Spacecraft Structures Basics

Introduction

Structural analysis is one of the oldest engineering professions. The math was created to answer questions like, “Will the house support the roof without it falling apart?” and “Can a wheel support the wagon without the wheel cracking?”. This is also one of the most visible engineering fields where the public will know about your success and your failure. For instance, almost all rocket explosions have come down to some sort of structural failure and you know people love watching that stuff.

A Bit More Detail

Source: NASA's DART spacecraft structure before everything is installed

One of the most often ways spacecraft have failed over the years is from something breaking, snapping off, getting crushed, or not opening like it’s supposed to. The analysis methods have gotten a lot better now, but there were whole decades where many structures didn’t do what they should have and the mission had to be ended. Even in 2021, there have been failures that made the news like solar panels not opening on the ISS. This also happened to one of the solar panels on NASA’s Lucy mission.

A lot of what structure engineers use for satellites has been taken from civil engineers who build walls and bridges and buildings. These types of analysis have to do with “static” loads, which means fairly constant loads like gravity and slowly changing temperatures. A lot of the equations for static analysis can be done by hand or with simple computer models.

Dynamic analysis makes sure structures are not shaken to pieces during a loud, chaotic launch or any kind of action-packed event on orbit. A lot of the equations for this type of analysis come from electrical and audio engineers, specifically from Bell Labs in the mid-1900s. Doing this work usually requires computer models to be accurate.

There are lots of other interesting things structural analysts work on including mechanisms, materials, and optics. Mechanism analyses are things like switches and latches and motors and anything else that moves on a spacecraft. A satellite worth hundreds of millions of dollars that took years to build is often successful or fails completely because of how one mechanism does. It’s a bad day when you hear a “clunk” of something falling off when they simulate a rocket launch!

Engineers

People who specialize in structures are called “Structural Analysts”. It used to be that some of them would specialize in static or dynamic analysis, but a lot of companies have combined them now so that you do everything.

They spend a lot of their time making math models in computer software. An example of a model being used to test a collision is shown below. Building a good model you can trust is a bit of science and a bit of art. It’s almost like playing a video game and knowing when and where to click, but you do need to understand the strategy of analysis since you sometimes have to correct the computer.

Many people are able to be outstanding structural analysts with a bachelor’s degree. Usually, it’s in aerospace or mechanical or civil engineering, but it doesn’t have to be. People who want to continue to specialize in this field often have a Master’s degree, usually in something like dynamics or materials. However, while there are Ph.D. degrees, they tend to be fairly rare and those folks tend to focus on improving the tools or solving unique and rare problems.

Materials

Source: NASA flies material experiments outside the space station to see which ones survive the extreme environments

Structural analysts are also material experts. They need to know which materials can take the loads, which ones won’t melt in the heat of the sun, and which ones need special treatment. Here are a few points:

• Almost all satellites are made of some sort of aluminum. It ticks the three boxes you care about: it’s strong enough, stiff enough, and light enough. It’s either machined out of a plate or people use something called “Honeycomb” panels.
  
  
• Carbon fiber is used too, although only when you really need to save weight or you care a lot about temperature changes warping your structure. Most people try to avoid it since it takes more time and money and more things can go wrong with it.

Curated Videos

1. https://www.youtube.com/watch?v=3DHYf_jW60k
   The US Air Force put together a series of videos on different space topics for their cadets. The one on structures covers all the basics quickly and then works a few example problems.
   
   
2. https://www.youtube.com/watch?v=9FSZv4rTYus
   The lecturer talks for a solid 90 minutes about the different types of structural topics. Interestingly, they cover many things that have failed on expensive spacecraft over the years and the lessons learned. Worth a watch if you’d like to learn more about what to watch out for.
   
   
3. https://www.youtube.com/watch?v=FNjqOPhdFos
   This is the first in a series of fast lectures on aerospace structure topics. You can zip through the different videos fairly quickly to pick up a flavor of the topics being discussed.

Curated Links

1. https://en.wikipedia.org/wiki/Roark%27s_Formulas_for_Stress_and_Strain
   This is the Wikipedia link that talks about one of the most famous books structural analysts use. It’s packed with tables and equations you can do hand analyses of your structures with. A lot of the best analysts do this before building models so they know if they’re getting good model results or not.
   
   
2. https://llis.nasa.gov/lesson/819
   NASA has written a “how to” of structural analysis including steps and reasons why you do things. It’s a lot of text with no pictures, so may not be for everyone trying to learn. It’s great for people looking for something more like a handbook.
   
   
3. https://pressbooks-dev.oer.hawaii.edu/epet302/chapter/4-6-structural-analysis/
   The whole online textbook is a great reference and highly recommended. The chapter on structural analysis covers many of the same topics as the NASA link above and includes images.