2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/9092We present a ballistic description of the propagation of the working surface of a relativistic jet. Using simple laws of conservation of mass and linear momentum at the working surface, we obtain a full description of the jet flow parametrised by the initial velocity and mass injection. This analysis will soon be applied to particular cases of time-dependent injection of mass and velocity into the jet.2 pages, To be published in the proceedings of the conference "XXII Texas Symposium on Relativistic Astrophysics"Astrophysicstext