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Burst Diaphragm Test

This Idea, like so many ideas started out with a different function.  The idea of burst disks started out as a noisemaker.  I had taken my old "Green Hornet" gun and  put 2 or 3 layers of paper paint masking tape over the opening on the male chamber threads and screwed a short (2 ft) barrel onto the chamber.  As a result the paper had to be blown up for the gas to exit.  

This made an extremely loud noise, so I posted it on the spudtech forum.  Freefall, a user at the forum posted this: "Sounds like a burst diaphragm gun. Perhaps this would allow higher speeds from combustion guns by ensuring more complete combustion before the spud begins to move?..."

This post really got me thinking about burst diaphragms and how they would effect a spud gun.  I did a preliminary test with 2 different barrels, and the burst disks seemed to dramatically improve the distance the spud traveled.  This preliminary test really got me excited about the possibilities of burst diaphragms.  

I bought some spuds(30 lbs, only used 20 lbs) and supplies for four different barrels.  Barrel 2 would be the "magic" C:B ratio of .8:1.  Barrel 2 was 1.5:1, barrel 3 was 3:1 and barrel 4 was 6:1.  

Sunday morning came and testing was scheduled to take place.  I loaded all of my potato gun testing supplies into my dad's car.  I had 4 barrels, one of which was sticking out the window due to it's length, 2 ramrods, and 30 lbs of spuds in the front passenger side floorboard.  My launcher, with a stun gun attached to the side was setting in the front seat, and my propane meter was setting in plain sight in the back seat.  I took the following pic before I left home, I thought it looked funny at the time.

Here is a pic of the inside of my car.

I started rolling down the street of my small town and was about 2 blocks away from my house when what do I see?  An Iowa State Patrol car behind me.  I think "Oh Crap!"  He turns on his lights but quick thinking, I pull into the local post office and get out of the car.  I started walking up to the door when he stopped me.  He said he pulled me over for a faulty license plate.  The plate was in the window instead of on the front bumper.  I told him that it was my dad's car and I had just borrowed it to check the mail (it was Sunday haha).  So he says he is just going to give me a warning since it's not my car.  He asked me for my license and I got it out of the console between the front seats.  I was playing it cool the whole time, then he looks directly at the potato gun and supplies.  I was sweating bullets.  He went to his car, wrote the warning ticket, came back, gave it to me and said" Have a good day and be sure to have your dad fix the plate."  HOLY CRAP!!!  I was lucky!  Either he was really dumb, really nice, or I just got lucky!  I had a freekin' mobile spud launching cache and he didn't say a word.  I knew he saw them too!

Here is a pic of me with my warning ticket!

After that whole ordeal I arrived at the testing grounds....  

For the test I had decided to record the hang time of the spuds when shot straight up.  Hang time is not a good indication of projectile speed, but is a great test to compare performance with little equipment.  The four barrels mentioned above would be shot with four different variables on burst disks, with 5 tests of each variable.  Each barrel would be tested with no burst disk, for a control, 1 layer, 2 layers and 3 layers of  burst disks.  After looking at the results this should sound much clearer.

The procedure for loading was the same with every launch, with fuel being consistent and mixed with chamber fan for 5 seconds.  after each shot the chamber was ventilated for 10 seconds.

The burst disks were simply a layer of MANCO brand Easy-Mask Painting tape.  To see how to make the burst disks click here.

I predict that the burst disk will increase hang times by a higher percentage in the tests with higher C:B ratios.  Once the C:B ratio becomes more ideal I believe the effect will be minimal, if even noticeable.  I believe the 1 layer tests will increase hang time, while 2 layers will increase hang time even more.  I believe 3 layers will also increase hang time, but far less drastically.  I believe 2 layers will be ideal for all C:B ratios.  


The first conclusion is that the control test helps to confirm latke's work with C:B ratios.  While this test cannot completely confirm the testing, since only four lengths were used, it does point to similar results.  It appears that burst disks (burst diagram) do improve performance of higher C:B ratios.  Barrel 4 was most greatly improved with one layer,  greatly improved with 2 layers, and barely improved with 3 layers.  Barrel 3 was improved somewhat with both 1 and 3 layers, while much improved with 2 layers.  Barrel 2 was improved a small amount with one layer, improved even less with 2 layers, and actually worsened hang times with 3 layers.  Barrel 1 was negatively effected by any use of burst disks.  One layer worsened the least, 2 layers worsened it more, and 3 layers worsened it the most.  The time lost was not extremely great, but the burst tape was detrimental to the hang times of Barrel 1.

It appears that overall shorter barrels (higher C:B ratios) are helped greatly with the use of burst tape.  This means one could make a launcher with a less than ideal C:B ratio, but counteract the effect by using burst diaphragms.  As you go down the C:B ladder, and reach more ideal C:B ratios the burst disks do not help as much.  This makes sense.  The more ideal a C:B ratio, the more efficiently the fuel is used.  The main purpose of the burst diagram was to trap the combusted gasses until they had built up enough pressure to burst the disk, sending a quickly moving gas to accelerate the potato.  The longer barrels of the more ideal C:B ratio, allow the gasses to expand and accelerate the spud for a longer duration.  

As with any spud gun tests, the results pose more questions. 

Mike, official spud testing helper


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