380. Measurement of the Moment of Inertia of the Orbiting Astronomical Observatory

Paper

Abstract

This paper was presented at the Twenty-second Annual National Conference of the Society of Aeronautical Weight Engineers at St. Louis, Missouri, April 29-May 1, 1963. The precise stabilization and control requirements of the Orbiting Astronomical Observatory (OAO) require an exact knowledge of the spacecraft’s mass moments of inertia. In addition to the normal calculations, a carefully conducted test program was taken to measure these moments of inertia. After a literature survey to investigate the measurement available, the bifilar torsion pendulum was chosen as simplest and most exact.
The design of the bifilar hardware and test instrumentation is presented in detail. The three axes mass property measurements (weight, center of gravity, and moment of inertia) made on the OAO are described. Particular attention is given to the measurement of tare inertia. This tare inertia arises primarily from the adapters necessary to affix the OAO to the suspension filaments, and also from the additional apparent mass effect caused by air drag of the spacecraft’s large solar paddles. Both effects were measured directly by the bifilar pendulum. The adapters were swung as a unit, without the OAO, and their combined weight, c.g., and moment of inertia were recorded. The drag effect of the air on the oscillating paddles was determined by swinging two large open frameworks of the same size and shape as the actual OAO paddles. These frameworks were then covered with lightweight mylar and re-swung. The difference between the two measured inertias was the additional mass effect caused by paddle air drag.
The bifilar facility proved to be readily adaptable to other vehicles associated with the OAO development.