The NORAD S-Series shelter is a rectangle. The ceiling ranges from 8 ft. to 10 ft. high. At the ceiling’s lowest point a 6 ft. tall person has 2 ft. of headroom. At the ceiling’s tallest point, a 6 ft. tall person has 4 ft. of headroom. NORAD shelter walls are straight up and down, not curved. Round shapes are not efficient to support people unless the walls are greater than 10 ft. in diameter. A 10 ft. diameter cylinder used for a shelter is 60% useful due to its shape. A 12 ft. diameter shape is more than 80% useful for people operating inside the shelter. NORAD rectangle shaped shelters allow for 100% useful space.