STAR: A UNIVERSITY STUDENTS DROP TOWER EXPERIMENT

The use of tethers in space has been proposed to carry out several different tasks, including, for instance, electrodynamic drag devices, spacecraft tugging, novel docking systems. Tether deployers developed and flown so far demonstrated lack of engineering knowledge during tether operations; in particular, a tether retrieval has never been performed by small automated spacecraft. In this context, Space Tether Automatic Retrieval (STAR) is an experiment carried out by five engineering students from the University of Padua, whose main objective was to design, build and test in microgravity a novel concept of space tether deployer with retrieval capability. The experiment has been selected by the Education Office of the European Space Agency for the 2016 edition of the Drop Your Thesis! educational programme. The Drop Your Thesis! programme offers university students the unique opportunity to perform scientific experiments in microgravity conditions using the Bremen Drop Tower facility operated by ZARM. To demonstrate the functioning of the deployer, a braided line has been successfully deployed and retrieved in each of the five tests that were conducted in microgravity conditions. The prototype of the deployer is inspired to passive deployers already developed for past space missions, such as SEDS-I and SEDS-II, integrated with an innovative reeling device to enable the tether retrieval capability. More specifically, the experimental setup was composed by four main subsystems: a spring-based launch device to start the deployment phase; a length and length rate measurement system to measure the amount of deployed tether by means of optical sensors; a dedicated active braking mechanism to control the tether deployment velocity by means of a feedback control on length-vs.-time reference trajectory; and a retrieval system to eventually rewind the tether around the spool and reset the system to the initial state. This paper presents motivation, conception and microgravity testing of the Space Tether Automatic Retrieval Experiment, and describes the educational return of the project.