Evaluation of materials for Venus aerobot applications

Abstract

During the last 5 years JPL has examined a number of concepts ‘, ’ for robotically controlled balloons or aerobots. The Venus Geoscience Aerobot (VGA) concept uses reversible fluid altitude control to carry out repeated descents to the surface of Venus. This concept uses frozen water to keep the electronics cool. The balloon must periodically go to high altitude so that the water can be refrozen. Balloon materials would be required to withstand the rigors of the Venus atmosphere, including passes through sulfuric acid clouds and survival at temperatures up to 460 “C. Polybenzoxazole (PBO) and polyimidobenzoxazole (PIBO), materials developed by Dow Chemical Corporation, appear very promising. For example? weight loss measurements of PBO and PIBO films have been made with less than 2-percent weight loss observed over a 4-hour period at 460 “C, and mechanical properties appear persistent up to high temperatures, although the high temperature limits have not been determined. We have acquired PIBO film and tested it under conditions that simulate the conditions that would be encountered by a balloon in the Venus environment, including temperatures up to 500 “C. It is also intended to fabricate composites made from high temperature coating/sealing material and a fabric made from PBO fibers, with PIBO as the high temperature composite matrix. A metalized PBO-based multilayer composite would be appropriate as a balloon material. Another Venus Balloon concept, the Venus Multiprobe Mission, which is described in a companion paper, involves higher altitude missions. While higher Undergraduate Summer Research Participant, University of Southern California. Los Angeles, CA 90089 altitude missions have only moderate temperature requirements, balloon materials must’ still survive passes through sulfuric acid clouds. To meet the materials requirements for these higher altitude missions. the focus was changed from PBO-based chemistry to fluoropolymers as balloon envelope materials themselves, as a protective layer in a multilayer composite, oi in a concept in which an outer “protecting” balloon is made from a fluoropolymer and the inner balloon is made from Mylar. To evaluate fluoropolymer candidates the tensile modulus and strength of several fluoropolymers has been studied. Preliminary sulfuric acid permeation test of various fluoropolymers was performed. Sulfuric acid exposure tests were also performed on candidate fluoropolymer films as well as on fluoropolymer protected Mylar film. In these tests the mechanical properties preand post-exposure were measured and compared. As NASA’s plans for in situ exploration and sample return from Mars progress, plans are proceeding for in situ exploration and sample return from Venus. Whereas balloons and aerobots play a support role to rovers and other vehicles in the exploration of Mars, at Venus balloons will play a central role for mobile exploration of the surface, acquisition of surface materials and deployment of high altitude earth return vehicles. For the Venus Surface Sample Return mission (VSSR), which does require a short IO-20 min Venus surface exposure (figure I), a potentially viable balloon envelope material is PBO or carbon fiber as the reinforcing element of a fluoropolymer composite. A fluoropolymer composite has the advantage of not Copyright