When a shaped charge jet impinges on a monolithic, infinite, metallic target, its initial strike velocity is typically above the classical "hydrodynamic limit". The hydrodynamic limit is the velocity above which the penetration depth is proportional to the length of the penetrator and the square root of the ratio of the penetrator density to the target density, to a first approximation. Increases in penetrator velocity above the hydrodynamic limit serve primarily to increase the volume of the penetration hole; they do not significantly increase the penetration depth.

As the jet or jet particle strikes the target, a "stagnation point of the flow" is formed at the interface between the two colliding materials in the frame moving with this interface. JETPEN calculates the penetration using the classical hydrodynamic penetration law based on Bernoulli’s equation for incompressible flow, applied to each element of the jet assuming steady state flow in the frame moving with the stagnation point - an approximation that works well. This approach gives very accurate results for metals and other materials of low compressibility. However, JETPEN can also take a more detailed look at the interaction between the jet and the target including the effects of compressibility.

If the penetration velocity is lower than the speed of sound in either the jet or target material, no standing shock wave will be set up in either the target or the jet material. Standing shock waves will be set up in those materials for which the speed of sound is less than the penetration speed. For sufficiently high speed jets, shock waves will be set up in the jet particle and the target materials, as shown in the figure below. The presence of shock waves in either the target or the jet can significantly reduce the predicted penetration depth. As material passes through these shock waves, it is compressed, increasing its density.

To calculate material properties at the stagnation point, it is again assumed the material flow toward the stagnation point is steady, and the material is continuously decelerated and adiabatically compressed. The motion of the stagnation point is described by a compressible form of the Bernoulli’s equation.

For both the compressible and incompressible cases JETPEN predicts the penetration of each segment of the jet striking the target, along with the radius of the cavity created by each segment. The impact depth and radius of each jet segment is recorded and summed to yield the total penetration depth and crater geometry.

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