Left: time evolution of the NS acceleration in model W15-2. The acceleration computed by the assumption of momentum conservation, i.e., by the negative derivative of the total linear momentum of the gas surrounding the NS (red; denoted by “mom.consv.”), is compared with the value obtained by the integration of all hydrodynamical and gravitational forces that act on a sphere of radius 1.3R_ns around the PNS (black; denoted as “total”). Also shown are the absolute values of the individual contributions of all terms in Eq. (8), namely the momentum transfers by downflows (light blue), outflows (green), anisotropic pressure forces (purple), and the gravitational attraction by the asymmetric ejecta (blue). The last contribution clearly dominate the evolution over long periods. Right: NS velocity components in all coordinate directions and total value of the NS velocity. The two lines in each panel, which correspond to our two methods of computing the NS recoil acceleration in the left panel, can hardly be distinguished from each other.