Holy Moly! Not to wish anything like this for Saturday’s launch, but I came across this video of the STS-68 launch attempt in 1994 that was aborted at the very last second. Watching it is enough to make your heart stop. Everyone involved must have experienced a tremendous rush followed by extreme let-down! The main engines had lit, but were shut down 1.9 seconds before liftoff when on-board computers detected higher than acceptable readings in a sensor monitoring the discharge temperature of the high pressure oxidizer turbopump in main engine #3. In the history of the shuttle program, five launch attempts were aborted under five seconds from the planned launch. STS-68 came the closest to hauling the mail before being aborted.
The main engines light at six seconds before launch, and within that time, the on board computer decides from hundreds of inputs and constraints if it’s safe for the three liquid fueled engines to keep burning and whether to light the two solid rocket boosters. Once the solids light, there’s no turning them off.
In STS-68’s case, the computers noted the anomaly and shut everything down in about four seconds of time.
Endeavour had to be brought back to the Vehicle Assembly Building to be outfitted with three replacement engines. A test firing of the original engines confirmed that a slight drift in a fuel flow meter in the engine caused a slight increase in the turbopump’s temperature. The test firing also confirmed a slightly slower start for main engine #3 during the pad abort, which could have contributed to the higher temperatures.
The other launches that aborted under five seconds were STS-41 D in June of 1984 which went down to 4 seconds before launch before calling it quits, and these three were stopped at 3 seconds before launch: STS-51 F in July, 1985; STS-55 in March 1993, STS-51 in August 1993.
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Here’s a link to a video with a compilation of all five launch aborts, (and includes STS-93 which aborted with 7 seconds to go) and here’s a link to an article that describes what a crew would have to do if they were forced to bail out of the shuttle before launch because of an expected explosion. Yowza!