SpudFiles

been designing an air cannon for a local high school student to enter into a competition that entails accuracy (targets at 30,60 and 90yards) as well as distance.
plan on drawing board= an 8 ft long rifled barrel, two 4inch x 4ft air chambers, a piston valve design (working on idea to have the release electrically accentuated so each firing is identical. may prevent jerking motion due to the operator pulling back on the trigger rod.
a home brew chronograph to measure muzzle velocity using a 18F452 PIC
a wind speed as well as wind direction measurement
a digital air chamber reading
using a gram weight scale measure the projectile weight then caculate projectile energy.
using all this data, be able to accurately hit the targets at will.
this is a design in progress, any input or advice or if you wish to have a copy of schematic and code(swordfish basic) so far then let me know

You could use GGDT to test some parameters of your gun to get an expectation on performance.

I wouldn't bother with a rifled barrel unless you're using a wad or an ammo soft enough to be forced through the riflings.

The actuation for the piston could be spring loaded which would achieve much better performance.

Consider any exploitable parameters in the competition; could you create a guide line to your target, or does it need to be set up and hit the first time? A dart is probably the best solution for fair accuracy over distances.

Use sprinkler valves to pilot your piston/s mate, they will vent faster and can be linked and fired simultaneously by either electronic or pneumatic actuation.

Edit: Just like this

You may wish to find your basic design accuracy before investing highly in precision. Sometimes buying a micrometer to use with a chainsaw is a poor investment. Start with the basic build and lob a few rounds. See if they land within 5 yards of each other, then you will know the level of accuracy you are working with in your launcher. Playing with an attempt at precision golf ball launching showed the first thing needed was getting the launcher to have high repeatability at all ranges. Dropping a golf ball into a bucket at 10 yards isn't easy.

Friction in a barrel adds a huge variability. I had to discard a foam sabot for that reason and went to a freely moving rigid sabot for repeatability.

For air a good precision pressure regulator is highly recommended. The tiny regulators supplied with cheap compressors are very temperamental in holding a set pressure. Use a regulator with a large diaphragm. I use a welding regulator.

Good luck

What are you shooting?

The design seems overkill for lobby something less than 100 yards. You could omit one chamber and cut the barrel length in half and probably still lob a tennis ball or golf ball or spud 100 yards. (A hand held combustion with a 30"x2" barrel will lob a half spud ~100 yards without any problem.)

Like Tech said, reproducibility is the key and friction between the ammo and the barrel may be the trickiest thing to get to be consistent.

Before you do anything you need to determine which kind of trajectory gives the best reproducibility in range. Which is better a fairly flat trajectory (like a rifle) or an arcing trajectory (like a howitzer)?

First what is GGDT?
the proposed projectile is a base ball.
hav'nt heard weather its a soft ball or hardball?
could even be a tennis ball?
the contest is accuracy at 30, 60 90 yards then distance.
thinking of a foam sabot (sabot = something like wadding in a black powder rifle?)
as I understand it, a long riffled barrel will add up to 50% to the accuracy?
points are to be awarded for inovotion, "green technology, accuracy, distance.
on the valve, just to be sure I am talking about the same animal, its the one Ive seen pictured with a gofe ball mounter on the end of a rod?
yes a spring to begin the triggering then the air flow will force the valve open as I understand it?

here is my first draft of trigger mechisim.
A fork is holding the spring against the cap.
the solnoid pushes the klever thus releasing the fork thus triggering the piston back.[img][img]http://i992.photobucket.com/albums/af44 ... rigger.png[/img]
curde but basic idea.[/img]

MrDEB wrote:First what is GGDT?

http://www.thehalls-in-bfe.com/GGDT

The concept is discussed here as well.

http://www.spudfiles.com/forums/qdv-tee ... 19158.html

It is referred to as QDV (Quick Dump Valve) as called and developed by Technician1002.

several questions
What are these link bars that Technician1002 talking about on his piston to keep the O rings on

Poland-spuds idea of a ported air chamber to trigger the rod to move back is good BUT maintaining the seal on the rod moving in and out? A rotating rod seal is easy but in and out, different animal for sure.
may just go with the spring trigger.
on the piston cylinder using galvanized pipe, I hope you are using a cylinder hone to get a smooth interior wall?
I was contemplating on using muffler pipe?
just need to do some welding.

ohh you mean the idea I meantioned in this thread??

acctually that was iknowmy3tables' design I just proposed that it could be done
I don't think that sealing the rod is such a big deal here... one can always use a coiled air hose... in that way you won't have to worry about seals at all


then just attach a small air cylinder to the rod and hook it up to the main chamber and put a simple two way valve in between (e.g. a blowgun) and you've got a full auto gun

MrDEB wrote:several questions
What are these link bars that Technician1002 talking about on his piston to keep the O rings on..

Here is an explanation. Note there are 3 of them.
Click to elarge

LINK BARS that explains it
At first I thought i was something on the piston itself.

Hmm why not go smaller i dont think you need that huge of an aircannon to get out to 90 yards or so. Why not go for a simple tee design piloted by a sprinkler or some thing? Also are you going to hit a target or have groupings on the ground?

the competition also involves distance
point awared for accuracy, distance, innovation, "green technology"
just imagin showing up with an air cannon that has several wires attached to the barrel(one cat 5 cable) , and a weather station
I know I would be imitated and maybe just go home (yea right)
I mean think about it.
plan is to have cannon covered but all set up.
a rifled barrel, a small black box with an LCD display, an anemometer/wind speed device (design is 10 feet tall (lets really imitate the competitors.
just need to complete the chronograph design, do the programming of chip, de bug chip, build cannon, build weather station, debug the weather station inputs, debug the air pressure input etc.
lots to do.
then the fun begins=LOTS OF TESTING