Calculating Stability

Project Overview

The fins on a rocket provide a corrective lift force which keeps the rocket flying straight. This is called stability, and is directly related to the center-of-gravity and the center-of-pressure.

This project is all about building a rocket, and evaluating the stability before launch.

Goals

- Learn about the parts of a rocket and the forces involved in a flight.

- Learn about stability, the center-of-gravity and the center-of-pressure.

- Build a rocket.

- Perform tests and stability calculations

- Launch their rocket and evaluate the results

Materials

- Apprentice Bulk Pack (1 rocket per student)

- Hobby Knife or Scissors

- Wood Glue

- Altimeter

- Cardboard

- Pencil & Ruler

Details

The stability of a rocket depends on the location of the center-of-gravity and the center-of-pressure. If the center-of-gravity is ahead of the center-of-pressure, the rocket will be stable in flight. The further ahead it is, the more stable it will be.

Day 1

- You will learn about the parts of a rocket, and the forces involved in a flight.

- You will build a rocket kit.

Day 2

- You will test and evaluate the stability of your rocket. You will find the center-of-gravity and center-of-pressure and calculate the stability margin.

Day 3

- You will launch your rocket and evaluate the results. Finding the Center-of-Gravity and Center-of-Pressure The center-of-gravity (CG) is where the weight of the rocket would balance on a fingertip. Finding the center of gravity can be as simple as that. Balance the rocket, mark the location, and measure from the tip of the nose cone to the marked point. The center-of-pressure (CP) is a bit more complicated. The true method for finding the center-of-pressure lies in the Barrowman equations, and can be pretty complicated. We will be doing a simpler technique called the cardboard cutout method. These results are less accurate, but should work well for our purposes.

Stability Worksheet

Center-of-Gravity:

Using the method listed above, find the center-of-gravity. What is the distance from the nose cone to the center-of-gravity? ________________________ Inches.

Center-of-Pressure:

Take the rocket, and lay it down on a piece of cardboard. Take a pencil and outline the rocket. Using scissors or a hobby knife, cut out the outline of the rocket from the cardboard, as seen above. Balance that piece of cardboard like you did on the rocket to find the center-of-gravity. The point where it balances is the center-of-pressure. Mark that location on the cutout.

What is the distance from the tip of the nose cone to the center-of-pressure? ______________________________ Inches.

Calculating the Margin of Stability:

The margin of stability is an evaluation of HOW stable a rocket is. It can be defined as the distance between the center-of-gravity and center-of-pressure relative to the diameter of the body of the rocket. In rocketry, we use a unit of measurement called calipers.

Measure the diameter of your rocket’s body tube. What is the diameter? ________________ Inches.

Using this equation, calculate the margin of stability. (CP - CG) / Diameter = Margin of Stability.

What is the margin of stability of your rocket? _________________________ Calipers.