RPT-EXCLUSIVE-Space firm founded by billionaire Paul Allen closing operations -sources

SEATTLE/ORLANDO, Fla., May 31 (Reuters) - Stratolaunch Systems Corporation, the space company founded by late billionaire and Microsoft Corp co-founder Paul Allen, is closing operations, cutting short ambitious plans to challenge traditional aerospace companies in a new "space race," four people familiar with the matter said on Friday.The company, a unit of Allen's privately held investment vehicle Vulcan Inc, had been developing a portfolio of launch vehicles including the world's largest airpl