A System for Suspending and Vibration-Isolating a  Large Spacecraft for Testing in Vacuum  David A. Kienholz*1 Abstract  A system is described for suspending from above a 15,900-kg (35,000-lb) payload for ground testing in a  vacuum chamber. The system provides ve ry low suspension frequencie s to isolate the payload from  ambient vibration. It also includes active capability to maintain a very stable ride-height and attitude of the  payload. Passive magnetic dampers are included to suppress pendulum-mode lateral oscillations.  Designated as the Large Suspension / Isolation System (LSIS) the system is described in terms of its  design, analysis, fabrication, and component-level testing. Introduction  Spacecraft are often required to maintain a very accurate line-of-sight for a beam-pointing instrument.  Doing so requires knowledge of the on-board disturbances, any external disturbances, and the dynamics  of the spacecraft as they relate to the line-of-sight. Verifying the first and last of these in ground testing can be quite difficult because of the need to isolate t he spacecraft from any external disturbances that  would be only artifacts of the ground test environment. The present case was particularly challenging  because of the shear size of the test article and the fact that the ground testing had to be done in vacuum. In response to this challenge, a unique vibration isolation system was developed.    The system design was derived from smaller versions developed originally under NASA sponsorship for  simulating the free-floating boundary conditions of space in ground dynamic testing, such as modal  surveys of very flexible spacecraft [1]. While a number of such systems have been built, including even  one designed for operation in vacuum [2], none approached the capacity needed for the present application. The primary accomplishment of this development was demonstrating that the design was  scalable to much larger sizes than built previously. In fact, experience reported here indicates that it can  be scaled to even larger sizes.   Requirements and System Architecture  Figure 1 shows a simple schematic of the test articles and the suspension system. There are actually two  separate test articles that must be held in accurate registration to each other during the test. The larger, designated Body C in the figure, was essentially two relatively rigid bodies connected by a trusswork. The  smaller, called Body A/B, was initially separate but was, in some tests, connected by trusswork to Body  A/B. The entire apparatus, payload and suspension devices, is inside a large vacuum chamber with the suspension devices mounted to a framework near the top of the chamber.  The defining characteristics of the development were (1) the schedule was very aggressive, and (2) very  little information about the payload was available to the suspension system designers. No hard  requirement was ever set for the vertical suspension frequency, usually a defining requirement. Early  simulations indicated that a value of 0.50 Hz or lower would be adequate so this was taken as the  allowable maximum. The ride height and orientation of the payloads had to be highly stable once they  were “floated” on the suspension and the test began. Again, no hard quantitative requirements were set  but predicted values of uplift force resolution give n later and the re sulting position resolution were  accepted as adequate.  * CSA Engineering, Inc, Mountain View, CA  Proceedings of the 39th Aerospace Mechanisms Symposium, NASA Marshall Space Flight Center, May 7-9, 2008  317