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

"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"(http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml).

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

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closed as off-topic by GdD, Pondlife, CGCampbell, TomMcW, David Richerby Oct 25 '17 at 18:26

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "This question does not appear to be about aviation, within the scope defined in the help center." – GdD, Pondlife, CGCampbell, TomMcW, David Richerby
If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ This looks like a good question for Space.SE $\endgroup$ – mins Oct 25 '17 at 5:35
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    $\begingroup$ It has already, by the same OP. @DecKonroyd, please do not double post. $\endgroup$ – GdD Oct 25 '17 at 8:57
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    $\begingroup$ You're trying to apply physics of laminar subsonic airflow to plasma encountered on reentry at Mach 50. From Design of an integral thermal protection system for future space vehicles: "It is required [...] the TPS be below certain limit in order to maintain the smooth aerodynamic profile of the vehicle. Excessive local deflection, such as a top surface dimpling, can lead to severe local aerodynamic heating, which may lead to catastrophic failures. $\endgroup$ – mins Oct 25 '17 at 10:08
  • $\begingroup$ Duplicate of space.stackexchange.com/questions/23490/… $\endgroup$ – Jan Hudec Oct 26 '17 at 11:52
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Golfballs are dimpled because the resulting turbulent boundary layer can follow the contour of the ball much better. Subsonic turbulent boundary layers have more drag than laminar ones, but fast separating boundary layers create high pressure drag, which more than compensates for the higher boundary layer drag.

So that is all great for subsonic golfballs. The re-entry vehicle though:

  • is air-braking upon re-entry and actually requires drag to slow it down;
  • enters hypersonically, with associated shock waves that have no intention of folding back or re-attaching over the back end of the sphere.

If the re-entry vehicle is a Space Shuttle with flat wings, at the end of the flight it does fly sub-sonic but even at this last bit is using speed brakes and deploys the landing gear as early as possible to slow down. For a re-entry sphere there seems to be no use for dimples: we want drag.

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    $\begingroup$ The point of re-entry vehicle it that it needs the drag, because it needs to slow down. So it is not optimized for lower drag, it is just optimized to minimize the transfer of heat from the compressed air to the body. Also keep in mind that vortex generators (dimples) work at subsonic speeds, but re-entry happens at hypersonic speeds. $\endgroup$ – Jan Hudec Oct 25 '17 at 8:13
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    $\begingroup$ @JanHudec Duh! Thanx for that, have re-written and will go quietly sit in a corner now. $\endgroup$ – Koyovis Oct 25 '17 at 9:57
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Near as I know, the key to designing good re-entry vehicles or shielding is large flat plates, primarily because the design ensures the energy transfer from kinetic and potential energy of the craft to thermal energy occurs mostly in heating the air flowing around the shape rather than the craft itself. It sounds kind of counter intuitive, but doing this ensures the spacecraft remains as cool as possible and the least risk to structural damage from heating exists. Therefore reducing drag is these applications is not a top priority.

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    $\begingroup$ Reducing drag is not just not a top priority. It is straight undesired. The thing is aerobraking*—it *needs high drag to slow down from the 7.9 km/s orbital speed down to safe landing. $\endgroup$ – Jan Hudec Oct 25 '17 at 8:17

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