• derivation of detailed Finite Element models;
• static and dynamic analysis of payloads and racks;
• micro-vibration analysis;
• vibration test predictions;
• post processing of launcher coupled dynamic analysis results;

The Structural Verification of ISS (International Space Station) Payloads is an important task needed to ensure the safety and the performance of the payload hardware during all ground and flight events to which it is subjected. It is a rather long process, especially if the payload is launched on board the shuttle NSTS (National Space Transportation System).

The structural design of a payload starts with the production, by the Payload Developer, of a Structural Verification Plan (SVP) that includes the definition of the different structural parts, and interfaces, the identification of the safety critical structural elements, the preliminary design loads to be considered for the design, the verification approach, the chosen model philosophy (proto-flight or prototype) and all the testing the structural hardware will undertake.

In line with the SVP, the Payload Developer develops a payload structural design based on existing preliminary (design limit) loads and initial mathematical models structurally representative of the payload. The design limit loads come from the launcher and ISS generic requirements and, since are based on former missions experience and measurements, are supposed to envelope the actual flight ones. This, as described below, must be verified prior to the flight of the payload. The loads affecting the payload design are of many types. They are listed below.

• Assembly and Installation
• Testing
• Ground Handling & Transportation
• STS Flight, lift-off, ascent, descent, re-entry, landing & emergency landing (inertial, transients, acoustics, random, constraints)
• ISS On orbit (ISS boost, vehicles docking)
• Crew applied (IVA & EVA)
• Pressure, related to pressurized systems and to pressurization-depressurization of the STS cargo bay and of the ISS elements
• Thermal

Once the structural design is accomplished all efforts are focussed on the payload verification.

The structural verification of STS and ISS payloads is performed by a combination of analyses and tests to demonstrate positive margins of safety of all safety critical structural elements. In most cases, the tests are performed to validate the initial mathematical models used in the analytical verification. This means that, for most cases, the structural verification is accomplished only by the results of the analyses performed using validated models. This is particularly true for metallic structures, whilst for composite structures additional tests, besides the ones required to validate the payload models, are needed.

One or more detailed mathematical finite element models of the payload are built and used to support the payload design and the analytical verification. Normally, only one model is built, although if the payload has different configurations for different mission phases (e.g., launch, landing, on-orbit operations), more than one model must be built and validated. The models must be checked for mathematical correctness and, at the final phase of the verification process, they are validated by dynamic (either resonance search of modal survey) and/or static tests of either flight or flight representative hardware. Typical analyses run using the payload models are the following:

• Modal (to check modes and frequencies)
• Static, thermo-elastic (to derive forces, stresses, displacements), stability
• Dynamic (either transient or frequency response, whenever time or frequency consistent results are needed to decrease conservatism)
• Multi-body dynamics for special cases
• Hand or spreadsheet calculations for post-processing and derivation of margins of safety.

As mentioned above, the transient loads used in the structural verification of STS and ISS payloads are evolving during the whole process. We have already described the design limit loads used for the structural design of the payload. Further in the development process of the payload, the payload mathematical models (not necessarily validated yet) are submitted to the launcher authority to perform system level analyses. New transient loads, affecting the payload, are calculated from the system level analysis performed by the launcher authority and called Design Coupled Loads Analysis (DCLA). A comparison between the initial design limit loads and the DCLA loads must be performed and, if the DCLA values exceed the limit loads, the payload design must be checked against the new loads. This check will decide on possible changes of the payload structural design.

Once the payloads models are validated by test and the launcher configuration (including the payload carrier) is known and frozen, the last system level analysis is performed by the launcher authority. This time is named Verification Coupled Loads Analysis (VCLA or VLA). It provides the final transient flight loads that, again, must be compared with the payload design limit loads. This is the last step of the payload verification process and is normally followed, once other mechanical disciplines like fracture control are cleared, by the formal certification of the payload to fly on board the STS and being operated on board the ISS.
 
 
The whole payload verification process is summarized in the figure below.