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| 20111012 WBR Backslider |
Backsliding is defined as “reverting to a worse condition.” But Seminole SECME is backsliding to move forward with rocketry. At the very least, backsliding should increase our stability.
What does it mean to be stable?
Stability: the tendency of the rocket to return to equilibrium, or balance.
This means if the rocket begins to wobble in flight, it will straighten back up on its own.
For a stable flight, the center of pressure must be below the center of gravity.
http://microgravity.grc.nasa.gov/education/rocket/rktstabc.html
Center of Gravity (or center of mass) is the point at which the entire weight of a body may be considered as concentrated so that if supported at this point the body would remain in equilibrium in any position. Center of Pressure is the point on a body where the sum of the total pressure acts. Pressure acting on a surface area causes a force.
One purpose of fins is to guarantee that the center of pressure stays below the center of mass of the rocket. See G. Harry Stine's “The Handbook of Model Rocketry”: It is generally agreed among advanced model rocketeers and has been generally confirmed by flight tests that the CP should be no less than one body diameter between the CG. This is known as 1-caliber stability (from the days when rockets were a part of the artillery corps of armies). If your model has more than 2 to 3 calibers stability, it may be overstable and suffer from excessive weathercocking (tendency to turn into the wind). For a two liter bottle, the diameter is 11 cm (4 ¼ inches).
Recovery Systems
- A recovery system is something that slows the rocket's decent.
- It can be active (like a balloon, a parachute or helicopter blades), or passive (like a backslider)
Backslider
A backslider is made by making the rocket’s CG close to the CP.
This prevents the rocket from turning over at apogee and instead floating down on its side.
The backslider encounters more air resistance because by design it encounters drag. It is not landing in an aerodynamically efficient fashion, with its nose first.
Fins on this type of rocket should be larger to help slow it down. This will also increase the stability of your rocket.
“Swing Testing” is a relatively simple test that you can use on a model rocket to determine the stability. Tie a string around the body tube at the location of the center of gravity. Then swing the rocket in a circle around you while holding the other end of the string. After a few revolutions, if the nose points in the direction of the rotation, the rocket is stable and the center of pressure is below the center of gravity. If the rocket wobbles, or the tail points in the direction of rotation, the rocket is unstable. You can increase the stability by lowering the center of pressure, increasing the fin area, for example, or by raising the center of gravity, adding weight to the nose. If the swing test fails, it doesn't necessarily mean the rocket is unstable. It may be proved stable using other methods: like the Barrowman Equations, or the RockSim software calculations. These other methods should be used when the swing test fails.
