One of the Pentagon’s four X-61A Gremlins is undergoing repairs after suffering damage during a July test -- the program’s second mishap in less than nine months -- and therefore will not participate in the long-awaited first experiment with the air recovery system that was delayed to this fall. The third phase of the multiyear Gremlins program -- cofunded by the Defense Advanced Research Projects Agency and the Air Force -- aims to demonstrate mid-air launch and recovery of four...