Rocket Aerodynamics

Objective

The objective of this project is to measure the change in rocket performance based on selected differences in the rocket's design.

Introduction

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
The equation for drag
Rocket stability: center of gravity, center of pressure

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. 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.

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

The National Association of Rocketry: http://nar.org/index.html
Apogee Components: http://www.apogeerockets.com/education/index.asp
Estes Educator: http://www.esteseducator.com/cfusion/publications.cfm

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:

Estes: http://www.estesrockets.com/
Apogee Components: http://www.apogeerockets.com/

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. Because wind and thermal lift can have a significant and unpredictable effect on duration, you need to either do several flight tests and use averaged values for each duration data point, or you need to do all your tests in absolutely identical weather conditions. It is best to use two people with stopwatches to collect each duration data point, in case one loses sight of the model or has a stopwatch malfunction. If your hypothesis has to do with measuring the performance of recovery systems, you will get less scatter in the data if you can do "drop tests" of the rocket and recovery system from a roof or tower 30 feet or more in the air, rather than flight tests.
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

Tim Van Milligan, an aeronautical engineer and the president of rocket manufacturer Apogee Components suggests, "The most common science fair project tries to find the best fin shape that yields the highest altitude. This project is useless, and doesn't yield any valuable data."(2) See the original source for why this is the case.

Instead, the National Association of Rocketry suggests these possible experiments(1):

Predicted rocket altitude vs actual altitude achieved. How good are your theoretical predictions vs tracked altitude, what are the factors that go into making an accurate prediction?
Rocket fin size and location vs stability. How big must fins be to make a rocket stable, and why? What difference does it make where the fins are located, and why?
Effects of spin on rocket performance. What change occurs in the tracked height that a rocket reaches or the straightness of its boost if the fins are placed at a slight angle so that the rocket spins in flight, compared to an identical rocket whose fins are not angled?
Parachute shape and size vs performance. Which performs better, a round parachute with many shroud lines or a polygon shape of the same area with only a few shroud lines? How about a round chute with a spill hole in the middle vs a slightly smaller round chute with no spill hole and thus the same total chute area?
Streamer shape and size vs performance. Fly the same rocket design with a series of streamers of different lengths and widths but the same total area. Or use a series of streamers of identical size and shape but different materials. Which stays up longest and why?
Rocket surface finish or shape vs altitude performance. What difference does a smooth surface finish vs a coarse one make to the drag of the rocket, and thus to its altitude performance? Or compare the effect of nose cones of different shapes [be sure to include flat and hemispherical shapes among those you select], or of identical fins with and without airfoil streamlining. [How does the diameter of the rocket (keeping the weight and shape equal) affect performance?]

Credits

(1) Barber, Trip. "Model Rocketry in Science Fairs." National Association of Rocketry. http://nar.org/pdf/science_fair_rocketry.pdf, accessed October 2, 2004.

(2) Van Milligan, Tim. "What Type of Fin Shape is Best?" Apogee Components. http://www.apogeerockets.com/technical_publication_16.asp, accessed October 2, 2004.