# Presentation: 498310

## Structural Engineering for the SNAP Telescope: Status ReportEric Ponslet and Roger SmithHYTEC Inc.Lawrence Berkeley National LaboratoryNovember 3, 2000

**Structural Engineering for the SNAP Telescope: ****Status Report****Eric Ponslet and Roger Smith****HYTEC Inc.*****Lawrence Berkeley National Laboratory******November 3, 2000***

## Outline

- Who is HYTEC?
- Experience with composites, space applications, and stable structures
- The SNAP Baseline Structural Concept
- HYTEC’s Role in SNAP
- Contractual commitments
- Outline of progress
- Assumptions and Requirements for Initial Studies
- Summary of Conceptual Design and Analysis Work
- Stable structure and material design
- Secondary Metering Structure
- Tertiary Metering Structure
- Spacecraft-Instrument Interface Structure
- Primary Mirror Substrate
- Primary Optics Bench
- Primary Baffle
- Conclusions and Special Issues Identified
- Future Work

## Who is HYTEC Inc.?

- Full service engineering company located in Los Alamos, NM
- End-to-end design, analysis, integration, testing, and procurement of physics instruments
- Thermal/Mechanical design of complex systems
- Stable structure design
- Advanced composite materials (GFRP, C-C)
- Vibration isolation
- Space-based applications
- Passive and active thermal design
- Prototype fabrication & testing
- Facilities for general mechanical, thermal, and vibrations testing
- Unique testing capabilities: TV Holography and X-Ray Computed Tomography

## Customers

**Customers**			**Projects**
- NASA			SBIR Phase 1: Space Qualified C-C Structures
- NRL			STEREO: Composite Optical Bench
- SLAC / NRL / UCSC		GLAST Gamma-ray detector: design and procurement
- CERN			Composite Support Structures for ATLAS/CMS
- DOE			SBIR Phase 2: HEP Pixel Detector Thermal Structures
- LANL			ATLAS (Pulsed Power Machine)
- LANL / UNM		PHENIX (High Energy Physics)
- LANL			SNS (Spallation Neutron Source)
- Caltech / MIT		LIGO Detector Structures: design and procurement
- LBNL			ATLAS Pixel Detector: Cooling Structures
- LANL CRADA		TV Holography
- LANL CRADA		Computed Tomography
- NIST			Ultra Stable Isolation Platform
- DOE			SBIR Phase 2: Low Cost Carbon Composites

## Advanced Composites Experience

- facing overhang

- tube extensions

- mounting
- points

- GFRP optical bench for the Sun Centered Imaging Package (SCIP) to be flown on STEREO (NRL)

- Carbon-Carbon composite support structure for ATLAS

- ATLAS detector for CMS: all composite pixel detector support and cooling structures

- Carbon-Carbon cooling tube for ATLAS
- (3.2 mm ID, 0.25 mm wall thickness)

- all GFRP and carbon-carbon sandwich Gamma-Ray tracker tray for GLAST

## Space-Based Instrument Design Experience

- SCIP Instrument Package for the STEREO Mission

- Shake test of tracker structure for GLAST

- Structural and system engineering for the GLAST Instrument

- Tracker and Calorimeter prototypes for GLAST

## Ultra-Stable Structures Experience 

## SNAP Telescope - Current Structural Concept

## SNAP Telescope - Current Structural Concept

## SNAP Structural Design Work at HYTEC

- HYTEC is under contract with LBNL to:
- Establish baseline structural design requirements
- Explore conceptual design options for all telescope structures
- Perform preliminary analysis as required for initial sizing
- Establish structural design baseline by February 2000
- May 2000 - present: 1 engineer + 1 part time CAD designer
- Adding second engineer on November 7
- Progress to date:
- Design requirement document was produced (1 document)
- Identifies design issues
- Lots of TBD’s, documents *assumptions* made in design studies
- Overall telescope layout in 3-D CAD models
- Background research on stable optical telescopes
- Literature about other projects and R&D efforts: telescope design, stable composites, stable structural design, etc...
- Secondary Mirror Metering Structure (3 reports)
- Conceptual design: direct/indirect support concepts, materials,...
- Direct support frames and trusses
- Sizing for dynamics, buckling, stresses
- Obscuration and diffraction issues
- Design trends

- Conceptual design and preliminary sizing:
- 	— Completed
- 	— In progress
- 	— Not started

## SNAP Structural Design Work at HYTEC

- Progress to date (continued):
- Tertiary Mirror Metering Structure (1 report)
- Easier design problem: no obscuration issues, smaller dimensions
- Maximize use of designs/components common to Secondary support structure
- Spacecraft – Instrument Interface Structure (1 report)
- Kinematic mount for strain decoupling
- Extreme stability *not* required
- Sizing for dynamics, buckling, and stresses
- Primary Mirror Substrate (report to be issued)
- Definition of a conservative baseline design
- Analysis of baseline design for dynamics, deflections, and stresses
- Support conditions for 1g figuring and launch
- Primary Optical Baffle (started)
- Dynamic modes of stiffened shell
- Primary Optics Bench (started)
- Examine design options: single piece/multiple pieces, bench/ring approach, construction concept
- Minimize axial space required (primary-folding distance)
- Analysis for dynamics, deflections, and stresses

## Assumptions and Key Requirements

- Dimensional Stability
- Metering structures
- Mirror assemblies
- Primary optics bench
- Instrument package(s)
- Launch Related
- Baseline Vehicle: Delta IV-M
- Payload Stiffness Requirements:
- Fixed-base *spacecraft*:
- f transverse > 10 Hz
- f longitudinal > 27 Hz
- Telescope sub-structures:
- f > 35Hz
- Pseudo-static design cases
- 0.5g transv. + 6.5g long.
- 2.0g transv. + 2.5g long.
- Sine Vibrations
- Acoustic loads
- Obscuration and diffraction
- Goal: < 3.5% obscuration

## Designing for Dimensional Stability

- A few “rules” of stable structure design
- Maintain as much symmetry as possible (including composite layups)
- Keep load paths short, simple, and well defined
- Use stable materials:
- low CTE, low CME, coatings, low outgassing, no aging issues, etc...
- In GFRP structures, design all critical joints to avoid relying on through-the thickness properties
- Strive for strain-free structures and assemblies:
- Kinematic interfaces
- Favor trusses over frames
- Minimize dissimilar materials
- Room temperature cured adhesives
- Also, if required
- Minimize temperature gradients and fluctuations:
- High thermal conductivity
- Shielding
- Active control (heaters,...)
- Orbit design
- Minimize sources of dynamic disturbances
- Vibration isolation of noise sources

## Dimensionally Stable GFRP Composite Materials

- Graphite Fiber Reinforced Plastics
- High modulus graphite fibers (negative CTE)
- Thermoset resin matrix (positive CTE)
- Select low moisture expansion (CME and saturation) resins (cyanate esters)
- Metal coatings slow absorption/desorption rates
- Low stress allowable to avoid permanent deformations from launch (microyield)
- Tailorable properties
- CTE “tuned” through fiber/matrix selection, fiber volume content, and layup design
- Variability in processing leads to somewhat variable properties
- Development must include thorough testing program

**Graphite Fiber Properties**

## Metering Trusses - Design Aspects

- Strut design
- Stiffness driven designs
- Buckling and stresses never found critical
- Minimize cross sectional dimensions (obscuration)
- Violin modes dominate dynamic response
- Tubular composite struts have highest violin frequencies
- Violin frequency function of material, length, and *diameter* only (*not* wall thickness)
- Pinned ends violin mode frequencies are 2.25 times lower than built in ends...
- End fittings
- Bonded to tube ends
- Low CTE materials
- Titanium, Invar, composite
- Provide pinned end conditions (flexures) to avoid initial strains
- Lock flexures after alignment to increase violin frequencies?
- Potting, bonded sleeve

- Typical violin mode of a support truss

## Metering Trusses - Materials

- Material design compromise:
- High longitudinal modulus
- Near zero longitudinal CTE
- Typical “zero-CTE” GFRP:
- 75 Msi fiber (P75, XN50, M55J), about 60% fiber volume content
- Quasi Isotropic (0/-45/45/90|ns)
- CTE~0,  E~100 GPa

- Assumed material:
- 250 GPa, 2220 kg/m3
- Easily achieved with high modulus fibers and custom layup
- CTE ~ -1×10-6/oC
- Density ~ 1800kg/m3
- Material development?
- Hybrid layup of high modulus GFRP layers and high CTE metal layers
- 250 GPa, 0 CTE, 2200 kg/m3 feasible on paper
- Manufacturing and thermal cycling issues

## Secondary Metering Structure

- Key requirements:
- Minimize obscuration (<3.5%) & interference spikes
- Dimensional stability
- 35 Hz minimum fundamental frequency
- Conceptual design options:
- Indirect support:
- Large structure outside of aperture + short radial spider
- Higher mass and complexity, lower modularity
- Higher stiffness, lower obscuration
- Direct support:
- Entire support structure sits in aperture
- Lower mass, simplicity, modularity
- Difficult obscuration / stiffness compromises
- Cross-braced direct support:
- Attempts to increase stiffness with cross-bracing
- Cross bracing in “shadow” of primary struts
- Trusses VS Frames
- Frames rely on bending stiffness of beams
- Require larger cross sections
- sensitive to bending deformations in struts (thermal)
- usually not kinematic

## Secondary Metering Structure

## Secondary Metering Structure

- Baseline design: hexapod truss with fixed end
- Simple design with low obscuration (3.5%)
- 6-spiked diffraction pattern
- Ø 23 mm by 1 mm wall tubular composite (250 GPa material) struts with invar end-fittings.
- “Optimal”: both violin and truss modes are critical No stress or buckling concerns
- Requires locking of end flexure after initial alignment (violin modes)
- Total mass ~ 4 kg

## Tertiary Metering Structure

- Key requirements:
- Dimensional stability
- 35 Hz minimum fundamental frequency
- Easier design problem than secondary metering structure
- Overall dimensions much smaller than secondary metering truss
- No obscuration concerns
- Use strut design from secondary metering structure (cost effective)
- Baseline design
- Ø 23 mm by 1 mm wall composite tubes
- Invar end fittings
- Total mass ~ 3 kg
- Fundamental mode = violin mode @ 58 Hz
- No stress or buckling concerns

## Spacecraft - Instrument Interface

- Key requirements and issues:
- Kinematic interface (spacecraft bus not dimensionally stable)
- High stiffness
- Require frequencies >14 Hz transverse and >38 Hz axial for fixed base interface (2x requirement for fixed base spacecraft)
- Strength and buckling safety
- Stability not a real issue  conventional materials (Ti or Al alloys)
- Baseline design
- Kinematic truss (6 struts) with pinned ends
- Both violin (38 Hz) and truss (17 Hz) modes near critical
- Overall stresses are low (SF from 13 to 75)
- Buckling near critical (SF from 2 to 7)
- Total mass ~ 37 kg

## Primary Mirror Substrate

- Key requirements and issues
- Dimensional stability
- High specific stiffness (1g sag, acoustic response)
- Stresses during launch
- Design of supports
- Baseline technology
- Multi-piece, fusion bonded, with egg-crate core
- Meniscus shaped
- Triangular core cells
- Maintains 0/120/240 symmetry of the entire telescope
- Little performance difference with other common cell geometries (hexagonal, square)
- Material
- Baseline = ULE Glass (Corning)
- Worst case: other materials such as Zerodur (Schott) have higher *E/***
- Allowable stress (macroscopic) must be conservatively low
- Brittle material, lots of pre-existing defects (cracks) at the bonds
- Design allowable = 600 psi / 4.1 MPa

## Primary Mirror Substrate

- Initial design for primary mirror substrate: 334 kg

## Primary Mirror Substrate

- Free-free modes
- Sag during 1g figuring
- Sag is too large (>0.1m) on simple supports (3 pt vertical, strap horizontal)
- Will likely require vertical axis figuring on airbag supports

- 1g sag on 3pt support
- vertical axis
- P-P Z deflection = 2.3 m

- 1g sag in 180o strap support
- horizontal axis
- P-P Z deflection = 0.5 m

- 1g front face ripple on perfect back-side support
- P-P Z deflection = 0.018 m

## Primary Mirror Substrate

- Stresses from pseudo-static launch load factors
- 6.5g axial, 0.5g transverse
- 3-point supports with Ø 6cm ×2 cm thick Invar pads
- Stresses are locally high: 5.5 MPa peaks (35% above allowable)
- Lessons learned
- Initial mirror design nearly acceptable
- Advanced mirror support required for figuring
- Conceivable that mirror could survive launch on simple 3 point kinematic support

## Primary Mirror Substrate

- “Improved” baseline
- Thinner face sheets (12 mm)
- Locally thickened web walls (10 mm)
- Thicker outer ring (8 mm)
- Mass is unchanged (320 kg)
- Fundamental mode drops to 333 Hz
- Pseudo-static launch case
- Stresses down to < 3.1 MPa
- Deflections < 20 m
- Conclusions
- 80% lightweighted design is workable
- 3 pt support *may* be usable for launch
- Vertical axis airbag support required for figuring
- More aggressive lightweighting would have little positive effect:
- Stresses and deflections unchanged (1st order)
- Increases technological risk?
- Reduces instrument mass...

- Deformations of mirror top face under pseudo-static launch loads: peak deflection = 20 m

- Modified design with locally thicker web plates
- Standard web thickness = 5 mm (orange)
- Thickened plates = 10 mm (red)

## Primary Optics Bench

- Key requirements and issues
- Dimensional stability
- Stresses (supports 600 to 900 kg of instruments and mirror)
- High stiffness
- Baseline technology
- Bonded eggcrate construction from flat laminates (cost effective)
- Invar fittings bonded to web plates for all interfaces
- Attachment points for secondary and tertiary metering trusses, spacecraft interface, and primary mirror close to one another (short and direct load paths)
- 0/120/240 symmetry

- Structural concept for Primary Optics Bench (top face removed)

## Primary Optics Bench

- Baseline material
- 75 Msi fiber (P75, XN50, M55J), about 60% fiber volume content
- Quasi-isotropic, symmetric, balanced layups, 4 mm thick
- E ~ 100 GPa typical, near-zero CTE, ultimate strength ~ 275 MPa
- yield stress ~ 1/3 strength?  Allowable for initial design ~ 75 MPa
- Just started simulations of initial design
- 20 cm total thickness, 90 kg total structural mass
- Assuming 540 kg (too much?..) of instruments distributed on bottom face, in addition to 320 kg mirror attached to top
- Vibration modes rigid enough but show significant deformations of the POB itself

## Primary Optics Bench

- Stresses under pseudo-static launch case
- Deflections ~ 1mm
- Stresses are acceptable: peaks near 30 MPa
- Lessons learned and issues
- Initial design is acceptable
- Stresses at least a factor of 3 below conservative allowable
- Stiffness is sufficient but marginal (longitudinal mode)
- 20 cm total thickness is about right
- Many other concepts to examine (ring shaped, honeycomb,...)
- Large unsupported spans of face plate, attachment points for instruments,...

- Average stresses from pseudo-static launch case 1: maximum stress ~ 30 MPa

- Deflections under pseudo-static launch case 1: peak Z deflection = 1.14 mm

## Primary Baffle

- Key requirement and issues
- Stiffness
- Optical properties of inner surface
- Dimensional stability requirement?
- Thermal conductivity
- Baseline design
- Based on early layout, smaller than current design
- Stiffened aluminum shell (HST)
- Use baffle rings as stiffeners
- 1 to 1.5 mm wall, about 125 kg (smaller design)

## Conclusions and Critical Issues

- Summary of baseline designs examined so far:

## Future Work

- Finish first round of analytical design studies: define complete baseline
- Primary optics bench
- Several other concepts to examine
- Primary baffle & support scheme
- Instrument package(s)
- Central baffle
- Secondary Mirror assembly and baffle
- Tertiary Mirror assembly
- Perform simulations of entire instrument
- Modal analysis
- Jitter from reaction wheels / damping requirements for metering trusses
- Pseudo-static loading
- First cut of dimensional stability simulations
- Establish work plan for development work
- Include *early* efforts in material development, prototyping, and testing
- Flat GFRP laminates
- Near-zero CTE, ultra-high modulus GFRP tubes
- End fittings
- Thermal design aspects
- Coordinate with mirror manufacturer (support conditions)