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They hope to be able to do a number of dress rehearsal countdowns prior to actual launch, which is what makes this schedule somewhat tentative.
While SpaceX has ample experience lighting all nine engines of the Falcon 9 simultaneously, with every Falcon 9 going through a full duration hot fire at McGregor followed by a static fire on the launch pad before all nine engines are lit a third time for launch, no company in the U.S. rocket industry has experience lighting 27 engines at the same time.
While each of the three Falcon Heavy debut cores – two flight-proven cores for the side-mounted boosters (boosters B1023 and B1025) and a new core for the central core (booster B1033) – have undergone hot fire testing at McGregor, they were all fired separately because the Texas test site is not built to accommodate three cores at the same time.
This means SpaceX will not gain a full understanding of how all 27 engines light until the more-crucial-than-usual static fire at LC-39A at Kennedy.