A system to damp the free piston oscillations in a two-stage light-gas gun used for hypervelocity impact experiments

Hypervelocity impact experiments that reproduce on-orbit collisions between micrometeoroids or orbital debris and space structures are commonly performed by means of propellant-driven twostage light-gas guns. Such devices accelerate projectiles using the thrust of a light propellant gas that is compressed to high pressure and temperature by a piston running in a pump tube. Though these guns have the unique capability of accelerating particles up to 9 km/s, many components of the gun must be checked and/or substituted after each shot making test sessions long and expensive. In order to have a lot of and many different types of hypervelocity impact data, the Center of Studies and activities for Space CISAS ”G. Colombo” of Padua University developed a high-shot-frequency two-stage light-gas gun that can increase the shot repetition rate of standard facilities by a factor of 5 or more and at the same time reduce the shot cost by a factor of 2 or more. This is made possible through the use of special mechanical and diagnostic solutions that were designed to operate the gun for more than 50 shots in sequence without having to carry out maintenance operations. This article presents the design and operation of the CISAS two-stage light-gas gun damping system, which is one of the subsystems that makes it possible to achieve high-shot frequency. The damping system is in charge of controlling the piston oscillations in the pump tube, making it possible for the piston to withstand more than 100 shots without any damage. In particular, the damping system avoids piston strikes onto the gun head at the end of each compression stroke and allows the piston to be positioned at the base of the pump tube after each shot. The sensitivity of the piston oscillations to the damping operations and main subsystem design parameters were identified using numerical simulations, carried out according to a model that describes every working phase of the gun. Moreover, in this paper, the technical solutions for the damping system implementation are presented and the numerical predictions are compared with experimental results. For the CISAS high-shot-frequency gas gun, an efficient damping system proved to be a fundamental requirement to reliably accelerate 100 mg projectiles above 5 km/s and 70 mg projectiles at 5.5 km/s with a shot frequency of 10 shots per day at least, including the time needed for replacing the target and pumping down the target vacuum chamber.