Mechanical Systems

1
GLAST Large Area Telescope Mechanical
Systems Martin Nordby Stanford Linear
Accelerator Center Mechanical Systems
Engineer nordby_at_slac.stanford.edu
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Outline

• Issues and Recommendations from previous reviews
• Technical status
• LAT analysis
• Structural analysis
• Thermal Systems Analysis
• Interface design
• Technical issues
• Schedule
• Cost issues

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LAT Mechanical Layout
Thermal/micro-meteorite shield (MLI and ceramic
blankets)
Anticoincidence Detector (ACD) surrounds TKR
Tracker (TKR) mounts to top flange of Grid
Grid support structure (aluminum)
Radiators mounts to Grid and Spacecraft (SC)
Calorimeter (CAL) inside Grid
Electronics (Elec) mounts to CAL bottom plates
Heat pipes on top of Grid (aluminum)
Spacecraft mount points (4x)
Radiator (aluminum)

• Radiator mounts to SC

LAT Model with Radiators and Subsystem Stay-Clears
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Issues from May 2001 Quarterly Review

• Transient thermal behavior of the LAT
• Issue 1 transient behavior is driven by LAT,
  Radiator, and SC operating scenarios, none of
  which are well defined
• Action GLAST operating scenarios being set and
  reflected in updated IRD Radiator and LAT
  operating scenarios becoming understood
• Issue 2 transient response of LAT has never
  been modeled
• Action 2 transient response of LAT and
  Radiators fully modeled, and design of thermal
  control system updated as a result More
  analysis in progress
• LATSpacecraft interfaces
• Issue design, structural and thermal interfaces
  are still immature
• Action some progress during SC study still an
  issue
• Grid prototyping
• Issue prototype testing must be done to
  validate FEA model details
• Action prototyping thus far shows design
  carries gt75 margin in CAL joint. Further CAL
  and Grid heat pipe prototyping underway

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Recommendations from the Feb 2001 Review

• Finish performance specifications by PDR
• LAT-SP-00010 LAT Performance Spec revision
  in-progress to reflect better understanding of
  operations and thermal requirements
• LAT-SS-00115-1-D4  Mechanical Systems Spec
  finished draft ready for approval by LAT project
• Missing Mechanical Systems Level IV specs for
  Grid, Radiators, Thermal Control System being
  drafted now (drafts complete by PDR)
• Finish interface control documents by PDR
• LAT-DS-00038-2-D3  LAT Instrument Layout
• LAT-SS-00273-1-D3  LAT-Cal Mech Interface
  Control Document
• LAT-DS-00233-1  CAL-LAT Interface Definition
  Drawing
• LAT-SS-00241-1-D2  LAT-ACD Mech Interface
  Control Document
• LAT-TD-00224-1-D1  LAT Thermal Design Parameters
  Summary 
• LAT-TD-00225-1-D1  LAT Dissipated Power for
  Thermal Design
• LAT-TD-00227-1-D0  LAT Thermal Operations
  Assumptions
• Missing TKR and Electronics Mechanical ICDs
  (drafts complete soon after PDR)

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Structural Analysis

• LAT2 finite element model delivered
• Modal analysis results
• Drum-head mode, F4 64.8 Hz
• IRD Requirement gt 50 Hz
• Structural analysis
• Bowing of Grid closes 2.5 mm gap between TKR
  modules by 0.39 mm
• LAT stiffness dependent on CAL bottom plate

Drumhead modeF4 64.8 Hz
-Torque test-Tensile test-Validate FEA model
with shear pull test
-Back out bolt forces-Design joint, pick bolt
Tensile Test ofCAL Joint
LAT Distorted-Shape Axial Deflection Contour Plot
Detail of FEA Model at CAL Mount
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Thermal Systems Development

• TSS reduced thermal radiation model delivered
• LAT on-orbit transient thermal model complete
• Used to characterize LAT performance due to
  varying environmental heat loads
• Radiator development
• Detailed Radiator thermal model complete
• Design decisions
• Two thermally coupled 2.7 m2 Radiators on
  opposing Y-sides
• Variable-Conductance Heat Pipes to regulate
  Radiator active area, compensating for variations
  in heat loading
• Active control system to operate VCHPs and keep
  LAT temperature stable to /- 3 oC
• Detailed mechanical design and structural
  analysis just beginning
• This is starting late, the result of delays in
  Radiator work start-up, and needed changes to
  their thermal design
• Planning to catch up to schedule this fall

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Thermal Systems Development (2)

• LAT steady-state thermal behavior
• 650 W of dissipated power
• Grid bows 173 mm due to temperature gradients
• TKR tips 22 arc-sec
• Requirement 30 arc-min
• LAT performance due to /- 3 oC swings in
  Radiator temperature
• Transient temp gradients tip TKRs
• Tipping 3.7 arc-sec
• Req max misalignment 7 arc-sec
• Response to time-varying process power draw
• TKR tip 3.0 arc-sec for DP 68 W
• Req tip lt 7 arc-sec for DP 350 W
• Design mod shunt electronics heat directly to
  Radiators

Tmin -0.3 oC
Temp Variation for 5.25 Orbits(red Rad
interface, blue CAL plate)
Tip 3.7 arc-sec
Distorted-Shape Temperature Contour Plot(red
hotter)
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Interface Design

• CAL joint
• Prototype testing underway Grid, bolt, joint
  design strength OK
• Interface looks solid, with no surprises in view
• TKR interface
• Flexure design in place and TKR is prototyping
  joint for EM tests
• Design looks good, but testing needs to resolve
  thermal, structural questions (how to align
  flexures affect of thermal gasket on structure)
• ACD
• Conceptual design in place, thermal interface
  design in-progress
• Interface may need modification if ACD tile
  coverage is modified
• Spacecraft
• Interface development held up due to
  export-control issues
• Legal hurdles recently cleared, and progress is
  now re-starting
• Will not be able to baseline an interface design
  with SC before PDR

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Technical Issues List

• Specs and ICDs
• Issue level 4 specs and ICDs are not yet
  complete
• Response expect completion of 90 of drafts by
  PDR
• Thermal systems development
• Issue 1 Radiator design is immature for PDR
• Response 1 Contractor is ramping up on
  personnel to develop conceptual design
• Issue 2 Electronics thermal design is very
  immature
• Response 2 Handling thermal transients by
  shunting power to Radiator is being baselined.
  Planning EM test and analysis of this
• Spacecraft interface design
• Issue Design has not matured adequately for PDR
• Response Export-control issues have been
  resolved, and progress is re-starting. We will
  miss PDR, but expect conceptual baseline by early
  CY2002

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Mechanical Systems Schedule

• Scheduling
• Mechanical Systems schedule is being
  final-checked for baseline
• Check links to IT
• Find and resolve budget phasing discrepancies
  between budget and cost-loaded PMCS schedule
• Status
• In general, we have slipped with respect to the
  schedule
• Short staffing of budgeted positions
• Late start of thermal contract
• Expect to make up schedule variances during EM
  development, since this program is being scaled
  back
• New IT Manager and Engineer will unload Mech
  Systems personnel from double-duty work
• Thermal contract in-place (possibly approved
  today)

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Cost Issues

• Budget vs proposal estimate
• Current budget of RYM8.5 is high by RYM3.5
• Areas where cost grew
• Corrections for lost system-level analysis,
  reserve, and overhead
• Addition of EM test program, Qual. Grid test
  program
• Issues in investigating de-scoping options
• Engineering model
• Current budget includes 700k for EM
  thermal-vacuum testing
• We are planning to scale this back to a minimal
  EM program with moderate additional risk
• Qual Grid
• Qualification test of Grid and Radiators
  (thermal, mass models)
• Budget includes 800k for Qual Grid fabrication
• This reduces technical and schedule risk of qual
  testing flight hardware at the end of the program