Early flights of the Space Shuttle, which used "Columbia", were with a crew of two, both provided with ejector seats, (STS-1 to STS-4) but the seats were disabled and then removed as the crew size was increased. "Columbia" and "Enterprise" were the only two Space Shuttle orbiters fitted with ejection seats. The Buran-class orbiters were planned to be fitted with K-36RB (K-36M-11F35) seats, but as the program was canceled, the seats were never used.