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Help: Need a physics teacher to calculate something

Old 05-25-2010, 03:36 AM
  #1  
Hookpilot
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Default Help: Need a physics teacher to calculate something

I'm going to fire the working electric torpedo from my Vosper MTB using compressed air. Need to know a few things to size tubing, air storage tank and working pressure to push a 1 pound torpedo to splash down 30 inches in front of the boat. Here's all the data:

Torpedo tube - 1.546" I.D. x 22" long
Torpedo - 1.500" O.D. x 17" long and 430 gram mass
The system must deliver enough force to propel the torpedo 30" to clear the boat's deck (measured from the front end of the torpedo tube to the back of the torpedo at impact with the water.) The torpedo tube center sits 7 inches above the surface of the water.
The torpedo must travel the first 12 inches as measured from the front end of launch tube to the back end of the departing torpedo without dropping more than 1 inch, or it will hit the deck.

Need to know:
Storage capacity of compressed air cylinder when released to shoot 2 fish clear of the deck, one at a time (thinking of the Robart Air Tank with volume of 43 cu-in at 150psi. I will fire two torpedoes from one air charge or mount two air tanks if I have to.
Working pressure of cylinder needed to shoot 2 fish clear of the deck
I.D. of tubing to deliver enough volume over time to do all of the above

Will be using a Clippard EV-2-6-H electrical pneumatic valve with a 0.060" orifice to blow down the compressed air tank. This valve opens very quickly so I'm told.

Any physics teachers or engineers out there who would like to tackle this engineering challenge? Otherwise I'll do it by trial and error and a good old fashion SWAG (Scientific Wild-Assed Guess) or two...

Thanks,

Mike J.
Old 05-25-2010, 01:58 PM
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Umi_Ryuzuki
 
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Default RE: Help: Need a physics teacher to calculate something

The maximum volume will be the length of the torpedo at it's full diameter.
Once the taper at the tail reaches the front edge of your launching tube, any
additional volume is wasted. Once you have your maximum volume calculated, then
it becomes a matter of pressure.



Now if you are shooting the "fish" consecutivley... one, then two...
Then what must be considered, is that the valves can ofter dump
fourteen cubic inches of air a second. If you have held the valve open
for two seconds to fire the first torpedo, you may not have the volume or
pressure to launch the second torpedo if you are using a fixed volume air resevoir.
Old 05-25-2010, 10:05 PM
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Hookpilot
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Default RE: Help: Need a physics teacher to calculate something


ORIGINAL: Umi_Ryuzuki

The maximum volume will be the length of the torpedo at it's full diameter.
Once the taper at the tail reaches the front edge of your launching tube, any
additional volume is wasted. Once you have your maximum volume calculated, then
it becomes a matter of pressure.



Now if you are shooting the ''fish'' consecutivley... one, then two...
Then what must be considered, is that the valves can ofter dump
fourteen cubic inches of air a second. If you have held the valve open
for two seconds to fire the first torpedo, you may not have the volume or
pressure to launch the second torpedo if you are using a fixed volume air resevoir.
Umi: So the calculated working volume of the torpedo (minus taper is 22.9 cu-inches) how do I determine how much pressure to apply to create an acceleration force great enough to move the 430gram torpedo from rest to strike a point 30 inches in front of the launch tube and 7 inches below the centerline of the torpedo tube which is the surface of the water? I might be able to do the ballistics calculation on the torpedo and then throw in some frictional loss as the torpedo moves down the tube. How would I then calculate the air pressure required to do this? I'll probably end up firing both torpedoes at the same time as they did on the full-sized boat. So I'll simply double air tank storage pressure and volume to do the job. I got a reply back from Scott Lamb, an engineer at Clippard and he suggested that the small pneumatic valve shown in an earlier post doesn't have the flow volume and gave a larger valve, MME-34ZES, with 3/8" or 1/2" NPT tubing as one that might do the job. That valve is large and requires 12V to operate it, but my boat has plenty of room to add the new valve and a small 12V batt. A spring would be so much simpler, but finding one is a challenge. Illinois Spring sent me 4 collapsed springs 1.5 in. in diameter and told me how to forcibly stretch it to the proper length and then heat treat it for 45 minutes at 500 degrees. I did that, but the spring changed in diameter just enough to no longer fit in the launch tube. Thanks for the input.

Mike
Old 05-26-2010, 11:21 PM
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Umi_Ryuzuki
 
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Default RE: Help: Need a physics teacher to calculate something

I don't think I can add any more hints to the concept than I have..
My college physics book is,... well I hope I haven't carried it around with me
all this time... I do have a trig book though.
Old 05-27-2010, 07:09 AM
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mfr02
 
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Default RE: Help: Need a physics teacher to calculate something

The important clever bit is making sure the torp doesn't foul the deck, and that means getting it forward so far before it drops by one inch at 1G, or 32 ft/sec/sec.  
My old school books have had nearly half a century to hide away, so I leave the actual calculation of muzzle velocity to someone younger and smarter, possibly the physics teacher.
Old 05-27-2010, 10:44 AM
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TCHedOff
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Default RE: Help: Need a physics teacher to calculate something

Here's a link to a balistics forum you could post your question:

http://www.absoluteastronomy.com/topics/Ballistics

Old 05-27-2010, 01:16 PM
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Hookpilot
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Default RE: Help: Need a physics teacher to calculate something

Since starting this part of the build, I've spent time digging around physics texts and can come up with simple formulas telling me how fast the torpedo must be traveling to clear the edge of the deck. But what is more difficult to determine is the actual rate of compressed air delivery, which I'm calculating to be at least 19 CFM per launch tube injected in less than a second. Then the final part of the puzzle is to find the correct air pressure at that flow rate to give me a launch velocity of around 3.6m/s. This will place the point of water impact .7 meters ahead of the launch tube. With the right valve, I'll be able to dial in the best pressure. It's been fun working this all out, and I thank all of you for your input. Once I dial in all the variables and get this thing working, I'll post videos of the testing and final result. It should be very impressive to see a 60-inch long Vosper MTB charging along with all three motors at max speed, fire a two fish spread and watch as the torpedo wakes race to their target, which will be me standing on the shore. I don't want to go swimming to retrieve these things. Our salt water pond is not the cleanest body of water around Honolulu.

Mike

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