The oft-glowing 14,000-capacity Hampton Coliseum was known to fans as the Mothership, owing to its resemblance to (and function as) a space vehicle. Outside, in one quadrant of the parking lot, Supercarnival ’81 is raging, a delight for Dead Heads to wander into after the show. But as the true Heads know, the wild times are inside as blastoff occurs. “Let It Grow” is among the songs to find a vivid new identity after the band’s 1974 road hiatus, a new vehicle for all-out double-drumming, usually part of the first set’s finale, tonight overflowing with detailed Garcia landscaping and accoutrements by Mydland’s chiming Dyna-Rhodes keyboard.