The appropriate design and test factors for a given mechanical or structural flight hardware element depend on several parameters, such as the materials used, attachment methods (e.g., bonding), and the verification approach (prototype or protoflight). In addition to the minimum factors of safety specified in this NASA Technical Standard, some structural and mechanical members may be required to meet other more stringent and restrictive performance requirements, such as dimensional stability, pointing accuracy, stiffness/frequency constraints, or safety requirements (e.g., fracture control).
4.1.1 Prototype versus Protoflight Approaches
The standard accepted practice for verification of launch vehicles and human-rated spaceflight hardware is the prototype approach in which a separate, dedicated test structure, identical to the
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flight structure, is tested to ultimate loads to demonstrate that the design meets both yield and ultimate factor-of-safety requirements.
An acceptable alternative for verification of spacecraft and science payloads is the protoflight approach, wherein the flight structure is tested to levels above limit load but below yield strength to verify workmanship and demonstrate structural integrity of the flight hardware.
The protoflight verification approach has the advantage that a dedicated test unit is not required, because qualification testing can be performed on the flight hardware. However, a protoflight verification approach does require that margin over flight limit loads be demonstrated by test; therefore, higher yield design factors of safety are required to prevent damage to the flight structure. Under a protoflight verification approach, yield and ultimate modes of failure or structural margins are not directly verified by test.
[FSR 6] A protoflight test shall be followed by inspection and functionality assessment.
Consideration should be given to development testing prior to committing to major test article configurations and especially prior to committing the flight article to protoflight test.
4.1.2 Test Verification Criteria
4.1.2.1 Test Methods
Strength verification tests fall into three basic categories: (1) tests to verify strength of the design (qualification); (2) tests to verify strength models; and (3) tests to screen for workmanship and material defects in the flight articles (acceptance or proof).
Strength verification tests are normally static load tests covering critical load conditions in the three orthogonal axes and, generally, can be classified as prototype or protoflight (see section 4.1.1).
In some cases, alternative test approaches (centrifuge, below resonance sine burst, saw tooth shock, etc.) are more effective in reproducing the critical load or environmental conditions and may be used in lieu of static testing if it can be demonstrated that the resulting loads in the test article are equivalent to or larger than the limit loads multiplied by the test factor.
- a. [FSR 7] The strength verification program shall be approved by the delegated Technical Authority.
- b. [FSR 8] The magnitude of the static test loads shall be equivalent to limit loads multiplied by the qualification, acceptance, or proof test factor.
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- c. [FSR 9] Strength model verification, if required, shall be accomplished over the entire load range.
Strength model verification is normally performed as part of the strength verification testing.
Verification of the strength model over the entire load range is especially important if the response of the test article is expected to be nonlinear.
Strength model verification may not be required if the load path is easily determined and straightforward and the flight loads are well characterized.
- d. [FSR 10] The test article shall be instrumented to provide sufficient test data for correlation with the strength model.
- e. [FSR 11] Each habitable module, propellant tank, and SRM case shall be proof pressure tested.
- f. [FSR 12] Departures from test plans and procedures, including failures that occur during testing or are uncovered as part of post-test inspection, shall be documented by a non-conformance report per the approved quality assurance plan.
4.1.2.2 Test versus Design Factors of Safety
When using the prototype structural verification approach, the minimum ultimate design factors are the same as the required qualification test factors for both metallic and composite/bonded structures, except in the case of discontinuity areas of composite/bonded structures used in safety critical applications.
- a. [FSR 13] When using the prototype structural verification approach, metallic structures shall be verified to have no detrimental yielding at yield design load before testing to full qualification load levels.
- b. [FSR 14] When using the protoflight structural verification approach, design factors shall be specified to prevent detrimental yielding of the metallic structure or damage to the composite/bonded flight structure during test.
4.1.2.3 Test versus No-Test Options
Structural designs generally should be verified by analysis and by either prototype or protoflight strength testing. For metallic structures only, it may be permissible to verify structural integrity by analysis alone without strength testing.
- a. [FSR 15] Analysis shall be provided with an acceptable engineering rationale for the "no-test" option.
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- b. [FSR 16] To use the “no-test” approach, project-specific criteria and rationale shall be developed for review and approval by the delegated Technical Authority.
Projects that propose to use the "no-test" approach generally have to use larger factors of safety than for tested hardware.
Increasing the design factors of safety does not by itself justify a "no-test" approach. Some examples of criteria on which to base such an approach are as follows:
- The structural design is simple (e.g., statically determinate) with easily determined load paths; the design has been thoroughly analyzed for all critical load conditions; and there is a high confidence in the magnitude of all significant loading events.
- The structure is similar in overall configuration, design detail, build quality, and critical load conditions to a previous structure that was successfully test verified, with good correlation of test results to analytical predictions, and for which the same level of process control has been maintained.
- Development and/or component tests have been successfully completed on critical, difficult-to-analyze elements of the structure, and correlation of the analytical model to test results has been demonstrated.
4.1.3 Probabilistic Methods
Current standard NASA structural verification criteria are deterministic, and experience has shown these deterministic criteria to be adequate. The probabilistic method uses knowledge (or assumptions) of the statistical variability of the design variables to select design criteria for achieving an overall success confidence level.
[FSR 17] Any proposed use of probabilistic criteria to supplement or as an alternative to deterministic factors of safety shall be approved by the delegated Technical Authority on an individual-case basis.