RSLS aborts occur after the engine start command given at T minus 6.6 seconds but before the redundant frangible nuts and SRB ignition firing chain is triggered at T Zero. The twang motion continues on for quite some time follow the conclusion of a Flight Readiness Firing(FRF) and/or a Redundant Set Launcher Sequencer(RSLS) Aborts. Twang motion at the External Tank top end, where the Gaseous Oxygen beanie cap resides before launch. As it turns out, the timing of those events did not go quite as expected or planned for STS-1. The other day I just happened to have a discussion over lunch about thrust buildup with some of the folks involved in evaluating/quantifying the “Twang” pre and post STS-1. "While it is true that keeping the engine bells from colliding is an important consideration, the “start position” (as it was explained to me) is really about loads management of the entire stack (i.e. They might bang together if they were in the flight position." "They are spaced apart to allow for motion during the start transient. Ī quote from JIM discussing flight and start SSME positions. The SRB joints elusively allow for the "twang" movement as the SRB nozzles are bolted hard-down to the MLP. An effect was a reduced "twang" effect which at the same time reduced that amount of movement in the SRB joints. And while having engine 2 and 3's nozzles splayed further apart for the "start" position was perhaps a small part of the reason, primarily the start position was to control thrust vector in an attempt to reduce of center loading of the stack. Way upthread, I was making the same discussion about SSME in Position 2 and 3(left and right engine positions).
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