Soyuz 7K-ST No.16L, sometimes known as Soyuz T-10a or Soyuz T-10-1, was an unsuccessful Soyuz mission intended to visit the Salyut 7 space station, which was occupied by the Soyuz T-9 crew. However, it never finished its launch countdown; the launch vehicle was destroyed on the launch pad by fire on 26 September 1983. The launch escape system of the Soyuz spacecraft fired six seconds before the launch vehicle exploded, saving the crew. As of 2022, it remains the only time a launch escape system has been fired before launch with a crew aboard. Crew Mission highlights The crew was sitting on the pad awaiting fueling of the Soyuz-U booster to complete prior to liftoff. Approximately 90 seconds before the intended launch, a bad valve caused nitrogen pressurisation gas to enter the RP-1 turbopump of the Blok B strap-on. The pump began spinning up, but with no propellant in it, the speed of rotation quickly exceeded its design limits which caused it to rupture and allow RP-1 to leak out and start a fire which quickly engulfed the base of the launch vehicle. Titov and Strekalov could not see what was happening outside, but they felt unusual vibrations and realized that something was amiss. The launch control team activated the escape system but the control cables had already burned through, and the Soyuz crew could not activate or control the escape system themselves. The backup radio command to fire the LES required 2 independent operators to receive separate commands to do so and each act within 5 seconds, which took several seconds to occur. Then explosive bolts fired to separate the descent module from the service module and the upper launch payload shroud from the lower, the escape system motor fired, dragging the orbital module and descent module, encased within the upper shroud, free of the booster with an acceleration of 14 to 17g (137 to 167 m/s²) for five seconds. According to Titov, "We could feel the booster swaying from side to side. Then there was a sudden vibration and a jerking sensation as the LES activated". Just after the escape tower pulled the descent module away, the booster exploded. Its remains burned on the pad for nearly 20 hours. Four grid fins on the outside of the shroud opened and the descent module separated from the orbital module at an altitude of , dropping free of the shroud.