A deployable and restowable dome made of flexible material with semirigid reinforcements has been developed to protect the main engines of the Cassini spacecraft against impacts by micrometeoroids during the long cruise to Saturn. The design of the dome could readily be adapted to terrestrial applications in advanced, easily erectable temporary structures like tents and portable sheds.
In its appearance and basic mechanical function, the dome bears some resemblance to covers on some vintage baby carriages and cloth rooftops on convertible automobiles (see Figure 1). When fully deployed, the dome is a hemisphere approximately 2.1 m in diameter; when fully stowed, it becomes a crescent wedge less than 15 cm thick. The flexible dome material is sewn together from gore-shaped pieces with accordionlike pleats to obtain the hemispherical shape. The dome shape is supported by graphite/epoxy semicircular hoops called stays, sewn into pockets within the material. In the original Cassini spacecraft application, the flexible material is an advanced, multilayer, thermally insulating blanket that is made electrically conductive to prevent the accumulation of static electric charge. Other, less-advanced flexible materials could likely be used in terrestrial applications like tents and sheds.
Two semicircular bows made of aluminum tubing, forming a circle on the dome at the "equator," meet at two hubs that allow them to pivot (see Figure 2). The ends of eight of the stays meet at the hub and slide in grooves, while other stays sewn into the material only support the shape of the dome. The flat shape of the stay end fittings allow the dome shape to collapse to a compact stowed position. The lollipop-shaped fittings on the ends of the stays slide freely in the slots in the hubs, with sufficient slack to allow for folding of the flexible material during stowage. One of the bows is fixed; the other is driven in rotation about the diameter to produce motion needed for deployment or stowage (see Figure 2). The drive mechanism, located at one hub, is nonbackdriveable and redundant; it includes two independent, electronically commutated dc brushless motors and two paths of 20:1 spur-gear and 605:1 harmonic output gears. In the other hub, the driven bow is retained by an idler mechanism that includes gears to transfer the rotary motion of the driven bow to a potentiometer for measurement of the angle of deployment. The rotation of the driven bow is limited by adjustable stops. A miniature lever-actuated switch at each stop provides an electrical indication that the driven bow has reached the stop.
This work was done by Donald Sevilla, Lori Shiraishi, and Randal Foehner of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Mechanics category, or circle no. 187 on the TSP Order Card in this issue to receive a copy by mail ($5 charge). NPO-20156