Bottle Rocket

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Bottle Rocket
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Division B Champion Solon Middle School
This event was not held last year in Division C



Note: This information is for the 2014-2015 season

In Bottle Rocket participants design, construct and launch up to two rockets made from two empty plastic carbonated drink bottles, which hold 2 liters or less, to remain aloft for the maximum period of time possible.

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. Here is some of what has been learned over the past few years, but 'there is no substitute to experimenting with your own designs!'

Note: 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, think about the standard design for a bottle rocket:


A.The bottle: a standard 2L or less bottle of any shape or color that is not modified in any way

B.The nose cone: cone shape on top of bottle that is beneficial and necessary to achieve winning times with good backsliding, etc

C.The fins: used to add stability to the rocket, generally 3 or 4 of them

Second, the objective of this event is to get your rocket to stay up as long as possible. With the removal of parachutes in this year's rules, it is nearly impossible to predict the times. Some predictions for nationals say slightly over 30 seconds. There are 2 basic factors that will get you time. These are height, and the cone (backsliding). 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 backslide or having a rocket with great backslide that doesn't go that high. This will get you some success but to get the great times you will need a good mixture.

Contents

Time Factors

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. One suggestion is adding a penny at a time until the rocket's height at apogee starts to lower. This should 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 (Cone)

The rules state that "Rockets must not change shape or deploy and type of recovery system." This bans parachutes and any other non-rigid recovery devices. A few of the systems you should experiment with are backsliders and gliders. Backsliders are the rockets that are modified so that at apogee, in stead of flipping over and heading for the ground, they float back in a higher drag configuration than a nose-dive. A great guide to backslider construction can be found in the links below. Next, are the gliders. You will want to be very careful when designing your wings as to prevent violation of the definition of a parachute or the change of shape rule. See below under components for more.

Components

Rocket Pressure Vessel

  • An unmodified standard 2 liter or less plastic carbonated beverage bottle of any shape or color is to be used as your rocket's pressure vessel. The inside of the vessel must be able to be inspected, i.e. it must not be spray painted or covered in opaque duct tape.
  • Labels can be removed, but need to be presented to the judges to prove the size and other information about the bottle.

Recovery System

Passive

Passive recovery systems help slow the descent without changing the profile of the rocket.

  • Backsliders are the rockets that are modified so that at apogee, instead 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 category with plenty of designs to try out. A great guide to backslider construction can be found in the links below.This is the most common recovery design and, if built correctly, can achieve a hang time of over 30 seconds.
  • Gliders have a lot of potential. The issue with gliders this year is that the most successful variations of gliders will deploy the wings at apogee and glide to the ground, which involves a change of shape which is now illegal. If you plan on building a glider you will have to have fixed wings at launch which brings up a whole new list of challenges. 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.

Active

Note: Active recovery systems such as parachutes are not permitted for the 2014-2015 season.

Fins

Fins usually add to the stability of the rocket, 3 or 4 work fine. Tape is the only material allowed to secure fins to the rocket.

Wood is commonly used as material for the fins. However, finding the right thickness of the wood may be tricky. If the wood is too thick it will weigh down the bottom of the rocket and shift the center of balance/pressure, which will affect the trajectory of the rocket's flight. Thin wood is very vulnerable to cracking and splitting, especially if your rocket does not land smoothly. Based on the rules, if your fins (or any other part of the rocket) break off during its flight then that is a violation.

There are many different materials that can be used as fins for your rocket. Your best bet is to find and test different materials.

The shape of the fins is something that is often debated. Generally speaking, the shape of the fins should not affect much especially on the way up, but on the way down during say the backsliding stage, more area can mean more drag for longer times, to a certain extent. Experiment with different shapes, but for the most part, any of the simple designs below will give you fine and equal results.

Fins.png

Nose

The nose (or nose cone) is the main part of your rocket. It is connected to the bottle. Your nose will determine the quality of the flight of the rocket. The tip of the nose must be rounded enough for a standard 2 liter bottle cap to be placed on it with no part of the nose touching the inside top of the bottle cap.

There are many ways and materials you can use to build a nose. Usually, the nose will be mostly conical (hence the name "nose cone") with an opening at the top where you can place your non-pointed object (ping pong balls, etc.). It is important that you can easily re-create your nose cone as they are very accident-prone.

When attaching the nose to the bottle, make sure it is secured. A good way to test this is to try and wiggle it. If you can wiggle the nose after it's attached to the body then it's not attached well enough.

Launcher

If at all possible try to test your rockets with the same launcher that will be used during competitions. Sometimes the event supervisor or director will announce the launcher that will be used during the competition, so if you can obtain that exact launcher then you will have accurate results. The Aquaport II Water Rocket Launcher is a very good (but costly) launcher and was used during the 2011 National Tournament.

Strategy, Balance, and Stability

In addition to the two time factors stated above, there is one more: the weather. Colder weather will have a negative effect on your rocket's performance. Try to waterproof your rocket. In combination with other factors, this will get you the time desired. One possibility is adding balls of clay one at a time until the rocket's height at apogee starts to decrease. This will 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 do not appear to have a large influence on height as long as there are even fins. Again, testing will determine this.


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 maximum 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 balls of clay one 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 do not appear to have a large influence on height as long as there are even fins.

Parachutes (Not Permitted in 2015 Event)


Past Results

Past Results


Tips

Remember, the most important part of Bottle Rockets is repetition, and an excess of about 70+ hours of launching tests is recommended (meaning, several hundred runs for maximum efficiency). Be sure to test all available options and configurations for the best rocket.

Chance is an important factor in bottle rockets. Prevailing winds, humidity, temperature, and other weather conditions can all affect rocket performance. While these factors cannot be completely mitigated or predicted, bottle rockets can be made to function in as many environments as possible through practice and testing.

Links

Backslider Construction Information
Past National Event Chair's web page, includes his past national winning designs
Original active deployment with an airspeed flap
Rockets Away- simulation for testing rockets
A great site for rocket construction
Links and Concepts