In Boeing's OFT-2 mission, an Atlas V is the launch vehicle. The solid rocket booster burns out at T+ 1:35 minutes, but the boosters are jettisoned at T+ 2:40 minutes. Why are the boosters not jettisoned immediately after burnout?
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$\begingroup$ Related, and possibly a duplicate: Why is there a "coasting" phase in some space launches? $\endgroup$– David HammenCommented May 20, 2022 at 6:06
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2$\begingroup$ I don't think the advantages of a coast phase are applicable here. Just guessing, so not an answer: The Atlas V has a one minute delay between booster engine out to jettison to ensure that the boosters are indeed completely out of propellant. Suppose the engine out signal was erroneous; that some unburnt propellant remained inside. A premature jettison could shake things up and result in re-ignition. That might well result in a catastrophic collision between a rapidly accelerating unattached and uncontrolled booster with the launch vehicle. I suspect the delay was a safety issue. $\endgroup$– David HammenCommented May 20, 2022 at 6:16
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1$\begingroup$ @DavidHammen This is not during coast phase. Engines are burning throughout that 1 minute. $\endgroup$– AshvinCommented May 20, 2022 at 7:54
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$\begingroup$ I realize that the Atlas booster operates throughout that minute. My guess, which was incorrect, was that the delay was to avoid recontact with a re-ignited SRB. The delay, as properly identified in @TrySCE2AUX's answer, is to avoid inducing extra vibrations during a period when vibrations are already high. SRB shutdown happens shortly after maxQ, so it's best to wait a bit. $\endgroup$– David HammenCommented May 20, 2022 at 11:50
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$\begingroup$ The coast phase that occurs after MECO, with the main engine still attached for three minutes after MECO, does apply. But you didn't ask that question. The video claims that the vehicle is out of the atmosphere at that point. It is not. It is out of the thick parts of the atmosphere. The atmosphere extends for several hundred kilometers above orbit insertion altitude. The atmosphere is very thin at orbit insertion altitude, but it is still there. $\endgroup$– David HammenCommented May 20, 2022 at 12:12
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1 Answer
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The solid rocket boosters are left attached because they don't want to separate them during the phase of highest dynamic loading of the vehicle. The risk of something going wrong during this critical maneuver is just lower when the forces are lower.
It's only a few Words in the Atlas V Launch Services User's Guide on page 33:
The SRB jettison sequence is initiated after SRB burnout. SRBs 1 and 2, if applicable, are jettisoned at a predetermined time dependent upon the dynamic pressure constraint. SRBs 3, 4, and 5 are jettisoned 1.5 seconds later, if applicable.
"dependent upon a dynamic pressure constraint" are the magic words here.
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6$\begingroup$ I replaced the screenshot of the relevant text with the relevant text itself. Otherwise, the answer is unchanged. $\endgroup$ Commented May 20, 2022 at 12:14