How to add elliptical fins in OpenRocket

OpenRocket lets you attach elliptical fins by opening the Fin Set dialog, choosing "Elliptical" from the Shape dropdown, then setting your desired dimensions — root chord, tip chord, and height. The fin automatically generates with an elliptical profile once you confirm. You can then position it on your rocket body, set the number of fins, and adjust the cant angle if you want them angled for spin.

The process takes about two minutes once you know where the controls are. Most of the work is deciding what dimensions you actually want, not clicking buttons.

Key Takeaways

  • Elliptical fins are created in the Fin Set dialog under the Shape menu, not drawn separately or imported.
  • You need to specify root chord (the wide part at the body), tip chord (the narrow part at the point), and height (how far the fin sticks out).
  • OpenRocket automatically calculates the elliptical curve between those measurements once you enter the numbers.
  • You can add multiple fins at once by setting the fin count, and they will space evenly around the rocket body.

Opening the Fin Set dialog

Start by clicking on the body tube where you want the fins to attach. In the left panel under "Components", you will see the tube listed. Right-click it and select "Add fin set", or use the menu: Edit > Add component > Fin set.

A dialog box opens with tabs across the top: General, Fin shape, Position, and Appearance. You are starting on the General tab. This is where you set how many fins you want and what they are called. Leave the count at 4 unless you have a reason to change it — most model rockets use four fins arranged in a cross pattern.

Selecting elliptical shape and setting dimensions

Click the "Fin shape" tab. At the top, you will see a dropdown that says "Trapezoidal" or whatever shape was last used. Click it and select "Elliptical".

Three fields appear: Root chord, Tip chord, and Height. Root chord is the width of the fin where it meets the rocket body. Tip chord is the width at the pointed end. Height is how far the fin extends outward from the body. All measurements are in the units you set for your rocket — usually millimeters or inches.

For a typical model rocket, try root chord 40 mm, tip chord 10 mm, and height 80 mm. The preview pane on the right shows your fin shape as you type. Adjust until it looks right to you. OpenRocket calculates the smooth elliptical curve automatically between the root and tip.

Positioning fins on the body tube

Click the "Position" tab. This controls where along the rocket body the fin set attaches. The "Position" field shows how far down from the top of the tube the fins start. For a typical rocket, fins go near the bottom — often 80 to 90 percent of the way down the body tube.

Leave "Rotation offset" at 0 unless you want to rotate the entire fin set. If you set it to 45 degrees, all four fins rotate together by that amount. This is useful if you want to stagger them instead of placing them in a perfect cross.

Adjusting cant angle for spin

Still in the Position tab, look for "Cant angle". This tilts each fin slightly so that air pushes against it at an angle, causing the rocket to spin in flight. A cant angle of 2 to 4 degrees is common for model rockets. Set it to 0 if you want fins that are perfectly perpendicular to the body.

The cant angle applies to all fins in the set equally, so they all tilt the same way. OpenRocket shows you the effect in the 3D view at the bottom of the window.

Viewing and saving your design

Click "OK" to close the dialog. Your fins now appear on the rocket in the main design window. You can see them in both the 2D side view and the 3D view. If they do not look right, right-click the fin set in the left panel and select "Edit" to go back to the dialog and change the dimensions.

Save your file using File > Save. OpenRocket saves the fin shape as part of your rocket design, so you can come back and edit it later. If you want to export the design to build a physical rocket, you can print the fin template or export it as a PDF for cutting.

Frequently Asked Questions

Can I make the fins different sizes?

No — a fin set in OpenRocket makes all fins identical. If you need different-sized fins, you would create multiple fin sets on the same body tube, each with different dimensions. This is rare for model rockets.

What is the difference between root chord and tip chord?

Root chord is the width where the fin attaches to the body. Tip chord is the width at the pointed end. If both are the same, you get a rectangular fin. If tip chord is smaller, you get a tapered fin. Elliptical fins taper smoothly from root to tip.

How do I know what fin dimensions to use?

Start with a root chord about the same as your body tube diameter, a tip chord about one-quarter of the root chord, and a height about twice the body tube diameter. Then adjust based on how the rocket looks and how it performs in simulation. OpenRocket includes a stability calculator that shows whether your fins are large enough.

Can I add fins to multiple body tubes?

Yes. Each body tube can have its own fin set. Create one fin set, then click on a different tube and add another fin set with different dimensions if you want. This is how you build rockets with fins at different locations.

What does the preview pane show?

The preview pane on the right side of the Fin shape tab shows a side view of your fin as you adjust the numbers. It updates in real time, so you can see whether the elliptical curve looks right before you confirm the dialog.