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Abstract: In the LISA Pathfinder mission, the grabbing positioning and release mechanism will perform a critical phase, which is the injection of a test mass into a perfect free-fall condition (also called geodesic trajectory). A possible failure is the test mass remaining adhered to the mechanism or released with an excessive velocity, which would hinder the subsequent capacitive control to the desired geodesic status. In space engineering it is recognized that mechanisms constitute critical subsystems, because they may have single point failures which can seriously affect the mission. In order to limit the risk, a good practice is to test the mechanism critical phases in an environment which simulates the conditions that will be experienced in operation. In the LISA Pathfinder ground testing plan a combined experimental-analytical strategy has been conceived to test this critical phase. The momentum transfer from an actuator to an adhered test mass upon quick separation is measured by means of a dedicated experiment, characterizing the behavior of adhesion under dynamic failure. In parallel, a mathematical model of the release mechanism electro-mechanical dynamics is experimentally validated. The mathematical model of the test mass in-flight injection into geodesic is built, combining the adhesion model with the identified model of the release mechanism. The compliance of the resulting injection with the requirements is presented and discussed. [Copyright &y& Elsevier]

LISA Pathfinder (LPF) was an ESA mission which was launched to test the most relevant technologies for the forthcoming mission, named LISA, which will be the first gravitational wave observer in space. The LPF payload includes two gravitational reference sensors, each one containing the sensing body of the experiment, i.e. a cubic-shaped test-mass (TM). The mission successfully fulfilled the requirements on the maximum noise level affecting the relative acceleration between the TMs. However, the mechanism which was responsible to inject them into free-fall condition presented some criticalities: the telemetry data showed an out-of-nominal TM state at the release, which may be explained with unexpected impacts between the TM and the release mechanism (GPRM) end effector. Experimental tests are performed on-ground to confirm this hypothesis and to provide an interpretation of the possible causes of the impacts. A 3D lumped-parameters multi-body numerical model of the mechanism is developed to extend the analyses results and a classifier is employed to distinguish between pre-release conditions in which impacts may occur or not. The analyses here presented give important guidelines for the design of the GPRM units for LISA. [ABSTRACT FROM AUTHOR]