麻省理工学院：《Satellite Engineering》Lecture 14 Structures in Space Systems

Structures in Space Systems Roles o technologies Shielding Multifunctional structures Thermal, radiation, glint Deployment and geometry Maintaining System Geometry maintenance Carrying loads Deployable booms o applications Mesh antennas Power and thermal management Membrane structures Aperture forming Inflatables Spacecraft backbone Tethers ssues Formation Flight(virtual structure Light-weighting Structural dynamics Thermal distortion

Structures in Space Systems Structures in Space Systems  Roles — Shielding — Thermal, radiation, glint — Maintaining Syste m Geometry — C arrying Loads  Applications — Power and thermal m anage m ent — Aperture forming — Spacecraft backbone  Issues — Light-weighting — Structural dynamics — Therm al distortion  Technologies — Multifunctional Structures — Deploym e nt and geo metry maintenance — Deployable booms — Mesh antennas — Membrane structures — Inflatables — Tethers — Formation Flight (virtual structure)

Deployment and geometry Maintenance o Deployable membranes Used for solar arrays. sunshields decoys Being researched for apertures starting at rf and eventually going to optical o Inflatables First US satellite was inflated(ECHO D) Enables a very large deployment ratio deployed over stowed dimension Membranes stretched across an inflated torus Outgassing and need for gas replenishment has led to ultra-violet cured inflatables that rigidize after being exposed to the UV from the sun

Deployment and Geometry Maintenance Deployment and Geometry Maintenance  Deployable Membranes — Used for solar arrays, sunshields, decoys — Being researched for apertures starting at RF and eventually going to optical  Inflatables — First U S satellite was inflated (ECHO I) — Enables a very large deploy m ent ratio — = deployed over stowed dimension — Membranes stretched across an inflated torus — Outgassing and need for gas replenishment has led to ultra-violet cured inflatables that rigidize after being exposed to the UV from the Sun

Sa Deployment and Geometry Maintenance o Truss structures High strength to weight ratio due to large cross-sectional area moment of inertia Moment= el ax o Deployable booms able engineering a bearing ring at the mouth of the deployment canister deploys pre- folded bays in sequence EX SRTM mission on Shuttle Handout gives key relationships between I, El and truss diameter °ota/ system mass canister mass fraction

Deployment and Geometry Maintenance Deployment and Geometry Maintenance  Truss Structures — High strength to weight ratio due to large cross-sectional area mom ent of inertia  Deployable Booms (ABLE Engineering) — A bearing ring at the m outh of the deployment canister deploys pre-folded bays in s equence — EX: SRTM mission on Shuttle Moment = EI ∂2 w ∂x 2 Handout gives key relationships between l, EI and: •truss diameter •total system mass •canister mass fraction

Deployment for Aperture Maintenance o aperture physics requires large dimensions for improved angular resolution 6.=1.22-= D B Large area for good sensitivity (SNR) o Options include Filled apertures Deployed membranes ( Courtesy of the European Space Agency. Used with permission. Deployed panels Sparse apertures Deployed booms Formation fown satellites

Deployment for Aperture Maintenance Deployment for Aperture Maintenance  Aperture physics requires: — large dim ensions for improved angular resolution — Large area for good sensitivity (SNR)  Options include: — Filled Apertures — Deployed m e mbranes — Deployed panels — Spars e Apertures — Deployed boom s — Formation flown satellites θ r = 1.22 λ D = λ B (Courtesy of the European Space Agency. Used with permission.)

Origins Telescope Dynamics and Controls PSD and cumulative RMS Total OPD(int I) Disturbance Contribution Frequency (Hz) Normalized Sensitivities of Total OPD (int I)RMS value w.r. t to physical parameters

Origins Telescope Dynamics and Controls Origins Telescope Dynamics and Controls

Integrated Model

Integrated Model Integrated Model #1 #2 #3 #4

Example Transfer Function RWA TX to Internal OPd#1: Reduced Transfer Function of RWATX to Int Met Opd #1 Reduced MIT (536 states) 10 10 TF Normalized to JPL Original 10 10 10 Frequency [Hz

Example Transfer Function ple Transfer Function RWA Tx to Internal OPD #1 : Reduced to Internal OPD #1 : Reduced 10−6 10−4 10−2 100 102 104 Magnitude [nm /Nm] Transfer Function of RWATx to Int. Met. Opd #1 Original JPL Reduced MIT (536 states) 10−1 100 101 102 103 10−4 10−2 100 102 TF Normalized to JPL Original Frequency [Hz]

SIM Dynamics and Control Block Diagram Pathlength metrics White Noise Input SIM Dynamics and Controls Diagram Star OPd#1-3 Int. Met. OPD#1-3 Appended System Dynamics 叫山: Ext. Met. Pathlength Optical Opto-Structural Plant Control 0= RMS OPD SYS 18x1 SYS Disturbances SYS r 6x1 Performances 13x1 (FSM, ODL) (RWA) 3x1 Attitude control Wavefront Tilt metrics 3x1 Star dWFT#1-3 (ACS) SYS_ paor FEC DWFT#1-3 RSS DWF Assume continuous time LTI system RWA are the only disturbance source at this point

SIM Dynamics and Control Block Diagram SIM Dynamics and Control Block Diagram ￾
     

 
           
  
  
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Dynamic disturbance Sources o Reaction Wheel assemblies RWA Radial Force Disturbance PSD: B Wheel (xdirection) (RWAS are comprised typically of four wheels Applying torque to the wheels za0 creates equal and opposite torques on the spacecraft As a result, the wheels spin Static and dynamic imbalances in Wheel Speed(RPM) equency(Hz) wheels cause 6-DOF forces/torques to be imparted on the structure at the frequency of the wheel rPm Typically place on isolators and operate in frequency regions where structural response is low o System design requires careful Ithaco rWa,s trade between wheel balancing, Isolator corner frequency, vibration (www.ithaco.comp control etc roducts. html)

Dynamic Disturbance Sources Dynamic Disturbance Sources  Reaction Wheel Ass emblies (RWAs) are comprised typically of four wheels — Applying torque to the wheels creates equal and opposite torques on the spacecraft — As a result, the wheels spin — Static and dynamic i mbalances in wheels cause 6-DOF forces/torques to be i mparted on the s tructure at the frequency of the wheel RPM. — Typically place on isolators and operate in frequency regions where structural response is low  System design requires careful trade between wheel balancing, isolator corner frequency, vibration control, etc. 0 50 100 150 500 1000 1500 2000 2500 3000 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 Frequency (Hz) RWA Radial Force Disturbance PSD: B Wheel (xdirection) Wheel Speed (RPM) PSD (N2 /Hz) Ithaco RWA’s (www.ithaco.com/p roducts.html)

Dynamic disturbance Sources o Cryocoolers o Fluid Slosh Mechanical compressors Liquid propellants and cryostats expanders undergo thermodynamic (liquid Helium for cooling cycles(e.g, Sterling cycle) to cool detectors)can exhibit fluid slosh detectors(cameras). Sometimes Difficult to model these dynamic called"cold fingers resonances since The moving piston induces gravity stiffens the fluid in 1-g vibration Surface tension stiffens in 0-g

Dynamic Disturbance Sources Dynamic Disturbance Sources  Cryocoolers — Mechanical compressors￾expanders undergo therm odyna mic cycles (e.g., Sterling cycle) to cool d e t e c t o r s ( camer a s ). S ome time s called “cold fingers.” — The m oving pis ton induces vibration  Fluid Slosh — Liquid propellants and cryostats (liquid Heliu m for cooling detectors) can exhibit fluid slos h — Difficult to model these dynamic resonances since — gravity stiffens the fluid in 1-g — Surface tension stiffens in 0-g