# Presentation: 449512

## SNAPElectrical/Mechanical/Thermal

** ****Michael Sholl**
**Space Sciences Laboratory, UCB**
**11 November 2003**

## SNAP Observatory

- Front cover

- Instrument
- Radiator

- Ka Band
- Antenna

- Solar
- Array

- Payload
- Attach
- Fitting

- Optical
- Baffle

- Secondary
- Mirror Assy.

- Secondary
- Mirror Support

- Stovepipe
- Baffle

- Primary Mirror

- Focal Plane Assy.

- Thermal Straps

- Radiation
- Shield

- Spacecraft

- Fold Mirror

- Tertiary
- Mirror

- Optical Bench

- Shutter

- Thrusters

## SNAP Electrical Block Diagram

## Electrical System Features

**System has no single point failures**
**Fully redundant and cross strapped on spacecraft side**
**Redundant Observatory Control Units and Large Memories on instrument side**
**Instrument Detectors and Mechanisms modularized for failure tolerance**
**Uses Industry Standard 1553 interface between instrument and spacecraft**
**Wide band downlink controlled directly by instrument**
**Memory Capability incorporated into the Instrument (Instrument Memory Unit)**
**No need for SSR in spacecraft**
**Spacecraft consists of 3 independent modules connected by a 1553 bus**
**Power system**
**ACS System**
**C&DH System**

**Notes:**
Really need to read 1553 tutorial before Friday

## Observatory Control Unit  (OCU)

**Observatory Control Unit  (OCU)**

**Includes all interfaces to the detectors and instrument for both science data and housekeeping**
**A given unit connects to either the prime or the redundant interface on focal plane modules**
**Has redundant cross strapped interface to Spacecraft Electronics**
**Comprised of five major subassemblies**
**DPU**
**Interfaces to spacecraft C&DH via a single 1553 port on the DPU**
**Provides command and engineering data interface**
**IP design implemented in a rad hard FPGA**
**Memory Control / Formatting**
**Manages Instrument data storage memory**
**Provides 300 Mhz science data output which directly drives the Ka band transmitter**
**I/O and Control**
**Operates motors and mechanisms**
**Collects instrument housekeeping**
**Performs power distribution to focal plane electronics**
**Thermal control**
**Sets up control for 1024 heater elements **
**System uses a total of 32 wires coming**
**Instrument power services**
**Provides regulated power to telescope systems**

## Instrument Memory Unit  (IMU)

**Two 200 giga-byte units used in a hot redundant organization**
**Custom stacked chip technology to achieve high density**
**Segmented to provide graceful degradation and SEU tolerance**
**Parallel organization supports an average data rate of 400Mbs**
**Array of high speed DMA pointers supports rapid reprocessing of science data by OCU**
**Includes modulation circuits to support 300 Mbs output data rate to Ka band transmitter**

## CCD Module

**CCD Module**

**Visible detector (0.35-1.0****μm)**
**16-bit dynamic range, 12-bit conversion**
**Fully depleted, back-illuminated CCD**
**CCD clock signals are generated by a custom hybrid chip mounted on the module**
**Four video output from the CCD are conditioned and ADC’ed by a custom ASIC also mounted on the module (CRIC)**
**A  voltage regulator on the module provides power for clocks and video circuits**
**CCD data are output via a high-speed LVDS serial interface: 6.4Mbits/s (800kB/s)**

## Infrared Detector Module

**Infrared detector (0.9-1.7****μm)**
**16-bit dynamic range**
**Analog to digital conversion performed by Rockwell SIDECAR chip**
**Voltage regulation performed within module**
**LVDS interface to instrument memory controller**

## Spectrograph Detector Electronics

**Spectrograph operates over the 0.35-1.7****μ****m**** band**
**spectral resolution: ~150**
**Small FOV (3x3”) allow targeting of individual supernovae**
**The Spectrograph Detector employs two sets of CCD and IR modules, identical to those used by the telescope.**
**Electronics in the Spectrograph interface to the Prime and Redundant OCU’s**
**The Spectrograph Electronics also control the shutter and any other mechanisms in the Spectrograph as well as implementing commands and collecting engineering data**

## Guider CCD Module

**Four trackers are located in opposite corners of focal plane**
**Windowing around individual guide-stars employed to reduce data throughput requirements**
**Centroids computed on DPU**
**Centroid data sent to ACS system at 10Hz, via 1553 Bus**
**1-frame latency used in ACS computations**
**RS-422 link direct to ACS node backup possible in the event 1-frame latency proves difficult via 1553 (C&DH system is bus master)**

## Focal Plane Electronics Summary

**The complete system includes:**
**36 Visible CCD modules**
**36 IR Detector modules**
**4 Guider CCD modules**
**2 complete Spectrograph Electronics Assemblies**
**Focal Plane circuits operate at 140 K**
**Each circuit module may operate from either the Prime or Back-up OCU**
**Power is provided and monitored by the OCU, and regulated locally where necessary**
**High-volume data sent directly to IMU via LVDS**

## Spacecraft C&DH System

## Spacecraft C&DH System Notes

**The C&DH system includes the C&DH controllers,  the S-band transponders,  S-band antennas,  the Ka band transmitters,  high gain antennas,  and gimbals.**
**Either of the redundant independent controllers can control either set of telemetry components**
**Each controller has a pair of redundant 1553 interfaces to provide full cross strapped redundancy to the spacecraft 1553 bus.  **
**The C&DH controller is the bus master.**
**The C&DH controller tasks include**
**Command decoding**
**S-band antenna switching**
**S/C Engineering T/M collection and transmission**
**Control of high gain antenna gimbal**
**Modulation of the Ka band transmitter is done directly by the OCU**
**High-gain data downlink rate: 300MBytes/s**

## Spacecraft Power System

**System operates solar array,  performs battery charge control,  and supplies switched,  limited,  and monitored  +28 volt  +/-2% power to OCU’s and spacecraft systems**
**Includes a pair redundant independent controllers,  either of which can control all solar array elements and both batteries**
**Each controller has a pair of redundant 1553 interfaces to provide full cross strapped redundancy to the spacecraft 1553 bus**
**Every powered subsystem  (OCU’s and S/C components)  has a redundant set of protected power input interfaces**

**Notes:**
Look up LET, and all that

## ACS / Propulsion System

**Pointing requirements: 20mAs 1-******
**Sensors: Tracker, 3X CT-602, 2X HRG, 2X SS**
**Actuators: 4X reaction wheels, 12X thrusters**
**Reaction wheel control only (FSM not baselined)**
**Slosh mitigated via diaphragm and baffles**

## ACS / Propulsion System Notes

**The ACS / Prop system includes the ACS controllers,  Reaction Wheels,  Coarse Star Trackers,  IRU’s,  Sun Sensors,  Propellant Tanks,  Thrusters,  and propellant valves and lines**
**Each controller has a pair of redundant 1553 interfaces to provide full cross strapped redundancy to the spacecraft 1553 bus**
**Interface between the ACS  subsystem components is done by a separate redundant 1553 bus**
**All ACS components shall interface via the 1553 bus**
**Interface to the propulsion components is done by a dedicated custom board in the controller**
**The propellant system includes 6 redundant fuel tanks connected to a redundant pair of manifolds with a redundant set of thrusters**
**The ACS / Prop system controller tasks include**
**Safe-Hold mode on initial power up**
**Orbit injection and station keeping**
**ACS control algorithms including:**
**Coarse (~2 arcsec)  pointing of the spacecraft**
**Fine pointing using Fine Guider centroid data received from the OCU via 1553 bus**
**Reaction wheel momentum dump**
**Spacecraft disposal**

## System Modularity

**Command and data handling system**
**Power system**
**ACS/Propulsion**
**These systems are completely stand alone with pairs of dedicated control units which are completely independent mechanically and electrically from the spacecraft Power and C&DH systems.  The only connection between them and the other S/C systems is the 1553 interface and the power service lines.**
**There shall be no aspect or feature of the system which limits the ability of the SNAP Project to procure the ACS / Prop system from a different vendor than those supplying the other two major spacecraft subsystems.**

## Mechanical Requirements

**Launch Environments (Delta IV)**
- Stiffness: > 25 Hz (axial) > 10 Hz (lateral)
- Strength: +6.5, -2 Gs Axial; 2.0 Gs Lateral
- Minimum Required Safety Factors
- Two for ‘Ordinary’ Engineering Materials
- Four for Composite Materials
- Ten for Glass & Ceramic Materials
- Vibro-Acoustics: *mostly for Boxes *
**Mission Environments**
- Stiffness > 5 Hz to Minimize ACS Interactions
- ~ ppm Optics Path Stability, periodic Focus is acceptable
- Low Outgassing; Low Water Absorption; Low *(particulate)* Contamination
**Integration & Test Environments**
- Handling Fixture Accommodation on OTA
- Needed (Large) Inventory of Handling Fixtures

**10Gs**

**100 kg**

**MASS ACCELERATION CURVE**
- FOR SPACECRAFT STRUCTURAL DESIGN
- JPL Document: JPL D-5882
- by: Marc Trubert	dated Nov 1, 1989

**Notes:**
Go through Atlas V requirements, and add to this slide.

## Telescope Structure

**SECONDARY MIRROR SUPPORT STRUCTURE IS THE CRITICAL ITEM**- Secondary Mirror is the most Sensitive Element in TMA Optics- Strong Science Desires to Minimize Obscuration- Remote Location Presents a Unique Design Challenge- Desire: verify alignment throughout I&T phase (BITE)

**REQUIRED MATERIAL Properties**** ***(driven by ppm Optics Rqm’t)*- Cyanate Ester matrix for low CME (moisture)- M55J High Modulus & Conductivity Fibers for sub-ppm/K stability- Quasi-Isotropic Patterning highly desired, given the non-linear Geometry - Re-focus secondary via hexapod after 30-60 day dryout period- On Orbit Temperature Controlled (few K) Telescope Structure- ***This technology is mature, and has extensive flight heritage***

**Notes:**
Quantify structure shrinkage, and show how is may be mitigated through secondary motion.

## Current Telescope Incarnation

- Secondary
- Mirror
- Baffle

- Secondary
- Mirror
- Strongback
- & Motorized
- Hexapod

- Secondary Mirror
- Support Struts

- Optical Bench

- Stovepipe
- Baffle

- Primary Mirror

- Spectrograph

- Coldplate
- (Focal Plane)

- Shutter
- Mechanism

- Fold Mirror
- Hexapod
- (Fixed)

- Tertiary
- Mirror
- Hexapod
- (Fixed)

## Built In Test Equipment

- Point source illuminators on focal plane
- Collimated at pupil-plane
- Reflected back into system via pair of mirrors
- Look for relative change between returned spots

## Focal Plane

- Molybdenum coldplate (140K)
- -Low CTE
- -High thermal conductivity
- -Large diameter
- 36 HgCdTe detectors
- 36 Silicon detectors CTE mismatch
- Design in place
- Focal plane guiders
- Calibratino sources
- Radiation shield
- -Bellows connection to coldplate
- -5K cooler than coldplate (CC)
- Spectrograph mounting scheme
- -Reduce height of  spectrograph
- -Thermal isolation from coldplate
- -Increased thermal strap capacity

## Finite Element Analysis (R. Besuner)
Quantifies gravitational distortion WFE budget
Offloader actuator trades
Stress and dynamics modeling
Optimization
ASAP, BEAM 4, MATLAB TMA-63 models developed
Hexapod model
Converts actuator resolution to angular/linear resolution
Converts alignment focus needs to actuator stroke requirements
Determines strain (stress) in hexapod flexure pivot bearings
On-orbit secondary/tertiary focusing algorithm
Quantifies PSF across focal-plane, using simulated stars
Variable metric algorithm employed to adjust 5-DOF secondary and/or tertiary
Used to develop wavefront error budget
Used to convert alignment WFE budget to mechanical tolerances
Used with hexapod model to determine stroke requirements

**Quantifies gravitational distortion******** WFE budget**
**Offloader actuator trades**
**Stress and dynamics modeling**
**Optimization**
**ASAP, BEAM 4, MATLAB TMA-63 models developed**
**Hexapod model**
**Converts actuator resolution to angular/linear resolution**
**Converts alignment focus needs to actuator stroke requirements**
**Determines strain (stress) in hexapod flexure pivot bearings**
**On-orbit secondary/tertiary focusing algorithm**
**Quantifies PSF across focal-plane, using simulated stars**
**Variable metric algorithm employed to adjust 5-DOF secondary and/or tertiary**
**Used to develop wavefront error budget**
**Used to convert alignment WFE budget to mechanical tolerances**
**Used with hexapod model to determine stroke requirements**

**Telescope Optomechanical Analysis**

## Mirror Design Status

**Wavefront error budget **
**Established for all mirrors (See telescope specification for details)**
**Currently performing system trades**
**Reduced Strehl (70%) during ground test**
**Mechanical trades performed**
**Zerodur –vs- ULE**
**Open back, partial open back, closed back**
**Hexagonal core –vs- triangular**
**Edge mount –vs- 2/3 mount**
**Preliminary results:**
**ULE preferred for PM, SM and TM**
**Zerodur works, but less desirable**
**SiC possible for FM and TM**
**SM/TM not challenging or extremely long-lead: final figure adjustments based on as-built PM**

- Fold Mirror

- Tertiary Mirror

- Primary Mirror

## Secondary Mirror Hexapod

**Secondary adjustment required: 5-DOF Stewart Platform**
**Gravity sag during ground testing**
**Misalignment budgets for theodolite initial alignment of telescope**
**Secondary**
**Fold Mirror**
**Tertiary**
**Focal Plane**
**Composites water loss on-orbit**
**Bulk thermal changes**
**Thermal gradients**
**Design tolerates failure of one actuator**
**Moog Rubicon-based actuator meets requirements**
**100nm resolution (40x margin)**
******1mm (5x margin)**
**500N survival load (bearing Brinelling)**
**Ø5cm X 15cm L**
**Only needed on secondary**
**Additional trades were performed**
**Tertiary only correction: does not work!**
**Tertiary and secondary hexapods**
**1********m resolution!**
******1mm**
**More mechanisms, but more reliable?**

- LBT, MMT, etc., heritage

## Shutter Mechanism

**Accuracy requirements:**
**1% photometric uncertainty ******** 0.1% to shutter?**
**Driven by short exposures (~1s)**
**≤****1s shutter open/close**
**Actuator baseline:**
**Redundant, toroidally wound Limited angle torque motor (Aeroflex, etc.)**
**Sensor baseline:**
**14-bit resolver (Honeywell, etc.)**
**R2D (Analog, etc.)**
**Control options:**
**Voltage lookup table (counter, PROM and DAC)**
**Type II integrator**

**Shutter Mechanism**

**Location: Behind Folding Flat**
**Back surface Spectralon ****ε****=0.02**
**290K operational temperature**
**Reduandant motor windings**

## Stray Light Baffle Cover

**“****ONE SHOT” BARN DOORS****- Ribbed Split & Edge Attachments****- Domed Center & Stiffening Ribs****- COMPOSITE shell construction****- Spring Preload Against Baffle ****  When Open for >10 Hz****- Disposable option at L2**

**Stray Light Baffle Cover**

**NOTE:** *the Baffle Cover is not a Science Requirement, but an Anticipated Need for the Integration and Contamination Control*

**FRANGI-BOLT Release Mechanisms****-  Shape Memory Alloy actuator fractures notched titanium bolt****-  Activated by (Redundant) Electric Heaters****-  Reusable by Cooling & Resetting with a Hydraulic Press ****-  Up to 5000 lb Tensile Rating****-  Very Minimal Debris Generated, No Gas Release****-  Extensively Flight Qualified**

## Telescope Thermal

**Telescope Thermal**

**OPTICS: Build,Test, & Fly Warm**

**KEY DESIGN FEATURES**
** ****High Earth orbit (HEO) helps to minimize IR Earth-glow loading**
** ****GaAs cell - OSR striping of the (hot) solar array panels**
** ****Front surface heat rejection OK**
** ****Optical Solar Reflectors are back silvered Quartz tiles (*****a****** *****~ 8%, *****e****** *****~ 80%)**
** ****Thermal Isolation mounting**
** ****Emissivity control: Where possible, exposed surfaces are insulated (MLI), or radiators**

**Notes:**
Add temperatures to this autocad drawings

## Thermal Modeling

**Lumped parameter and analytical models in place**
**Radiation/conduction balance**
**Heater sizing**
**Coldplate/shield thermal strap sizing**
**Emissivity definition********thermal tapes and MLI**
**Detailed SINDA modeling will refine predictions**

## Conclusions

**Straightforward electronics system**
**High data-rate science data transferred directly from IMU to HGAs**
**Modular S/C systems on 1553 bus: C&DH, ACS and Power systems are not vendor-specific**
**Required telescope structure not state-of-art**
**Secondary hexapod and shutter mechanisms requirements developed, solutions exists**
**PM/SM/FM/TM: trades underway (ULE/Zerodur/SiC)**
**Thermal system: 7-node model in use for system trades**
**Temperature profiles are manageable**
**R&D phase proving valuable for identifying system issues prior to system design**