  Growing observational evidence supports the proposition that gamma-ray bursts
(GRBs) are powered by relativistic jets from massive helium stars whose cores
have collapsed to black holes and an accretion disk (collapsars). We model the
propagation of relativistic jets through the stellar progenitor and its wind
using a two-dimensional special relativistic hydrodynamics code based on the
PPM formalism. The jet emerges from the star with a plug in front and a cocoon
surrounding it. During its propagation outside the star, the jet gains high
Lorentz factor as its internal energy is converted into kinetic energy while
the cocoon expands both outwards and sideways. External shocks between the
cocoon and the stellar wind can produce $\gamma$-ray and hard x-ray transients.
The interaction of the jet beam and the plug will also affect both of them
substantially, and may lead to short-hard GRBs. Internal shocks in the jet
itself may make long-soft GRBs.
