  Variable mass systems are a classic example of open systems in classical
mechanics. The reaction forces due to mass variation propel ships, balloons,
and rockets. Unlike free constant mass systems, the angular momentum of these
systems is not of constant magnitude due to the change in mass. In this paper,
we show that the angular momentum vector for such a system has a fixed
direction in space and, thus, is partially conserved for both rigid and
flexible, torque-free, variable mass systems. A potential use of this result is
that it provides a suitable stationary reference frame against which the
orientation of variable mass system could be measured.
