Abstract: Gamma Ray Bursts are the most luminous events in the universe and are associated with the death of massive stars. GRBs are powered by relativistic jets from a newly-formed compact object at the center of a star. The jet then punches through the outer layers of the star before eventually emitting gamma-rays. However, the extent to which passage through the star effects the jet is not well understood. I will present new simulations of propagation and long-term evolution of jets with time-varying central engines and discuss the observational implications of the results.