How does stability affect a rocket

To understand how to place fins and how large to make them, it is important to understand about centre of mass and centre of pressure. The centre of mass of an object is the point at which all of the mass of an object can be thought to be concentrated.

To find the centre of mass of a rigid object such as a water bottle rocket, balance the rocket on your finger so that the rocket is horizontal. The centre of mass is a point directly above your finger.

The centre of mass can be moved closer to the nose cone end of a rocket by adding some mass near the nose cone. This will increase stability. The single point at which all of the aerodynamic forces are concentrated is called the centre of pressure.

To find the approximate position of the centre of pressure, draw an outline of the rocket on a piece of paper. The centre of the area of the outline shape is approximately the centre of pressure. For a rocket to be stable, the centre of pressure needs to be closer to the tail end than the centre of mass. If the centre of pressure is at the same position as the centre of mass, the rocket will tumble. Stability increases as the distance between the centre of mass and the centre of pressure increases.

Figure 1, Example B This time the point about which it rotates will be closer to the weighted end. If you take the weighted stick and balance it across a sharp edge you will find that the point about which it balances its center of gravity is the same point about which it had been rotating when tossed into the air. Figure 1, Example C. This simple explanation should aid you in understanding how a free body in space rotates around its center of gravity.

If, for any reason, a force is applied to the flying rocket to cause it to rotate, it will always do so about its center of gravity. Rotating forces applied to rockets in flight can result from lateral winds, air drag on nose cones, weights off center, air drag on launch lugs, crooked fins, effect of exhaust gasses emitted at an angle, motor mounted at an angle of slightly off center, unbalanced drag on fins, unequal streamlining, etc.

Obviously, some of these factors are going to be present in all rockets. Therefore, since rotating forces will be present, your rocket must be designed to overcome them. Nearly all model rockets are stabilized by air currents. By stabilized, we mean that all rotating forces are counteracted or overcome.

This means that for each force trying to make the rocket rotate we must set up an equal and opposite force to counteract it. How is this accomplished? Ask any rocket expert and he will simply say to design the rocket so that the center of gravity is ahead of the center of pressure. From studying our first experiment it is easy to see how we could find the center of gravity by simply balancing the rocket on a knife edge as shown in Example A of Figure 3.

But what and where is the center of pressure? The following experiment should aid you in understanding more about the center of pressure of a rocket. Suppose we take the same 2 foot long piece of dowel used in our first experiment and place it on a low friction pivot as shown in Example A of Figure 2. Then suppose the dowel is held in a uniform air current wind of 10 or 15 miles per hour.

If the pivot has been placed in the center of the dowel and if the dowel is uniform in size area the forces exerted by the air pressure will be equal on both sides of the pivot and the air current will produce no rotating effect. See Example B, Figure 2 This will cause the dowel to rotate so that the end away from the vane points into the wind.

Does the CP location change when you upscale a rocket? Read what you'll have to consider when you upscale your favorite old kit. Designing Rockets with Asymmetrical fins - An asymmetrical rocket is one where the fins are not evenly spaced around the perimeter of the tube.

Read this article to learn how to design models like this using the features in RockSim. What is "Static Margin? But this number can be misleading. You could end up designing a rocket that goes unstable when you launch it. Read this article that explains the number, and how it should be used to design rockets. However, they are all missing one important piece of information that this report contains. For articles on rocket stability check out the Peak of Flight index.

This is a list, by topic, of all of the great articles the Peak of Flight put out over the years. Understanding Rocket Simulations From the RockSim Software A report that covers the cardboard cut-out method, and computer simulations. Exploring the Cardboard Cut-out Method. An article that explains how the cardboard cut-out method works, and how it differs from the Barrowman Method of calculating stability.

Wind Caused Instability A report by Bob Dahlquist on how wind affects the stability of rockets, and gives a procedure for calculating the rod length or rocket speed necessary for a stable flight. This report answers that question. College level K Download. Submit a teaching idea or a link. Please be patient, as the Wayback Machine can be slow If you find more links that are needing to be moved here, please let us know! PDF file. Pitts, Jack N.

Nielsen, George E. A method is presented for calculating the lift and centers of pressure of wing-body and wing-body-tail combinations at subsonic, transonic, and supersonic speeds. This was the report that was used by Jim Barrowman to come up with the classic method of determining the stability of rockets University Level. Visit Us: Directions. Online Tech Support Check the bottom of your browser window to see if we're online to chat.

After working on a custom design, I ordered the parts, and the scheduled delivery was just in time for a build meeting. The parts fit as advertised Read More.