Selection of Materials and Processes for Space Use Ruben Edeson, Mechanical Engineer, SSTD.
Materials Selection for Space Mechanical properties, ie specific stiffness Thermal and optical properties Cost and ease of use Outgassing properties Heritage
Why is outgassing bad? Adsorption on optics and detectors Changes emissivity of thermal surfaces Contaminating science data Power reduction in Solar Cells Structures change size and shape ESD
Optics Some products absorb or scatter light Silicone absorbs in the UV and EUV Water in some IR bands Hydrocarbons complex longer chains unpredictable effects Polymerisation and blackening under UV
Stable Structures Outgassing can cause ultra-stable structures to shrink in space CFRP: HM fibres ~ 0CTE CME Outgassing rate Eutectic barriers
Quantifying Outgassing VCM (0.1%) TML (1%) RML (1%) (0.1%/0.01% for critical areas)
Outgassing Information http://outgassing.nasa.gov/ http://esmat.esa.int/services/out gassing_data/outgassing_data.ht ml Heritage plays a big role. Manufacturers datasheets
Mechanisms Lubricant contamination Lubricant loss Solid lubricants not always the answer.
Electronics PCBs and coatings Wires Thermal fillers Solder and solder-resist
Manufacture Avoid silicone cutting fluids Weld porosity Adhesives Coatings/paint MLI
Cleaning/Storage Cleaning schedules Bakeout Minimise moisture ingress N2 purge or vacuum storage.
Testing of materials for space applications Giles Case Space Science & Technology Department Assembly Integration and Verification (AIV) Group
Building for Space Not ideal materials or build conditions Has to be built by (dirty!) people and tested in various environments Mixtures of materials and coatings Can spend long durations out of vacuum Very tight outgassing limits for optical instruments Outgassing path well defined away from sensitive surfaces Some coatings sensitive to silicone contamination
Micro VCM Test Setup What looking for TLM, CVCM Total Mass Lost (TML) <1%, Collected Volatile Condensable Mass (CVCM) <0.1%
Micro VCM
Micro VCM
Micro VCM
Outgassing information allows you to make an informed choice on materials Built instruments are assembled from clean parts in-house aqueous cleaning and vacuum bake outs Majority of instruments when being tested in vacuum will have an RGA on the chamber to monitor gas species RGA is only good for looking at ratios of gases not absolute levels Need a way to quantify absolute outgassing rates in representative conditions
TQCM Test Setup Thermoelectric Quartz Crystal Microbalance Can measure tiny amounts of condensed material Can be used to determine type of condensed material during warm up phase
TQCM
Typical Bakeout Procedure Install into test chamber. Evacuate to at least 10-5 mbar Heat to Bake o C Dwell for 72 hour (typically) Once RGA partial pressures >32amu have levelled out cool to Outgassing o C Monitor outgassing rate using a TQCM running at Cold o C Return to ambient Remove from chamber
Record frequency rate at 0.5 hourly intervals Record rate of change at five hourly intervals 2 d f ( t) = 2 dt ( f ( t 0.5hr ) f ( )) df ( t) t = dt 0.5 df ( t 5hr) df ( t) dt dt df ( t 5hr) Bakeout complete when rate of change < 0.05 over 3 hours dt
Bake out of big/complex items MIRI Operational temperature 6K to 40K Cryogenic QCM used at operational temperatures Tested and ougassing rate verified in 50m 2 vacuum chamber pumped with cryopumps (total pumping speed 60,000 l/s) Maximum bake temperature 47 C MIRI Outgassing Rate = 9.87 x 10-10 g/s
MIRI
MIRI
MIRI
MIRI OM Outgassing Final Run 300.0 14 TQCM Frequency / Rate Second Derivative (%) 12 10 8 6 4 2 250.0 200.0 150.0 100.0 50.0 Frequency Rate (Hr/hr) 0-14/10/2008 10:00 14/10/2008 14:00 14/10/2008 18:00 14/10/2008 22:00 15/10/2008 02:00 15/10/2008 06:00 15/10/2008 09:59 15/10/2008 13:59 Time and Date TQCM Frequency (khz) Rate Second Derivative (Hz/hr/hr) Frequency Rate (Hz/Hr)