cev structures

Structures: 

Structures Charles Vaughan AA420 Space Design

Outline: 

Outline Driving Issues and Requirements Types of Structures Types of Materials Fasteners Interfaces/Attachments Tools Design Approach Testing References: Sections 10.4, 11.6, 12.4, 18.3 of Larson and Wertz Sarafin, Spacecraft Structures and Mechanisms: From Concept to Launch Blevins, Formulas for Natural Frequencies and Mode Shapes

Basic Structural Functions: 

Basic Structural Functions Transmit Loads launch (acceleration, shock, vibration), deployment, propulsion settling of structure vibration - isolate instruments Provide Surfaces for Mounting Functional hardware electronics thermal propulsion payload power Shield Sensitive hardware from the Space Environment radiation (i.e. charged particles, cosmic, Van Allen) solar radiation (i.e. thermal, UV, also Earth albedo) micrometeoroids atomic oxygen Pressure Vessel

Driving Issues and Requirements: 

Driving Issues and Requirements Mass and mass concentrations Acceleration loads from launch Vibration and acoustic loads from launch Natural frequency profile Shock loads from deployment, launch Thermal stresses, material differences (next lecture) Interface attachments ejection system component attachments (bolts, adhesives) Safety Cost Many of these flow down from higher level requirements to constrain the designer to a smaller area.

Driving Issues and Requirements: Static Acceleration Loads: 

Driving Issues and Requirements: Static Acceleration Loads Maximum during launch Really dependent on launch vehicle thrust and total mass Usually in terms of yield stress ultimate stress factors of safety Factors of safety usually depend on safety plan analysis alone (1.6) analysis and test (1.2) Margin of Safety = Usually not a big driver in the structural design Best tool: Finite element analysis yield stress ultimate stress stress strain AllowableStress SafetyFactor*ActualStress - 1 must be > 0

Driving Issues and Requirements: Dynamic Loads: 

Driving Issues and Requirements: Dynamic Loads Acoustic loads are very important at lift-off (ground reflection), with large affects on thin materials Aerodynamic and internal pressure loads are also important Most important is the vibration loads due to primary rocket engine, ground handling, etc. Power spectral density is usually the unit of measure This is usually the biggest driver in the structural design Best tool: Testing and a ANSYS or MATLAB analysis Max PSD given launch loads PSD G2/Hz frequency 50 Hz 200 Hz Area is RMS2 of acceleration

Driving Issues and Requirements: Natural Frequency Profile: 

Driving Issues and Requirements: Natural Frequency Profile Usually coupled with dynamic response Objectives: make sure launch loads do not overly excite first flex mode make sure there is little coupling with launch vehicle The first objective is usually met with a minimum frequency requirement, and the dynamic analysis/test The second objective is usually met using a minimum frequency requirement Best tool: Finite element analysis and testing

Random Variables: 

Random Variables Given a random signal u(t), the mean, variance, standard deviation, and other statistics can be calculated. The 3 value is usually a bound where 99% of the random variable values lie within the boundary. For the above example: dt=0.5; N=1028; u=randn(N,1)/sqrt(dt); MATLAB functions: mean(u)=0.03; var(u)=2.03; std(u)=1.43;

Stochastic Analysis for Sine and Random Noise: 

Stochastic Analysis for Sine and Random Noise Sinusoids have all power at one frequency Random (white) noise has power at all frequencies - a fictitious mathematical tool Sinusoidal Signal Random Noise PSD Signal/Hz T = 1/f = 1/(2) A  f = 2 A 2t PSD Signal/Hz A2/2(t) frequency frequency time time

Random Vibe Analysis and Test: 

Random Vibe Analysis and Test Analysis can be done given the input spectrum & plant model. MATLAB: Given Suu() and G(j), then Syy=abs(G).^2.*Suu; Testing passes random (white) noise through a filter to create the given input spectrum Suu(). Input Suu() Plant G(j) Output Syy() Input Filter Plant G(j) Output Syy() Random Noise Analysis Test Area under PSD is the variance!

Example: PSD Analysis: 

Example: PSD Analysis Time Based Approach: y=lsim(…,u,t), xf=fft(x), Sxx=xf.*conj(xf)/T; TF Approach: Sxx=abs(G).^2.*Suu The filtered random noise changes the energy in the output Random Noise Filtered Random Noise m c Input accel output accel

Driving Issues and Requirements: Shock Loads: 

Driving Issues and Requirements: Shock Loads Usually caused by pyrotechnic events (ejection) launch (second stage, etc.) ground handling Approach is similar to Vibration Best tool: Testing and a MATLAB analysis Max shock Peak Response G’s frequency 1,000 Hz 10,000 Hz

Types of Structures: 

Types of Structures Also: Honeycomb panels inflatables deployables

Example: Isogrid of the UW Dawgstar: 

Example: Isogrid of the UW Dawgstar Right Hexagonal Prism 18” dia, & 12.625” high Aluminum 6061-T651 isogrid Total structure mass 2.2 kg TOP SIDE

Example: Honeycomb Panel: 

Example: Honeycomb Panel Aluminum panels Honeycomb structure in between Benefits: lightweight strong Drawbacks machining/pricing

BSAT-2: 

BSAT-2 Systems Delivered: 3 Propellant Load: 464 lbm (210 kg) Hydrazine Total in 2 Tanks 400-100 psia (27.5-6.9 bar) Blowdown Operation 12 MR-103G 0.2-lbf (1 N) Thrusters 4 MR-501B Electrothermal Hydrazine Thrusters (EHTs) Used for Orbit Raising and Attitude Control (GEO Spacecraft)

Ikonos: 

Ikonos Ikonos Facts Integrator: Lockheed Martin Launch Mass: 1600 lbm (725 kg) Launched September 24, 1999 from VAFB on an Athena-2 Rocket into Sun-Synchronous Orbit Mission Objective: Commercial Earth Observation Satellite (1-m Monochrome and 4-m Multispectral Resolution) First Ikonos Spacecraft was Lost During April 27, 1999 Launch Vehicle Failure Aerojet Participation Hydrazine Propulsion System Used for Spacecraft Attitude Control Aerojet Fueled the Spacecraft at the Launch Site and Provided Launch Support for the Athena-2 Rocket

Ikonos: 

Ikonos Systems Delivered: 2 Propellant Load: 83 lbm (37.6 kg) Hydrazine 307-72 psia (21-5 bar) Blowdown Operation 6 MR-103G 0.2-lbf (1 N) Thrusters Used for Attitude Control (LEO Remote Sensing Spacecraft) Aerojet Integrated System on Customer-Supplied Structure Aerojet Fueled Spacecraft at Launch Site (VAFB) Propulsion Schematic

Truss Structure: 

Truss Structure Benefits: Strong Drawbacks: Not many attachment points for small satellites JPL Interferometry testbed

Pegasus Launch Vehicle : 

Pegasus Launch Vehicle Pegasus Facts Prime Contractor: Orbital Sciences First Privately-Developed Space Launch Vehicle Air-Launched from L-1011 Aircraft at 40,000 ft 30 Flights Since 1990 (Including 6 Pegasus XL w/HAPS) Payload Capability of 1000 lbm (450 kg) to Low Earth Orbit Aerojet Participation 4th Stage Hydrazine Auxiliary Propulsion System (HAPS) on Pegasus XL Provides Improved Performance and Injection Accuracy 6 Successful Flights

Pegasus Hydrazine Auxiliary Propulsion System (HAPS): 

Pegasus Hydrazine Auxiliary Propulsion System (HAPS) Systems Delivered: 9 Propellant Load: 130 lbm (60 kg) Hydrazine 450-90 psia (30.9-6.2 bar) Blowdown Operation with Aerojet- Designed Low Cost AF-E-332 Bladder Tank 3 MR-107K 50-lbf (220 N) Thrusters Used for Final Orbit Trim (4th Stage) for Pegasus XL Aerojet Integrated System on Customer-Supplied Structure Propulsion Schematic

Types of Materials: 

Types of Materials Aluminum Magnesium Titanium Beryllium (rarely used) Composites Section 11.6 has a good discussion of the trades Aluminum has been the norm, but this is changing (composites) Problems with composites: dependent on human manufacturing, so it must be tested more so than metallic structures - i.e. higher risk attachments (epoxy is difficult, holes introduce stresses) thermal differences

Types of Fasteners: 

Types of Fasteners Aerospace fasteners structural fasteners must be consistent with NAS and MIL standards UW Nanosat purchased them from NASA GSFC These are #10’s at a minimum Flat washers are usually used to prevent scratching of finishes Torques are usually specified in requirements, and enforced with a torque wrench Threaded fasteners require a locking mechanism staking - an epoxy patch on fastener head vespel pellets - polyester patches on screws Welding Adhesives

Attachments and Mechanisms: 

Attachments and Mechanisms Attachments and mechanisms drive structural/system design more than one would think ejection system and attachment inner wiring inner electronics box inner battery box deployable antennas deployable booms Smart loads analysis can help with this design

Good Example of Loads Analysis and Requirements Definition: ION-F Deployment: 

Good Example of Loads Analysis and Requirements Definition: ION-F Deployment Stacked with USU: 6” UW: 12.625” VT: 12.625” Lightband system selected simple little surface area will be qualified

Lightband deployment system: 

Lightband deployment system

Flexible Frequency Requirement: 

Flexible Frequency Requirement From the Shuttle SHELS platform 100 Hz if only using analysis 50 Hz if using analysis and test for first mode 35 Hz minimum requirement - requires full modal analysis to show how it may couple with Shuttle (15 Hz first mode) TWO STACKS SHELS ejection attachment

Flexible Frequency Requirement: 

Flexible Frequency Requirement First Requirement from AFRL: 50 Hz for each nanosat A few months later: 50 Hz for each stack (~90 Hz for each nanosat) A few months later: 50 Hz for whole platform, 150 Hz for each stack TWO STACKS SHELS ejection attachment

Flexible Frequency Requirement: 

Flexible Frequency Requirement Dawgstar analysis of stack: currently at ~55 Hz AFRL analysis of each stack: ~140 Hz Why?: ejection system modeling (watch how your loads move!) 1/2” inside end plate UW AFRL

Tools: 

Tools Finite Element Analysis (static and dynamic loads) ANSYS (on AA system) Nastran MATLAB for dynamic loads analysis (on AA system) CAD or like packages for drawing, CNC machine preparation Unigraphics (on AA system) IDEAS (on Nanosat Lab system, can also do thermal analysis with the same model!) Autocad etc. Dawgstar IDEAS model

Design Approach: 

Design Approach List design drivers, associated calculations, narrow requirements Fill out more detailed specification based on template and mass, test, and other requirements Examine types of materials to see the benefits and drawbacks to each (Al, composite) Examine types of structures to see the benefits and drawbacks to each (isogrid, Honeycomb sandwich, truss) This includes price, lead time, whether we can develop it here, mass, etc. i.e. all of the design drivers! List and examine each of the interfaces (see previous slides) After narrowing the options to 1-2, do a first cut model in ANSYS to analyze static, dynamic requirements Move to IDEAS/Unigraphics for the prototype All during this process, update specs and keep systems engineers informed of changing designs, options, conclusions, etc.

Testing: 

Testing Random Vibe: a vibration table, similar to the one at Primex for vibrations in three DOF. Sine Sweep: similar, but stepping through a series of sinusoidal excitation frequencies - good for nonlinear systems. Shock: Impact testing, similar to a hammer Static: simply add an even distribution of weight to the structure. 15 kg 15*10 kg +x 10 G loading