Simulation and experimental investigation of a ballistic compression soft recovery system

A soft recovery system is used to arrest a supersonic object over a limited distance in a controlled manner. This may be achieved through ballistic compression of gas. This work explains the motion of a supersonic object passing through a ballistic compression decelerator i.e., pressurized gas column initially sandwiched between two diaphragms. The accompanying mechanics is complex and includes diverse effects such as separation of shock from the supersonic object, travelling shocks, shock reflections, creation of a new shock, emergence and dissolution of contact discontinuities and expansion waves, and shock-shock interactions. In this work, these phenomena have been numerically and experimentally studied. While the method of characteristics was used to solve Euler's equations in continuous regions, jump conditions derived from control volume considerations were used to obtain solutions across discontinuities. In this way, a duly validated finite difference method computer program was developed to analyze the problem. Finally, simulation predictions were validated by conducting experiments on a 7.62 mm soft recovery system tube. Our results showed that, an object having an entry velocity of 880 m/s, left the SRS with a velocity that was lower by 47% from simulation predictions. Further analysis showed that friction between the object and tube was a major contributor to this gap. Post accounting for friction, the difference between numerical analysis and experimental data got reduced to about 5% at most locations, and to 17% at the end of the SRS. We attribute this residual difference between observations and simulations to build up of pressure at a location post passage of shock by it. Our 2-D finite volume study results, which are consistent with earlier research, as well as with our experimental data, show that such a phenomenon is prominent particularly in narrow tubes due to development of significantly thick turbulent boundary layers.