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NEXUS Spacecraft Integrated Modeling and SimulationOlivier L. de Weck July 2001 Sponsor: Gary E. Mosier, NASA GSFC |
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Background and MotivationSubsystem Requirements Science Requirements Functional Requirements HST - 1990 Requirements Flowdown Process 5 year wide-angle astro- metric accuracy of 4 masec for 20th Magnitude stars Science Interferometer OPD < 10 nm RMS Fringe Visibility > 0.8 for astrometry Space-Based Observatory Multipurpose UV/Visual/IR Imaging and Spectroscopy “Performance” NGST-2008 SIM-2006 NEXUS-2004 TPF-2011 Deployable Cold Optics NGST Precursor Mission Faint Star Interferometer Precision Astrometry Nulling Interferometer Planet Detection Lightweight 8m-Optics IR Deep Field Observations Achieve stringent requirements in a cost-effective manner with predictable risk level. |
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Nexus Case StudyNexus Spacecraft Concept Demonstrate the usefulness of Isoperformance on a realistic conceptual design model of a high-performance spacecraft Integrated Modeling Nexus Block Diagram Baseline Performance Assessment Sensitivity Analysis Isoperformance Analysis (2) Multiobjective Optimization Error Budgeting Purpose of this case study: on-orbit configuration launch configuration The following results are shown: NGST Precursor Mission 2.8 m diameter aperture Mass: 752.5 kg Cost: 105.88 M$ (FY00) Target Orbit: L2 Sun/Earth Projected Launch: 2004 Details are contained in CH7 OTA Sunshield Instrument Module Deployable PM petal Delta II Fairing Pro/E models © NASA GSFC |
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Nexus Integrated ModelLegend: Spacecraft bus (84) Design Parameters 8 m2 solar panel m_bus (I/O Nodes) RWA and hex isolator (79-83) sunshield I_ss K_rISO 2 fixed PM petals Instrument (149,169) (207) K_yPM Cassegrain Telescope: PM (2.8 m) PM f/# 1.25 SM (0.27 m) f/24 OTA SM spider t_sp deployable PM petal (129) SM (202) K_zpet m_SM Structural Model (FEM) (Nastran, IMOS) |
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Nexus Block DiagramNumber of performances: nz=2 Number of design parameters: np=25 Number of states ns= 320 Number of disturbance sources: nd=4 |
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Initial Performance Assessment Jz(po)23.1 Hz Critical Mode Time [sec] |
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Nexus Sensitivity AnalysisGraphical Representation of Jacobian evaluated at design po, normalized for comparison. RMMS WFE most sensitive to: Ru - upper op wheel speed [RPM] Sst - star track noise 1s [asec] K_rISO - isolator joint stiffness [Nm/rad] K_zpet - deploy petal stiffness [N/m] RSS LOS most sensitive to: Ud - dynamic wheel imbalance [gcm2] K_rISO - isolator joint stiffness [Nm/rad] zeta - proportional damping ratio [-] Mgs - guide star magnitude [mag] Kcf - FSM controller gain [-] |
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2D-Isoperformance Analysisjoint isolator strut CAD Model E-wheel K_rISO [Nm/rad] Ud=mrd [gcm2] |
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Multiobjective Design OptimizationPerformance Jz(piso) =Jz,req Cost Objectives Jc Risk Objectives Jr Control effort Implementation Cost (mid-bound) System Mass Dissipated Power Closeness to “cheap” bound Stability Margins (SISO) max SV of sensitivity function / mvar Nyquist Sensitivity of performance to parameter variations Knowledge Error Since solutions piso in the isoperformance set I do not distinguish them- selves via their performance, we may satisfy additional objectives: Can scalarize NLP problem with preference order (weightings) h,Qcc,Qrr as shown on the left or can look for efficient set E of pareto-optimal solutions, piso*. |
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Nexus Multivariable Isoperformance np=10Design A Design B Design C Performance Cost and Risk Objectives Pareto-Optimal Designs p*iso Best “mid-range” compromise Smallest FSM control gain Smallest performance uncertainty Jz,1 Jz,2 Jc,1 Jc,2 Jr,1 Design A 20.0000 5.2013 0.6324 0.4668 +/- 14.3218 % Design B 20.0012 5.0253 0.8960 0.0017 +/- 8.7883 % Design C 20.0001 4.8559 1.5627 1.0000 +/- 5.3067 % |
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Nexus Error BudgetingLTI System , Jz,req, p_bounds, p_nom (Design A) Isoperformance Toolbox C A Error Source Contributions Plot Error Contribution Sphere Budget B Allocated Budget piso Capability Budget Note: ACS sensor noise contributions not shown var_contr 0.8 0.6 FGS 0.4 0.2 0 0 0.2 0.2 0.4 0.4 0.6 0.6 0.8 0.8 RWA Cryo LOS Budget Capability |
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Nexus Initial po vsFinal Design p**iso Initial Parameters Final Ru 3000 3845 [RPM] Us 1.8 1.45 [gcm] Ud 60 47.2 [gcm2] Qc 0.005 0.014 [-] Tgs 0.040 0.196 [sec] KrISO 3000 2546 [Nm/rad] Kzpet 0.9E+8 8.9E+8 [N/m] tsp 0.003 0.003 [m] Mgs 15 18.6 [Mag] Kcf 2E+3 4.7E+5 [-] Improvements are achieved by a well balanced mix of changes in the disturbance parameters, structural redesign and increase in control gain of the FSM fine pointing loop. secondary hub tsp SM Dsp Deployable segment Kzpet +Y SM Spider Support +Z +X Spider wall thickness 50 Centroid Jitter on Focal Plane [RSS LOS] 40 T=5 sec 30 20 10 Initial: 14.97 0 m m m m Centroid Y Final: 5.155 -10 -20 -30 -40 -50 Centroid X -50 0 50 |
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