The sub-assembly system does not 'weld' your parts together. They are still all there separate part. Meaning that if you did not strut them, it will most likely wiggle and wobble. The sub-assembly system only saves parts, it doesn't make them one giant part. There is a plugin that will weld your parts together to make one part. It saves ram and cpu usage, but it has not been worked on since .21 I believe. Don't quote me on that. I haven't seen it since .21 so I'm assuming it's not been worked on. I could have just missed it. Anyway, if this is just a question and not an add-on development thread such as a WIP, Development Thread, or Planning Thread, this should not be posted in this part of the forums. It belongs in General KSP: Gameplay Questions and Tutorials forum, or the Development and Support: Support and Bugs forum. If this is just a question, you need to ask the mods to move it. If it is a legitimate development thread for a plugin, then you need to edit the first post and the title to say that.