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During reentry, why is the orientation of the spacecraft where the heat shield side leads the vehicle the most stable?

I'm not sure how accurate KSP is, but when I reenter the atmosphere headshield side down (picture 1), it is pretty much impossible to reorient the spacecraft any other direction.

However, if I reenter the atmosphere pointy end first (picture 2), I am able to reorient the spacecraft the way it is in picture 1.

I am having trouble reasoning why this is the case.

Picture 1

Picture 2

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    $\begingroup$ If it were heatshield up, the vehicle would be subjected to a Rapid Unplanned Disassembly. $\endgroup$
    – Jacob Krall
    Dec 30, 2022 at 3:35
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    $\begingroup$ Also this answer space.stackexchange.com/a/19808/40489 $\endgroup$
    – blobbymcblobby
    Dec 30, 2022 at 9:28
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    $\begingroup$ Also, ksp is not a fully realistic simulation...] $\endgroup$
    – Rory Alsop ♦
    Dec 30, 2022 at 9:46
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    $\begingroup$ Maybe the title is better worded as why is the heatshield on the wide end rather than the pointy end and why is the wide end down stable? $\endgroup$
    – DKNguyen
    Dec 30, 2022 at 19:46

3 Answers 3

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We’re accustomed to seeing things travel pointy-end-first (bullets, rockets, arrows, Lamborghinis) so it seems “natural” that Entry Vehicles (EV) should be most stable traveling pointy-end-first as well. Not so.

Pointy-end-first is chosen when drag coefficient is the prime design criteria. Engineers then need to add tailfins, spin, fletching or gimbaled engines to overcome the inherent instability of this choice.

For example, bullets are inherently unstable since their Center of Gravity (CG) is behind their Center of Pressure (CP). They only achieve adequate static stability due their extremely high spin rate (hundreds of thousands of RPM).

enter image description here

An airgun pellet (they sometimes travel supersonic) has static stability since the CG is ahead of the CP.

enter image description here

A sphere has static stability due to its symmetry.

enter image description here

A portion of a sphere has a similar shock wave to a complete sphere. As long as the CG is ahead of the center of spherical curvature, the object is statically stable.

enter image description here

The static stability of a spherical section is assured if the vehicle's center of mass is upstream from the center of curvature

https://en.wikipedia.org/wiki/Atmospheric_entry#Entry_vehicle_shapes

An EV has a similar shape to the pizza-pie section above, but rounded at both ends. The relationship between its CG and CP is similar to an airgun pellet.

enter image description here

If this EV is travelling pointy-end-first, the curvature which is generating the shock wave has a much shorter radius. This places the CP in front of the CG and creates static instability, just like a bullet.

enter image description here

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    $\begingroup$ It would be neat to test this by dropping a piece of foam in this shape. I suppose it could be something denser too but you would drop it in water in that case. $\endgroup$
    – DKNguyen
    Dec 30, 2022 at 19:49
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    $\begingroup$ @DKNguyen ... give it a try and let us know. A nice feature of foam is you can embed a dense weight in it to position the CG where you want. I don't know how the results would apply to supersonic flight. $\endgroup$
    – Woody
    Dec 30, 2022 at 21:54
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    $\begingroup$ Supersonic aerodynamics are very different from subsonic, so I don't know if easy-to-perform tests would work well. Incidentally, aren't airgun pellets stabilized by rifling as well? $\endgroup$
    – ikrase
    Jan 1 at 5:10
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    $\begingroup$ To make the answer even better, you might want to explain what Center of Pressure exactly means. $\endgroup$
    – asdfex
    Jan 1 at 13:46
  • $\begingroup$ @ikrase ... correct. Airgun barrels are rifled. And most are subsonic. $\endgroup$
    – Woody
    Jan 1 at 18:14
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Air resistance will slow a part of the spaceship according to its surface area (and pointiness), but gravity and inertia are proportional to its mass.

The capsule's mass is 800 kg, and the heatshield is 300 kg, however when the spaceship turns sideways, the surface area of the capsule is much larger than the heatshield. This means the capsule is pushed back with a stronger force than the heatshield, reorienting the spacecraft to go heatshield first.

If you reenter pointy end first, you may be able to keep the craft pointed that way for a while because drag is equal on all sides, but as soon as you turn a bit, you will get flipped over.

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    $\begingroup$ Stability of an entry vehicle does not come from the area of the capsule. A capsule with a smaller surface area than the heat shield would still have static stability. Stability comes from the relationship between center of mass and center of pressure. $\endgroup$
    – Woody
    Dec 30, 2022 at 17:56
  • $\begingroup$ Interestingly, I have done many a pointy-end-first entries in KSP and they seem not only stable, but resistant to being turned during the high speed passage. (You would never want to do it for real this way, but in KSP it works quite well for aerobraking. Turn tail on the last orbit where your apoapsis is outside the atmosphere. This trades time for allowing for less heat shield when coming in fast.) $\endgroup$
    – Loren Pechtel
    Jan 2 at 22:19
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Throw a hammer in the air: you'll see it lands head first no matter how you initially launch it. That's why your ship leads the descent through the heaviest side.

For control as mentionned it's because of the massive reentry airflow, in KSP the command pod's reaction wheels are very powerful so when in terminal velocity it will be possible to reorient easily the capsule.

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    $\begingroup$ Your hammer analogy is not true. Did you try it? I did. 6 out of 10 tries, the handle landed first. Evidence trumps intuition in this case. $\endgroup$
    – Woody
    Dec 30, 2022 at 17:49
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    $\begingroup$ Entry vehicle stability comes from the relationship between center of mass and center of pressure. Reaction wheels are not needed. $\endgroup$
    – Woody
    Dec 30, 2022 at 17:51
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    $\begingroup$ @Woody If you're throwing and catching the hammer safely, you're probably not throwing the hammer far/fast enough for wind resistance to be dominant. If you drop it from an apartment block so it's properly able to interact with the air you should see the predicted result. But if you really feel the need to conduct this experiment please find a cliff with nobody around and perform it safely. Alternatively, use a larger, lighter object such as a badminton shuttlecock. Your airgun pellet example is excellent. $\endgroup$
    – Level River St
    Dec 30, 2022 at 19:28
  • $\begingroup$ @Woody: I think the point about KSP having very powerful reaction wheels is that's why it's possible to flip out of the aerodynamically stable orientation at all. A real vehicle with realistic reaction wheels couldn't do that, and would just be stuck in the stable shield-down orientation. (Unless it mounted powerful RCS thrusters) $\endgroup$
    – Peter Cordes
    Jan 1 at 13:52

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