Eddy current torques,air torques,and the spin decay of cylindrical rocket bodies in orbit

This paper considers the torques causing spin decay in cylindrical rocket bodies in orbit. Eddy current torques, due to the Earth's magnetic field, are estimated using Smith's (1962) model—a hollow cylindrical conducting shell, tumbling about a transverse axis. Air torques are estimated by numerical integration of aerodynamic moments over the rocket surface. It is shown that for Cosmos rockets, 7.4 m long and 2.4 m in diameter, eddy current torques outweigh air torques by several orders of magnitude at altitudes near 500 km, and that they are dominant at altitudes down to 160 km. Visual observations of several such rockets illustrate a variation of spin decay time with altitude which supports this conclusion. The same observations suggest that a few Cosmos rockets may be special cases, different from the rest in construction.