1. GLAST Large Area Telescope:
Gamma-ray Large Area Space Telescope
GLAST Large Area Telescope:
Mechanical Systems Peer Review
March 27, 2003
Section 3.2- Changes since
Delta PDR
Marc Campell
SLAC
Mechanical Systems Mgr.

2. Topics Agenda Mechanical design changes since ∆PDR
Impacts on Subsystem design
Studies since Delta PDR

3. Design Changes Since Delta PDR
X-LAT to E-Box thermal joint
WAS: Rigid bonded joint with flexures at CAL-TEM interface
IS: Rigid interface at CAL-TEM with compliant joint at X-LAT plate and a thermal velvet material (Vel-therm) used to fill gap between bottom box & X-LAT
X-LAT plate
WAS: honeycomb panel; IS: .125” plate
CAL-Grid bolts:
Use #8’s everywhere except at TRK cables pass-thru’s to increase clamping force on CAL plate

4. Design Changes Since Delta PDR
Refined S/C interface stiffener (wing) to reduce grid distortions
S/C attach method
WAS: onto –Z surface of Grid
IS: tang that protrudes down from wing
S/C stayclear & center EMI shield design modified to accommodate above

5. Design Changes Since Delta PDR
E-box harnesses
WAS: bulkhead connectors in EMI skirt
IS: 8 Connector patch panels into EMI skirts that E-box harness mount directly to
Added 2 TCS external box & 2 brackets – mounting accommodated on Radiator Mount Brackets
Added vents to EMI Electronics enclosure

6. Design Changes Since Delta PDR
Radiator Heat Pipes
WAS: U shaped bend to Grid
IS: S shaped bend to Grid
Radiator panel has top stepped for integration access
Radiator – SC interfaces have been finalized
SA Boom cutout size & location, strut locations
Radiator reservoir size
WAS 300 cc; IS: 75 cc
Radiator VCHP extrusion
WAS 1.75” wide; IS: 2.0” wide
LM will design, fabricate and test X-LAT plates

7. Impact of Changes on Subsystem Designs
LAT
No impact
TKR
CAL
Baseplate design has been modified for #8 fasteners
ACD
Electronics
Improved reliability by reducing bulkhead connection
External TCS boxes have been accommodated
Conclusion: Changes consistent with PDR to CDR design maturity. Design changes have been incorporated within the Mechanical Subsystem

8. Studies since Delta PDR
X-LAT to Electronics Thermal Joint Design
Optimize joint design to accommodate
Maximum thermal conductance
Repeatable and verifiable thermal joint
Ease of integration & removal to service E-boxes
Compliance in X-Y plane required either at E-box to CAL or E-box to X-LAT
Tolerance stack up of E-boxes, EMI skirts & CAL plates
RTV, thermal gaskets, spring fingers, thermal straps and Vel-Therm traded
Compliant joint at E-box to X-LAT interface with Vel-Therm material in between selected.

9. Studies since Delta PDR
CAL-Grid interface shear load capability
No bulk movement of CAL with 70 Fasteners & μ=.01
Isolated concern as small local motions of Grid wall underneath grid tabs
Optimize wing design to reduce loads (FEM)
Examined friction of various surface treatments
Examined Tungsten Carbide flame spray as a way to guarantee high coefficient of friction at interface
Maximized clamping force available from fasteners
Examined backing bars on top of CAL tabs to create “double shear” joint
Examined perimeter clamping bars that increase preload on all CAL tabs around the perimeter of the Grid
Developed maps from FEM showing required coefficient of friction at each bolt location for a given clamping force

10. Studies since Delta PDR
CAL-Grid interface shear load capability (cont)
Summary:
A friction joint will prevent bulk slippage of the CAL and maintain the LAT natural frequency.
89% of fasteners require μ < 0.2. Plan to run an analysis to show that load redistribution for the remaining 11% does not adversely affect the LAT natural frequency.
GSFC has recommended that a bolted-pinned joint be adopted.
Design implementation study and impact study would be the next step.

11. End of
Section 3.2