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 Basically, a rockets center of gravity (CG) should be qt lest a body diameter in front of its center of pressure (CP). To achieve this, fins are added to put the CP further back, and nose weights are added to put the CG further up. Dragging fins The best thing would to have the fins behind the nozzle of the rocket. This is not possible for a pyro rocket, because it would burn them off, but for a water rocket, it is highly recommended. It is very simple to do, find something to use as struts to hold the fins below the bottle, one can use thin strips of wood, plastic, or metal. I used the shafts from lawn flags (used to mark sprinkler heads and invisible dog fences) these are thin and flexible. Glue the fins onto the struts with hot melt glue, then glue the struts to the bottle with polyurethane adhesive. The easiest and best way (I think) is instead of using individual fins, cut off the bottom and top of a bottle so that it is a tube, then glue it to the struts with hot melt glue, and use PL Premium, Goop, or another polyurethane adhesive glue the struts to the rocket, making sure that the struts are evenly spaced to keep the rocket balanced. Leave about five or six inches between the tubular fin and the rocket body. This type of fin provides the best results for stability, as long as nose weights are added. Traditional Fins If one is going for the traditional rocket look, three of four fins can be added to the body, they should be evenly located, so that the rocket is balanced. The fins can be made of other soda bottles, wood, or plastic. Any shape will work, as long as it provides resistance to horizontal but not vertical movement. Gluing the fins is harder than taping them, because it requires something or someone to hold the fin on while the glue sets. The other alternative is to tape the fins on, then if the rocket crashes and is destroyed it is possible to re-use them. There are pros and cons to both, but with a little practice, it is very easy to make them. Nose Weights Anything can be used as a nose weight; something very dense is best, so that there is still room for the recovery system. Lead fishing weights can be used, as long as they are cushioned so that they don’t cause a great amount of damage if the rocket hits something. Batteries should not be used because they could crack and leak. The important thing is to have the weights evenly distributed. They should be heavy enough to bring the CG at least two inches above the CP. See the section below about finding the CP and CG Nosecones Cutting the top off of a bottle and taping it on a rocket can make the simplest nosecone. Other ways are to cut a tennis ball in half and use it, or shape a piece of foam; these would have a great amount of drag but would be perfect if the rocket has no recovery system. Some people shape the bottles using heat and pressure, these have low drag, but are very tricky to make and require practice. Finding the CP and CG This is fairly simple; all it requires is your rocket, pencil, and a large piece of paper. To find the CG simply balance the rocket on your index finger and thumb. The place where your fingers are is the approximate CG. To find the CP trace your rocket onto the paper, and find the center of gravity of the tracing. Also, it is possibly to find out weather your rocket is stable without knowing the center of pressure, just tie a string around the rocket at the center of gravity and swing it around in a circle, if the nose points forward, then it is stable. |
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Rocketss 112 Thomashire Ct Richmond, Va 23229 (804) 741-2391 |
