Why should or shouldn’t a rocket fins leading edge bevel run from the fin leading edge to the quarter chord line?

That’s an excellent question, as it gets to the heart of fin design for different flight speeds.

The short answer is: it generally shouldn’t.

You are correct to link a fin’s bevel to a specific chord line, but for a high-speed rocket, the quarter-chord (25%) line is the wrong one.

• A leading-edge bevel on a supersonic rocket fin should run to the mid-chord (50%) line.
• The quarter-chord (25%) line is the critical point for subsonic airfoils (like on a passenger jet).

Here is a detailed breakdown of why this is the case and the difference between the two designs.

The Supersonic Fin: Bevel to 50% Chord
For rockets intended to break the sound barrier, the primary goal is to minimize wave drag. This is the intense drag created by the shockwaves the rocket generates.

To do this, you use a specific shape called a double-wedge or diamond airfoil.

• What it is: A double-wedge profile is sharp at the front, has straight, beveled edges running to a “shoulder” in the middle, and then tapers back to a sharp trailing edge.
• Why the 50% Chord? The “shoulder,” or thickest part of the fin, is placed at the 50% chord line.
  • Drag Reduction: This shape creates a small, attached oblique shock at the sharp leading edge and another at the 50% chord shoulder. This is vastly more efficient than the massive, high-drag bow shock that would be created by a rounded (subsonic) fin.
  • Center of Pressure: At supersonic speeds, the fin’s aerodynamic center (its center of pressure) shifts backward from the 25% chord position to the 50% chord position. It makes aerodynamic sense for the fin’s thickest point and its center of pressure to be at the same location.

So, for a supersonic rocket fin, you should bevel the leading edge in a straight line to the 50% chord line to create a diamond profile.

The Subsonic Fin: Thickest at 25% Chord
The quarter-chord (25%) line you mentioned is the king of subsonic flight (like model rockets, small drones, or airliners).

• What it is: A subsonic airfoil has a rounded leading edge, and its thickest point is typically located around the 25% chord line.
• Why the 25% Chord?
  • Drag Reduction: At subsonic speeds, the main enemy is flow separation (turbulence). The rounded leading edge and gentle curve back from the 25% chord “high point” help the air stick to the fin smoothly, reducing drag.
  • Center of Pressure: As you know, the aerodynamic center (where the pitching moment is stable) for a subsonic fin is at the 25% chord line.