of reaching an emergency runway for various SSME failure scenarios. Ejection escape systems An ejection escape system, sometimes called a "launch escape system", had been discussed many times for the shuttle. After the Challenger and Columbia losses, great interest was expressed in this. All previous and subsequent U.S. crewed space vehicles have launch escape systems, although none have ever been used for an American crewed flight. Ejection seat The first two shuttles, Enterprise and Columbia, were built with ejection seats. These two vehicles were intended to be part of the shuttle test program and would fly with a crew of two test pilots or astronauts. Subsequent shuttles Challenger, Discovery, Atlantis, and Endeavour were built for operational missions with a crew of more than two, including seats in the lower deck, and ejection seat options were deemed to be infeasible. The type used on the first two shuttles were modified versions of the Lockheed SR-71 seat. The approach and landing tests flown by Enterprise had these as an escape option, and the first four flights of Columbia had this as a crew abort option as well. With STS-5 marking the end of Columbia's test flight program, and as an operational mission with four crew members, the two cockpit ejection seats had their rocket motors removed for the flight. Columbias next flight (STS-9) was likewise flown with the seats disabled in this manner. By the time Columbia flew again (STS-61-C, launched on January 12, 1986), it had been through a full maintenance overhaul at Palmdale and the ejection seats (along with the explosive hatches) had been fully removed. Ejection seats were not further developed for the shuttle for several reasons: Very difficult to eject seven crew members when three or four were on the middeck (roughly the center of the forward fuselage), surrounded by substantial vehicle structure. Limited ejection envelope. Ejection seats only work up to about and . That constituted a very limited portion of the shuttle's operating envelope, about the first 100 seconds of the 510 seconds powered ascent. No help during a Columbia-type reentry accident. Ejecting during an atmospheric reentry accident would have been fatal because of the high temperatures and wind blast at high Mach speeds. Astronauts were skeptical of the ejection seats' usefulness. STS-1 pilot Robert Crippen stated: The Soviet shuttle Buran was planned to be fitted with the crew emergency escape system, which would have included