Solid Motor Rocket Propulsion


Objective

The objective of this project is to measure changes in a rocket's performance based on differences in the rocket's motor.

Introduction

Intro image

Model rockets utilize small, commercially-manufactured rocket engines to enable speeds of up to several hundred miles per hour, while reaching altitudes as high as several thousand feet. By following the National Association of Rocketry, Model Rocket Safety Code, you can experiment with the aerodynamics of these rockets with almost complete safety. And, there are many possible experiments you can undertake (see "Variations" below).

Model rockets can make for an extremely fun and exciting science fair project!

Terms, Concepts and Questions to Start Background Research

To do an experiment in this area, you should do research that enables you to understand the following terms and concepts:

  • The four forces in flight: weight, thrust, drag, and lift
  • Key concepts having to do with rocket engines: combustion chamber, propellant, nozzle, F=ma, specific impulse, total impulse
  • The equation for thrust (advanced students)

In addition, study the Model Rocket Safety Code and the proper means to construct a rocket.

Bibliography

Be sure to study the model rocketry sections (among others) of NASA's Beginner's Guide to Aeronautics. This excellent NASA Web site includes a rocket simulator called RocketModeler as well as a nozzle simulator. http://www.grc.nasa.gov/WWW/K-12/airplane/guided.htm

Stine, G. Harry, and Stine, Bill. Handbook of Model Rocketry, 7th Edition. John Wiley & Sons, 2004. This book is the bible of model rocketry, containing a wealth of information on rocket design, construction, and competition.

This is an excellent introduction to model rocket motors: http://www.lunar.org/docs/handbook/motors.shtml

You can find a wealth of general information at these sites:

Altitude tracking is important for many experiments in rocketry. These links contain excellent information about how to measure your rocket's altitude:

Materials and Equipment

Model rocketry supplies can be purchased at many hobby stores. Two of the primary manufacturers are:

Experimental Procedure

The National Association of Rocketry offers these tips for experimentation(1):

  • Plan to do at least three flights of identical rockets with identical engines for each variable that you want to test. There is a lot of "scatter" in the data from rocket-based experiments, and you will get much better results if you use the average of three or more flights for a data point rather than a single flight. This scatter is the result of a combination of experimental error (such as in measuring altitude), weather-based variations (such as in measuring parachute flight duration), and/or slight differences in the construction of the rocket or the motor. If you understand statistics, having multiple data averaged into a single point gives you the opportunity to impress the judges with an analysis of standard deviations and confidence intervals in your data.
  • Measuring a rocket's maximum altitude accurately is not easy, but is generally the best way to show conclusively how differences in rocket characteristics affect performance. Altitude measurement should be done using data from at least two trackers who look at the flight from different directions but about the same distance, and who communicate by radio to make their measurements at the same moment in the rocket's trajectory. This is generally either at the exact highest point or "apogee" or (this is easier) at the moment of parachute ejection. Using the more complex tripod-mounted trackers that measure both horizontal "azimuth" angle as well as vertical "elevation" angle gives far more accurate results than simple hand-held elevation-only trackers.
  • Measuring a rocket's flight duration is fairly easy, but the data is generally only useful for demonstrating differences in the performance of recovery systems (such as parachutes of various sizes) rather than the rocket....
  • Make sure that you vary only one variable between flights. The height a rocket reaches depends on the engine type and delay time; the smoothness of the surface finish on the rocket; the weight of the rocket; and the shape/size/alignment of the rocket and all its parts (fins, launch lug, nose, etc.). How long it stays up depends on how high it goes, plus on the type and size of the recovery system, the weather conditions, and whether the recovery device deploys fully and properly. If your hypothesis is that rockets with one shape of nose go higher than with another shape, for example, make sure the rockets you test are identical in design, liftoff weight, and surface finish and fly them in the same weather conditions off the same launcher. Make sure that the nose cone difference is the only difference. And use identical motors (preferably from the same pack or with the same date-of-manufacture code on the casing) in all your tests of the two different rockets.

Variations

The National Association of Rocketry suggests these possible experiments dealing with propulsion(1):

  • Rocket engine average thrust vs altitude. What difference does it make in tracked altitude performance if the same rocket is flown with two engines of very different average thrust levels (like the Estes A3 or A8) but the same total impulse and liftoff weight?
  • Multi-staging vs single staging. Which goes higher, a two-stage design with a B motor in each stage, or a single-stage model with a C motor having the same total impulse as the combined total of the two B motors?
  • Rocket weight vs altitude. How much difference does the weight of a rocket (with variable weights in its payload compartment) make in how high it goes with a given engine?
 

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