During an hour-long event on Thursday, Abel Avellan, the founder and chief executive of the telecommunications company AST SpaceMobile, laid out his vision for developing a large rocket to launch his Bluebird satellites into orbit. Avellan emphasized, over and over again, that AST plans to build a "simple" rocket. He paraphrased SpaceX founder Elon Musk several times, saying, "the best part is no part."…