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The best of us get stupid ideas which run through our heads day in and day out without finding an answer. That's why I came here:

A golf ball has dimples to reduce drag and increase flight distance (As you can read here). This works by creating a small "windshield" which causes the air to travel around the ball more smoothly and by doing this reduces drag.

What would happen if we were to apply this to a rocket or a lander? Does air pressure at Max Q change the dynamics so much that something like this won't have an effect? If it wouldn't work on rockets, could it improve airplanes?

A possible problem is the structural integrity being lowered or it even being impossible for the relatively thin hulls of a rocket body. The effect on a rocket might also be small since, as the article states

This allows the smoothly flowing air to follow the ball's surface a little farther around the back side of the ball, thereby decreasing the size of the wake.

which implies that the effect comes from the end of the ball creating less turbulence (not where such dimples could be created in a rocket so maybe that's better for a plane?).

A German video shows this used in an engine block's air intake system to increase static pressure and air flow. So the spin of a golf ball is not needed for the effect.

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    $\begingroup$ see also space.stackexchange.com/questions/23490/… $\endgroup$
    – user20636
    Sep 5, 2019 at 9:56
  • $\begingroup$ @JRCM oh that question didn't get shown when I searched for golf ball and dimple. Thanks! $\endgroup$
    – Flewrider
    Sep 5, 2019 at 10:06
  • $\begingroup$ I remembered it, but it took me a few goes to actually find it. $\endgroup$
    – user20636
    Sep 5, 2019 at 10:09
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    $\begingroup$ A slightly more accurate handwaving explanation of both dimples on spheres and vortex generators on airplane wings: they convert a few large turbulent eddies into many small ones, thereby reducing drag (and smoothing over the knee in the lift-drag curve). $\endgroup$
    – Camille Goudeseune
    Sep 6, 2019 at 1:00
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    $\begingroup$ Mythbusters actually did this experiment with a car, and it had positive results if I recall. It would be pretty useless for anything traveling in the vacuum of space, but might have some effect on planes within an atmosphere. Just not sure if it's worth the extra weight and structural instability that might result. (MB did it by covering the entire car in about 3 inches of clay, significantly increasing its weight.) $\endgroup$
    – Darrel Hoffman
    Sep 6, 2019 at 19:02

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The dimples on a golfball is structurally the simplest iteration of a vortex generator. Vortex generators are most definitely used in practice to improve aircraft performance. (for some usage examples, see the tag over at Aviation.SE)

Turbulence will happen, that's the fundamental nature of aerodynamics. Controlling the transition into turbulence is the fundamental consideration in aerodynamic design, hence the inclusion of controlled vortex generators is naturally also a part of rocket design. (but they may not end up looking like dimples)

For specifics on how high velocity and pressure (Max Q happens around Mach 1-2) affects this in the context of rockets, the paper you are looking for is An Investigation of Cavity Vortex Generators in Supersonic Flow

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  • $\begingroup$ I like that for their simplicity they can double the distance that is covered! And thanks for the paper! $\endgroup$
    – Flewrider
    Sep 5, 2019 at 9:25
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    $\begingroup$ Maybe worth making explicit that golf balls spend very little of their time supersonic. $\endgroup$
    – Russell Borogove
    Sep 5, 2019 at 16:12
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    $\begingroup$ @RussellBorogove - And apparently with current golf club (and ball) technology, video here it is not possible to hit a golf ball at supersonic speeds. $\endgroup$
    – Glen Yates
    Sep 5, 2019 at 20:44
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    $\begingroup$ An important difference when comparing balls to planes is that planes are expected to fly in a known orientation while balls aren't. Therefore, designers can place vortex generators in the chosen part of a plane, but they need to be spread evenly on the surface of a ball. $\endgroup$
    – Pere
    Sep 6, 2019 at 10:37
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    $\begingroup$ You could imagine a ball with a tail - like when playing bedminton - which in turn would orient the ball. Of course the drag lessens the flight time of the ball and you'd have to argue about the definition of a ball. But good point @Pere $\endgroup$
    – Flewrider
    Sep 6, 2019 at 12:48
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When talking about air flow there are some different types/definitions to know:

  • Laminar (uniform, smooth) flow
  • Turbulent flow

You're correct when you say

This allows the smoothly flowing air to follow the ball's surface a little farther around the back side of the ball, thereby decreasing the size of the wake.

The reason that works is because the dimples trip the air flow around the ball into turbulent flow. If the ball was perfectly smooth, it would be more laminar.

For a blunt object, turbulent flow causes the trailing wake to be smaller. This reduces drag. For a non-blunt object, typically you want laminar flow to reduce drag, not turbulent flow. What happens when an airplane stalls? The flow over the wing goes from laminar to turbulent, and when it does it separates more from the wing. This causes a sudden loss in lift (because obviously the wings generate the lift for the airplane to stay in the air). Airplane wings are designed mostly for laminar flow (and yeah there are some strategically placed vortex generators as the other poster mentioned).

You don't want a blunt object flying at high speed or above the speed of sound. When you fly above the speed of sound, you form shock waves in the surrounding air and have much higher drag forces than when flying slower. Look at high speed aircraft. They're all much "pointier".

Rockets fly at like mach 3-20 or somewhere between 3 times to 20 times the speed of sound (Mach 13 is 17000 km/hr). An airliner flies at about Mach 0.86 (or maybe 550 mph).

So you can't really take an aerodynamic technique that works on a low speed blunt object (golf ball dimples), and apply it to a high speed streamlined object. The physics are much different and there are many factors to consider to reduce overall drag.

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  • $\begingroup$ Hypersonic re-entry capsules are blunt. $\endgroup$
    – Organic Marble
    Sep 6, 2019 at 20:00
  • $\begingroup$ @OrganicMarble Right, they are blunt and they're meant to slow the capsule down as much as possible while at the same time surviving extremely high temperatures. That's a special use case that doesn't really apply to the main point of the discussion. Most of the time you want your aircraft to be faster and more efficient, not purposefully slower with higher drag as is the case with a re-entry vehicle. $\endgroup$
    – echo
    Sep 25, 2019 at 19:42

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