TESTING AND OPTIMIZATION OF A STRUCTURE FOR PRESSURE AND TEMPERATURE MEASUREMENTS IN PLANETARY ATMOSPHERE "Dipartimento di lngegneria Meccanica, Universita di Padova Via Venezia 1,351 31 Padova, ITALY 'Dipartimento di lngegneria Industriale, Universita di Parma Viale delle Scienze, 43100 Parma, ITALY ABSTRACT The STUB subsystem, part of the HAS1 (Huygens Atmospheric Structure Instrument) experiment on board the Cassini Mission, will measure pressure and temperature parame- ters of Titan's atmosphere during probe entry and descent. The objective of the optimization of the sup- porting structure, STEM, is double: first to reduce the structure mass much below the requirement of 120 g. In this way, more mass budget is free for sensors with an improve- ment of their stiffness and reliability; as sec- ond, to increase structure stiffness (frequency much higher than 80 Hz) to re- duce disturbances to sensors due to vibra- tions of the support. This paper illustrates design optimization using as main variables material properties and geometrical parameters. The concept evolution, starting with material selection, structure shape definition, FE validation and test qualification, is decribed. This procedure leads to an optimal structure with a cylindri- cal shape linked to the rectangular flange on the probe via a conical-elliptical section.The optimized STEM has a mass of 85 g, a first natural frequency of 134 Hz, capable of en- suring sufficient reliability and stiffness of sensor to accurate measurements of Titan's atmospheric parameters. The conical shape, in particular, resulted from the uniform stress distribution in the structure. Experimental testing of the optimized structure for valida- tion and qualification of the numerical model is performed. Copyright c 1994 hy the authors 11 INTRODUCTION The aim of the Cassini Mission, scheduled for flight in October 1997, is to put a space- craft in orbit around Saturn and to deliver the Huygens probe to the surface of Titan, its major satellite (Ref 1). HAS1 scientific goals require measurements of temperature profiles in the range T = 70- 300 K with an accuracy of 0.5 K, atrnos- pheric pressure in the range 0-2000 mbar with an accuracy of 1%, and the acoustic environment over a frequency range of 0-5 kHz, accuracy within 5%.(Ref.2) Fig.1 shows the flight configuration of the STUB. Temperature (TEM) pressure (PRE) and acoustic (ACU) sensors are mounted on brackets fixed to the STEM. The STUB, housed under a thermal shield during interplanetary flight, will be exposed to Titan's atmosphere following a cruising pe- riod of 7 years, after the release of the Probe front shield and back cover. (Ref.3) 3) OPTIMIZATION CONSTRAINTS The optimization process is complicated by the great number of requiremets imposed by the mission complexity and duration. The constraints can be divided in three major categories: basic project constraints, peculiar environmental constraints and integration constraints. (Ref.4) In the following a review of each class of constraints is presented. Puhlisced by American Instituts of Aeronautics and Astronautics. Inc. All rights reserved.