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Step 1
Acquire an ordinary ballpoint pen. Anything that has a straight tube (without a non-removable clip) and that can be disassembled will do, but since you will be destroying the pen, you might as well get something cheap.
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Step 2
Disassemble the pen. Unscrew or yank out the top and bottom plugs, and remove the ink cartridge and any other contents, so that you've stripped the pen down to a bare, empty tube.
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Step 3
File down the inside writing end of the tube using a knife, scissors or a nail file. The idea is to reduce the tightness which the writing end cap fits into the pen tube. You want it snug enough to stay in, but loose enough to fly out under pressure.
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Step 4
Repack the pen. When you point the pen away from you and click it down, the end should shoot off. If it doesn't, you need to shave the inside of the tube more.
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Step 5
Power-up your rocket pen by putting more springs into the mechanism. You can just cannibalize other pens for them.
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Step 6
Make a real rocket pen by purchasing a miniature rocket engine and stuffing one inside the tube, along with a fuse. Glue small cardboard fins onto it to stabilize its flight and also keep the pen upright prior to launch.
