  Motivated by recent work on the structure of the singularity in inhomogeneous
Tolman-Bondi collapse models, we investigate the behaviour of null geodesics in
the particular case where the collapse is self-similar. The presence of the
homothetic Killing vector field implies that the geodesic equation can be
described by an integrable Hamiltonian system, and exploiting this fact we
provide a full qualitative picture for its phase flow.
