SCIENCE FAIR 2
Smooth off the face of the coupling with very fine abrasive paper. This face will mate with the mouth of the rocket and to make the contact air tight it must be perfectly smooth. Put the abrasive paper on a flat surface and rub the garden hose coupling over it with a circular motion.
Cut a hole through the centre of a PET bottle top. The hole must accept the shoulder of the hose coupling. For me the hole needed a diameter of 19 mm. If you use a twist drill the hole may not be properly round so use an 18 mm drill bit and carefully shave off extra plastic with a sharp knife. Remove what remains of the plastic seal from inside the bottle top.
Make up a gasket from the thin plastic. Cut a circle of the material to the same diameter as the hose coupling and then cut a hole in the middle of it. The gasket helps to make a good seal between the coupling and the mouth of the PET bottle as well as protects the coupling from being scratched as you screw it on and off the bottle.
Put the hose coupling, the bottle top and the gasket together as shown in the photo. No glue or sealant is necessary. This becomes the nozzle for the rocket and can be screwed onto any rocket that you care to make.
With a hacksaw, cut one of the PET bottles into three parts as shown in the figure. The top part will become the nose cone, the middle will become the fin and the bottom can be discarded.
Push the nose cone over the base of the second PET bottle. Tape it in place with some duct tape.
Using the duct tape, tape the struts to opposite sides of the second PET bottle. The struts should point down and extend at least, say, 100 mm below the mouth of the second PET bottle.
Tape the cylindrical fin section from the first PET bottle to the struts with the duct tape. The rocket is complete and should now look something like the one shown in the figure. Note that the longer and further back that you make the fin the more stable the rocket will be.
Testing and Launching the RocketWARNING: There is enough energy stored inside a pressurized PET bottle to seriously injure yourself if it explodes. And the rocket flies with enough speed to break bones if it should become unstable in flight and head back to earth. Please adhere to the safety precautions given below. You need a car foot pump to pressurize the rocket. It needs to be sturdy to withstand the amount of use it will get. And it needs to have an in-built pressure gauge so you know how far it has been pumped.
Pressure TestThe pressure test will allow you to check that there are no leaks in your system and that the bottle will withstand the pressures required.
Fill the rocket completely with water. It must be full of water so that it cannot explode if the PET bottle ruptures.
Attach the rocket to the launcher. Keep yourself and other people clear of the launcher.
Pressurize the bottle to 3 bar (45 psi) and check all joints and connections carefully for leaking water. Make sure that the pressure reading on the pump's pressure gauge does not fall; it should stay constant if there are no leaks.
Increase the pressure to 7 bar (110 psi) and check again for leaks. You will not pressurize the rocket beyond 6.5 bar (100 psi) when you launch it.
Launching the RocketLaunch your rockets only in large open areas where there are no people around. The best rocket flights go straight up and then come straight back down. If the rocket is unstable it can fly in any direction. It will often fly horizontally. Rockets have been reported to fly as high as 90 metres so if it is windy they may land a large distance from the launch pad. Your launch string should be at least 10 metres long.
Push the launch pad into the ground so that the nails hold it stable. Make sure the rocket sits level.
Fill the rocket 40% with water.
Attach the rocket to the launcher and make sure that it is securely held and pointing directly up.
Pressurize the rocket. Use 3 or 4 bar (50 psi) for your first launch in case the rocket is not stable. Do not go higher than 6 bar as the bottle may be at risk of exploding. Keep yourself (and everyone else) as far from the bottle as you can while you are pumping it.
Warn everyone that you are about to launch and check that everyone is paying attention to the launch.
Count down to 1 and pull the launch string (from 10 metres away). If the rocket does not go straight up it may be unstable. To improve the stability of the rocket you must either increase the length of the cylindrical fin or move the fin further back (by increasing the length of the struts).
posted by Patricia @ 3:01 PM
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