When constructing a rocket the rotate and translate gizmo has a limit on how far you can translate a part along it's respective axis. This limit inhibits rocket design.
Example:
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In the attached image the spaceplane boosters have shifted the center of mass below the center of lift. To fix this the boosters need to be translated up on the Z axis, however, the maximum Z translation has been reached and the booster cannot be translated up any further.