Physics of Model Rockets

Forces

In its most simple form, a rocket is a chamber enclosing a pressurized gas. With a small opening at one end of the chamber allowing the gas to escape, the rocket is given thurst and propelled into the air. Slowing the rocket as it glides into the air is air resesistance while pushing the rocket back towards the ground is gravity. Controling the rocket's motion is the three major forces involved in model rocketry: thrust, gravity, and air resistance.

A scientific explanation of these forces involved in model rocketry is based in a book published in 1687. It was this year that the English scientist Sir Issac Newton published his fundamental work, Philosophiae Naturalis Principia Mathematica. In the book, Newton formulates three main laws of motion:

1. Objects will continue in their state of rest or motion unless acted upon by an unbalanced force.
2. Force is equal to mass times acceleration.
3. For every action there is an equal and opposite reaction.

Newton's First Law of Motion

Rest and motion are opposite concepts. Rest is when an object is not moving in relation to its immediate surroundings while motion is when an object does move in relation to its immediate surroundings.

In model rocketry, like a ball in a hand, forces are acting upon the object. When the rocket is on the launch pad or the ball is at rest in the hand, the forces acting upon the objects are balanced. The surface of the pad pushes the rocket up while gravity pushes the object down just as gravity pulls the ball down as the hand of a person holds the ball up.

Thrust, however, unbalances these forces. When the enginges of a rocket are ignited, the thrust from the rocket pushes the rocket more than gravity pulls it down. When this condition is reversed and the fuel of the rocket producing the thrust runs out, the gravity of Earth's pulls outbalances the thrust and pushes the rocket back to the ground.

Newton's Second Law of Motion

This law of motion involves mass (m), acceleration (a), and force (f). As a formula, it is represented by f = ma.

This formula can easily be applied to cannons and rockets. Force is the pressure created by the explosion inside the rocket's or cannon's enginges. It is that pressure that accelerates the gas one way and the rocket or cannon ball the other. The greater the mass of fuel burned, the faster the gas produced can escape, and thus the greater the thurst given onto the engine.

Newton's Third Law of Motion

This law allows for a rocket to push off the gas and the gas to push off the rocket. This can be demonstrated by ridding a skateboard when the rider jumps off (action) and the skateboard continues to move in the opposite direction (reaction). With rockets, this same principal holds true for rocekts. The action is the expelling of gas out of the engine, while the reaction is the movement of the rocket into the sky.

Stability

For a successfull rocket, stability in flight is a must. A stable rocket is one that flies in a smooth and mainly uniform direction. An unstable rocket is one that flies in an erratic path, sometimes unable to predict in the direction that it will go.

While a rocket is in flight, instability can take place around one or more of the three axes. The point where all of these axes meet is called the center of mass. The center of mass is important in the science of rocketry because it is around this point that an unstable rocket will tumble. The pitch axis and the yaw axis are the most important to insure stability across because even the slightest movement in either of these two directions can easily cause the rocket to go offcourse. The roll axis is least important because movement across it will not affect the rocket's flight path.

Another important indicator of stability is center of pressure. When air is moving past the rocket in flight, pushing against its outer surface, sometimes it can cause the rocket to begin moving around of its three axes. It is important that this center of pressure is behind the center of mass. If they were in the same place or the center of pressure was in front of the center of mass, the rocket would try to rotate about the center of mass in the pitch and yaw axes, causing a potentially dangerous situation for those involved.

Model Rocket Parts

Various parts are present inside a a model rocket.

• Nose Cone - the conical head of rocket which reduces drag.
• Payload - the part of the rocket which is empty and can hold anything.
• Body Tube - the middle part of the rocket.
• Recovery System - the system which triggers the parachute and allows the rocket to come back to the Earth at a slow speed.
• Launch Lug - the lug which allows the launch rod to hold the rocket straight while it is placed on the launch pad for lift-off.
• Recovery Wadding - the protective covering which protects the recovery wadding from being burnt by the engine.
• Fins - the wings at the bottom of the rocket that balance and provide stability to the rocket during flight.
• Engine Mount - the part that holds the engine firmly in place.

Rocket Engine Parts

There are many parts that make the engine an intergal component of any rocket.

• Clay Nozzle - the piece at the end of the engine that focuses the exhaust crating.
• Propellant - the chemical designed to combust and allow the rocket to take off the gound.
• Coast Phase - the part of time where thrust is no longer provided by the engine.
• Paper Casing - the paper wrapping around the engine that pushes out the rocket nozzle.
• Ejection Charge - the charge that emits gas which activates the rocovery device.
• Clay Retaining Cap - A cap used to hold in the combustion gasses.

Flight Stages

The following are stages in a rocket's flight:

• Liftoff - in this stage, the engine starts to the rocket which is being guided by the launching rod into the air.
• Thrust Phase - in this phase, the engine continues to accelerate the rocket and the fins take over as the main stabilization device of the rocket in allowing it to follow a straight path.
• Coast Phase - in this phase, the rocket is allowed to continue to travel up, even though the engine is no longer creating thrust.
• Ejection - in this last stage, the engine ejects its recovery system.

Predicting Height and Duration of Flight

From the information inputed into the Model Rocket Information Database and predicted by myself after figuring a possible change in direction:

Predicted Height: 74.5 meters
Predicted Duration: 2.00 seconds

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