The launch was the first to use a newly developed high-performance motor for the solid rocket boosters, which gave approximately 7% greater thrust. Post-flight analysis later showed there was nearly a burn-through of the rocket casing, a significant problem that later doomed the 51-L mission (see "Post-flight safety analysis" section below for more). This launch was also the second-to-last to use the original standard-mass steel casings for the boosters. These had been replaced by a thinner case, saving some , on STS-6 and STS-7, but because of safety concerns the next two flights used the conventional cases.