Unmanned carrier rocket launches also undergo TCDTs, when countdown procedures are followed.
2.
Launch day countdown procedures went without any major problems; however, " Atlantis " encountered two minor issues.
3.
To maintain proficiency, launch controllers rehearsed countdown procedures four times during the summer's shuttle flight hiatus, but without the crew.
4.
Launch of Apollo 1 was planned for February 21, 1967, when the crew entered the spacecraft on January 27, mounted atop its Saturn IB booster on Launch Pad 34 at Cape Kennedy, for a " plugs-out " test of the spacecraft, which included a rehearsal of the launch countdown procedure.
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