EFFECTS OF THE VENUS GROUND AMBIENT ENVIRONMENT ON MATERIALS

Principal Investigator: Linda Del CastilloCo-Investigators: James Polk, Michael Pauken, Elizabeth Kolawa

Abstract:

Due to their unique characteristics, the Venus lower atmosphere and ground ambient environments provide particularly challenging conditions for materials. Such conditions include an ambient temperature of 470°C an ambient pressure of 92 bars and an atmospheric composition of 96.5% CO2+3.5% N2 + trace gases at levels ≤ a few 10s of ppm. To address these challenges and to mitigate the risk of failure due to materials degradation resulting from interactions with the Venus ground ambient, a Venus Materials Test Facility (VMTF) that is capable of simulating measured Venus surface ambient conditions, including temperature, pressure and chemical composition has been developed. This work describes the test methodology developed as well as the results of preliminary evaluations performed on a variety of materials and small subsystems. Pre-exposure and post-exposure analyses include gravimetric measurements, optical microscopy of the material surface, and scanning electron microscopy as needed. Application specific properties were also included in the evaluation.

Venus Materials Test Facility (VMTF):

This facility includes all of the power, high pressure gas delivery system, high temperature pressure sensing system, instrumentation and control software, high temperature, high pressure, chemically compatible electrical feedthru systems, and a robust set of test equipment. The development of this facility was complicated by the extremely challenging test conditions and degradation of systems used for the test facility, such as gas seals, heaters and thermal insulation. Lessons learned were critical for the development of a robust test facility and will be implemented into screening guidelines documentation.

Test Conditions:The figure below shows a CO2 phase diagram. Included within the diagram is the descent profile for a Venus lander as well as the minimum pressure required to achieve Venus surface conditions and the maximum pressure achievable with the room temperature saturation pressure of CO2 (57 bar). Currently the chamber is filled at room temperature and sealed. Increases in the temperature of the chamber yield Venus surface pressure conditions at 470°C. The gas delivery system is designed to support a variety of gas combinations, and is used with pure CO2 as well as the full Venus surface ambient composition.

Feedthru System:The chamber is designed with six electrical feedthru ports. Each port can support up to four individual wires. These feedthrus are used for the in-situ evaluation of electrical subsystems within the chamber. In addition, the feedthrus are used to place thermocouples within the chamber for the evaluation of the internal chamber conditions and thermal insulation materials. Chamber Instrumentation:Currently the chamber is instrumented with 14 external thermocouples and a minimum of 1 internal thermocouple. In addition, a high temperature pressure transducer is attached to the sidewall of the vessel to allow for the direct measurement of the internal pressure. A back pressure regulator can be set to maintain the chamber at the desired pressure to allow for greater control during testing.

Control:The VMTF allows for remote control of instruments being tested within the chamber. Cables, which include 24 electrical control lines and 15 thermocouple lines, are run to a control room in a separate building.

Upgrades:The chamber is being upgraded with sapphire sight glasses (through the currently existing side ports), as well as a remotely controlled camera system. In addition, a high pressure CO2 pump is being procured to allow for long-term operation of the system.

National Aeronautics and Space Administration

Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena, California

www.nasa.gov

Copyright 2010. All rights reserved.

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10

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-100 0 100 200 300 400 500

Pure Carbon Dioxide

Liquid/Vapor Saturation LineMelting LineVenus DescentChamber Pressure LowChamber Pressure High

Pres

sure

(bar

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Temperature (C)

Critical Point

Supercritical Fluid

Vapor

Liquid

31.1oC, 73.9 bar

Photographs of the Venus Materials Test Facility

Carbon dioxide phase diagram with the Venus Lander descent profile and chamber test conditions for the VMTF.

Photographs of various test specimens, including thermal insulation material and high

temperature motor assembly.

Evaluation of Materials and Sub-systems:

Preliminary screening was performed on several materials of interest for Venus Lander concepts, such as feedthru materials, thermal insulation materials, window materials (sapphire) and electrical insulation materials for cables. The tests were performed to determine the chemical compatibility of these materials with the Venus atmosphere.

Environmental exposure had essentially no influence on the optical properties of the sapphire window materials; however, environmental exposure did influence the thermal conductivity of the porous silica insulation (see the “Effects of the Venus Atmosphere on Thermal Insulating Material Performance” for details). Preliminary work on the electrical characteristics of the wire insulation revealed no obvious issues, but a more detailed evaluation is currently underway.

In addition, a high temperature pressure sensor, mineral insulated and shielded temperature sensors, a high temperature position sensor, and a high temperature motor for use within the Venus ground ambient environment were evaluated. Each of the sensors performed well in the target environment. The results of the motor test were inconclusive and testing will continue later this year. Photographs of the thermal insulation material and the high temperature motor assembly following exposure to Venus surface ambient conditions are shown.

Acknowledgement: The research described in this poster was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.