7th ESA Workshop on Advanced Space Technologies for Robotics and Automation ASTRA 2002 Robotic Assembly of Large Space Structures: Application to XEUS R. Licata(1), M. Parisch(1), I.J. Ruiz Urien(2), M. De Bartolomei(3), G. Grisoni(4), F. Didot(5) (1)ALENIA Spazio,Turin, Italy; [email protected] (2)SENER, Bilbao, Spain; [email protected] (3)Tecnospazio, Milan, Italy; [email protected] (4)Media Lario, Lecco, Italy: [email protected] (5)ESTEC, The Netherlands; [email protected] INTRODUCTION In-orbit robotic assembly and/or maintenance is an enabling technology for large space structures that, due to their substantial mass or dimensions, cannot be assembled on ground and hence be transported into space with conventional space vehicles The objectives of the Robotic Assembly of Large Space Structure (RALSS) ESA-Study, conducted by Alenia Spazio in the years 2001-2002, as Prime Contractor, with SENER and Tecnospazio as Sub- contractors, are related to this important technology or topic development, starting with some detailed analysis of the first important example of this type of activity, that is the on- going International Space Station (ISS) in-orbit build-up or assembly. During the ESA RALSS study development, detailed investigation has been conducted for the design and the application of the robotic assembly on the ISS of the required Mirror Sectors (MS) for the large Mirror Spacecraft Figure 1: XEUS Mission Illustration needed for the second phase of the XEUS mission, called · to perform cinematic simulations of these XEUS hand- XEUS-2. over and assembly operations at ISS. The XEUS mission, in fact, is planned as a long term X-ray · to formulate preliminary design concepts for the mirror observatory to be grown in orbit, from an initial sector mechanism(s), to trade-off and selected the configuration of a mirror spacecraft, MSC-1, with a 1 keV attachment mechanism design, before manufacturing a mirror sectors effective area of 6 m2 to a second phase representative prototype. configuration of the mirror spacecraft, MSC-2, with an · to perform dynamic simulations for proximity and effective area of 30 m2, by the assembly of extra sectors to contact operations of the mirror sector removal from its be firstly transported into orbit at the end of a first mission Transport Container and its installation onto the docked phase, while the Mirror Spacecraft is docked at the ISS. MSC. Some of the main development activities performed during · to demonstrate the feasibility of the robotic assembly of the presently illustrated study may be summarized as: the XEUS-2 mirror with the attachment mechanism · to provide an overview of in-orbit assembly techniques prototypes on representative mock-ups and a laboratory for large payload guiding and fixation, in particular demonstration of proximity and contact assembly considering constraints imposed by robotics means of operations the Space Station and the ERA manipulator. · to perform mission feasibility analysis, including system · to review in detail of the in-orbit assembly scenario of configuration, sizing, launcher accommodation and XEUS Mirror Spacecraft at ISS. mass and power budgets. · to establish essential requirements for the XEUS-2 mirror installation interfaces associated in particular XEUS MISSION with the mirror sector removal from the Transportation The XEUS mission will build upon the European leadership Container by the ERA arm, while manipulated by the in X-ray astronomy that XMM will provide by supplying SSRMS arm (hand-over operation) and its installation or unique opportunities for studying the distant universe, hot mounting onto the mirror spacecraft MSC-1, while is plasmas, material under extremes of density, temperature docked at the ISS Service Module docking port. and pressure and subjected to the effects of strong gravity. 1 XEUS Scientific Goals The XEUS mission is a potential follow-on to the already launched ESA Cornerstone X-ray Multi-Mirror (XMM mission) and is envisaged by the ESA Horizon 2000 Survey Committee as a major astrophysics facility within the Space Station Utilization Programme. The mission is planned as a long term X-ray observatory to be grown in orbit, from an initial configuration, called XEUS-1. Figure 3: XEUS Spacecraft Configuration Figure 2: XEUS Spacecraft Approach to the ISS A new detector spacecraft, DSC-2, will also replace the XEUS is planned as a long term X-ray observatory designed detector spacecraft, DSC-1, with next generation instruments to meet a sensitivity to measure a broad-band spectra of and technology. sources as faint as ~ 10-17 erg cm2 s-1 in the 0.5 - 2.0 keV energy range and a photometry limiting sensitivity of 10-18 erg cm-2 s-1, in order to address some fundamental questions in astrophysics. Currently, X-ray astronomy can only detect the most luminous of AGN, quasars, to a redshift of about 4. With comparable energy resolution, the sensitivity of XEUS will be much dramatically better than provided by the AXAF and XMM dispersive spectrometers. As well as allowing the detection of much more distant objects, XEUS will pioneer the field of detailed X-ray spectroscopy of these distant AGN. To reach a signal-to-noise ratio of about 30 with the above stated spectra in 10 6 sec (flux) requirements on mirror area and angular resolution are imposed (i.e. an effective mirror area of tens of square meters around 1 keV and a high enough resolution, ideally as high as 2 arcsec HEW). Figure 4: XEUS MSC-2 Spacecraft Assembly After assembly at ISS, the second mission XEUS2 will XEUS-2 Assembly Scenario initiate by the MSC-2 returning back to observation orbit The initial configuration (XEUS-1) consists of a mirror FTO to continue its astrophysical program. spacecraft, MSC-1, with a 1 keV effective area of 6 m2 and a The robotic assembly operations are assumed to be carried separate detector spacecraft, DSC1, to be aligned by active out at the ISS, by means of the available robotics facilities, control to provide focal length of 50 m with an accuracy of 1 comprising the Shuttle Arm, the ISS SSRMS arm and the mm3, as depicted in Figure 3 below. ERA manipulator to be installed and available mounted onto After several years of operations with this first configuration, the Russian Service Module. XEUS-1 will visit the ISS, where from the MSC-1 the ISS facilities, in particular, will allow the performance of the second mirror spacecraft configuration MSC-2 will be robotic transportation of mirror sectors delivered in a assembled by the addition of extra mirror sectors, as container by the Shuttle, on one side of the ISS, to the illustrated by the picture in Figure 4 to obtain an effective Russian Sector side of the ISS, as illustrated in the above area of 30 m2. Figure 5, where the XEUS mirror spacecraft will be docked and the ERA arm installed. 2 Figure 5: MS Delivery and Assembly Locations on ISS Mirror Sector Design While the configuration of the XEUS MSC-2 Mirror Sector was depicted in the Figure 4 above, the eight Mirror Sectors to be integrated on the external rim of the basic mirror spacecraft (MSC-1) are in principle similar and with the preliminary structural configuration given in Figure 6. The mirror sector core elements are the mirror plates, which are integrated in a total of 12 petals, containing 45 to 122 plates each. The mirror petals are attached to the mirror support structure via mechanisms in order to enable the in- orbit alignment of the individual petals. In addition the Figure 6: MS Preliminary Structural Design mirror petals are foreseen to be equipped with entrance and The individual petals will be mounted into the mirror support exit doors, serving in-orbit also as baffles. The mirror structure via specific actively controllable alignment support structure connects the complement of mirror petals mechanisms allowing adjustment of the alignment as and provides the interface to the mirror spacecraft. necessary. Additional equipment will be attached to the mirror support Dimensions Weight structure such as the solar panel sub-system and the RCS and F = 4.5 m balance sub-system (replacing mirror petals at specific MSC-1 12,400 Kg locations). During launch the solar panels are folded and (outer diameter) F = 10 m stored at the outer rim of the Mirror Sectors and will be MSC-2 25,000 Kg deployed in orbit. The deployed panels serve as sun shield (outer diameter) TC and baffle at the same time. 12.9 x 4.6 x 4.6 m 15,735 Kg The present baseline of the structure is a Nickel sandwich (loaded) structure in order to achieve a uniform thermal expansion of TC 12.9 x 4.6 x 4.6 m 3,135 Kg the entire petal assembly, avoiding misalignments and image (empty) errors due to thermally induced deformations. Table 1: XEUS Vehicles Mass & Envelope Budgets The baseline concept foresees that the mirror plates are attached to the radial plates of the petal structure along the Transport Container Design and Requirements corresponding edge of the mirror plates. The XEUS-2 Mirror Sectors Transport Container (TC) shall The first petal of the Mirror Sector (Ring 3 of the fully transport into orbit the eight mirror sector elements just integrated telescope) consists of 122 mirror plates, the illustrated and shall incorporate a Grapple Fixture (FRGF) second petal (Ring 4) consists of 62 mirror plates and the on the topside to allow the NSTS SRMS to grapple and third petal (Ring 5) consists of 45 mirror plates. Each Sector extract the TC from the Shuttle Cargo Bay, as shown in contains 4 petals of each ring, so that the 8 sectors together Figure 7 below.