Building an AIR CANNON, question 1

[ithemark]

Considering pneumatic air cannons:

When the valve is opened and the air travels from the compressed air chamber to the barrel, is it desirable to have the valve/barrel openings the same or should it choke at the barrel?

Would a choke make the air move faster thus satisfying the momentum equation, or is this necessary at all?

 

[ivymike]

uh, what do you mean about satisfying the momentum equation?

 

[JStephen]

I think it would be desirable to have all the openings as large as possible, as smooth as possible.

 

[smcadman]

You might want to post a question on a forum dedicated to spud guns that can answer most all your questions, the Spudgun Technology Center:

Above is a picture of the massive custom air cannon I recently built for the US Dept. of Agriculture. It's a tornado simulator designed to fire a 12 foot long, 15 pound 2 x 4 at a wall at 100 MPH.

http://www.spudtech.com/

Incidentally, I noticed this is your first post here on Eng-Tips, and you might want to keep the questions work-related. I got red-flagged many moons ago for a question on air cannons.

Flores

 

[WilliamH]

smcadman,

Do you think this site may be monitored by Dianne Feinstein and, or Sara Brady?

If it is, we have problems in my opinion . . .

 

[ivymike]

That sounds like a joke, but just in case - the site is monitored by you and me and the other ~500,000 members. Many of us prefer to find our work-related help quickly and easily, without wading through tons of threads about hobbies, broken cars, doing homework, etc. The red flag button is at the lower edge of this message, if you find the message or the whole thread inappropriate (for the site), please feel free to click it. I'm guessing that someone will (I've had a few of mine go that way too, and I've certainly clicked the button dozens of times this month, mostly for student or non-engineer posts). If Dianne Feinstein or Sara Brady were members of this site, they could click the RF button - and I expect they'd have trouble leaving posts of their own that didn't end up RF'd.

 

[bklauba]

Perhaps this is the type of air cannon used to dislodge undesirably cohesed bulk material in a large hopper or similar vessel. Coal is one of the more common materials this is applied to. Thus it might be work related.

Only the OP knows.

BK

 

[MikeHalloran]

Or, the question might be related to a particularly large "air rifle". Which would be a good search term to use on "Google For Gun Nuts". Air rifles are mature and well documented armaments, so the answer is available, somewhere.



Mike Halloran
Pembroke Pines, FL, USA

 

[Ataloss]

I thought it was just asking about homework questions that was frowned upon.

A lot of times engineers (and non-engineers too) come up with some really creative and well engineered things at home for their hobbies, and sometimes they get stuck and need to ask for help. I've seen quite a few well engineered RC airplanes, weekend road racers, even home brew setups/tools, and if someone has a question about something like that, they should be able to ask it, as long as it's engineering related.

 

[Pud]

Hi,

Not a trivial question indeed....

We have customer who has got an air cannon which fires an aluminium slug 500g (I think), at 400mph! It is used to test laminated ballistic screens for trains. The air reservoirs are about 6 ft diameter and about 15ft long x2. Great fun!

Rgds


Harry

 

[berkshire]

ithemark (Civil/Environme)
The Federal Aviation Administration had an air cannon to shoot chickens at aircraft wind shields for certain tests.
It pales in comparison to the things the fanatics called Pumpkin Chuckers are producing now. This is some serious if light hearted engineering.

http://www.geocities.com/zbig10inch/

B.E.

 

[rorschach]

air cannons are also used to generate seismic pulses for subsea seismic work. So I would not assume this is not work related.

 

[Darken99]

F=P*A so if your choke is the same size as the tube, you will have the same force coming out of the barrel opening.

Although a smaller barrel will have more air friction loss then a larger barrel, but the choke will have more air friction loss then a smaller tube.

To go into more detail, the size of barrel and pressure would need to be known.

 

[GBor]

Darken makes a good point and, perhaps more importantly, gets back to the original question. The entrance into the choke being the same size as the tube would minimize friction, but only maintain pressure. If you narrow as you exit, you should pick up pressure/speed, I would think.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group

 

[smcadman]

Objection your honor, leading the witness! Tell them ithemark, this is work related isn't it? Will this turn into another thread where there is 20 replies, and the original poster doesn't return?

Flores

 

[Jordonlaw]

ithemark,

I had some dealings with an air gun (cannon?) used for ballistic testing (3grms at 3km/s). This was two-stage so the first stage 'projectile' was a plastic plug that was forced down a tapering tube. This increased the pressure and hence speed applied to the second stage which had the specimen/sabot assembly. Guess you could play with the numbers to get the characteristics you want.

I played with the bird guns as well; 1kg at 480kts.

 

[ornerynorsk]

Hey, are you guys lawyers or engineers ?!?!?! LOL.

 

[smcadman]

trebuchets are a no-no also...DAMHIK.

Flores

 

[machmech]

Hi ithemark

I don't think you should choke the barrel unless you
want to compress the wad holding a multitude of
projectiles to maintain a smaller pattern at a
given range, this is what a shotgun does. For accuracy
I would rifle the barrel, this of course would depend
on the projectile following the groves but smooth bore
is much easier. Maintaining minimal friction, maximum
gas seal and pressure throughout barrel travel is what
you need.

Chuck

A government which robs Peter to pay Paul
can always depend on the support of Paul.
- George Bernard Shaw

 

[ornerynorsk]

Speaking of tapered barrels, the germans had an anti-tank shotgun known as a squeeze gun during wwii. the barrel tapered from 28 down to 20 mm. There is a point where you would get negative return on this effect, dependent on the projectile. Just my 2 cents, I think this is a very appropriate posting. There is a lot of physics behind air cannon, and they are used in industry. Who hasn't messed about in the shop with the blow gun and various projectiles ???