ISBN 9783843947619

978-3-8439-4761-9, Reihe Raumfahrt

Yannick Lammen
Development, Integration and Inflight Testing of an Advanced Secondary Mirror Mechanism for SOFIA

223 Seiten, Dissertation Universität Stuttgart (2020), Hardcover, A5

Zusammenfassung / Abstract

The Stratospheric Observatory for Infrared Astronomy (SOFIA) consists of a 2.7 m infrared telescope integrated into a Boeing 747 SP. Maintaining a stable image while the optics are exposed to heavy wind loads and structural vibrations at stratospheric flight altitudes is extremely challenging. It can only be achieved through a cascaded system of active damping and pointing correction mechanisms. One of these mechanisms is the Secondary Mirror Mechanism (SMM), providing active steering capability for image stabilization and infrared chopping. Limited by a strong structural resonance, the SMM failed to meet its required steering bandwidth. This thesis describes the development of an improved SMM and its integration into the operating observatory. A ring-shaped reaction mass was identified as the structural mode's causing component based on experimental modal surveys.

New design concepts were evaluated under flight conditions using an end-to-end simulation tool, including a finite element model of the mechanism and a controller model. A segmented reaction mass design made from tungsten and AlSiC (i.e., improved stiffness and strong mass redistribution) showed optimal performance.

A prototype was manufactured and thoroughly tested on a full-scale engineering model of the SMM, confirming the predicted steering bandwidth improvement of 78%. Subsequently, a flight unit of the improved SMM was manufactured and assembled. The unit was tested against performance requirements and environmental loads. After the permanent integration into the aircraft, the improved steering capability was confirmed using preliminary controller settings during a test flight campaign. Future controller adjustments are discussed and were successfully tested on ground, delivering a mirror steering bandwidth improvement of up to 128%. This is enough steering capability to achieve the image stability requirements. Structural modes of the SMM are no longer an observatory performance limitation.