Garrett Reisman, in testimony made this comment on page 4:

Additionally, a movable ballast sled allows the angle of attack to be actively controlled during entry to further provide precision landing control.

The comment is made in terms of controlling the reentry of the Dragon V2 crewed vehicle.

Do other capsules use a similar system, or is this approach unique to SpaceX?

  • $\begingroup$ Soyuz did this to rotate the lift vector and thus providing a small amount of cross range. I can't seem to find a reference though. $\endgroup$ – Erik Mar 2 '15 at 14:27

To first answer your question I think it is prudent to identify why SpaceX is moving the Center of Mass(CoM) in the first place.

Space capsules since the Mercury days have needed heat shields for reentry. The high speeds and relatively flat shapes cause compressive heating which leads to exceedingly high temperatures. While heat shield materials like AvCoat and PICA-X are marvelous materials that can withstand high temperature, extended exposure to reentry plasma can cause the structure to fail. Engineers identified that a ballistic reentry from Lunar Return velocities(11km/s) would create unmanageable heating effects. Their solution was to change the ballistic trajectory into a semi-controlled descent. The Apollo Command Module had a CoM offset from its axis of symmetry. That created an aerodynamically stable "edge" of the capsule that could be controlled by rotating the craft. This allowed the capsule to generate lift and slow the acceleration from reentry.

Apollo used its Reaction Control System(RCS) to change the orientation of its CoM.

Atmospheric Reentry
Apollo EDL Plan


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