Would a dimpled heat shield reduce heat transfer during reentry?

Question

Would a large ceramic golfball absorb less heat on reentry than a standard sphere?

From aerospaceweb.org:

The difference in the flowfields around a smooth sphere and a rough, or dimpled, sphere can be seen above. Since the laminar boundary layer around the smooth sphere separates so rapidly, it creates a very large wake over the entire rear face. This large wake maximizes the region of low pressure and, therefore, results in the maximum difference in pressure between the front and rear faces. As we have seen, this difference creates a large drag like that seen below the transition Reynolds number.

Would the reduction in heat absorbed be significant enough to warrant a larger payload via lessening the weight of the heat shield?

Answer 1

The dimpled sphere may produce lower drag (though as Uwe points out I’m not sure if that is true in hypersonic flight). However, this does not mean that it will be better at surviving reentry.

Higher drag implies a faster reentry and shorter exposure to extreme temperatures. Conducting heat to the core takes time, so even if the outer layer is ablating, the core may be less affected by brief exposure to extreme temperatures than by long exposure to moderate temperatures.

The wake is relatively cool and doesn’t need much shielding - if the hypersonic flow does resemble that diagram, the second ball would need approximately double the shielding of the first.

Turbulent flow is generally avoided in heat shield design - turbulence implies convection, providing a way for more heat to reach the surface of the sphere.

The dimpled sphere has a larger surface area. More surface exposed to hot air means more heat absorbed.

Answer 2

Assuming that the spin of a golf ball has nothing to do with effect of its depressions (a reentry vhicle certainly would NOT be spinning), although the smooth ball has a larger surface area, the recesses would not be subject to the friction flow so that the surfaace area actually being subjected to atmospheric friction would only be the higher areas; the depressions would be 'vacumn" areas created by the higher areas deflecting the flow. The higher areas would be subject to no more friction than the smooth ball. Assumming that the higher areas compose half the surface area, the reentry vehicle should be subject to nearly half the friction and lower heat. A "dimpled" or "wavy" type tile oriented in the most effective direction should be tested...I think it would reduce drag, heat and friction.