1. Telescope - Mechanical
Albert Lin
The Aerospace Corporation
Mechanical Engineer
(310)
6/27/05

2. Problem Statement The CRATER Telescope must:
Hold three pairs of thin-thick detectors
Hold two samples of TEP
Be configured to meet geometry for science mission
Hold interface circuit boards
Subject to:
Positive stress margin for all environments
Minimum first fundamental frequency
Weight constraints of overall instrument

3. Telescope Requirements Mechanical Requirements Design Details
Overview
Telescope Requirements
Mechanical Requirements
Design Details
Trade Studies

4. Telescope Requirements
From Level 2 Mission Requirements Document
Section
Requirement
3.1.1
Pairs of thin (approximately 150 micron) and thick (1000 micron) Si detectors
3.2.1
0.030” thick aluminum wall on both ends of the telescope
3.3.1
A-150 TEP of 27 mm and 54 mm in length
3.5.1
30 degree FOV zenith, 80 degree nadir
All requirements incorporated into model

5. Telescope Geometry All Requirements Met
Pairs of thin (~150 micron) and thick (~1000 micron) Si detectors used
0.030” thick Aluminum on top and bottom apertures
A-150 TEP of 27 mm and 54 mm in length
30 degree FOV Zenith
80 degree FOV Nadir

6. Telescope Requirements Mechanical Requirements Design Details
Overview
Telescope Requirements
Mechanical Requirements
Design Details
Trade Studies

7. Mechanical Requirements
From Environments, 431-RQMT
All components must have positive stress margin with an appropriate factor of safety used for the material analyzed
Requirement
Description
Levels
3.1.2
Net cg limit loads
Superceded by Random Vibration
12 g
3.4.2
Sinusoidal Vibration Loads
Frequency: Hz
Protoflight/Qual: g
Acceptance: g
3.5
Acoustics
Enclosed box without exposed thin surfaces
OASPL Protoflight/Qual: dB
OASPL Acceptance: dB
3.6.1
Random Vibration
See next slide
4.2.1
6.0
Minimum Fundamental Frequency
Minimum > 35 Hz
Recommended > 50 Hz
Will not provide FEM model > 75 Hz

8. Random Vibration Random Vibration will drive most of the analysis
For resonances in the Random Vibration Spec, Miles’ Equation shows 3 sigma loading on the order of g

9. Stress Margins Load levels are superceded by random vibration spec
Factors of Safety used for corresponding material (MEV 5.1)
Metals: Yield, 1.4 Ultimate
Composite: 1.5 Ultimate
Margin of Safety = (Allowable Stress or Load)/(Applied Stress or Load x FS) – 1
Description
MS yield
MS ultimate
Bolt Interface Loading
+2,662
+5,615
Detector Boards
brittle
+18.6
Silicon Detector*
+31.5
TEP Clamp
+0.91
+1.21
All components have positive Margin of Safety
*Assumes an ideal 3-point mount, to be discussed later

10. First Fundamental Frequency
First Fundamental Frequency at 1,410 Hz
Much greater than 75 Hz frequency where the FEM model will not need to be supplied

11. Telescope Requirements Mechanical Requirements Design Details
Overview
Telescope Requirements
Mechanical Requirements
Design Details
Trade Studies

12. Design Overview
Telescope is assembled using card guides for the circuit boards and screws for the TEP holders

13. Overall Dimensions
Weight = 2.7 lbs

14. How to Mount TEP Limited Material Properties information on A-150 TEP
Need to mount TEP to account for
Minimal deformation of material during assembly
Allowance for thermal contraction
20 lbs preload to withstand random vibration
Springy Clamp
Cross Section
TEP
Solution: Oversized mounting hole to allow for radial thermal expansion with a thin, springy clamp to hold in TEP. With differential thermal contraction at -40°C, spring still pushes with 7.4 lbs force

15. Mounting Detectors
Detectors mounted using three point mounts on circuit boards to minimize stress caused by circuit board vibration
Further investigation needed for the effectiveness of the three point mount interface
Thin Detector
Wires strain relieved away from mount to minimize stress from vibration
Thick Detector on Underside

16. Purging and Venting
Purge Inlet
Purge and Vent Outlet
Detector Mounts suspended above circuit board allows for gaps that equalize pressure

17. Telescope Requirements Mechanical Requirements Design Details
Overview
Telescope Requirements
Mechanical Requirements
Design Details
Trade Study

18. Trade Study
A limitation to the current design is the uncertainty of the detector mounting scheme in minimizing the effects of circuit board vibration
An alternative design is to clamp the detectors in a stiff structure and decouple it from the circuit board using cables
The detectors are tested at the manufacturer in a similar configuration but there are issues with using Rigiflex cables ?

19. Telescope – Mechanical
Albert Lin

20. Backup Slides

21. Bolt Interface Loading
Assuming worst-case loading at 1410 Hz fundamental frequency
3 sigma load = 125 g
A286 CRES Bolts at Interface
Worst Case Bolt
Mechanical Engineering Design, by Shigley
RP-1228 NASA Fastener Design

22. Detector Board Resonance
First Mode: Hz
Total nodes: 60546
Total elements: 33546
COSMOSWorks 2005

23. Detector Board Stress Using Miles Equation, assume Q = 15, FS = 1.5
3σ g loading = 133g
Max Stress = 1,527 psi
MS ultimate = 45,000 psi / (1.5 * 1,527 psi) - 1 = 18.6

24. Thin Detector Analysis
Assuming Detector is mounted on an ideal 3 point mount, the silicon behaves linearly, and Q = 15
Fundamental Frequency = 1795 Hz, which yields 3 sigma load of 111g
Margin of Safety = (17,400 psi / (1.4 * 382 psi) – 1 = 31.5

25. TEP Thermal Contraction
Analysis of Beryllium Copper clamp
TEP CTE assumed to be same as polyethylene, 77.8 μin / in-°F
During launch, temperature is ~70°; TEP clamp exerts 20 lbs to resist vibration
During cold survival mode at ~-40°, TEP clamp still exerts a preload of 7.4 lbs. The preload is lower due to the relative thermal contraction
All stress margins are positive

26. Sensitivity Analysis Preceding calculations used a nominal Q of 15
This table shows how the 3 sigma g-loads vary with Fundamental Frequency and Q
Most structures have Q between 10 and 20

27. Factors of Safety Used

28. Material Properties MIL-HDBK-5J1 1 1 2 3
MIL-HDBK-5J
Silicon as a Mechanical Material, Proceedings of the IEEE, Vol 70, No. 5, May 1982, pp
Boedeker Plastics via