# Presentation: 056804

## Integrated Modeling for the James Webb Space Telescope (JWST) Project: Structural Analysis Activities 

**Presented by: Mark McGinnis/Swales**
**John Johnston, Gary Mosier, Joe Howard, ****Tupper Hyde, and Keith Parrish (NASA/GSFC)**
**Kong Ha (Jackson and Tull)**
**Frank Liu**** and Mark McGinnis (Swales Aerospace)**
**May 6, 2004**

## Overview

**JWST Overview**
**Observatory Structural Models**
**Integrated Performance Analysis:**
- Performance Budget
- Linear Optical Analysis
- Structural-Thermal-Optical
- Optical jitter dynamics
**Future Work and Challenges**

## JWST Mission Concept

**JWST Mission Concept**

- Measure the luminosities, morphologies, and environments of galaxies within the spectral band 0.6 – 10 μm
- Measure the spectra of 2500 galaxies over the redshift range 1 < *z* < 5
- Obtain a total observing time of at least 1.1x108 seconds. JWST is designed for at least a 5-year lifetime.

**Constraints**
- Launch by 2011
- Cost capped
- Significant International Contributions
- Spacecraft from Prime Contractor (IRT Finding)
- Use existing Launch Vehicle Capabilities

**Science Requirements**

**Key Mission Trades**
- Orbit, Method to Orbit
- Launch Vehicle/Shroud Configurations
- Filled vs Partially-Filled Apertures
- Thermal Management
- Instrument Packaging
- Sky Coverage
- Communications Strategy

** ****NIR Imaging Camera [NIRCam] **
-  8 square arc minutes field of view
-  Spectral resolution R ( ≤ 100
-  Wavelength range 0.6-5 μm
** ****Multi-object spectrograph [NIRSpec]**
-  Observing > 100 objects/observatory pointing
-  9 square arc minutes field of view
-  *R *~1000 over wavelengths 1-5 μm
-  *R *~100 over wavelengths 0.6-5 μm
**MIR instrument [MIRI]**
-  Imaging and spectroscopy
-  2 square arcminutes field of view
-  *R *~1500 spectroscopy over wavelengths 5-28 μm.

**Science Instruments**

## Observatory Architecture

**Optical Telescope Element**

**Integrated Science Instrument Module (ISIM) Element**

**     ****Spacecraft Element**
**Sunshield           Spacecraft Bus**

## Observatory Structural Model

## Integrated Performance Analysis

**Overview**
- Multi-disciplinary analysis
- Thermal, Optical, GN&C, and Structural
- Tight requirements drive the project toward more integrated analysis
- Performance budget
- Northrup-Grumman Space Technology (NGST) has adopted a very detailed optical performance budget allocating wavefront error
- Seek to place the project in a position to intelligently comment on this budget as the contractors estimate the telescope’s performance
- Linear optical model
- MATLAB-based tool to allow non-optical engineers to estimate wavefront error
**Baseline Analyses:**
- STOP
- Jitter

## Performance Budget

**NGST allocates and tracks optical performance with a spreadsheet**
**Rooted in project Strehl ratio and Encircled Energy requirements**
- Calculations translate these into total allowable WFE
- Allocated into 3 spatial-frequency bands (cycles/aperture)
- Allocations for both beginning and end of life
**Two main branches divisions at top level**
- Active control
- Stability
**Geometry errors of optics divided into “figure” and “alignment”**
**Temporal performance is allocated to either “drift” or “vibrate”**
**Lowest-level requirements often related to equivalent mechanical requirements**

## Performance Budget

**Performance Budget**

- Top Level

- Alignment Drift

## Linear Optical Analysis

**Provides accurate estimate of OPD wavefront error for perturbed systems (within the limits of the model)**
**Coefficients created by ray-tracing runs in OSLO**
- 10nm (nrad) motion introduced in each of optical DOF
- 100x100 array showing OPD at exit pupil generated in MATLAB for each optical perturbation
**Arrays scaled and summed in MATLAB based on actual motion in each of the 132 DOF**
- Displacements multiplied by appropriate array
- OPD maps summed
- FSM manipulated to minimize RMS wavefront error
- Results are reported as “Best Fit Plane” with global piston offset removed

## Linear Model Accuracy

## Structural-Thermal-Optical (STOP) Analysis

**OSLO**

**Observatory Structural Model**

**Observatory Thermal Models**

**TSS and SINDA, or IDEAS/TMG**

**Thermal Transient Analysis**

**Temperature vs Time (thermal)**

**Generate Interferograms**

**Map Temperatures to Structure**

**NASTRAN**

**SigFit**

**Temperature vs Time (structural)**

**Displacements of optical elements and surfaces**

**Wavefront Error ****(OPD Map & RMS value)**

**Static Loads Analysis**

**Optics Model**

**Ray-Trace Analysis**

**Linear Optical Tool**

**MATLAB**

- Prelim.

- Final

**PATRAN EXCEL**

## STOP Analysis – WFE Predictions

**STOP analysis of slew maneuvers requires pairs of linear statics runs**
- Calculate delta between displacements of two room to operational thermal-loaded runs
**Most STOP analyses use linear optical tool for WFE prediction**
- Current generation thermal models rarely include PM segment details
- Beryllium PM segments not expected to develop substantial gradients

## Optical Jitter Dynamics (Jitter) Analysis

**Integrated Jitter Model**

**Jitter Model Validation**

**Eigenvectors & Eigenvalues**

**from normal modes analysis**

**Validation Results & Documents**

**Optical Sensitivity Matrix**

**from optical systems analysis**

**Dynamic WFE**

**LOS Error**

**Disturbance Models**

**FSM Model**

**ACS Model**

**FGS Model**

**from controls model**

**Dynamics Model Reduction**

## Jitter Analysis – Modal Analysis and Damping

**Jitter Analysis – Modal Analysis and Damping**

**The structures discipline provides frequencies, mode shapes, and modal damping values for use in integrated modeling (IM) and attitude control system (ACS) studies:**
- Mode shapes (mass normalized) are partitioned based on DOF corresponding to predefined reference points (optics, RWAs, etc).
- Modal damping values are either:
- Uniform
- Variable (Based on group participation determined using modal strain energy fractions)

- 40 Modes in 0-100 Hz Frequency Range
- First flexible mode = 0.42 Hz

## Jitter Analysis: Mode Shapes

**Secondary Mirror Support Structure**
**Bending Mode @ 8 Hz**

**Backplane Twisting Mode @ 12 Hz**

## Jitter Analysis – LOS and WFE Predictions

**Reaction Wheel Assemblies (RWAs) are largest jitter disturbance source:**
- Harmonic disturbances
- Excite spacecraft and telescope structural modes when the RWA spin speed or harmonics align with the lightly damped structural modes.

## Challenges and Future Work

**Future Work:**
- Program plans on following a schedule of analysis cycles:
- STOP/Jitter/Launch analyses
- First such cycle is underway (6 month duration)
- Need to verify budget allocations by means of integrated modeling
- Government team performs independent modeling analysis to validate prime contractor
- Performance predictions
- Requirements placed on subcontractors/partners
**Challenges:**
- Constant pressure exists to create accurate, detailed models while keeping run times tolerable:
- Need for high-fidelity (multi-million DOF solid element) structural model anticipated for CDR distortion analysis.
- Superelement approaches under investigation
- Need to understand sensitivity of results to variations in material properties
- Need to expand linear optical tool to calculate WFE at multiple field points and FOV locations