Rarely mentioned, but I think the foam is polyisocyanurate. I think it changes to the rust colour due to exposure (it's typically light yellow). Anyone got more

Rarely mentioned, but I think the foam is polyisocyanurate. I think it changes to the rust colour due to exposure (it's typically light yellow). Anyone got more info? https://ntrs.nasa.gov/citations/19890003374 https://ntrs.nasa.gov/citations/19890003374