Temple Assembly Site
Video needed.
Discovered by Frozen Mr. Toastbrot.
I found out, that you can get the Missile Expansion in Temple Assembly Site before the main Missile Launcher (right after loosing your items). The trick is to jump onto the small pillar that is the most left when you enter the room from Collapsed Tunnel. It is quite tricky to get onto the pillar with a single jump, but it is still possible. You need to use the small slope on the ledge in front of the pillar (I hope you'll understand what I wrote) to get a ghetto effect. It also seems to help if you start the jump a bit right of the crack in the slope (the crack is next to the wall, see this sketch). To get onto the pillar, you need to hit it's broken edge and use it to push yourself up onto the pillar ((see another sketch). Once on the pillar, jump onto the platform left of it and proceed to get the Missile Expansion.