Water Rocket


We had to make a rocket out of a bottle powered only by water and air.


-Guidance System - A guidance system may be a combination of sensors, computers, GPS, and communicators. The guidance system has two main roles during the launch of a rocket; to provide stability for the rocket, and to control the rocket during maneuvers.

-The longitudinal (roll) axis runs lengthwise down the rocket. The rockets tends to twist about this axis.

-The lateral (pitch) axis runs side-to-side. Think of being on a seesaw and moving up and down. That up and down movement is your pitch and it occurs along a lateral axis.

-The vertical (yaw) axis runs top-to-bottom. On a rocket, yaw is very similar to pitch except that the nose moves side-to-side.

On a "real" rocket, movement along these axes is controlled by adjusting the thrust direction, placing rudders on the fins, and other techniques. For a water rocket, we have to pay close attention to the location of the center of gravity and center of pressure (more on that later).

-Drag - Drag is created by air resistance. Imagine running down a track with a small parachute attached to your back. You would notice that it is more difficult to run with the parachute pulling you back versus running with no parachute. The parachute, in this example, causes considerable drag. When a rocket flies through the air, drag pushes back - or resists the rocket's forward motion. A rocket's drag is affected by shape, texture, velocity, as well as other factors. (fins)

-Structural System - The structure of the rocket typically consists of a nose cone, body, frame, fins, and other components depending on how the rocket will be used. These components are usually lightweight yet very strong. Each component has a separate and important function. Here are just a few.

  • The rocket's skin helps to control the internal temperature. This is critically important because the fuel and oxidizer must be kept cool. This is a tough job when rockets often reach extreme temperatures. The space shuttle was designed to handle an external temperature of 3,000 °F on atmospheric reentry!
  • The shape of the nose cone is used to help reduce drag. Also the nose cone may protect the payload, guidance, and recovery system for the rocket.
  • The fins of the rocket are important for maintaining stability. Without the fins the rocket would rotate uncontrollably about all three axes.
  • The frame, or body, helps to maintain the structural integrity of the rocket.

- Weather is very vital to how a rocket flies.

  • Another factor to remember is that conditions can be different not only due to weather but also due to the different levels of atmosphere.
  • Aerodynamic designs are altered based upon the layer of atmosphere a rocket flies through.
  • Propulsion System - In the picture it is easy to see that the rocket consists of mostly the propulsion system. The propulsion system must produce enough thrust to overcome weight and drag to achieve lift-off. There are two main types of rocket propulsion: solid and liquid fuel. The above picture is a liquid fuel propulsion system. Both types of rocket propulsion are still used
  • For a water rocket, we have to pay close attention to the location of the center of gravity and center of pressure (more on that later).
  • This is the force that produces lift-off, or upward movement of a rocket. Thrust is produced by engines or some other form of propulsion system. For lift-off to be achieved, thrust must exceed weight.
  • A combination of factors affect the weight of a rocket and can include cargo, passengers, fuel, parts of the rocket, materials, and other items. Weight is related to mass, but also includes the gravitational pull of the Earth.
  • A force is anything that can influence a change in speed or direction of an object. For rockets, there are four key forces that affect how a rocket flies.
  • A rocket must generate enough thrust to overcome mass, gravity, and drag to achieve a sufficient velocity to exit the atmosphere.
  • While traveling through the atmosphere a rocket must deal with a rapid change in temperature, lack of oxygen, and other atmospheric conditions.
  • Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it
  • Force is equal to the change in momentum (mV) per change in time. For a constant mass, force equals mass times acceleration (F=ma)
  • Thrust is an excellent example of this law. The rocket's movement off the launch pad is a reaction to an action created by the motor. In the case of a water rocket, the rocket reacts to water being forced out of the bottle by moving in the opposite direction
  • Drag acts through the center of pressure and is related to air density, velocity, and the shape of the rocket. Drag always opposes forward motion
  • Thrust acts through the center of gravity, and provided it can overcome the force of weight will propel the rocket forward (or upward).
  • Roll is a twisting motions about the roll (longitudinal) axis. Roll movement is not really a problem on a water rocket since it does not tend to alter the rocket's flight path
  • Pitch refers to an up and down movement of the rocket's nose. This movement can take the rocket off of its intended flight path and could result from a gust of wind. Careful attention to the location of the center of gravity and center of pressure will ensure pitch stability
  • Yaw refers to the side to side movement of the rocket's nose. This movement can take the rocket off of its intended flight path and could result from a gust of wind (very similar to pitch). Careful attention to the location of the center of gravity and center of pressure will ensure yaw stability
  • All rockets will rotate about their center of gravity during flight. A stable rocket will be able to recover from this rotation and return to its normal flight path. This rotation is typically characterized as roll,pitch, or yaw.
  • Weight pulls vertically downward through the rocket's center of gravity (CG)
  • weight opposes thrust
  • heavier objects take more effort to pick up
  • Lift and drag are mechanical forces, meaning the rocket has to be in physical contact with the air to generate the force.
  • a gravitational force is a field force. A field force means that two objects do not need to be in physical contact to generate a force between them.
  • weight = mass x gravity
  • mass = density x volume
  • volume = thickness x width x length
  • Thrust is a force that produces lift-off, or upward movement of a rocket. Thrust acts through the center of gravity and must exceed weight in order to move the rocket off of the launch pad.
  • momentumwater = masswater x velocitywater
  • For every action, there is an equal and opposite reaction
  • momentumwater = momentumrocket ormasswater x velocitywater = massrocket x velocityrocket
  • The variables that you can actually control to affect the velocity of the ejected water are bottle pressure, water volume, and bottle size.
  • Drag slows down the rocket.
  • In order to maintain stable flight, rotation around all three axes - especially yaw and pitch - must be prevented.
  • controlling rotation is more important around the yaw and pitch axes is because instabiliy about these axes would change the intended flight path
  • center of gravity is the "balance point" of the rocket.
  • The center of gravity is at the point were the mass on the nose end is equal to the mass on the tail end of the rocket.
  • To move the center of gravity towards the nose of the rocket, mass should be added to the nose
  • The center of pressure is the point through which the drag force acts and any wind forces.
  • center of pressure is the point where a force acts to cause a rotation around the center of gravity.
  • The way you can change the location of the center of pressure is by adding more material to the nose or tail of the rocket
  • most important method for controlling stability around the pitch and yaw axes is to control the location of the center of gravity and center of pressure.
  • Changing the static margin changes the stability of the rocket.
  • distance between the center of gravity and center of pressure is the static margin
  • A good rule of thumb is that the static margin should be greater than or equal to the bottle diameter (although this is very hard to do with a bottle rocket). A key point to remember is that the center of gravity should be closer to the nose of the rocket than the center of pressure.
  • Active recovery systems tend to be better for heavier rockets, and provide a safer recovery. However, active recovery systems are often more complex, have a higher chance of failure, and add weight to the rocket.
  • The parachute is an active recovery system. When the rocket reaches maximum altitude, gravity causes it to begin its descent back to earth. Since the ping pong ball is not attached firmly and is itself a high drag shape, the ball descends more slowly than the rest of the rocket, thus pulling the parachute out. The parachute then opens and produces extra drag which slows the rocket's descent even more. Thus, the parachute increases total flight time.
  • Prior to launch, the parachute and string are placed inside of the nose cone tube. The string is then attached to the ping pong ball. If the parachute works properly, the ping pong ball will descend more slowly than the rocket and pull the parachute out of the tube.
  • Look for items in the Recovery system work area called Deploy Volume andTube Volume. If the tube volume (what you have actually designed) is less than the required deploy volume, then the parachute will not deploy. (Basically, the parachute is crammed into such a small space that it sometimes will not come out). If this happens, you should increase your actual tube volume by selecting a tube with a larger radius, or increasing the length of the tube.
  • The mass of the ping pong ball may be too small. This is related to the issue described above in that the parachute needs to be pulled out of a small area. Adding clay to the ball will increase its mass and provide more force to pull the parachute out of the nose cone tube.
  • You should design your recovery system to minimize the falling velocity of the rocket. (less weight more parachute diameter)

                                                            Criteria and Constraints:

-Parachute Recovery, Bottle Size, Fin/Launcher Interference, Fin Validation, Parachute Deploy Status

-Air Pressure: 70psi, Nose Length: 228mm, Number of Fins: 4, Parachute Size: 279.4mm, Cost: $6.00


The lighter the bottle with the right amount of pressure made it go higher. Putting the fins close to the neck of the bottle added static margin making the rocket have a good center of gravity and not wobble as much. Adding the parachute and weight to the nose added some weight to the rocket all together but it also increased its distance.


                                                     Key: # = 1 * = 2 @ = 3

                                    P&F Rocket          F325                Lil E               

Cost:                           $2.20@                $2.92*              $2.25*

Nose Length:              218.6*                208.3@              211.2*

Air Pressure:              70psi@               70psi@               70psi@

Parachute Size:            279*                  279.4*                279.4*

Number of fins:            4@                      4@                       4#

Total:                               13                       13                     12


After testing my rocket for the first time with the parachute deploying while it went up, I repacked the parachute and it stayed in the air longer and the parachute slowed the fall.

                                                                Refining the design:

To make my design better I would need to make my parachute just a little bigger, my fins would need to stay on better, a sturdier bottle would be better if possible.


To put my rocket on the market I would need a lot of advertisement, a big warehouse, employees, lots of materials, and I would try to find a way to make the rocket easier/safer to launch.

                                          Video of the F325 water rocket in action:

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