The transient behavior of an active thermal protection system consisting of a semi-infinite layer of a porous medium with gas injection is considered. During the early stages of heating, thermal penetration depth, surface temperature and surface heat flux do not depend on the gas injection velocity. As the heating time increases, the thermal penetration depth of the active thermal protection system reaches a constant value for each gas injection velocity, and the surface heat flux vanishes keeping the back face at a low temperature. However, the thermal penetration depth of conventional insulation without gas injection never reaches a constant value. Transient solid temperature distributions in the layer for two different surface boundary conditions are almost identical, except in the vicinity of the front surface.