"Space Mail" and Tethers sample return capability for Space Station Alpha
- Paper ID
IAF-95-T.4.10
- author
- company
‘ESA/ESTEC,.automation informatics department
- country
The Netherlands
- year
1995
- abstract
Frequent and rapid sample return capabilities (~ 1-2 times 10 kg/week) for the Space Station has been studied by ESA in 1986 resulting in small re-entry capsules (88 kg) that used rocket engines (15 kg) to reenter. For the presently planned International Space Station Alpha (ISSA) the significance of such a return capability for the progress of science, particularly for biology and material science is unchanged and high. In this paper it is shown that using a swinging tether instead of a rocket engine for the required AV can have several advantages such as safety, lower mass overhead and a more accurate landing spot. Various deploy strategies (path and time) and corresponding control laws were investigated, as to minimize the final landing site dispersion, The deploy process was simulated using the'Star!rack' program that simulates the full process from start of deploy, tether cut, re-entry and landing. The program also includes an active control module Good_Module and a dispersion generation module Bad_Module. The latter produces fluctuations, variations and errors that can be caused by the tension, tether properties, and measurements and control operations. It was found that some critical stages during the deploy can be identified, where timing can greatly enhance the landing accuracy and where the disturbances due to the tether and system components can be compensated for. As a result a two stage deploy strategy is proposed in which the tether is first deployed to a relatively small length (1.5 km) after which at a chosen time the second phase deploys the tether to a large angle (68 deg, 35 km) before inducing the swing back for the release to re-entry. Particularly successful is proven to be an energy feedback control together with a special brake profile that dampens the tether longitudinal oscillations at the end of the deploy. Only two inputs are sufficient for robust control: accurate measurements of time and tether length. Use of GPS seems not necessary. The resulting dispersions of the landing site range remains typically below a few km for the realistic case studied (ISSA). This work is performed in support of the Tether Assisted Transfer Study under contract by ESA's directorate of Manned Space Missions, the result of which will define a demonstration mission with Progress M and a tethered re-entry of the Raduga capsule.