“In order to access the missile silo, maintenance crews had to pass through "formidable mechanical barriers" in a process that often took up to an hour. First, a weather cover was opened, a combination entered, and the vault door removed. This allowed for retraction of the locking shaft, and operation of the hydraulic controls used to slowly raise the steel and concrete primary door. The crew could then descend a few feet down the cylindrical shaft and enter another combination into the secondary door (B-plug), and retract the locking bolts. Following a preset, timed interval, the large steel B-plug would slowly lower to the level of the upper equipment room. The crews could then climb down into the equipment room surrounding the missile silo, lower their equipment, and begin maintenance tasks. See, “Chapter 5, Maintenance Crew” available from: http://www.nps.gov/mimi/historyculture/upload/The Missile Plains Section II Chapter 5.pdf. In addition: Bruce Ecker’s spherical panoramic image of the Delta-9 LF at Ellsworth AFB shows the personnel access doors open. (In 1991, Ellsworth AFB, SD, was deactivated and Delta-1 LCF, and Delta-9 LF incorporated into the National Park System. At Delta-9, a glass enclosure has been placed over the top of the Minuteman missile in its silo for public viewing). Available from: http://nonplused.org/panos/minuteman/html/delta09.html. Also, an image in the lower equipment room and the retracted B-plug is available from: http://nonplused.org/panos/minuteman/html/delta09_silo4.html.