# How is Starship longitudinally stable during reentry?

During IFT-4, it was noted that the flaps rarely moved. But I don't understand how Starship would be stable at that attitude. Stability requires the center of pressure to move rearward with increasing angle of attack. And in order to achieve this in a lifting attitude, the the rear surfaces need to have a lower pitch than the front surfaces. But as far as I know, Starship body flaps are always parallel to the longitudinal axis.

Edit: for those claiming imcorrectly that Starship did not produce lift

• It's a strong indication that the vehicle is unstable if the control surface is in the neutral position but the AoA is not neutral. Commented 22 hours ago
• @user3528438 It's a strong indication that the vehicle is stable if the control surface is not moving at all Commented 19 hours ago

Longitudinal stability, or pitch control, is not really the same in an aircraft as in the case of a bellyflopping Starship. The Starship is not experiencing aerodynamic lift in a traditional sense and not attempting to maintain level flight. Most of the force on Starship is drag, so it's comparable to a skydiver rather than an aircraft.

The center of mass is, more or less, in the center of the falling body -- somewhere between the navel and sternum, if it's a person. Like the skydiver with their hands and feet held out and behind the torso, the Starship with flaps in a half-extended position puts the center of pressure slightly dorsal to the center of mass, which makes the vehicle inherently stable. All they need to do is adjust the angle slightly if it starts getting too far head-first or tail-first, but it shouldn't need too much maintenance to stay close to the intended orientation.

For the record: Lift force isn't "away from the ground". Lift is a force produced that's perpendicular to the airstream. When an airplane is flying forward, it's moving parallel to the ground, and the lift force is pushing up, at 90 degrees from the airflow. The force that pushes back parallel to the airstream is drag. In an airplane, that's tail-ward, but in Starship the airstream is going up, so the drag force is going up.

When a skydiver sideslips by angling their body, technically that's a lift force, but typically we don't talk about it like that, using "lift" to refer specifically to the effect of streamlined flow over an airfoil (or hydrofoil, for that matter). Starship isn't an airfoil, but even if it was, it would be in a permanent stall.

Personally, I was also surprised by how little adjustment was needed to keep Starship stable, but apparently, it works pretty well -- even when the flaps are getting progressively chewed off by high pressure plasma.

• It absolutely does generate lift, that's the whole point of the belly flop position, it generates lift and slows re-entry to reduce heating Commented 23 hours ago
• @AlanBirtles Lift is a force perpendicular to the airflow. The force that's slowing down Starship is drag. In the context of a falling bluff-body like Starship or a skydiver, lift would be parallel to the ground -- sideslipping. Which I'm sure it can do, so technically that sort of maneuvering would be using lift, but in context of this question, considering the Starship like it's an airfoil is a misunderstanding of what it's doing. Commented 15 hours ago
• What surprised me was how once the ship got into the thicker atmosphere it seemed to fly almost level for nearly three minutes, which makes me wonder if the telemetry was correct. But I never heard any commentators either SpaceX or otherwise mention it. It took 33 seconds to drop from 69 km to 68 km, but then starting at T+50:41 it stayed at 68 km for 2 minutes 53 seconds before reaching 67 km. It then took only 23 seconds to get to 66 km. I thought maybe the telemetry was frozen but during the time at 68 km the speed dropped from 24,900 km/h to 22,200 km/h. On the coms it said it was at 0.5 g Commented 12 hours ago
• Possibly because it was reentering at a moderately shallow angle, that's the effect of the ground sort of curving away from it? There must be a point in the reentry where the increasing curvature of the trajectory nearly matches the curvature of the earth, I guess. Commented 10 hours ago

Those four surfaces are meant to adjust or trim the location of the center of dynamic pressure with relation to the center of mass.

These relative locations are set to control the intended lift vector direction.

Once that direction set, the ship remains passively stable about roll and pitch. (If it wasn't, the surfaces would continuously correct their deflection in an active manner)

Passive stability about roll axis is the result of the four surfaces deflected "up" creating a lateral dihedral angle.

Passive stability about pitch axis works in the same way, by creating a slight longitudinal dihedral angle:

If rear surfaces' hinges axes are parallel along the cylindrical body, front surfaces' hinges axes are not.

When set at an "up" deflection, front surfaces do create a longitudinal dihedral angle, between them and the rear surfaces.

This geometrical property creates a virtual curvature that ressembles a more conventional capsule's heatshield shape, looking more like an elliptic paraboloid (dome), than like a cylinder or worse, an hyperbolic paraboloid (saddle, that would be unstable).

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