Fin Sets Basics

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Supported Fin Sets

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.

Component	Function
	So what's a trapezoidal fin . . . a quadrilateral having two parallel sides. Yes, but what it means to you is that the fin's root chord (the edge that's against the body tube) is parallel to the tip chord (the edge that is furthest away from the body tube). The leading and trailing edges can be at whatever angle or length you want and it will still be a trapezoid; in OpenRocket a triangle is considered a trapezoid too. OpenRocket allows you to change the leading and trailing edge angles and lengths quickly and easily. And, if you want, with the click of a button you can even convert trapezoidal shapes into freeform form fins for even more design flexibility.
	If your passion is competition flying, the elliptical fin likely will be your shape of choice. Essentially, an elliptical fin has the shape of a nose cone, oriented perpendicular to the body tube. Elliptical fins have the lowest induced drag because the shape of the fin keeps more of the fin’s lift force close to the body tube, where the fin is longer. And, just as you can with trapezoidal fin shapes, the click of a button converts elliptical shapes into freeform form fins.
	What a freeform fin is almost speaks for itself. In the fin shape editor, on the left you have a table for horizontal and vertical points and on the right a grid. You can either manually type in the points or use a mouse and click on the grid, or both. The number of points (for all intents and purposes) is unlimited. You can essentially “put on paper” so to speak, whatever you can think of.
	Generally speaking, tube fins are short sections of body tube that are glued around the main body tube, near its bottom. The maximum number of tube fins of the same diameter as the main body tube is six, but OpenRocket allows you to either increase or decrease the diameter of the tube fins (relative to the main body tube) so that any number of variations are possible. That said, support for tube fins is currently basic in nature, meaning that the edge of a tube fin in OpenRocket is perpendicular to the body tube (it has a flat end). Those curving shapes you sometimes see are not yet an option.

One thing to keep in mind as you design, your rocket is that, if you are going to be flying at extreme speeds and velocities and intend to verify the OpenRocket simulation results using RASAero II, then you can only use a trapezoidal fin set.

Limited Support and Unsupported Fin Sets

Ring Tail Fins

Although ring tail fins can easily be visually added to an OpenRocket model using a body tube, 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 easily visually simulate a ring tail virtually at any point up or down the airframe using an inner tube. But, again, there’s a catch, inner tubes are aerodynamically ignored. OpenRocket does not support and simply WILL NOT accurately simulate ring tail fins at this time.

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; however, the simulated results with grid fins have not been verified and should not be relied upon without actual flight test verification and Cd adjustments of your own.

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