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The On-Orbit Environment

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Since the first U.S. spece shuttle launch. morę thought has been given to spece stations and other larga orbiting spece structures. We are now entering a new era in shock and vibration. The design and testing of the new breed of large spece structures presents a new set of dynamie problems to be dealt with.

First is the question of materials properties. Today, there are many new composite materials available to the aerospace design engineer, such as metal-matrix. graphite-epoxy. and Kelvar-epoxy. The damping properties of many of these composites arent available. even from the manufacturers. And. even if there are damping data evailable on coupon or beam samples. there is the ques-tion of the usefulness of this data in the prediction of the model damping of a built-up structure where the damping in the joints exceeds the inherent materiał damping.

Second is the problem of weightlessness. The new structures are designed to be constructed or deployed in spece. Some are so flimsy that they won t even support their own weight on earth. How. then. can a model or a vibration test be performed on these structures? One technique is to use scalę models which are dropped in large vacuum chambers or flown in parabolic trajectories in aireraft to simulate the zero-g environment. Other ground-based techniques have been developed where the spacecraft is hung from bungee cords or springs to simulate the zero-g environment. Ali of the earth-bound techniques have problems associ-ated with them. however. which may make their results invalid. The measured value of the stiffneas of a structure suspended from soft springs is the property most suspect. Gravity causes individual structural members to sag which reduces the stiffness of the structure. Also. the stiffness across joints in deploy-able panels is affected by gravity. In a recent space shuttle flight. the stiffness of the solar panel which was deployed in the cargo bay was much higher than the predictionsl

To summarize, I suggest new techniques will have to be developed to qual-ify these structures for the on-orbit environment. Such techniques might include sophisticated scalę model tests. morę powerful analytical prediction methods. and in-orbit testing of structures such as the solar panel flown on the shuttle. Also. morę work needs to be done to tabutate the available data on the damping properties of the newer composites materia Is.

JGS

DTIC

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DEC 7    1984