Two new ballistic limit equations for hypervelocity impact on homogeneous and composite-orthotropic materials have been developed for a velocity range above 6 km/s. The methodology used to develop the ballistic limit equations involves Kirchhoff's plate theory for a two plate fundamental structure comprising a shield and back plate. The Boundary Element Method is used to calculate the deformation and the moments when the load, is uniformly distributed over a circular area of the back plate, and is applied quickly so that the momentum transferred to the loaded area is equal to twice the momentum of the original projectile. The Von Mises yield criterion is used to account for elastic-plastic deformations into homogeneous materials and the Tsai-Hill yield criterion is used to account for elastic-plastic deformations into composite-orthotropic materials. The ballistic limit equations developed are compared with existing ballistic limit equations based on empirical and semi empirical formulations. It can be seen that our results are in good agreement with experimental measurements of spherical projectiles impacted on a two-plate shield at hypervelocity.