Spacecraft Designer is meant to create spacecraft for the Explorers repository. https://github.com/VallesMarinerisExplorer/Explorers/tree/main
This is a workbench for FreeCAD. To use it, first install FreeCAD for your system. This tool was developed with Windows so Linux and Mac users might need to adjust filepaths etc.
FreeCAD will likely create a directory for you:C:\Users\USERNAME\AppData\Roaming\FreeCAD\Mod
In the Mod directory, unzip this folder as C:\Users\USERNAME\AppData\Roaming\FreeCAD\Mod\SpacecraftDesigner. You should then be able to open FreeCAD and see the workbench show up. Create an issue if you have an issue.
This workbench is by no means complete, so if you see errors or have questions feel free to share your opinion and thoughts!
Here is a link to the documentation: https://docs.google.com/document/d/17-0f0Di_PyV1-UBev-qz00vglF6IMoDuzYaq7braegs/edit?usp=sharing