The results of a study on the hydrogen sound jets penetrating a cross flow at Mach 4 and their influence on the flow structure in a backward facing step (BFS) channel are presented. It is shown that jet penetration in a channel differs from penetration in a free stream and cannot be predicted using the well-known empirical relations. Due to the influence of the reflected shock wave and its interaction with the wall boundary layer, an increase in the hydrogen jet-to-crossflow momentum flux ratio leads to a change in the flow structure and pressure distribution in the jet injection area and downstream after the backward facing BFS.