Considering Starship atmospheric entry, belly dive and "bellyflop", latest iteration displays four aerodynamic surfaces, two aft fins and two carard fins. Soon will this configuration be put to a 15km altitude test, hopefully with SN8.

Reading this "How hard will it be to actuate the fins during reentry?", I wondered why attitude contol had to be done the way it is, having flaps hinged along longitudinal axis of Starship.

The proposition below also goes with one degree of freedom, yet a different one (slide along longitudinal axis) which suppresses the need to fight using brute actuating power, against the aerodynamic forces encountered during belly dive.

The two flaps illustrated below have a constant fixed dihedral angle, which provides inherent roll stability, the same way a shuttlecock stabilizes itself.

Aerodynamic loads are actually withstanded by linear rails and bearings, two per flap, compressive rail on the upper surface of the flap, tensile rail on the lower surface of the flap.

With one fixed attitude axis (roll), the two remaining axis are enough to steer Starship anywhere it has to during belly dive.

Pitch attitude is controlled by sliding both flaps in the same direction along longitudinal axis, shifting center of pressure relative to center of mass. Short before landing it pitches Starship up during "bellyflop".

Yaw is controlled by sliding both flaps in opposite direction, inducing yawing moment around center of mass, the same way a paperclip helicopter spins.

The power required to actuate these sliding flaps is quite low and should allow weight savings. (battery size, actuator size) Going from four to two aerodynamic surfaces also reduces moving parts. Nosecone is freed from canard fins, which adds payload configurations and deployment options.

The sliding rails are where tubings and wiring already pass, becoming a fairing for them.

Question is, what are the issues with this design alternative, is it viable?

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enter image description here (paperclip helicopter)

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    $\begingroup$ The biggest immediate concern is that mixing pitch and yaw could induce roll which is only counteracted by the dihedral, but this might not be an issue if you design and balance it right. $\endgroup$ – Dragongeek Nov 4 '20 at 15:57
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    $\begingroup$ I doubt confident in the hypersonic stability of any design would be enough for a manned reentry without dedicated roll authority. $\endgroup$ – Anton Hengst Nov 4 '20 at 19:02
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    $\begingroup$ Fantastic concept though, and you've clearly put effort into mocking it up. Nice question. $\endgroup$ – Anton Hengst Nov 4 '20 at 19:03
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    $\begingroup$ @AntonHengst, Apollo capsules were missing two attitude axis authority during reentry, yaw and pitch, yet were able to virtually build these attitudes using only roll to offset center of mass and lift vector. thanks for nice comment! $\endgroup$ – qq jkztd Nov 4 '20 at 19:37
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    $\begingroup$ I have the same concerns about sealing the slots and about the amount of volume the slot-based mechanisms take up as I did back then. $\endgroup$ – Organic Marble Nov 5 '20 at 1:53

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