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Design limit loads and Structural design process

Design limit loads and Structural design process

Design limit loads and Structural design process


I have a question about how strength should be assessed in satellites structures and their payloads. I have seen different ways of doing the task, and it is not clear to me what is right and what is wrong. Some people calculate stresses directly from dynamic loads caused by random and sinusoidal vibrations using frequency response analyses, whereas others calculate stresses only from static analysis with limit load factors (obtained, for example, from a Mass acceleration curve). Which methodology is more adequate? Are the "design limit loads" always defined as equivalent static load factors and do they include the dynamic effects, covering sine, random, etc.?

I would also like to ask another question: How is the structural design process carried out from the structural analysis point of view? Would the following sequence be valid?:
1. Creation of a structural (static) math model and preliminary sizing and strength assessment using generic limit loads from the launcher authority or given requirements.
2. Creation of a dynamic math model.
3. Correlation of the dynamic math model with test results. (Is a modal survey or a low level sine sweep enough for this purpose?)
4. Reduction of the correlated dynamic math model to be provided for the CLA.
5. Strength analysis of the static model using the updated limit loads obtained from the CLA (how are those load factors obtained?)
6. Base-driven frequency response analysis of the correlated dynamic model using transfer functions from the CLA to get responses in several points of the model and define limit loads for subsystems or payloads. (Should this step happen before?)

Thanks a lot!

RE: Design limit loads and Structural design process

I'm surprised no one has responded to your subject but this Spacecraft Engineering forum has few visitors.
I've been retired from aerospace for almost 15 years. Working primarily as a stress analysts in the military
missile "racket" and various military and NASA manned systems over 40+ years, the criteria for establishing design
limit loads, that I'm familiar with is the latter you wrote: "Are the "design limit loads" always defined as
equivalent static load factors and do they include the dynamic effects, covering sine, random, etc.?"

Suggest you try the aircraft forums for quicker responses. Do you have a loads department? Most big operations do.
Oh, ask NASA or the USAF about structural design criteria for satellites. Back in Star War days, I worked on a "Hit-to-Kill" test
vehicle were I used the criteria above... nearest thing to a satellite that I worked.

Smooth Sailing,
G-pa Dave

RE: Design limit loads and Structural design process

Dear GrandpaDave,

Thank you very much for your answer.

It sounds like you had a very nice professional career. Hopefully one day I will be able to say the same! For the moment, though, I am just a beginner. :)

So static load factors... I suspected that answer, but it is something I cannot fully understand. Is it acceptable to assume that the stress distribution and deformed shaped caused by dynamic loads are equivalent to those caused by static cases?

Kind regards.

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