The rear end of the Heavens Grace sat on the ground to the side of the factory, having broken off after impact  while the rest of the ship sat lodged awkwardly inside the factory at a roughly forty five degree angle. Bon Bon wasn't the kind of pony to question the princesses, but couldn't Luna have seen fit to set the vessel on fire before she lobbed it at the factory? It would have negated at lot of the destruction work Bon Bon needed to carry out since she was in charge of making sure the airship was destroyed.