Objective: To launch a soda bottle rocket, achieve maximum time of flight, and safely land a payload (tennis ball).

Transcription

1 Bottle Rocket Project Objective: To launch a soda bottle rocket, achieve maximum time of flight, and safely land a payload (tennis ball). Materials: 2 liter plastic soda bottle (carbonated beverage bottle) and any safe material you think you can use. You cannot use helium filled balloons or commercially made parts. Timing: Timing of the rocket starts when the rocket leaves the launch pad, and stops when the tennis ball carrying part of the rocket hits the ground, when the rocket disappears from sight, or when the rocket impacts or gets entangled in an object (e.g. the rocket collides with a tree.) Project Guidelines: You can work with one additional person or work by yourself. Each rocket s pressure vessel must be made out of a single 2-liter plastic carbonated beverage bottles with a neck/nozzle opening approximately 2.2 cm inside diameter. The structural integrity of the pressure vessel may not be altered. Examples of altering structural integrity include but are not limited to physical, thermal or chemical damage (e.g. cutting, sanding, using hot glues, or super glues on the bottle that will be pressurized). Adhesive may be used to attach fins and other components but must be limited to glue such as silicone adhesive, polyurethane based adhesives and others that do not damage the structural integrity of the pressure vessel. Rockets may not use sharp or pointed metal components or a leading surface with a rigid spike Fins and other parts added to the bottle must be 2 cm above the level of the flange on the bottle s neck. All energy imparted to the rocket must originate from the water/air pressure combination. All rockets will be launched at psi. Once the rocket is pressurized, contestants may not touch or approach the rocket. WHAT MAKES THE BOTTLE ROCKET WORK? 1. Fuel - Water is poured into the pressure chamber. Air is then pumped into the bottle to pressurize the air in the pressure chamber. 2. Newton s Third Law of Motion - When the pressurized rocket is released, the pressurized air forces the water downward, out of the bottle which provides an upward thrust 3. Newton s Second Law of Motion: To get your rocket to fly to great heights you will need to minimize the rocket s mass while maximizing the amount of force. 4. Stability - Be careful when minimizing mass since if it is too light, the rocket will lose stability as soon as the water is expelled and it will tumble end over end. Whether a rocket is stable or unstable depends upon its design. To be stable, the center of mass must be closer to the top of the rocket than the center of pressure. Construction quality is just as important as design style. Be sure to do a good job while building you rocket for best results. Smooth lines equal less drag. SAFETY CONSIDERATIONS DURING LAUNCH: Stand clear of launch area. Spectators should be 10 m from the rocket when being pressurized. All spectators should pay attention and track the flight of the rocket. DUE DATE: The rocket and pre-launch analysis sheet is due on. This is launch day. 1

2 Water Rocket Structure The basic anatomy of the rocket you will build is presented here. Remember that your objective is to design a water rocket to stay aloft for the greatest period of time and that will carry and land a tennis ball safely. Our rockets consist of an inverted lower 2 liter bottle that is partially filled with water (the fuel) and fitted on the launcher base. This engine bottle is held in place with a metal clip and pressurized to pounds per square inch with an air compressor. When the retaining clip is pulled, the pressurized air and water escape downward out of the bottle, forcing the rocket upward (Newton s 3 rd Law). Students have used a variety of rocket body styles in the past. Feel free to modify the basic designs as needed; however, you must not do anything that would put a hole or weakness in the pressurized, engine bottle. Various types of rockets: a. An engine bottle (2L) with a nose cone. The nose cone holds the tennis ball plus any recovery system. b. An engine bottle (2L) fitted with a second bottle whose base has been removed. The upper bottle slides snugly onto the intact engine bottle (lengthening the rocket can improve the rocket s stability). The tennis ball can be placed in either the upper bottle or nose cone and there is room for a recovery system. c. & d. Both designs feature lengthening the rocket to increase stability by using an intact upper bottle. Fins: Fins are necessary to keep rockets stable by moving the center of pressure towards the back of the rocket (see later section). There are several fin shapes to try. Streamers can also be used, but they also provide a lot of drag on the way up. Nose cone: Maximum height results from three things: maximum thrust, the smallest amount of weight that will provide stability, and minimal air resistance. A strong, smooth and symmetrical nose cone can - decrease air resistance, - aid stability by moving the center of mass towards the front (see later section). - house a recovery system and payload. Recovery System: - A recovery system is something that slows the rocket s decent (parachute, streamers, helicopter, backslider that redistributes the center of mass or pressure so that the falling rocket tumbles). A recovery system will protect the rocket and payload on landing. It is important that you design a parachute that i) opens effectively and functions to slow the descent of the tennis ball ii) folds compactly to stow away properly so that it does not increase drag on the flight up and that deploys and opens after the rocket reaches peak height. iii) You need to test out the parachutes you design 2

3 Rocket Performance THRUST To make the rocket shoot high into the air, an upward net impulse (net force x time) must be exerted on the rocket. In other words, the thrust force must exceed the sum of the drag and gravitational forces on the rocket. In a water rocket, compressed air serves as the propellant. When the rocket is launched, the compressed air expands, and as the rocket pushes the expanding air down and out of the bottle, the air propellant pushes upward on the rocket providing an impulse or thrust. The more compressed air present, the longer the thrusting phase lasts. Adding water to the bottle can significantly increase the impulse of the propellant because the larger exhaust mass directed downward produces a larger impulse upward on the rocket : Ft=mv). However, too much water in the rocket leaves too little compressed air to effectively expel the water. Finding the right air-to water ratio for maximum impulse during launch is essential for good performance. DRAG and WEIGHT Both drag and the rocket s weight limit the rocket s maximum altitude. The drag force depends on the geometry of the rocket and the smoothness of its surface. A more aerodynamic rocket has less drag force acting on it, helping to maximize the upward net force on the rocket during the thrusting phase of the launch (see left figure) and helping to minimize the downward net force on the rocket during the upward coasting phase of the flight (see right figure), in which the rocket slows. So the goal is to always minimize drag. Be sure to do a good job while building your rocket. Smooth lines equal less drag. The relationship between weight and performance is more complicated. Given the same impulse, a heavier rocket will not reach as great a velocity by the end of the thrusting phase as a light rocket. However, according to Newton s 2 nd law a low-mass rocket will quickly slow due to drag forces while a more massive rocket will not slow as rapidly (And remember that the acceleration due to gravity is mass-independent). So the goal is to make a rocket not so heavy that it compromises the thrust acceleration and not too light that the drag force rapidly slows a low mass rocket. STABILITY Even if the mass of the rocket has been optimized, if the rocket mass is in the wrong place, the rocket will not fly stably or nose-first; rather it will tumble and not ascend very high. Throughout the year in physics we have mostly dealt with particle models of objects. A rocket, however, cannot be treated as a point particle because the forces acting on it cause it to rotate around its center of mass () and the torque that each force exerts depends on where the forces are exerted on the rocket. In the force diagram to the right, although both forces, drag and weight, point down, the fact that they are of different sizes and act at different places will tend to rotate the rocket around the center of mass, resulting in either tumbling and an unstable flight (bottom, left figure) or corrective straightening and stable flight (bottom, right figure). If a stable rocket begins to veer, it will straighten back up on its own. More air pressure will be exerted on the lower end of the rocket than on the upper end. This keeps the lower end down and the nose pointed up! To be stable, the center of mass (where gravity acts) must be closer to the top of the rocket than the center of Thrusting Phase Rocket in flight will veer away from vertical F g UNSTABLE Destabilizing net torque causes rocket to rotate further away from vertical and tumble Coasting Phase F g STABLE Stabilizing net torque causes rocket to rotate back to vertical 3

4 pressure (where drag force acts). Whether a rocket is stable or unstable depends upon its design. How to Determine Rocket Stability A rocket that flies straight through the air is said to be stable. A rocket that veers off course or tumbles is said to be unstable. Whether a rocket is stable or unstable depends upon its design; you must design it so that the centers of mass and pressure are in the correct places. All rockets have two centers. 1. Center of mass (). This is a point about which the rocket balances or rotates. The force of gravity acts at the. You have learned that if the center of mass of an object is directly above or below a support then the object is stable. You can find the rocket s by finding the position where it can be supported stably. The picture to the right shows a rocket suspended from a string. When the string is placed so that the rocket hangs horizontally, it is positioned exactly beneath the rocket s center of mass. (This rocket looks like it should hang with its tail section downward. What you can t see in the picture is a mass of clay placed in the rocket s nose cone. This gives the left side as much mass as the right side. Hence, the rocket balances.) The lies along the line directly above (or below) a support point when the rocket is stable (the force and torque due to gravity is balanced by support). is in geometric center. The center of mass is important to a rocket. If the rocket is unstable, it will tumble around the center of mass in flight the way a stick tumbles when you toss it. 2. Center of pressure (). This is a point where all the aerodynamic drag forces act. It is the position where half of the surface area of the rocket is on one side and half on the other. The center of pressure of a rocket is the middle point. Air strikes the surface of the rocket as the rocket moves. You know what this is like. If you stick your arm outside a car window when it is moving, you feel pressure from the air striking your arm. The center of pressure is different from the center of mass in that its position is not affected by what is inside the rocket. It is only based on the rocket s shape. Depending upon the design of the rocket, the center of mass () and the center of pressure () can be in different places. - When the center of mass is in front of the center of pressure (towards the nose end), the rocket is stable because the torque due to drag acts to straighten the rocket to the vertical (see figure at right: bottom, right). - When the center of pressure is towards the front, the rocket is unstable because the torque due to drag causes the rocket to tumble around the (see figure at right: bottom, left). A simple way to accomplish stability is to lower the by placing fins at the rear of the rocket and raise the by placing extra mass in the nose. Rocket in flight will veer away from vertical. Only drag force exerts a torque around. UNSTABLE Destabilizing net torque causes rocket to rotate further away from vertical and tumble STABLE Stabilizing net torque causes rocket to rotate back to vertical 4

5 FOR YOU TO DO: Determine Your Rocket s Stability Look at the rockets on the left. One is stable and the others are not. is shown with a back dot and with a blue Scale Diagram dot. Rocket B is the most stable rocket. Rocket C will definitely tumble in flight. Rocket A will probably fly on a crooked path. Any cross winds encountered by the rocket as it climbs will cause it to go off course. 1. Draw a scale diagram of your rocket on the graph paper provided. Make it exactly like the shape of your rocket as seen from the side (see figure at right). 2. Center of Mass Determination: a) Tie a string loop snugly around your rocket so that you have one long end to hold. Except for the water needed for launch, your rocket should be set up exactly as it will be during launch. b) Slide the loop until the rocket hangs horizontally. When it hangs horizontally, the string is at the rocket s center of mass (see fig at right). Mark that spot in the middle of your rocket on the scale diagram (see fig of scale diagram). 3. Center of Pressure Determination (film this to verify): a) Cut out a silhouette of your rocket from a piece of cardboard. Make it exactly the same shape and size of your rocket as seen from the side. b) Balance the silhouette on the edge of a ruler. The center of pressure of your rocket is where the ruler is located (see fig at right). Mark that spot in the middle of your rocket on the scale diagram. 4. If the center of pressure is before (towards the rocket s nose) the center of mass, add some additional weight to the nose of the rocket and/or increase the size of the fins. Repeat the tests until the is in front of the. 5. Swing Test: Verify your design results by doing and filming a swing test. a) Balance the rocket again with the string. Use a couple of pieces of masking tape to hold the string loop in position. Determination of Rocket Determination of Rocket Cardboard cutout of silhouette of Rocket Tie string around rocket (without water) and find position of string where rocket balances horizontally. Trace and cut cardboard silhouette of rocket (same shape and size). Find position where cardboard silhouette balances horizontally. b) Stand in a clear area and slowly start the rocket swinging in a circle. If the rocket is really stable, it will swing with its nose forward and the tail to the back. On flight day, hand in the scale diagram (pg.8), the PreLaunch and Launch Analysis Sheet (pg.7) and submit the film of the determination and Swing Test. Other references to consult: 1. NASA site with LOTS of information about rocket design and flight: Recovery systems 5

6 Bottle Rocket Check List and Score Sheet Rocket Construction Criteria (2 pts each) Pressure Vessel is 2 liter carbonated beverage bottle. Pressure vessel has NO punctures/holes so that it holds pressure. No commercial rockets or parts, all parts 2 cm above the level of flange Rocket design and craftsmanship shows careful planning and creativity Fins to stabilize rocket flight are well designed (smooth and uniform) Nose Cone to stabilize Rocket flight Recovery system or something to slow descent Pre-Launch and Launch Analysis Sheet submitted. Stability Scale drawing completed and submitted Videos of Swing test and determination submitted Rocket Flight Score Stable Flight. Time to peak: 0-1.5s (2 pt), s (4 pt) s (6 pt) >3.5s (7 pt) Recovery: Time from Peak to Landing (0.5 points per second) Final Score /Max in class 6

7 Pre Launch Analysis (to hand in on flight day) Rocket Engineer Name: Rocket Engineer Name: Picture of Your Rocket Rocket Specifications Total Mass: g Number of Fins: Total Length: cm Length of Nose Cone: cm Width (widest part): cm Circumference: cm Recovery system: Rocket Stability Center of Mass () Distance from Nose: cm Distance from Tail: cm Center of Pressure () Distance from Nose: cm Distance from Tail: cm Distance of from : cm Did your rocket pass the swing test? Hand in the scale diagram and silhouette used to determine your and Time Trial 1 Time to peak: Time Trial 2 Time to peak: Time Trial 3 Time to peak: Launch Analysis Total time in air: Total tine in air: Improvements made: Total time in air: Improvements made: 7

8 STABILITY: Scale Drawing of Final Rocket with and Indicated (to be handed in on flight day) 8