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Rockets

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Observations About Rockets

• Plumes of flame emerge from rockets

• Rockets can accelerate straight up

• Rockets can go very fast

• The flame only touches the ground initially

• Rockets operate well in empty space

• Rockets usually fly nose-first

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History of Rockets

• The Chinese have used rockets for hundreds of years.

• The Chinese used rockets which inspired Francis Scott Key to write the Spangled Banner.

• Now the US military uses rockets around the world to keep peace. Most rockets are remote controlled and go several hundred miles to an exact location.

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Momentum Conservation

• A rocket’s momentum is initially zero

• Momentum is redistributed during thrust– Ship pushes on fuel; fuel pushes on ship– Fuel acquires backward momentum– Ship acquires forward momentum

• Rocket’s total momentum remains zero

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Question:

If there were no launch pad beneath the space shuttle at lift-off, the upward thrust of its engines would be

• approximately unchanged.

• approximately half as much.

• approximately zero.

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Rocket Engines

• Chemical reactions produce hot, high-pressure gas

• Gas speeds up in nozzle

• Gas reaches sonic speedin throat of de Laval nozzle

• Beyond throat, supersonicgas expands to speed up further

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Current Rocket Technology

• X-Prize Rockets• Single State to Orbit

Rockets• Improbable Dreams

– Rockets that rarely require refueling

– Rockets that can land and leave large planets

– Rockets that can turn on a dime in space

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Summary About Rockets

• Rockets are pushed forward by their fuel

• Total rocket impulse is the product of exhaust speed times exhaust mass

• Rockets can be stabilized aerodynamical

• Rockets can be stabilized by thrust alone

• After engine burn-out, rockets can orbit

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Energy Conversion

• Sun gives off electromagnetic energy• Plants capture sun’s energy turn it into chemical

energy.• Plants die and decompose creating fuel which is

chemical energy.• Fuel pushes rocket turning chemical energy into

mechanical (kinetic energy).• Mechanical Energy is turned into thermal energy

as the rocket passes through the atmosphere.

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Key Points• Reduce air friction.

– As a rocket goes into the air you are fighting 2 main forces. Air friction and gravity

• Newton’s 2nd Law (acceleration = force/mass)• You want to keep you mass down. The force will be

100psi for all rockets so the lighter your rocket the higher it will go.

• Newton’s 3rd Law (action/reaction)• The right amount of fuel (water) is needed to push

you rocket up. • Too much water will weigh your rocket down.• Not enough water will not give you adequate initial

force.• Objects fall toward the center of the Earth @ 9.8 m/s• No matter what they weight is objects fall to earth at

the same speed.• The only weigh to keep your rocket in the air longer

is to increase air resistance.

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Helpful Hints

• Keep weight to a minimum. Every rocket has an optimal weight. Small and simple single bottle rockets may sometimes be under their optimal weight, and adding a little weight to the rocket may increase altitude. Due to construction techniques, larger rockets typically come in above their optimum weight and as a result need to be built as light as possible.

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• Streamline the body of the rocket to reduce FRICTION. Avoid any unnecessary protrusions into the air stream. Keep the body of the rocket as smooth as possible, avoiding sharp transitions. The ideal shape is an elongated teardrop. Depending on the construction materials available, a minimal diameter rocket can reduce drag significantly, at the cost of volume. A smaller diameter rocket can also hold higher maximum pressure.

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• Use a launch tube on the launcher. A launch tube can have a significant effect on the apogee of a rocket. The longer the launch tube the better. The diameter of the launch tube should be as big as possible and should be about the size of the nozzle to reduce the amount of water loss as the rocket accelerates up the tube. Note that a maximum sized nozzle may not be the most optimum size after the rocket leaves the launch tube. Consider using a T-nozzle for better efficiency. The 2 liter bottle already is equipped with a t-nozzle.

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• Use the right amount of water. While a third the volume may be a good approximation, every particular rocket will have an optimal water fill based on its weight, drag coefficient, pressure, nozzle size etc. Use a simulator to predict the best amount of water to use for each rocket configuration. Go to this website: to help you determine how much water is optimal for your rocket: http://cjh.polyplex.org/rockets/simulation/

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• Use multiple stages. Correctly designed multi-staged rockets can increase the altitude of the sustainer over single stage designs. Consider your highly optimized rocket sitting on top of a booster. Example: making a parachute or a detachable nose

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• Optimize stage release timing. Releasing the next stage of a multi-stage rocket is critical in maximizing the altitude reached. Use of real-time in-flight measured flight parameters for initiating staging can achieve best results. The best time to release the next stage is just after booster burn out just as the booster starts slowing down.

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• Allow the air to cool inside the pressure chamber.  As air is compressed inside the rocket it is heated. As the air cools, the pressure will drop in proportion to the temperature decrease. You can trickle fill the rocket before launch to make sure the optimal pressure is achieved.

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• Streamline the leading and trailing edges of your fins. To reduce the profile drag of your fins they should have an aerofoil profile. Use fins that are aerodynamic and will not increase drag.

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• Use 3 fins instead of 4 or more. If the launcher allows it, and the rocket is otherwise designed to be stable, the use of less fins should result in less drag and less weight on the rocket.

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• Use optimally shaped fins. The fins should have an optimal shape. See this document for more details. The optimal shape will vary based on the rocket design and the rockets flight profile.

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• Use optimally sized fins. While having the correct fin profile and shape is important, it is also important to not make the fins too large. Fins that are larger than what they need to be add to the drag and weight of the rocket. Large fins may also cause the rocket to be over-stable.

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• Use a rounded nosecone. Parabolic nosecones are the most efficient for water rockets as they travel well in the subsonic range. Here is a nosecone shape comparison document detailing common nosecones used by model rockets.

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• Align the fins properly. Misaligned fins can cause more drag and potentially excessive rotation of the rocket. The rocket looses energy due to drag and some of the energy goes into the rotation of the rocket. Fins should also be as rigid as possible to prevent fins fluttering.

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• Make the rocket stable. Your center of mass should not be in the middle of the fuel cell. You should be able to balance you rocket near the tip on your index finger. You may need to add mass to your nose cone. Go to this website for help: http://www.tclauset.org/21_BtlRockets/BTL.html

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• Point the launcher as vertically as possible. All things being equal, a rocket that flies 2 degrees away from vertical will fly about 0.5% lower, and a rocket that flies 5 degrees away from vertical will fly about 2-3% lower.

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• Grease the launch tube for less friction. If you are using a launch tube with your launcher that has a relatively tight fit on the nozzle, make sure friction is reduced by lightly lubricating the launch tube. Less friction will result in higher take-off velocity.

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Rocket Construction• Step 1: Permit Test

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Step 2: Design• You must draw your

design and show the blueprints to Mr. Butler.

• Include shape of wing and tape

• Include shape of nose cone

• Possible Designs

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Step 2 cont.: More designs

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Step 3: Cut out wings

• Design a template and cut out the wings.

• Be careful when cutting.

• Use tape.

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Step 4: Nose Cone Design

• Design a nose cone for your rocket.

• New research = round nose, greater height

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Step 5: Parachute• Parachute use is optional.

• The best parachutes are garbage bags.

• Wal-Mart sacs do not work

• Parachutes increase air time.

• The key is for the parachute to open at the correct time.

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Here’s how it should fly in t+he air!