep you nice and safe. If you want a smaller cannon that has higher performance, you can go with the sprinkler valve, and most likely not incur any problems.

The choice it ultimately yours, but if it helps you decide, I will drop an egg into one of my sprinkler pneumatics and see what happens. (With some foam around it, it should come out just fine.)
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Unread postAuthor: jimmy101 » Thu Mar 13, 2008 10:58 am

You really don't need padding on the egg so much as you need an egg shaped support.

Padding does relatively little to the G forces that the ammo will experience. Think about it for a minute. The acceleration is dV/dt. All padding does is spread the initial dV/dt out by the additional distance that the padding compresses. Adding even a couple inches of padding will have essentially no affect on the G forces the egg experiences as it moves through a barrel that is several feet long.

What you need to do is spread the force out over as much of the egg's rear surface as you can. If you can perfectly distribute the force then the egg won't break until you get to high enough G forces that the top of the egg collapses.

Perhaps a sabbot cast around the bottom of the egg made from spray foam insulation. You can robably cast the sabbot around the egg in a short length of barrel using plastic wrap to keep the foam from bonding to the barrel.

Long barrel, largish chamber, slow valve like an unmodified sprinkler and you're good to go. Personally, I would make the barrel and chamber bigger than needed and then control the muzzle velocity with the air pressure.

I wouldn't use a ball valve, to much shot to shot variability. Since this is for a class there is a good chance that it'll be fired more than once and it sure would be nice if it behaved the same way every time it is fired.
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Unread postAuthor: BC Pneumatics » Thu Mar 13, 2008 2:28 pm

I really don't think there will be much of a difference. As long as nothing hard comes in contact with the egg, it really shouldn't break anyway. Like I mentioned in my post, all the padding really has to do is keep the egg from running into anything hard. (I.E. the barrel walls) It is acting more like dunnage than padding really.
I still think the most obvious way to go about this, instead of typing out paragraphs and paragraphs, is to drop an egg down a barrel.
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Unread postAuthor: D_Hall » Thu Mar 13, 2008 3:02 pm

Jello in a cup would likely make one hell of a padding. After all, it's molded to the shape of the egg!

(Or is there a requirement for an egg to "fly free"?)
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Unread postAuthor: sade » Thu Mar 13, 2008 4:12 pm

Wow, thanks alot guys! I didn't expect this much help.

Ok, so I'm gonna stick with a decently long barrel, somewhat large chamber, and a slower valve for a starting point. I'll be able to work out the kinks in time, since the project is due in 2 and a half weeks.

Once again, thanks for the awesome responses.
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