# Why are hexa airfoils used on rockets despite having more drag that diamond airfoils? [closed]

I was doing a study on airfoils shape for a rocket. I tested the diamond and the hexagonal airfoil with Missile DatCom, RasAero and the Shock Expansion theory and from my results it seems like the diamond is better (little more lift and noticeable less drag). Looking at other rockets design I found out that many actually uses hex airfoil, but I don't get why. Am I missing something? Any reason for that?

Also like 99% of rocket uses a sweep angle. If I remember correctly, having a sweep angle reduces the Mach number normal to the L.E., so using a sweep angle means that I'll have a subsonic flow normal to the L.E. even if the fligh Mach is supersonic. So, the L.E. should be rounded. Since all these rockets uses thin sharp airfoils, wouldn't the wing work better with no sweep angle?

• I don’t think this is off topic here since it has to do with aerodynamics but you might try asking on the space exploration stack exchange if you don’t get the results you are looking for here. Mar 21, 2019 at 17:16
• This question is also asked on space exploration. Mar 21, 2019 at 18:22

## 1 Answer

A rocket stabilized by fins alone needs a certain amount of lift at the rear to ensure that if it is disturbed from a nose-first attitude, it returns and damps out the resulting oscillation. Adding mass at the rear of the rocket increases the amount of lift (hence fin area) required, as well as increasing the inertia factor that counters damping.

If a diamond fine provides little if any more lift, but weighs more (as it most likely does) than a flat-sided hexagonal section, it's probably a net negative in this compromise. The added mass makes it harder to damp oscillation, and requires more area, which adds more mass. Drag, on the other hand, helps a little (since it's near the rear of the vehicle, it tends to pull the tail behind the nose).

End result, for a rocket, it's likely that both on a lift/mass basis, and in consideration of drag, the flat-faced hexagonal fin shape is superior for overall performance.

• Thank you for your answer! I'm beginning to understan the problem now. But why are you saying that the diamond weights more than the hex? Taken both with the same max thickness, wouldn't the diamond be lighter?
– gbos
Mar 21, 2019 at 18:20
• With the same thickness (and chord), the diamond would have flimsier edges.
– amI
Mar 21, 2019 at 18:23
• Or with the same strength (which was my assumption) it would be heavier. Mar 21, 2019 at 18:42