Sarah Brennan

Particle Damping Applications for Shock and Acoustic Environment Attenuation

Shock and acoustic environments can have significant impact in the overall design of a typical spacecraft. Often, mass constraints and other restrictions pose a considerable challenge in the design of a bus and a payload that can survive these environments. An approach to reduce launch loads is to provide attenuation by increasing damping, and several acoustic, shock and random vibration mitigation concepts were tested. Particle damping can be introduced without adding considerable mass or requiring hardware redesign, and it has been implemented extensively on several aerospace applications to reduce hardware vibration during launch. Several particle damping concepts were evaluated, including impact dampers, discrete particle dampers and tuned mass dampers. Other concepts tested evaluated the effect of open area and perforations on composite panel acoustic loads, and a damped joint concept was also considered. Acoustic test panel size was selected such that its fundamental frequencies would be below 50Hz, since it is challenging to reduce panel loads in low frequency range. The damping concepts evaluated in this paper show significant attenuation of shock, acoustic and random vibration loads can be obtained by implementing these simple, inexpensive mitigations. Since damage to spacecraft components due to environmental loads can result in significant cost and schedule delays, these damping solutions can minimize risk of hardware damage during verification testing and launch. Nomenclature eq ξ = equivalent critical damping factor for system with particle dampers s ξ = critical damping factor for system without dampers n ω = natural frequency for system with particle dampers n ˆ ω = natural frequency for system without dampers α = best-fit parameter for damping loss factor proportional to square of velocity β = best-fit parameter for damping loss factor proportional to velocity

Damping and Isolation Concepts for Vibration Suppression and Pointing Performance

Current state-of-the-art space exploration missions have stringent wavefront errors and pointing requirements in the few nanometers and miliarcsecond range. Achieving this level of precision is not an easy feat. Optical telescope designs often employ very large primary mirrors which can be susceptible to jitter and acoustic loads during launch. Reaction-wheel induced disturbances must be mitigated using a layered approach to isolation and damping, and structural resonances contributing to line-of-sight vibrations must be attenuated. Several damping and isolation approaches are discussed, including viscoelastic constrainedlayer damping and magnetic tuned-mass damping of a mirror segment, passive isolation of spacecraft disturbances, and active optical telescope pointing control. Results show that a layered approach may be required for the next generation of space telescopes.

Viscoelastic Damping of Structural Joints for Disturbance Isolation and Vibration Attenuation

Dam ping is a n important consideration in state -of -the -art space missions, as it is often us ed to mitigate launch loads and low -amplitude spacecraft disturbances . Although damping treatments are very effective, their implementation may be limited due to mass , cost and schedule constraints , or due to added complexity to the system design and verification efforts . Damped joints are a simple and cost effective damping option which can be easily incorporated into a spacecraft design . This approach can be used at bus panels, equipment interfaces, isolator ball joints and flexures, and other structural joints . In addition to increased damping, the addition of viscoelastic material to structural joints results in improved joint performance due to removal of nonl ine ar effects of friction and gaps . A viscoelastic damped ball joint was developed and demonstrated on three applications: 1) tuned -mass damping , resulting in 30 -50% critical damping to fixed -base damper modes; 2) acoustic test panel supports, resulting in up to a factor of 4 reduction of acoustic loads ; and 3) reaction wheel isolator struts, resulting in 5 -10% critical damping of isolator modes (and higher at strut modes) . An overview of the damped joint design, analysis and testing will be presented in th is paper.

New Particle Damping Applications

In its simplest form, a particle damper consists of a cavity box of various shapes and sizes that are partially filled with particles. This particle filled container is attached to the structure which needs to be damped. Particles can be metalli c, ceramic, polymeric, composite or a mixture of various materials. The particle size may vary from application to application any may vary from a few micro -inches to a tenth of an inch or more. The shape of the particles may be near spherical, cylindric al or irregular in shape. The shape, size and material of the particles will influence flow characteristics of the particles in the cavity. The work described in this paper documents experiment al work and performance results on three particle dampers used to suppress excessive vibration of spacecraft structural subsystems of various sizes subjected to random vi bratory disturbances . It is experimentally demonstrates that the performance of these devices are highly amplitude dependant. The first applicatio n is a revisit of a previously designed particle damper for a 23 Hz spacecraft appendage . An identical appendage is tested here with a shorter boom length with a resonant frequency of 32 Hz. It is shown that the previously designed damper for the longer b oom is also very effective with the shorter length appendage. The second particle damper application deals with performance comparison of two different shaped dampers. One of the dampers is cylindrical in shape with lead particles; the other damper is squ are in cross section but containing tungsten shot. It is demonstrated that the square damper provides better performance compared with the cylindrical damper. In a recent communication, a particle tuned mass damper (PTMD) was developed and tested. A tuna ble -length aluminum cantilever beam was used to model the stiffness portion of the damper. In the current application, the particle damper was built in a cylindrical form with adjustable cover which permitted to vary the volume of the cylinder containing the particles. The damper is housed in a cylindrical shaped machined spring thus providing a compact PTMD. F or a given amount of particles, the size of the gap could be controlled between the top of the particle bed and the cover. The structure to be

Particle Tuned Mass Dampers: Design, Test, and Modeling

The objective of this research is to design, build, and test a particle tuned mass damper (PTMD) and to develop prediction capabilities for particle dampers in both 1g and zero-g environments through experiment and computer simulation. The applications of this technology are quite diverse because particle dampers are inexpensive to build, may be adapted to accommodate almost any geometry, and are unaffected by environmental changes, specifically temperature, which has a dramatic effect on the performance of the visco-elastic materials often used for vibration isolation and attenuation. In recent years successful implementation has raised considerable interest in the applications and simulations of particle dampers. Here a modified particle system, a particle tuned mass damper (PTMD), is introduced that combines the desirable characteristics of both particle dampers and tuned mass dampers (TMD). It is known that the performance of particle dampers is not very sensitive to frequency, whereas a tuned mass damper is very sensitive to frequency and highly efficient at the tuned frequency. Thus, by combining the merits of TMDs with those of particle dampers it is anticipated that these new PTMD devises will be more robust and perform more effectively with built-in damping adjustability. An adjustable frequency PTMD was designed, fabricated, and tested. The device consists of an adjustable length cantilever beam to the end of which a particle damper is attached securely. The damper cavity is cylindrical in shape and is partially filled with particles. For tuning purposes, the distance from the top surface of the particle bed to the top of the enclosure is also adjustable. The damper will be excited with a sine sweep force using a shaker. The following parameters will be adjusted: (a) force amplitude, (b) beam length, and (c) cavi ty volume for a fixed amount of particles. The test results will be presented with in the form of transfer functions.

Cryo Magnetic Damping: Material Characterization and Damping Demonstration

The aim of this research is to develop analytical techniques that are supported by experimental data to predict the behavior of a passive magnetic damper in a cryogenic environment. The applications are far reaching--projects with high precision pointing requirements, such as JWST, operate in harsh environments where traditional damping techniques may be limited or ineffective. Therefore, we are interested in developing new technologies that can be used passively to isolate vibration disturbances effectively at a temperature of 40 K, under conditions that offer only a few nanometers of relative motion. Magnetic damping relies on the principle of motional EMF. As a conductor passes through a magnetic field, eddy currents form to produce a force that opposes the direction of motion. The magnitude of this force depends on several factors including the characteristics of the conductor and the magnetic field, which may be highly dependent on temperature. Current research is focused on material characterization, and a simple test cell has been designed to evaluate the damping, quantify the temperature effects, and expand our predictive capabilities. Several material combinations have been successfully cycled from 295K to 15 K, and the results, which illustrate damping from 1% to 100% critical damping, will be presented and compared to model predictions.