1. Structures and Mechanisms
Safety Concerns
Getz, Jen
Lowery Anthony
Shilling, Tim
*All slides are here, I am just not used to how power point works (it only shows what is on a slide if you have modified it.)
*Got failsafe stuff on here twice, since it’s my part, I’d like to keep mine, but, if you like yours (Anthony's?) better we can keep either one.
*please only delete the notes on this specific page, I need the others. Thank you. (jen)
February 23, 2019

2. Overview Structural Systems Encouraged Design Aspects
Discouraged Design Aspects
Factors of Safety / Margins of Safety
Failsafe Parts
Stress Analysis
Stiffness
Mechanisms
Classifications
Requirements
Fracture Control
General Considerations
Low-Risk Fracture Classifications
Composites
More Failsafe
Pressure Systems
Material Usage
Corrosion Resistance
Outgassing
Composites
Fasteners

3. Structural Systems Encouraged Design Aspects
Practice When Possible-
Use machined, all metal structures
Threaded fasteners with backout protection, locking helicoils or lock wire is preferred.
Use multiple fasteners, so that the failure of one does not cause a hazard.
Implement fracture control in accordance to NAS-STD-5003 and insure there are no fracture critical components.
Before purchase of any components, obtain a material list and ensure they meet outgassing requirements.

4. Structural Systems Discouraged Design Aspects
Avoid When Possible-
Non-metallic composite structures
Adhesives, epoxies or tape to join structural components
Use of such materials for required redundancy of fasteners and structure
Use of cords, cables, plastic or other “soft goods” in the primary structure, for retaining deployable mechanisms or when failure would result in hazards
Friction devices for retention, such as crimps, worm gears, lead screws and motor detent torques
*Pressure vessels
All components shall have adequate venting per section of the AFRL Internal Cargo Unit User’s Guide.
Liquids, gasses or any material that may change phase during launch or on-orbit
Welded joints or cast metallic components
**Parts or assemblies who's safety depends on the build process, such as non metallic composites or deployables

5. Structural Systems Factors of Safety
Material FOS
*Design requirements for Nanosat structures including expected loading and required safety factors:
*Yield Strength (limit loading) - 2
*Ultimate Strength
*Structural Integrity will be verified by the AFRL
*During this test, no detrimental permanent deformation or ultimate failures shall occur.
*These accelerations should be applied through the center of mass of the analyzed component.
*Thermally induced loading, including on-orbit thermal loading will be included in the expected loads.
*These tests take into account the worst case launch and landing load environments.
*Structural integrity verified by at sine burst testing at 20 g (1.2 x limit load) to verify structural strength, sine sweep test to verify the natural frequency, and a random vibration test.
*Because the Sine Burst Test is conducted at the ICU/Nanosat System level, the universities are not required to subject the individual Nanosat to a Sine Burst Test.
*CG: (0.25” from centerline & 12” or less above the SIP)
*A combination of steady state, low frequency, transient loads, and high frequency vibration loads.

6. Structural Systems Margin of Safety
University is required to perform stress analysis testing to show that the design FOS are met and/or exceeded, and that a margin of safety (MS) of zero or greater exists for both yield and ultimate stress conditions.
All stress analysis shall incorporate methods and assumptions consistent with standard aerospace practices.
Allowable mechanical properties of structural material shall be obtained from document #MIL-HDBK-5D Metallic Materials and Elements for Aerospace Vehicle Structures.
Buckling, crippling and/or shear failures will be considered ultimate failures.

7. Structural Systems Failsafe Parts
Failsafe: Redundant structural part shown to be a nonfracture-critical component by meeting the requirements of (defined below)
*In order to be considered failsafe, a components failure must be either:
*A) able to withstand the redistributed loads forces and be considered unflawed. OR
*B) documented technical reasoning may be used if there is sufficient structural redundancy for fail-safe classification, or failure of the part clearly would not create a catastrophic hazard and it can also be considered unflawed.
*It must be shown by analysis or test that, due to structural redundancy, the structure remaining after any single failure can withstand the redistributed limit loads with a safety factor of 1.0.

8. Structural Systems Stress Analysis
*Worst case combined stresses depend on the magnitude and direction of the combined stresses.
*For case and time consistent conditions, both the maximum positive stress and the maximum negative stress will be evaluated based on six possibilities:
*Positive/Negative stress associated with:
*Pressure and Thermal
*Mechanical and Thermal
*Pressure and Mechanical
*Alternatively, a max-on-max, non-case consistent, non-time consistent maximum positive and negative stress conditions may be used to envelope all stress cases.
*When stresses are derived from automated stress analysis systems (e.g. finite element models, post-processing programs,) a method will be available to demonstrate that proper signs and safety factors were used for each combined stress case.
*Important Note:
*Stresses can be either additive or relieving.
*not all stresses are calculated, depending on whether the stress is additive or relieving.
Minimum ultimate FOS for stresses due to combined loads (I.e. mechanical,pressure and thermal) will be determined in a thorough manner and will be equal to or greater than 1.4.
If pressure is additive, it will only be calculated at minimum guaranteed value (1.0) because you don’t want to count on it alleviating stresses
If thermal stress is alleviating, it will not be included at all in total stress calculations, it will be used only when additive.

9. Structural Systems Stiffness/Thermal
The Nanosat shall have a fundamental frequency above 100 Hz given a fixed-base condition at the SIP.
This will be tested by a sine vibration test to cover both structural integrity and simulated flight environment.
The spacecraft structure will be of sufficient strength and stiffness to maintain and withstand all ground testing, handling, transportation,launch and mission orbit environments.
Thermal:
The Spacecraft thermal control design shall maintain all Spacecraft subsystems and components
This includes instrument interfaces at temperature levels, thermal gradients, and temperature transition rates consistent with the mission lifetime for all Satellite operations and orientations.

10. Mechanisms Requirements
Binding/Jamming/Seizing
Quick Release Pins
Considered movable mechanical systems when used in safety critical applications
Springs
In designs and applications where spring failure would result in a hazard, the springs shall be redundant or designed, evaluated, and used under an acceptable fracture control program
Fastener Retention
Meet All Fracture Requirements
Positive Indication of Status
Torque/Force Margins
Testing

11. Composites CTD Composite Hinges
Review CTD’s documentation and verification of space worthiness of hinges
Other Composites
Avoid
Comply with:
NSTS 14046
NASA-STD-5003
CONFIGURATION/DOCUMENT MANAGEMENT AND QUALITY ASSURANCE PLAN, UNIVERSITY NANOSAT PROGRAM

12. Fracture Control General Considerations
All Fracture Critical Component Must Have:
Definition of environments, load spectra history, and stress analysis results.
Detailed design and assembly drawings.
Mechanical and fracture properties of materials in the appropriate environments.
* Assumed that pressure vessels and composites will be avoided, if need be documentation exists on these safety concerns

13. Fracture Control Non-Fracture Critical
Low Release Mass
Satisfy one
Component or released mass may not exceed .25 lb
Total mass in pounds supported by the part is no more than 14/h, where h is the part’s travel distance in feet to the aft bulkhead of the Space Shuttle cargo bay.
It can be shown that release of part will not cause catastrophic damage to the shuttle or payload
Low fracture toughness and tension loaded parts
May release fragments at high velocity as a result of failure
The total released mass may not exceed 0.03 pounds (14 grams).
A part shall be considered to have low fracture toughness when its material property ratio KIc/Fty < 0.33 in.1/2 (1.66 mm1/2)

14. Fracture Control Non-Fracture Critical
Contained Parts
Released pieces of failed component that violate the low mass requirement must be completely contained in the payload and not capable of causing a catastrophic hazard to the Space Shuttle as a result of subsequent damage to the payload in which it was installed.
Must be a Fail-safe Part

15. Material Usage Corrosion Resistance
Corrosion resistance rated as ‘A’ or ‘B’
‘A’ requires no coating
‘B’ coating required
Ratings found in MSFC-HDBK-527
Anodizing will be used for all Aluminum (Rated ‘B’)
Other metal components coated as required

16. Material Usage Outgassing
Components must be low-Outgassing
CVCM < .1%
TML < 1%
Material out-gassing data from GSFC database:
Outgassing materials will be coated
CVCM: Collectable Volatile Condensable Material
TML: Total Mass Loss

17. Material Usage Fasteners
Source for fastener selection and use – NASA, GSFS 541-PG
All safe life (STS) or single point failure (non-STS) flight hardware bolts shall be procured from GSFC-approved manufacturers
All flight hardware fasteners will be screened per the requirements of this document
All safe life or single point failure flight hardware nuts and bolts must be size #10 (5mm) or larger.
Manufacture material test reports are required for all STS and non-STS components.
Traceability including vendor, manufacturer, lot number, and screening level are also required.
Specialty fasteners will be handled as designated in section 8.6 of said document.
*** Safe life (STS) or single point failure (non-STS) – An approach where the failure of one element could create a hazard. NASA-STD-5003 and GSFC require that the largest undetected flaw that could exist in a safe life part will not grow to failure when subjected to the cyclic and sustained loads and environments encountered in four mission lifetimes.

18. References -
United States. National Aeronautics and Space Administration. Payload Verification Requirements: Space Shuttle Program, NSTS 14046, Revision E. Lyndon B. Johnson Space Center. Houston, TX.. March 2000.
United States. National Aeronautics and Space Administration. Fracture Control Requirements For Payloads Using The Space Shuttle, NASA-STD-5003, NASA Technical Standard. Lyndon B. Johnson Space Center. Houston, TX.. October 7, 1996.
United States. USAF, Research Laboratory: Space Vehicles Directorate. AFRL Internal Cargo Unit, User’s Guide. University Nanosat-3 Program. Kirtland AFB, NM. March 2003.
Additional references can be found at