Thanks, everyone!
Animated by hand. The rocket body is wobbling around its approximate center of gravity. The string is a two-point spline with one end attached at the surface of the rocket body. This gives the impression that the string is being jiggled by the rocket's mass. At first, you'd think the rocket should move around the point where the string attaches, but a rocket has much more mass than a string so it tries to turn about its own center