Fin Sets Basics
Fins serve to stabilize a model rocket while in flight, and to guide the rocket on a safe and an intended trajectory. Most rockets, without the stabilizing forces acting on the fins, would tumble in midair soon after clearing the launch guide. Aside from their aerodynamic factors, fins are often what gives your rocket the character that others remember.
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Ring Tail Fins
Although it's easy to add a ring-tail to an OpenRocket model, there's a catch, the ring tail must start at the exact aft-end of the component it visually surrounds, and the fins must trail to support it; this is because external airframe components in OpenRocket follow a strict linear pattern. You can also visually simulate a ring tail virtually at any point up or down the airframe using an inner tube, but, again, there’s a catch, OpenRocket will not accurately simulate the flight. At this time, OpenRocket is limited in simulating ring tail fins.
For a more detailed discussion about Ring Tail Fins, go to Advanced Rocket Design
Grid Fins
Fins, generally speaking, are aerodynamic control surfaces. Where grid fins differ from other fins is that they are not essentially just flat surfaces, but more of a frame with a grid inside. Usually made of metal (such as aluminum), not only can grid fins be light, they can be very strong (the very thin, spaced grid surfaces don’t generate much drag). Although OpenRocket doesn’t directly support grid fins as a single component, you can use the pod and fin set components can be used to create them.