GLAST Large Area Telescope: Mechanical Systems Peer Review March 27, 2003 Section 5.2 – Engineering Modeling Marc Campel

1. Gamma-ray Large Area Space Telescope GLAST Large Area Telescope: Mechanical Systems Peer Review March 27, 2003 Section 5.2 – Engineering Modeling Marc Campell / Giang Lam SLAC Mechanical Systems Mgr. / Mechanical Engr. marcc@slac.stanford.edu gianglam@slac.stanford.edu

2. Topics Agenda • EM Test Plans & risks retired (purpose) • Test Data samples • Test results • conclusions • design changes resulting from EM tests • Further Work

3. EM Test Plans and Development Risks • Engineering Modeling (EM) Tests are performed for: • Retirement or mitigation of moderate & high risk areas • Validate key mechanical design assumptions & design implementation • Minimize impacts of design changes by early implementation • Validate finite element models • EM test plans developed for: • Mechanical interfaces • Thermal interfaces • Critical fabrication & assembly processes

4. Mechanical - CAL-Grid Joint Testing • CAL-Grid joint is a friction joint between the CAL baseplate tabs and the Grid walls. • 70 fasteners around the perimeter of the CAL • 54 #8 , 16 #6 fasteners and 2 - .156” dia alignment pins • Helicoils used in Grid walls • To maximize the load carrying capability of the joint • Maximize bolt clamping force • Maximize friction at interface • Tests performed • Friction Characterization • Bolt – Helicoil pair characterization tests • Pull Tab Coupons

5. Friction Tests • Friction characterization tests • Purpose: Investigate static coefficient of friction for various surface finishes. • RHR 8, 16 & 32 and rough sanded • Bare and alodined • Tungsten carbide flame spray • Test set up evolved • Lower clamping force • Faster UTM head speed • Load cell washer measures clamping force • Record loading & unloading

6. Friction Tests (cont) • Results • Not sensitive to RHR (can get cold welding on smoother finishes) • Too rough gave low values (Alum peaks too soft) • Flame spray gave consistently high values (ms > .9) • 32 RHR alodine finish selected (ms= 0.73) • Producible finish • Corrosion protection • Electrically conductive

7. Friction Tests (cont) • Further Work • Repeat selected configuration with a statistically valid sample size (30-50)

8. CAL-Grid Joint Testing (cont) • Bolt – Helicoil pair characterization tests • Purpose: optimize bolt clamping force applied by examining combinations of fine & coarse threaded bolts and Helicoil finishes CRES, silver plated, dry film lube and Teflon coated (Heli-guard). These were compared to bolts lubricated with Bray-cote grease in CRES Helicoils. • A lubricated Helicoil is desirable because of the potential to contaminate the friction surface with the Bray-cote grease from the bolts.

9. Bolt – Helicoil tests (cont) • Test set up • Load cell washer used to record load. Digital torque wrench captured peak torque. • Aluminum plates with 10X each Helicoil type

10. Bolt – Helicoil tests (cont) • Findings • The internal wrenching feature of a socket head cap screw would cam out at the high torques we need to apply (60+ vs 15 in-lb usual for #8). Torx Plus head will be required • At these small diameters, large parasitic torques can result from Helicoil installation or plating variations • Bolt – Helicoil must be torqued up several times to seat the Helicoil before the bolt preload becomes repeatable. • Similar results for fine and coarse threads • Coarse threads easier to work with; less chance of cross threading • Fine threads provide better locking (GSFC recommendation)

11. Bolt – Helicoil tests (cont) • Findings (cont) • 2x diameter minimum Helicoil length to prevent pullout • Black oxide coating on fastener is a good lubricant • Design changes from test • Heli-coil drill tap size callout (use steel sizes) • Helicoil seating procedure incorporated • Torx Plus head fasteners • Further work • Tests on final fastener & Helicoil will be performed with a statistically valid sample size • Flight fastener final selection: A-286 200 ksi UTS, black oxide coating, Torx Plus head with Nylok patch locking feature

12. CAL-Grid Joint Testing (cont) • Pull Tab Coupon Tests • Purpose: Demonstrate tab joint shear load capabilities. • Investigate backing bar design as a means to create “double shear” in joint. • Examine backing bar material and thickness effects on load capability • Test Set-up • Representative geometry of CAL tabs and Grid wall with Helicoils used • Load vs deflection recorded

13. Pull Tab Coupon Tests (cont) Applied Force Applied Force Cal Plate Cal Plate Grid Rib Displacement transducers ( 4Places) 3 Tab 2 Tab

14. Pull Tab Coupon Tests (cont) • Findings: • Test configuration distorts unlike the flight configuration due to the axial freedom of the grid rib • Backing plates reduce axial displacement of grid rib and increase shear capability of the interface – acts more like a joint in double shear • Distortion of the test hardware and the high local loading of the joint does not permit application of measured friction data to the flight configuration

15. CAL-Grid Planned Tests • Grid 1 Bay Tests • Originally planned as a dynamic test of 1 Grid bay with a CAL mass simulator. • Test deleted because it did not simulate predicted tab loading • Distortion of Grid wall from opposing shear loads in adjacent CAL tabs • Z axis Grid distortions superimposed on lateral dynamic loads • Difficult to measure slippage in single tab • Test could demonstrate no bulk slippage

16. CAL-Grid Planned Tests (cont) • Cantilever Beam test • Originally planned to Validate FEM of Grid (predicted stresses, and deflections). Test set up would be modeled with similar techniques used to model Grid in LAT model. • Test was deleted. The 1 x 4 Grid provides a higher fidelity test setup. Test setup model taken directly from Grid model. Cantilever Beam EM Test Set-Up

17. 1 x 4 Grid Planned Tests • Purpose: Validate finite element model used for LAT predictions to date • Model created from full up model • Full scale CAL-Grid interactions • Test Set up: 1 x 4 Grid design is flight like • Partial bays with partial CAL plates provide interleaving CAL tabs • Load hydraulically applied 6 places • Reacted out at corners • Deflections measured along length • Record load vs deflection 1 x 4 Grid

18. 1 x 4 Grid Planned Tests (cont) • Load case 1 – Z axis loading • Case 1a no CAL plates for max Grid stress • Case 1b Cal plates installed for CAL-Grid I/F loads

19. 1 x 4 Grid Planned Tests (cont) • Load case 2 – Twist may be performed • Provides good shear loading of CAL-Grid interface • Coarse mesh model does not predict large twisting very well • OK since LAT does not twist

20. Planned Process Tests Grid Heat Pipe bonding process qualification test - planned • Objective • Qualify tooling & procedures for bonding heat pipe into Grid Top Flange • Risks mitigated • Process problems, poor thermal bond

21. Planned Process Tests (cont) 1 x 4 Grid unit fabrication • Objectives • Demonstrate Grid manufacturability (currently has UNC inserts • Demonstrate purge gas system • Provide unit to I&T group for I&T EM testing • Risks mitigated or retired • Fabrication errors, process problems (cost & schedule impact)

22. Engineering Modeling Thermal Tests Giang Lam

23. Engineering Modeling Thermal Tests • Thermal Tests • X-LAT to E-Boxes Thermal Joint Test • X-LAT EM Heat Pipe Thermal Performance Test • 3-way Heat Pipe Joint (Downspout, X-LAT, and Radiator) Performance Test

24. X-LAT to Electronics Thermal Joint Design • Objectives • Identify feasible thermal joint that meets thermal, structural and I & T requirements • Verify material thermal conductance property for feasibility study • Risks mitigated • Ease of assembly and servicing boxes at LAT level • Repeatable thermal joint for servicing boxes after environmental testing • Ease of CAL-TEM component level verification • Minimum delta T from electronics to heat sink in X-LAT

25. X-LAT to Electronics Thermal Joint Design Drivers

26. Joint Candidates Tested • “Wet” adhesive or gasket joints • Thermally conductive silicone adhesive, Nusil CV2946 • SilPad, VO Gap Pad • Low contact pressure joints • Off-the-shelf EMI gasket products, i.e. BeCu spring fingers, electrically conductive elastomers • Graphite velvet pad (Vel-Therm from ESLI) • No contact pressure joints • High conductivity materials mechanically fastened at both ends • Formed Copper sheets (30 mils) • Pyrolytic Graphite Sheet (4 mils thick stock)

27. EM Thermal Conductance Test Setup Chilled Plate Heater Plate Chiller Inlet/Outlet Chiller Inlet/Outlet Vacuum Chamber Thermocouples, 3 per side Chiller Chamber Lid Removed Heater Power Supply Material Compressed Gap Vacuum Pump Note: Chamber shown is semi-vacuum, attained 60 mTorrs minimum during tests Close-up View of Setup

28. “Wet” or Gasket Material EM Conductance Results

29. Low-Contact Pressure EM Conductance Results Vel-Therm data per AIAA-2001-0217 paper, “Carbon Velvet Thermal Interface Gaskets”, 60 mils sample compressed from 5 mils to 25 mils

30. Low-Contact Pressure EM Conductance Results (cont.) • 2 inch square, 80 mils thick Vel-Therm sample was EM tested at SLAC to verify properties • For single layer sample, thermal conductance is constant at varying gap distance • For two layer sample meshed together, conductance is same as single layer sample at larger varying gap distance • Additional verifications at ideal conditions is needed (i.e. high vacuum chamber, consistent application of thermal adhesive, input power proportional to design load and sample size)

31. Thermal Straps EM Conductance Test Results Copper sheets (30 mils thick): • Thermal and structural needs conflict with each other • 1st iteration strap test setup could be optimized to meet thermal conductance numbers • Strap designed for optimum thermal conduction (30 mils thick, short bolt to bolt length) does not allow lateral flexibility • Thinner foils (10 mils thick) and longer bolt to bolt distance for bend reliefs allow relatively more lateral give but poor thermal conductor Pyrolytic graphite sheet (4 mils thick stock): • Poor thermal conduction for X-LAT application due to large power output from boxes and small cross-section area for conduction • Thin material easily tears from handling and mechanical packaging

32. Thermal Joints EM Evaluation Summary Note: √ = Poor, √√ = Medium, √√√ = Excellent

33. Thermal Joints EM Evaluation Summary (cont.) • Vel-Therm material is best candidate among those evaluated to meet all design drivers • Thermal data indicate material can meet required conductance • Material is bonded to one surface prior to installing X-LAT plate, minimal fasteners required • Allow sliding contact for lateral flexibility • Can bridge varying gaps from stack to stack with different stock thicknesses up to .125 inch thick max for single layer • Two Vel-Therms can be meshed together face-to-face for larger gaps up to .24 inch max

34. Further Work • Integrated thermal analysis (including boxes stackup, heat paths and X-LAT thermal joint) to identify and alleviate “hot” spots (Electronics) • Final detail design of EMI Shields, X-LAT Plate, Special Electronics box bottoms to accommodate Vel-Therm joint design – stack height • Additional EM testing needed to retire risks • Conductance in high vacuum for dry tip contact • Sliding verification without generating loose fibers • Quantify contact pressure versus brush compression

35. Further Work (cont.) • Proposed additional EM thermal joint test configuration for Vel-Therm material • Mock up box stackup configuration with heaters on sidewalls to simulate electronics heat source • Install cold plate and Vel-Therm sheet to simulate X-LAT and thermal joint • Flight like installation procedure for Vel-Therm to identify process problems and errors • Perform Random Vibe and Thermal Vacuum tests to retire risks associated with contamination, thermal conductance at min & max fiber compression levels

36. X-LAT EM Heat Pipe Characterization Tests • Objectives • Verify thermal performance of EM X-LAT heat pipe in simulated on-orbit thermal cases • Risks mitigated • Demonstrate adequate design margins • Assure that heat pipe performs as predicted

37. Heatpipe Thermal Source Simulation Uniform Hot Case Test Case 1, 2, 5, 6 Non-uniform Hot Case Test Case 3, 4, 7, 8 • Power shown are per LAT-TD-00225-03, dated 6/14/2002. • EM Heatpipe was tested to ½ the predicted power shown since two heatpipes run across these bays shown.

38. Heatpipe Characterization Test Setup (cont.) Note: Cold zone = ON means chiller is turned on to keep temperature at 5 deg C.

39. Heatpipe Characterization Test Setup Cold Zone 2 Chilled Water Source Heat Zone 4 Heat Zone 3 Heat Zone 2 Heat Zone 1 Cold Zone 1 Tilt Meter CCHP Heatpipe Chamber Close-up of Cold Zone

40. Typical Test Data Heater Power, Each Heater Zone Temps Cold Zone Temps Avg W/C Delta T from Cold Zone

41. Typical Test Data (cont.) Heater Zone Temps Cold Zone Temps Heater Power, Each Avg W/C Delta T from Cold Zone

42. X-LAT EM Heat Pipe Characterization Tests Results • Cases 1 and 2: • EM Heatpipe functional at 65 Watts uniform heating, with and without sun load on one side. • Cases 3 and 4: • Heatpipe functional at 70 Watts non-uniform heating, with and without sun load on one side. • Case 5: • Heatpipe functional at 65 Watts uniform heating, with cycled sun load at both ends. • Case 6: • Heatpipe functional at 65 Watts uniform heating and 1.5-inch end to end tilt. No heatpipe dryout. • Case 7: • Heatpipe functional at 70 Watts non-uniform heating and 1.5-inch end to end tilt. No heatpipe dryout. • Case 8: • Heatpipe functional at 70 Watts non-uniform heating and 2.0-inch end to end tilt. No heatpipe dryout.

43. X-LAT EM Heat Pipe Characterization Tests Results (cont.) • Per current LAT dissipated power predict, LAT-TD-00225-04-D3, CCHP heatpipe has tested margin of 37% at 70 Watts non-uniform heating over EPU and PDU box stackup • Heatpipe was not EM tested by SLAC to expected power output of GASU stackup of 82W, but prior LMMS testing has exposed CCHPs to higher power output without problems

44. 3-Way Heatpipe Thermal Joint Conductance Test 3-Way heatpipe thermal joint conductance test • Objectives • Determine thermal joint that can meet conductance and ease of assembly requirement • Develop design-specific empirical conductance values for actual bolted joint configuration • Risks mitigated • Validates thermal model based on empirical thermal conduction values

45. 3-Way Heatpipe Thermal Joint Conductance Test Setup Configuration • Three 10-inch long 6061-T6 aluminum heatpipe simulators • Bolt spacing at 1.75-inch, #6 fasteners, torqued to 10 in-lbs • Radiator heatpipe simulator kept at 5 ˚C using chilled water • X-LAT and Downspout heatpipe simulator has internal heaters independently powered • X-LAT to Radiator heatpipe interface include a 60 mils aluminum shim simulator • Total of eight thermocouples monitored surface temperature on each heatpipe simulator (3 each on Radiator and X-LAT and 2 on Downspout) • Thermal joint materials tested are: • Bare aluminum to aluminum contact • 10 mils thick SilPad 2000 sheet • 20 mils thick SilPad 2000 sheet • 5 mils bondline of CV2946 thermally conductive adhesive

46. 3-Way Heatpipe Thermal Joint ConductanceTest Setup (cont.) Aluminum Shim(.06-in thick);CV2946 on Radiator HP side only Radiator Heatpipe (keep cold at ˜ 0 ˚C) SilPad Thermal Isolator Downspout Heatpipe(heat source #2)15 W constant X-LAT Heatpipe(heat source #1)15 W, 30 W, 50 W Leave 10 mils gap b/w X-LAT and Downspout HPs

47. Test Setup in Vacuum Chamber Chilled water to Radiator HP Radiator HP simulator SilPad 2000 X-LAT HP simulator – front Downspout HP simulator – far side Thermally isolated mounting base

48. 3-Way Heatpipe Thermal Joint Conductance Test Results Note: Thermal contact area is 72.58 cm2 for each heatpipe simulator

49. 3-Way Heatpipe Thermal Joint Conductance Test Results (cont.) • SilPad and dry joint resulted in delta T larger than 2 deg C allocated per thermal analysis • Baseline 5 mils thick thermal adhesive Nusil CV2946 to meet maximum delta T across the 3-way heatpipe joint • Adhesive requires on both side of 60 mils max thickness aluminum shim to further improve conductance from test setup • Incorporate bolt size, spacing and maximum shim thickness per test setup or better into flight detail design

50. Thermal Joint Further Work • Demonstrate Radiator installation process • 6 wet RTV joints in vertical orientation • Complete within pot life limits (bonded & torqued) • Verify bond line integrity (uniform thickness & % voids) • Demonstrate Radiator removal process • Separate 6 joints with limited access • No damage or distortions of heat pipes • Verify material can be cleaned up & surface prepared again for another bond