  We analyze the classical stability of string cosmologies driven by the
dynamics of orientifold planes. These models are related to time-dependent
orbifolds, and resolve the orbifold singularities which are otherwise
problematic by introducing orientifold planes. In particular, we show that the
instability discussed by Horowitz and Polchinski for pure orbifold models is
resolved by the presence of the orientifolds. Moreover, we discuss the issue of
stability of the cosmological Cauchy horizon, and we show that it is stable to
small perturbations due to in-falling matter.
