Bottle Rockets

BOTTLE ROCKETS

DESCRIPTION: Participants will design, construct and launch a rocket made from two empty one liter plastic pop (soda) bottle which will remain aloft for a maximum period of time.

This description hardly does justice to arguably the most fun science olympiad event in existence. Take a normal pop bottle partly filled with water on a launcher and add 60 psi of pressure and you get great times and great learning. To build a competitive rocket there is one key... EXPERIMENT!!!! The more rockets you launch the better idea you will get of what will work and what won't. It also helps to watch other team's rockets at competitions and see how they do. I will share some of what I have learned over the past few years, but there is no substitute to experimenting with your own designs!

WARNING!!!! Before you read any further you must understand that a lot of success in bottle rockets is chance, but also understand that chance favors the prepared mind. First, lets look at the standard design for a bottle rocket. a. the bottle, a standard 2L/1L bottle of any shape or color CAN NOT BE MODIFIED IN ANY WAY b. recovery system, in order to get a winning time you will need a recovery system that works well. c. fins, fins usually add to the stability of the rocket, 3 or 4 work fine

Second, the objective of this event is to get your rocket to stay up as long as possible. There are 2 basic factors that will get you time. These are height, and the recovery system. These in combination will get you the time desired. One strategy is to go all out on one of these by either having a rocket that goes almost out of sight but has a weak recovery or having a rocket with a great recovery system that doesn't go that high. This will get you some success but to get the great times you will need a good mixture.

Height- The most common misconception is that the lighter a rocket is, the higher it will go. This is not fully true. To achieve maximum height you must have stability. Maximum stability occurs when the center of gravity is ahead of the center of pressure. To find the center of gravity simply balance your rocket on a finger. The center of pressure is the point where you rocket would be balanced vertically if held in a strong wind. Achieving stability often means adding weight to the end of the nose cone. Again a good way of finding stability is by launching your rocket with different amounts of weight put in different places. My suggestion is adding a penny at a time until your height starts to decrease. This make it easy to find the optimum balance of your rocket. You will also want to have fins on your rocket, but the shape and size of the fins does not appear to have a large influence on height as long as there are even fins.

Recovery systems- By far the most commonly used (and won with) recovery system is the parachute. This is unfortunately illegal in division C now. So, if you are in division C, skip to the new division C recovery section. This old classic can really surprise you if you happen to be lucky enough to catch an updraft. Parachutes come in all shapes and sizes and materials The most common shape of the parachute is the circle. The circular design gives you the most surface area per material. Some teams have achieved great success with a semi spherical design but this is very difficult to make. The general rule for size of the parachute is as big as your rocket design can take. I have seen some teams with a parachute diameter of 7 feet. This is a bit extreme but it is the best way go for the good time, if you can make it work. Materials range from rip stop nylon (great but expensive) to the good old cut up garbage bag. Again, try different materials and see what works for you individual design. The most common material used on more successful designs is plastic from a drop cloth. You can get this at a local hardware store. Deployment- The hardest part of any parachute based system is getting the parachute to deploy at appogee consistantly. Two mainstream solutions to this problem have been developed. First and most simple, is the passive deployment system. This design uses the fact that the rocket tips over at appogee to make the nose cone fall off. This design usually consists of a nose cone that fits very loosly and is supported by either tabs or a margarin lid. This prevents the cone from being forced too far down at take-off. This way the tipping of the rocket seperates the nose cone and body. The next challenge is getting the parachute out of the nose cone. With parachutes getting increasingly larger, many teams are now using drouge chutes to pull out and open the main parachute. A drouge chute is simply a smaller parachute attached to the larger parachute. Many people also use talc powder to lubricate the parachute allowing it to leave the nose cone with less friction (it also makes a cool white cloud if you use enough). The other solution is active deployment. This involves a mechanical system that detects the appogee and pushes the parachute out of the nose cone. Most of these systems involve airspeed flaps and can be very finicky. More information on these designs can be found in the links below.

Other Recovery Systems - A few of the systems I suggest you experiment with are backsliders, gliders, and propellors. Backsliders are the rockets that are modified so that at appogee, in stead of flipping over and heading for the ground, they float back in a higher drag configuration than a nose-dive. This set includes true backsliders which actually fall vertically without tipping at all immediately after apogee and rockets that modify the aerodynamics at apogee to float. A true backslider can be made by taking your aerodynamically stable rocket and moving the CG back closer to the CP. Other ways of modifying the aerodynamics at apogee include removal of the cone or a ball, or tilting the fins for a "spin" recovery. This is a very broad cateogry with plenty of designs to try out. A great guide to backslider construction can be found in the links below. Next, are the gliders. This design also has a lot of potential. The most successful variations of gliders will most likely deploy the wings at apogee and glide to the ground. You will want to be very careful when designing your wings as to prevent violation of the definition of a parachute. Two things to watch are: 1. That there is a rigid border to the wings and 2. That the covering on the wings is reasonably taut. Finally, you can use one or several propellors to slow the decent. The hard parts about this design is getting the rocket to be positioned so that the propellor(s) will actually turn and hiding the propellors until apogee.

Good luck.