Water Rockets. Activity: Bottle Rockets and Propulsion Before you begin, print out a worksheet to use as you complete the activity. Background Information : Bottle rockets are excellent devices for investigating "Newton's Three Laws of Motion" : 1st Law - A rocket will remain on the launch pad until an unbalanced force is exerted, propelling the rocket upward.
Instructions : You will work in teams to construct two water bottle rockets with empty 2-liter soda bottles. You and your team members must decide 1 on the design you will use as a result of experimenting with the simulators and 2 on the materials you will use for the body, fins, and cone of the rocket which is placed over the empty 2-liter soda bottle before launch.
You will then predict how well your rocket will fly and record your prediction on the worksheet. You may want to customize your rockets by decorating them in some way. An example is shown below: One bottle rocket launcher is needed for the class.
While one team launches their rocket, another team can assist them by tracking the rocket, determining how high it flew, and recording the information on a worksheet. Follow this link to learn how to measure the altitude reached by your rockets: Model Rockets, Measured Altitude.
Water rocket enthusiasts have created all sorts of amazing water rocket designs, including this one shown in the video below which is a two-stage water rocket. Next I will get into some of the physics of water rockets. The analysis will be somewhat advanced, but it's a means to an end in which the end result will help you build and set up a water rocket that will reach a maximum height in the air. Water Rocket Physics Analysis The figure below shows a schematic for this analysis.
Note that the mass of the air inside of the rocket is small enough to be ignored C p is the center of pressure of the air drag force for the rocket when it is in flight A is the cross-sectional area of the rocket nozzle through which the water exits u is the velocity of the exiting water relative to the rocket T is the thrust exerted on the rocket, created by the exiting water s is the distance from the bottom of the rocket body to the center of mass G h is the height of the water, as shown v is the velocity of the rocket, with respect to ground The nose cone shown in the above figure reduces air resistance as the rocket flies through the air.
Also note that we are assuming a thin-walled rocket body in which the dimensions d , L , h , and s are approximately the same with negligible difference whether measured from the interior surfaces of the rocket body or from the exterior surfaces of the rocket body.
When the rocket nozzle is opened the water starts to exit at high speed due to the pressure inside the rocket body forcing the water out. As the water exits it accelerates downward due to a large downward force caused by the internal pressure.
By Newton's third law there is also an equal and opposite force pushing upward on the rocket, causing it to accelerate upward. As the water exits, the water height h drops. This causes the center of mass G to drop to a lower level relative to the rocket body, resulting in a lower distance s. As the water continues to exit the distance s eventually reaches a minimum value and then begins to rise again until no water is left inside the rocket body, and the distance s will correspond to the distance to the center of mass of just the rocket body.
As the rocket speed increases it encounters air resistance drag which can cause the rocket to tumble end over end. To prevent this fins must be placed on the rocket shown in the above figure which cause the resultant drag force from air resistance to act at a point underneath G , known as the center of pressure C p.
This will enable the drag force to keep the rocket aligned with its flight path, and tumbling will not occur. The fins must be located low enough on the rocket body so that the center of pressure C p is always below G the position of which changes as the water exits.
This will guarantee that the rocket will never tumble at any time during its flight, and it will therefore be stable as it flies through the air. The trade off of placing fins is that they are a source of air resistance and cause additional drag force on the rocket as a result, but without them the rocket would tumble as it flies through the air and not go very high as a result. We will now develop the equations to model the flight of a water rocket.
The thrust T for a water rocket is the same as for chemical rockets described in the rocket physics page. We can reasonably assume that, when the water is exiting, the volume of air inside the rocket body expands quickly enough so that it has no time to experience either heat gain from the external environment or heat loss to the external environment.
In thermodynamics this is called an adiabatic expansion or an isentropic process , and it can be represented mathematically by the following equation: Where: C is a constant k is a thermodynamic constant, which for air is equal to 1.
On the figure we show a generic launcher , although launchers come in a wide variety of shapes and sizes. The launcher has a base to support the rocket during launch. A hollow launch tube is mounted perpendicular to the base and is inserted into the base of the rocket before launch. The launch tube is connected to an air pump by a hollow feeder line. The pump is used to pressurize the inside of the body tube to provide thrust for the rocket.
We have attached a pressure gage to the feeder line to display the change in pressure in the system. This part of the system is very similar to the simple compressed air rocket. The other part of the water rocket system is the rocket itself.
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