Sorry for invading the airplane forum. I'm from the scale boat group and looking for a way to launch a 17-inch electric powered torpedo from my 60-inch torpedo boat and thought a compressed air retract valve and tank might do the trick. The torpedo is 1.5" in diameter and 17 inches long. It weighs just under a pound. My question is does a large scale airplane compressed air system have enough pressure to blow the torpedo clear of the PT boat? I've thought of using a spring, but finding one of the right performance specs has been difficult. I have a feeling that I'd just have to build and test a system to see if I can get enough pressure to push the fish out the tube and clear the boat. What kinds of air pressures do these things run on? Any help would be appreciated. See the boat in action on my web site at: web.me.com/hookpilot. Click on the "Hobbies" link to see build pictures and a video.

Thanks

Mike in Honolulu

Your torpedo is going to need to be SHOT out... the retract compressed air valves won't pass the air fast enough to give it any velocity.

There is enough air in a large retract air vessel for your purpose... the line simply is too small.

consider modifying a paintgun. Actuate the trigger with the servo.

paintball gun...
I'm having computer problems at the moment (FLASH error) and it won't let me edit the above post

Paintball CO2 based guns shoot an appx 1/2 inch dia glob of paint encased in a plastic shell at a high velocity. Should be enough impetus to shoot your torpwedo out of the tube and over the rail

Originally Posted by fhhuber506771

paintball gun...
I'm having computer problems at the moment (FLASH error) and it won't let me edit the above post

Paintball CO2 based guns shoot an appx 1/2 inch dia glob of paint encased in a plastic shell at a high velocity. Should be enough impetus to shoot your torpwedo out of the tube and over the rail

Thanks for the idea. I was afraid that the air volume over time issue would not work for this.

Mike in Honolulu

The number of shots per CO2 cartridge might be low by the time you get the system dialed in... But you only need one shot per tube per time you send the boat out. The real PT had no way to reload topredo tubes without going to the dock.

Or on a smaller scale to save weight you could try a C02 powered air pistol. They run on much smaller and cheaper cartridges and the guns are cheaper than paintball equipment. You would have to figure out if they have the power you need. But they do lauch 5-7 gram projectiles 400fps!

Originally Posted by Mike_Capflyer

Or on a smaller scale to save weight you could try a C02 powered air pistol. They run on much smaller and cheaper cartridges and the guns are cheaper than paintball equipment. You would have to figure out if they have the power you need. But they do lauch 5-7 gram projectiles 400fps!

Too small of an air chamber between the cylinder and trigger.

7 grams * 400 fps = 2800 gram-fps

(1 lb * (1/ 2.2kg/lb) * 1000 g/kg) = 454 grams (close enough)

2800 gram fps/ 454 grams = 6 fps ... maximum.... appx 4.5 mph. It might not make it out of the tube.

The paintball guns have a much larger air chamber which will deal with the added volume of the torpedo tube much better.

You should be able to find some kind of valve that will work with those festo press fittings. The RC industry is unlikely to have anything you need though besides a tank.

Keep It Simple: Use a spring...

Thats what I woudl do, Springs are made in a nearly infinite varieties of sizes and strengths. Worst case is make a sling shot type of launcher using some stretchy rubber tubing.

Check on a solinoid valve from an industrial supply store. They are available in 1/4" and I think you can get them in 12-24 volt operation, power on = open, off= close. One brand name is Asgo. They wouldn't weigh too much for a boat and we give you a rapid dump of air to propel the torpedo.

Originally Posted by whitewolf

Check on a solinoid valve from an industrial supply store. They are available in 1/4" and I think you can get them in 12-24 volt operation, power on = open, off= close. One brand name is Asgo. They wouldn't weigh too much for a boat and we give you a rapid dump of air to propel the torpedo.

Great input here. I was sent 4 springs from a big spring mfg. company. But they were totally compressed. The engineer from the company said to get two people to stretch the spring to the desired length and then heat it in the oven at 500 degrees for 45 minutes. I did that, but the spring grew in diameter just enough from the stretching process to no longer fit in the tube. I have 3 springs left. How do I pull them apart and keep the correct diameter???? This would be the preferred way to do this. Compressed air, valves, plumbing can be done, but I subscribe to the K.I.S.S. principle.

Thanks

Mike in Honolulu

One way to keep the diameter down on those types of springs is to stretch them while they are on a dowel smaller than your desired diameter, and while you and your friend are stretching, twist the spring.

this idea might be outside the box too far.
but if you get a spring and use it to power an air ram hidden in the hull, you should be able to duct the output through a hose of your chosen dia. to your tube. It would allow you to push a largish volume of air at a high velocity. you could if you chose to have an electric drive (ala servo) to retract the spring again or simply recock it manually.
This thought is based on spring powered air gun tech.
http://en.wikipedia.org/wiki/Air_gun
http://entertainment.howstuffworks.com/question649.htm
http://home1.fvcc.edu/~dhicketh/Diff...ngAirModel.pdf

the last link I think is best for explaining and showing pics.

nothing to stop you from using the high energy to move a heavy object slowly rather than a light object quickly, as the exchange is almost a 1:1

if you dont like the idea that ok just a thought. in my mind it was aluminum tube or PVC with a spring powered plunger. might take a bit of machine work to make it work well, something that doesnt disturb me as i have milling machine and lathe.

the other thought that came to mind is not knowing if the torpedo is self powered or not, if not and just for show, you could run a small dia tube up center of both the launch tube and the torpedo, make one fit inside the other and when you apply compressed air it would rapidly push the torpedo off of the rod.

hope one of these ideas helps you out.

Sean