- a. [FSR 18] The design factors of safety and test factors of this NASA Technical Standard are the minimum required values for NASA spaceflight structures and shall be applied to the limit stress condition, including additive thermal or pressure stresses.
- b. [FSR 19] If pressure or temperature has a relieving or stabilizing effect on the mode of failure, then for analysis or test of that failure mode, the unfactored stresses induced by temperature or the minimum expected pressure shall be used in conjunction with the factored stresses from all other loads.
Calculation of the portion of thermal stress or load which acts to relieve or stabilize stresses due to other applied loads is dependent on assumptions regarding boundary conditions and
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constraints between structural elements. In cases where a thermal stress or load acts to relieve or stabilize a failure mode, a conservative estimate (i.e., minimum value) of the portion of the thermal stress or load providing relief or stability should be used so as to not overestimate the beneficial effect of temperature.
- c. [FSR 20] Material selection and derivation of material design allowables shall follow the requirements defined in NASA-STD-6016, Standard Materials and Processes Requirements for Spacecraft.
Material allowables are to be chosen to minimize the probability of structural failure due to material variability. Considerations when specifying material design allowables include accounting for degradation of material properties under service environments and performance of sufficient material tests to establish values with an appropriate statistical basis.
- d. [FSR 21] The factored stresses shall not exceed material allowable stresses (yield and ultimate) under the expected temperature, pressure, and other operating conditions.
- e. [FSR 22] The hardware shall be designed to preclude any detrimental yielding under limit loads and, where applicable, under protoflight or proof test loads.
- f. [FSR 23] Applications of design and test factors to the development and verification of a structure shall be accepted by the delegated Technical Authority only when all the constraints and preconditions specified in section 1.4 are met.
If the proof test is to be used for fracture control flaw screening, higher factors than those listed here may be required for proof testing.
Factors of safety on yield are not specified for composite/bonded structures, glass, and bonds for structural glass.
4.2.1 Metallic Structures
Spaceflight metallic structures can be developed using either the prototype or the protoflight approach.
- a. [FSR 24] The minimum design and test factors of safety for metallic structures shall be as specified in Table 1, Minimum Design and Test Factors for Metallic Structures.
- b. [FSR 25] Workmanship verification shall be performed for the first flight build and follow-on structures under a prototype test approach and for follow-on structures under a protoflight test approach.
- c. [FSR 26] The workmanship verification program shall be approved by the delegated Technical Authority.
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Workmanship may be demonstrated by static or dynamic testing. Dynamic tests may be sinusoidal vibration, random vibration, or acoustic. The loads or responses generated in a workmanship test should be sufficient to verify the structural integrity of the flight hardware.
Table 1—Minimum Design and Test Factors for Metallic Structures
- Structure has to be assessed to prevent detrimental yielding during its design service life, acceptance, or proof testing. ** Propellant tanks and SRM cases only.
The strength qualification requirements under a protoflight verification approach may be satisfied with a dedicated flight-like qualification unit developed for strength testing but not intended to be used as a flight unit. For this type of strength qualification program, workmanship verification is required for the first flight build as specified in 4.2.1b.
| Ultimate Design Factor | Yield Design Factor | Qualification Test Factor | Proof Test Factor | |
|---|---|---|---|---|
| Prototype | 1.4 | 1.0 | 1.4 | N/A or 1.05 |
| Protoflight | 1.4 | 1.25 | 1.2 | N/A or 1.05 |
4.2.2 Threaded Fastening Systems
- a. [FSR 27] Threaded fastening systems shall be designed and analyzed per the requirements specified in NASA-STD-5020, Requirements for Threaded Fastening Systems in Spaceflight Hardware.
- b. [FSR 28] The minimum design and test factors of safety for metallic fasteners and internally threaded parts shall be as specified in Table 1.
The minimum design and test factors of safety for other components of a threaded fastening system not specified in 4.2.2b such as non-metallic fasteners, clamped parts or washers are covered by the appropriate section of this NASA Technical Standard.
4.2.3 Composite/Bonded Structures
[FSR 29] Composite/bonded structures, including bonded sandwich structures and bonded inserts but excluding glass, developed for NASA spaceflight missions, shall, as a minimum, use the design and test factors specified in Table 2, Minimum Design and Test Factors for Composite/Bonded Structures.
Metallic honeycomb (facesheets and core) is considered to be metallic structure.
For composite/bonded structures, each flight article has to be proof tested unless the requirements of section 4.6 are met.
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Table 2—Minimum Design and Test Factors for Composite/Bonded Structures
- Factor applies to concentrated stresses. For nonsafety-critical applications, this factor may be reduced to 1.4 for prototype structures and 1.5 for protoflight structures.
4.2.4 Glass/Ceramics
Because of their brittle nature and susceptibility to moisture-assisted crack growth, glass and ceramic structures pose a special challenge for design and analysis. The strength of glass and ceramics found in literature can be misleading and is almost never applicable to NASA applications. Knowledge of fracture toughness, crack growth characteristics, and environments are required to fully understand the ability of the structure to withstand a given stress for the required time. Fracture toughness and crack growth rates are basic material properties of glass and ceramics that have to be used in conjunction with environmental exposure to determine the adequacy of the structure. The environmental exposure has four components:
- • Stress.
- • Time.
- • Moisture.
- • Flaws/cracks.
Glass and ceramic structures are always imperfect and contain flaws either inherent in the material or created from manufacturing or use. Therefore, for glass or ceramic structures such as windows that have to carry pressure loads for extended times, fracture mechanics principles are required to assess strength.
| Geometry of Structure | Ultimate Design Factor | Qualification Test Factor | Proof Test Factor | |
|---|---|---|---|---|
| Prototype | Discontinuity Area | 2.0 | 1.4 | 1.05 |
| Prototype | Uniform Material | 1.4 | 1.4 | 1.05 |
| Protoflight | Discontinuity Area | 2.0 | 1.2 | 1.2 |
| Protoflight | Uniform Material | 1.5 | 1.2 | 1.2 |
4.2.4.1 Glass/Ceramics in Human-Rated Spaceflight Applications
[FSR 30] The design and verification of glass and ceramic structures for human-rated spaceflight application shall follow the requirements defined in NASA-STD-5018, Strength Design and Verification Criteria for Glass, Ceramics, and Windows in Human Space Flight Applications.
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4.2.4.2 Glass/Ceramics in Robotic Spaceflight Applications
[FSR 31] A non-fracture-based strength approach is possible for glass and ceramic structures in non-safety critical robotic spaceflight applications, such as mirrors and lenses used in science instruments, provided conservative strength allowables are used; however, approval by the delegated Technical Authority of the allowables and approach shall be obtained before implementing this option.
These options include, but are not limited to, the following:
- Determining the allowable through a Weibull distribution.
- Proof testing the actual article and using this proof test value as the ultimate strength.
- Using a "low" initial ultimate strength allowable of 1000 pounds per square inch (psi) for glass.
- Using a test verified threshold stress for the particular type of glass/ceramic chosen.
It should be noted that the traditional strength approach does not waive any fracture control requirements and that fracture-critical structures would still require a fracture mechanics assessment. Furthermore, the proof tests specified in this NASA Technical Standard are workmanship tests; fracture mechanics considerations may drive the project to a higher proof test factor.
- a. [FSR 32] The minimum design and test factors for pressurized and nonpressurized glass/ceramics shall be as specified in Table 3, Minimum Design and Test Factors for Glass/Ceramics in Robotic Applications.
Table 4—Minimum Design and Test Factors for Bonds in Glass/Ceramic Structures
4.2.5 Pressurized Structures, Pressure Vessels, Pressurized Components, and Habitable Modules
The design and analysis of pressurized structure is covered by this NASA Technical Standard. All relevant combinations of structural, thermal, and pressure loading are applicable.
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4.2.5.1 Pressure Vessels and Pressurized Components
- a. [FSR 38] Metallic pressure vessels and pressurized components shall be designed, qualified, and accepted per the requirements of ANSI/AIAA S-080, Space Systems – Metallic Pressure Vessels, Pressurized Structures, and Pressurized Components.
- b. [FSR 39] Composite overwrapped pressure vessels (COPVs) shall be designed, qualified, and accepted per the requirements of ANSI/AIAA S-081, Space Systems – Composite Overwrapped Pressure Vessels (COPVs).
- c. [FSR 40] ANSI/AIAA S-080 and S-081 shall be tailored such that MDP is substituted for all references to maximum expected operating pressure (MEOP).
The term maximum design pressure (MDP) can be used for design and testing of pressure vessels and related pressure components. The basic difference between MDP and MEOP is the degree of consideration of potential credible failure within a pressure system and the resultant effects on pressure of the pressure vessel(s) during system operation. MDP is associated with human-rated systems and is based on the worst case combination of two credible system failures. For robotic hardware pressurization due to failure conditions are not included and the terms MDP and MEOP are equivalent.
Habitable modules are not considered "pressure vessels."
Table 3—Minimum Design and Test Factors for Glass/Ceramics in Robotic Applications
Analysis Only*: Loading Condition=Nonpressurized, Ultimate Design Factor=5.0
- Not applicable to ceramic structures.
- b. [FSR 33] Structural integrity of all pressurized glass and ceramics shall be verified by both analysis and proof testing.
- c. [FSR 34] Proof tests of glass and ceramics shall be configured to simulate flight-like boundary conditions and loading.
- d. [FSR 35] For glass proof testing, the total time during unload shall be as short as possible and the whole test performed in an environment designed to minimize unrealistic flaw growth.
For pressurized glass/ceramics and nonpressurized ceramics, each flight article has to be proof tested unless the requirements of section 4.6 are met. For nonpressurized glass, proof testing is required, unless the requirements of section 4.6 are met, or if the unit can demonstrate a positive margin of safety using the analysis only “Ultimate Design Factor,” as defined in Table 3.
- e. [FSR 36] All glass and ceramic bonds shall be proof tested in the bonded flight assembly.
- f. [FSR 37] The design and test factors for structural bonds in glass or ceramics shall be as specified in Table 4, Minimum Design and Test Factors for Bonds in Glass/Ceramic Structures.
| Loading Condition | Ultimate Design Factor | Proof Test Factor | |
|---|---|---|---|
| Test | Nonpressurized | 3.0 | 1.2 |
| Test | Pressurized | 3.0 | 2.0 |
| Analysis Only* | Nonpressurized | 5.0 | |
| Nonpressurized | 1.5 | 1.2 | |
| Pressurized | 3.0 | 2.0 |
4.2.6 Softgood Structures
The design and test requirements for deployable decelerator systems such as parachutes, parafoils, airbags, inflatable heat shields and similar systems are not covered by this NASA Technical Standard.
4.2.6.1
[FSR 44] Static strength of all structural softgoods shall be test verified.
4.2.6.2
[FSR 45] The minimum design and test factors of safety for structural softgoods shall be as specified in Table 6, Minimum Design and Test Factors for Structural Softgoods.
Table 6—Minimum Design and Test Factors for Structural Softgoods
Prototype testing is required for softgoods to demonstrate qualification. Each flight article has to be proof tested, unless the requirements of section 4.6 are met.
| Ultimate Design Factor | Prototype Test Factor | Proof Test Factor | |
|---|---|---|---|
| Loss of Life or Vehicle | 4.0 | 4.0 | 1.2 |
| All Others | 2.0 | 2.0 | 1.2 |
| 1.4 | 1.6 | 1.2 |