L. Porter Davis

Vibration isolation and suppression system for precision payloads in space

This paper describes the design and performance testing of a vibration isolation and suppression system (VISS) which can be used to isolate a precision payload from spacecraft borne disturbances. VISS utilizes six hybrid isolation struts in a hexapod configuration. Central to the concept is a novel hybrid actuation concept which provides both passive isolation and active damping. The passive isolation is provided using a flight proven D-strut design. The passive design is supplemented by a voice coil based active system. The active system is used to enhance the performance of the passive isolation system at lower frequencies, and provide the capability to steer the payload.

Adaptable passive viscous damper: an adaptable D-StrutTM

This paper summarizes a development study that involved the design, fabrication, and test of a prototype adjustable viscous damper. The study, sponsored by NASA Langley Research, was performed by McDonnell Douglas and Honeywell, and addressed the need for an adaptable passive damping system for spacecraft by investigating methods of tuning the passive viscous damping device known as the D-StrutTM. The D-Strut is a flight-qualified device used for both compliant isolation systems and rigid structural damping systems. The goal of the study was to demonstrate a specific design that would allow peak damping to be adjustable between any frequency from 0.1 to 10 Hz. Several tuning methods were investigated. The tapered annulus concept was selected because it is mechanically simple and provides a broad range of damping adjustment. Results were predicted by modeling and design analyses. Characterization testing was performed using impedance test methods. High, low, and intermediate adjustments were made to validate range capability. Success of the project is clearly illustrated by impedance amplitude and phase plots.

Need for and benefits of launch vibration isolation

Spacecraft designs are driven by the necessity of the spacecraft to survive being launched into orbit. This launch environment consists of structure-borne vibrations transmitted to the payload through the payload attach fitting (PAF) and acoustic excitation. Here we present a discussion on the need for and benefit of isolating the structure-borne vibrations. If the PAF were replaced with an isolator with the correct characteristics the potential benefits would be significant. These benefits include reduced spacecraft structural weight and cost, as well as increased life and reliability. This paper presents an overview of the problem of vibration on a launch vehicle payload and the benefits that an isolating PAF would provide. The structure-borne vibrations experienced by a spacecraft during launch are made up of transient, shock, and periodic oscillations originating in the engines, pyrotechnic separation systems, and from aerodynamic loading. Any isolation system used by the launch vehicle must satisfy critical launch vehicle constraints on weight, cost, and rattle space. A discussion of these points is presented from the perspective of both a launch vehicle manufacturer and a spacecraft manufacturer/user.

Vibration reduction for commercial optical intersatellite communication links

The increasing demand for global communications and limitations on RF communication bandwidth has driven several corporations to baseline optical intersatellite communication links in their constellations. The use of laser communications over a long distance dictates the need for accurate pointing and jitter suppression in order to maintain signal. Vibrations on a satellite cause excessive line-of-sight jitter for optical performance. The solution to these vibration sources is a systems problem involving optical control of coarse and fine steering, vibration isolation of the optical payload, or reduction of the spacecraft disturbances. This paper explains the basics of tracking control of a laser communication package and details the systems trades for vibration isolation. Simulation results based on a vibration isolation and precision pointing of a hypothetical commercial LEO to LEO 4 Gbit/sec laser cross-link system are presented.

Structural control of a flexible satellite bus for improved jitter performance

The increasing demand for global communications and limitations on RF communications bandwidth has driven several constellations to baseline laser cross-links between the satellites within their constellations. The use of laser communications over a long distance dictates the need for accurate pointing and jitter suppression in order to maintain signal continuity. Vibrations upon a satellite bus or orbit come from several sources including: momentum systems, flexible appendages, motors and cryocoolers. Attenuation of these vibrations requires a combination of disturbance reduction, disturbance isolation, payload isolation, input command shaping, appendage damping and passive/active bus structural control. This paper addresses these techniques in a systems approach to satellite structural control. Experimental results from a representative flexible satellite truss structure using a series of integral D-Strut structural dampers is presented. The passive damping system is used to reduce resonant amplification of disturbances on precision optical equipment jitter. The use of different combinations of longitudinal, transverse and diagonal dampers is discussed to achieve specific modal damping. In addition, the design of the integral truss dampers is discussed along with their application to satellite bus construction.

Optimization of multiaxis passive isolation systems

Passive isolation design is often done in a single axis fashion, while most applications have performance requirements in multiple degrees of freedom. Optimization of six by six transmissibility requires a thorough understanding of base disturbances, payload performance needs, envelope constraints, and mass/cost weighting. Singular value analysis of the disturbance to performance transfer function matrix in the frequency domain is shown to be effective in creating a single curve for multi-axis isolation performance. The various limitations to passive isolation are enumerated as constraints on the optimization process. Analysis and design optimization results are presented for the kinematic hexapod (six strut) mounting of a hypothetical commercial laser communication terminal payload.