I saw doors like those at Camarillo, CA also at Sky Blue Air. They are old F-4 alert hangars...or some kind of old fighter jet. They use 55 gallon drums filled with concrete for counterweights. The whole assembly pivots down. It's kinda like the old garage doors that move out then back in when they're closed.
They are very impressive. I'd hate to see what it would take to open one if the counterweight cable broke...
