SRB Template

1
Performance Analysis of the NGST Yardstick
Concept via Integrated Modeling Gary Mosier,
Keith Parrish, Michael FemianoNASA Goddard Space
Flight Center David Redding, Andrew Kissil,
Miltiadis PapalexandrisJet Propulsion
Laboratory Larry Craig, Tim Page, Richard
ShunkNASA Marshall Space Flight CenterAugust
2000
2
NGST Yardstick Concept
3
Observatory FEM
Model contains 5400 DOF
4
IMOS Environment

• Integrated model was applied to investigate three
  focus problems during concept development
  phase
• thermal-elastic deformation of OTA
• line-of-sight stability (jitter)
• wavefront sensing and control (not really
  addressed here)

5
System Error Budget Overview
System EE , SR budget
Encircled Energy
WFSC
Post-WFSC optical aberrations
WF error
WF C subsystem WFE budget
OTA figure alignment
IM figure alignment
OTA actuator performance
Imaging performance
OTA structure
OTA optics
IM structure
IM optics
OTA mechanical
Non-WF C subsystem s WFE budget s
6
Thermal-Elastic Analysis

• Linear Systems Model
• Optics Model
• Thermal Model
• OTA FEM
• Results for launch-to-orbit cooldown
• Results for transient (attitude re-orientation)
• Results for transient with active thermal control

7
Linear Error Model for Thermal Analysis
xrb
Alignment and figure states
useg
w
udm
xfig
Wavefront sampled at N discrete points in
the exit pupil

• Linear optical model
• w0 Cx x Cu u0
• WF sensing
• west w0 dwest
• Control
• u1 -G west du
• G Cu CuTCu -1 Cu

Optical controls
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MACOS Ray Trace Model
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MACOS Spot Diagram
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Wavefront Error Design Residual
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Wavefront Error Segment Tilt
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Wavefront Error FEM Node Translation
13
OTA FEM

• recover 1044 DOFs (344 nodes on PM, translation
  only, plus SM and SI)

• 2.00mm thick face sheet by 4cm deep core
  orthogrid beryllium mirror shell
• cells are 14.5 cm on a side equilateral
  triangles,cell wall are 1.00 mm thick

• RBE2s used to attach SI kinematically to center
  main ring instead of CELAS
• Three OTA to S/C I/F points instead of four

• The petal reaction structure is a beryllium
  frame-work of I-beams
• The center segment reaction structure is a flat
  Beryllium frame with a 1.3M dia inner ring. The
  frame is composed of a 152 mm deep I-beam inner
  ring and 152mm by 100mm wide box section outer
  ring and spokes.

14
Observatory Thermal Model Steady State
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Steady State Temps Mapped on OTA FEM
Mapping made possible by one-to-one nodalization
!!!
16
Computing the Transformation from Nodal
Temperatures to Displacements

• Net Force Balance rnet 0 -Ku rTemp
• Where rTemp ? BT E ?0 dV Ku
• B standard strain-displacement matrix
• ?0 temperature induced strain vector, f
  (?,temp)
• We can factor out nodal temperatures, generating
  a temp to load transformation matrix
• rTemp rg Agg tg
• Where tg nodal temperature (and/or
  gradient) vector (g-size)
• rg nodal force (and/or moment) vector
  (g-size)
• Reduce Agg to f-set size and transform to Local
  (NASTRAN global) system
• Afg Tfg Agg
• Premultipy by the flexibility matrix Kff-1 to
  get the temperature to displacement
  transformation matrix G
• Gfg Kff-1 Afg
• Expand to g-set, and transform back to the basic
  coordinate system
• Ggg TfgT Gfg or
• Ggg TfgT Kff-1 Tfg Agg
• So we have the temperature to displacement
  transformation matrix
• ug Ggg tg

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Steady State Wavefront Error with Control
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Thermal Transient following 22.5 degree slew
Cold Petal (space-side)

• Initial attitude has sun normal to sunshield
• Final attitude is 22.5 degree pitch away from
  sun
• Thermal equilibrium takes DAYS to reach

DT -0.8 K
Hot Petal (sun-side)
DT -1.3 K
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Thermal Transient Wavefront Error no Control
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Thermal Transient Wavefront Error with Control
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Jitter Analysis

• Pointing Control Architecture
• Linear Systems Model
• Disturbance Model
• Compensation Model
• Results for parametric studies

22
The CSI Challenge for NGST

• Lightweight, flexible structure with very low
  damping limits ACS bandwidth
• FSM bandwidth limited due to guiding sensor
  noise
• Thermal environment presents challenges to
  smart structures solutions for active damping
  and vibration suppression

23
System Level Block Diagram
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State-Space Model
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Dynamics Model Sensor Actuator Locations
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Optomechanical Analysis
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Reaction Wheels are Dominant Disturbances
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Wheel Disturbances - Discrete Speed vs Swept Speed
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Reaction Wheel Isolation
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FSM Response Functions
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Linear Analysis - Nominal Response, Effect of
Isolation, Effect of Wheel Imbalance Amplitude
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How Much Isolation Is Required?
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Conclusions

• Development of end-to-end models using the IMOS
  environment was relatively painless, owing to the
  following factors
• translation from NASTRAN and SINDA was possible
  for FEM and TMM, as was output to FEMAP neutral
  format
• geometric and material properties were easily
  parameterized, as were all other significant
  entities in the models
• ray-trace code (MACOS) was open-source, so it
  could be integrated via Mex-function API
• Matlab is a matrix-oriented language/tool, with
  integrated graphics and visualization
• Questions remain about the ability to handle
  realistically-sized models within Matlab
  (eigenvalues, matrix inversion)
• None of these models have been validated, of
  course