Posted: 04/04/2009 19:01 PM Post subject: Quick Dump Valve
Senior Technician
Joined: 04 Apr 2009 Posts: 5010 16687.23 Spud Bux
I have been working on a different valve style to overcome some high speed air flow problems. I have a youtube video of the valve. I originally built a 2 inch version for a competition. I just completed a smaller version that shoots marshmallows. It explodes full soda pops. It punches holes through empty pop cans and busts out the bottom of 5 gallon poly pails all while using marshmallows for ammo.
Here is a video of how it works. The construction details will come later.
I am working on a larger one using a higher pressure tank, an old Propane tank.
This is a working model of the 2 inch version built for a t shirt launcher competition. I have the 2 inch prototype shown in the video. The school entry that won the competition is shown in this video. They show the breech loader more than they show the valve. Note the trigger is pulled just like the little model.
Nice valve. I would add in a spring and a sear so you could have a real trigger.
Could you build a piston valve cannon of the same size but with full porting and compare fps readings. I don't quite think that your design has the same flow as a regular piston valve. Unless you have two pipes and the piston kind of goes into the chamber and the two o rings sit in different pipes so you have full porting.
Very good catch. The original post is on Wikispaces on the official constest wiki. Because only instructors and mentors could post, I later joined crazybuilders and posted the t shirt cannon and the little brother the marshmallow cannon on there. Another user posted them here. To post videos on crazybuilders, (wikispaces could embed videos, sound, pictures, etc) I opened a youtube account to post the videos. Now that they are on youtube, i can put them here and on crazybuilders.
Edit; Crazybuilders dropped the forum, so it is no longer there.
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Waitaminute...
Tshirt launcher, Intel sponsoring, general cannon description...
I have this feeling your design arrived here earlier via another route.
The very beginning including construction photos can be seen at the original location here;
https://inteltrailblazerschallenge.wikispaces.com/message/list/home
The discussion is pretty dead as most of it moved to email as each team tried to keep secrets until the designs were turned in for judging. Use the nav menu on the left for the good stuff.
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What is the use of pressure in the chamber after the projectile has left?
The tank discharge time is 14 ms into an empty space. A full t shirt launch takes about 20 ms to go from stopped to exit. On an expansion and volume basis, if we launch at 25 PSI, we have about 10 left. If we launch at 100, we have about 80 left. Shutting this off at projectile exit is kinda hard.
jonnyboy wrote:
Nice valve. I would add in a spring and a sear so you could have a real trigger.
Could you build a piston valve cannon of the same size but with full porting and compare fps readings. I don't quite think that your design has the same flow as a regular piston valve. Unless you have two pipes and the piston kind of goes into the chamber and the two o rings sit in different pipes so you have full porting.
The porting was of extreme interest to us. The math is easier on the 2 inch valve, so here are the important bits. The area of a circle is Pi X radius squared. The 2 inch outlet pipe has a cross section area of 3.14 square inches. The 3 ports are 2 inches wide and an inch tall. That's 6 square inches of ports feeding directly into a 3.14 square inch tube. The porting is not a problem.
As evidence of flow, here is the little 54 cu in tank dumping thru a 1 inch valve into a 1.25 inch barrel. There is no projectile. This is just air flow.
Unlike most piston valves, this has the ports right at the outlet of the tank, so I can use various size screw on barrels. The distance from the piston to the outlet is only 2 inches in that cannon. We launched a 4 inch foam ball out the 4 inch tube at over 500 FPS. That said a lot for our airflow.
We launched an apple out the 2.5 inch barrel at 882 FPS using 80PSI if you are interested in speed.
Here is an impact with a water bottle from the 2 inch valve;
What is the use of pressure in the chamber after the projectile has left?
The tank discharge time is 14 ms into an empty space. A full t shrt launch takes about 20 ms to go from stopped to exit. On an expansion and volume basis, if we launch at 25 PSI, we have about 10 left. If we launch at 100, we have about 80 left. Shutting this off at projectile exit is kinda hard.
Don't worry about that bit, that is actually a piece of my forum signature which hides under all my posts, it's to do with a design I'm thinking of. It's there to get people to ask why its there
Good to see you finally saw the light and joined spudfiles!
I'm using this valve for a cannon, a much smaller one though.
15mm barrel and 28mm chamber coaxial design, with a tee leading off for a handle, simply because these were a few offcuts I had.
I think it lends itself more to a coaxil design, unless you connect up a mechanical trigger to the pilot rod.
882 fps with an apple at only 80 psi? that is so awesome!!!!!! I get it you could make a 1/4" coaxial one that would propel steel BB's at high speed too?
If you where to connect a piston at the back, wich gets pushed back by the outgoing pressure,closing the valve rapidly, you've got all the benefits, yet multishot capability.
Look down the page to the pipe for the 2 inch valve with the rectangular holes in it. In the marshmallow cannon, I put them even closer to the end of the pipe. The edge of the holes is only 1/4 inch from the base of the threads. This was designed for maximum airflow, and little restriction.
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What is the use of pressure in the chamber after the projectile has left?
As far as the pressure remaining in the tank, with a long barrel and low pressure, I can run the pressure negative. On this page http://inteltrailblazerschallenge.wikispaces.com/Barrel+length+trim+me thod
I have some in barrel acceleration graphs. Shown is a low pressure shot where the projectile accelerated, peaked, and then decelerated to about 1/2 the initial speed as it drew below 1 atm in the tank.
This happens when the volume in the tank at the operating pressure is less than enough to fill the volume of the barrel and projectile inertia is enough to carry it out anyway.
Shots with just enough pressure to fill the volume of the barrel are silent for truly stealth shots. A small chamber at a higher pressure can be very efficient with the air supply in this regard. Most people want the FPS, so they go larger on the chamber to keep the pressure up in the barrel during the shot.
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882 fps with an apple at only 80 psi? that is so awesome!!!!!! I get it you could make a 1/4" coaxial one that would propel steel BB's at high speed too?
Why not? Remember as the area decreases in the smaller diameters, the force drops. Friction becomes a serious enemy. Look for future posts on some design consideration for very low friction o ring use.
The simple soulution is to simply unscrew the 1.25 inch barrel and screw on a 0.25 barrel adapted to a 1 inch pipe fitting and start shooting.
I learned a lot in using o rings. Some learnings appear to be dead wrong at first glance, but that is for a later discussion.
Due to the thin metal of the disposable tank, we were hesitant at running at high pressure until we got a full hydrostatic test completed. That is why the apple was launched at only 80 PSI in the large cannon. After the higher pressure fruit launches, we felt safe to use the shirt launcher at 50 PSI and below.
At 20 PSI, I can hit the roof of the Rose Garden Arena with the long 3 inch barrel and a rolled up t shirt.
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If you where to connect a piston at the back, wich gets pushed back by the outgoing pressure,closing the valve rapidly, you've got all the benefits, yet multishot capability.
Speed is the issue, especially on smaller volume chambers. I haven't done the discharge time on the little tank yet, but the large 700 cu in tank completely empties in only 14 ms. Getting something to cycle the piston that fast without breaking the reset pin would be difficult as the acceleration forces would be insane. I suspect the little tank is able to completely empty in well under 10 ms. I'll do some pressure discharge graphing later to find out.
For multi shot, I have toyed with a way to make it work. After testing, I'll possibly start a thread on one.
There are 2 ways to test the discharge time of your valve cannon. 1 is high speed photography. Look at the fog, smoke, or mist in this sequence;
https://inteltrailblazerschallenge.wikispaces.com/Photos
The start of the flow is indicated by the label on the bottle waving in the breeze and the end when the flow stops. The other way is to use a high speed transducer. A very light piston in an air collum can be photo or magnetically detected as a fast pressure gage.
***EDIT The O Ring thread is started and the associated video shows the valve opening force*** http://www.spudfiles.com/forums/viewtopic,p,239004.html#239004
Last edited by Technician1002 on 09/09/2009 1:36 AM; edited 1 time in total
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