STS-51 was notable for having been scrubbed three times on the launchpad, each time after the crew had boarded the spacecraft: * On 17 July 1993, the launch was scrubbed on the pad due to a flaw in the pyrotechnic initiator controller that triggers the release of the solid rocket boosters from the mobile launcher platform. * On 24 July 1993, problems with a hydraulic power unit in one of the solid rocket boosters caused another scrub on the pad. Because of the Perseid meteor shower, the next launch window did not open until the second week of August. * On 12 August 1993, the count reached the T−3 second mark, at which point the Space Shuttle Main Engine (SSMEs) had ignited. A shutdown was then triggered by faulty fuel flow sensors in one of the SSMEs. * STS-51 successfully la