High Torque Wheels for agile Satellite Maneuvers - in Orbit Experiences and future Steps with Recuperation of Energy

Abstract

Agility is a necessary skill for several tasks. This applies in particular to remote sensing applications, but is also useful for optical communication from satellite to ground. While discussing a new reconnaissance satellite, the need for suitable actuators for agile maneuvers was identified several years ago. Developments for robotics at DLR (e.g. ROKVIS experiment on board the ISS (2004-2011) resulted in the possibility to further develop the reaction wheel principle for the special application "high torque" for fast accelerations. This was in competition with the widespread use of control momentum gyros (CMG). It was decided to equip the small satellite BIROS (launch 2016) with three “High Torque Wheels” (HTW) as additional technological payload. With this down-scaled version of the HTWs, the proof of the concept should be made under real conditions in space. This included various agile satellite maneuvers such as for “in track stereo”, scanning up to 5 parallel image strips on ground or switching between different targets on ground. This of course required changes to the ACS and the power supply system, which was a reuse of technology from the DLR small satellite TET-1. The attitude control system (ACS) uses several “attitude modes” for a comfortable and autonomous attitude control of the satellites in space. This was extended by a special “Fast Slew Mode”, which was developed for fast maneuvers using HTWs. The standard satellite slew rate is 0.5 degrees per second with a freely selectable single slew axis. A fast slewing maneuver intends to reorient the satellite by up to 30 degrees within 10 seconds. Actually however, the satellite achieved angular speeds of up to 10 degrees per second in space. A fast slew maneuver is finished, when all the HTWs are nearly stopped and the default ACS actuators took over the remaining angular momentum. This process of momentum exchange between the actuator systems was one important aspect of investigations. Another test case was the usage of these 3 experimental HTWs as actuator of the ACS. After successful completion of the first experimental phase in orbit the work will be continued by improving the fast slew maneuver algorithms and by preparing a second experimental phase in space including active payload cameras in the defined image scenarios. The HTW will be equipped now with an energy recuperation system storing electrical energy instead of kinetic energy within a spinning CMG.