SpudFiles

Come August 2010 the local High Schools have a technology fair.
This year they had Trebuchet contests. A good friend of mine who is in H.S built a trebuchet but was way out classed (he is home-schooled)
well I got to discussing next years competition = air cannons.
scoring is accuracy and distance. included is how "green" is the technology.
plan is two or three large air chambers, a 2 inch (shooting baseballs or tennis balls?) a piston QDV (saw a drawing somewhere on the web??) then intergrade a chronograph (a microcontroller with opto sensors) throw in a wind speed meter.
figuring muzzle velocity, muzzle energy for various weight balls, amount of air pressure should determine amount of air pressure to hit target every time (hopefully)
drawing of planned valve.
planning on two O rings to keep piston square and still have a wide gap between the barrel end and piston when open.
eliminated the bolt down the middle to cut down on weight.
thinking maybe a copper or EMT conduit cylinder?

You mentioned the Quick Dump Valve but drew a piston valve. The QDV uses 2 rings, has no face seal, and doesn't use pressure in a pilot area, so it won't fill from the pilot.

You have a drawing of a double ringed piston valve configured as a barrel sealer. The OD of the piston is nearly the same diameter as the sealing face on the barrel. If the ratio of diameters is too close to 1:1, then the chamber pressure has little to no surface area on the face of the piston to start to open the valve. Close ratio valves are known to be difficult to get to operate properly. They often fail to open either due to friction of movement is higher than the force trying to open it or there is enough leakage past the piston into the pilot area to keep the pressure high enough to exceed the force trying to open the piston.

If you decide to use a barrel sealer piston, having the face seal about 80-90% of the piston outside diameter (piston cylinder diameter) works well as the piston remains closed until a large portion of the pilot area is vented to a low pressure. Then when the piston unseats, it pops open quickly. A narrow ratio barrel sealer piston resembles a Quick Exhaust Valve in operation. They are noted for a characteristic hisss boom when fired.

As far as a wide gap, the area of the gap should be as large or larger then the area of the ID of the valve outlet. You will find the opening requirement is satisfied at an opening of only 1/4 the barrel diameter. This may help reduce the overall length of the piston to reduce the moving mass.

Look in the wiki for more information on the configuration of a Quick Dump Valve. They work well.
http://www.spudfiles.com/spud_wiki/inde ... Dump_Valve

I have already helped a HS as their official mentor in building a competition t shirt cannon. We won.

The shirt laucher contest site is here;
https://inteltrailblazerschallenge.wikispaces.com/

Edit;
Thin shell pistons are known to fail under high forces. A more robust piston would be in your best interest.

Does green technology count by using mostly recycled materials?

Technician1002 wrote:You mentioned the Quick Dump Valve but drew a piston valve.

A piston valve dumps air fairly quickly, it's all relative

which is better?
barrel sealer or quick dump??
if using a quick dump, how does one prevent the gun from moving slightly while pulling back on the rod.
thought about a solenoid or ??
If and when we get this thing built I hope to have working schematic and code for the PIC controlled chronograph.
it was mentioned on another form to place sensors the length of the barrel then measure the acceleration.
am after to eliminate most all the variables with as much info as possibly.

which is better?
barrel sealer or quick dump??

Depends what you are looking for. If you want power go for the QDV but if you are looking for more handheld accuracy go for the barrel sealer because they can be piloted with blowguns, which if mounted properly can be pulled like a trigger.

if using a quick dump, how does one prevent the gun from moving slightly while pulling back on the rod.

The only way i can think of is mounting it in a sturdy frame but it will be the same with the barrel sealer because that still has kick just from firing a golfball and jerking the pilot causing it to pull slightly in one direction.

I wonder why it is that no one has incorporated a trigger system for a QDV? It doesn't always have to be a pull rod to open it.

Hubb wrote:I wonder why it is that no one has incorporated a trigger system for a QDV? It doesn't always have to be a pull rod to open it.

Like what I had done with this?



Since there is no pressure differential on Tech's design, all you have to do is make the piston spring loaded.

Not exactly. I was thinking more along the lines of connecting a trigger to a rod and when the trigger is pulled, it pulls the rod to open the piston.

With your design, though, it would work with the right spring.