“It’s not unusual to have a series of failures at the beginning of a missile program,” Jeffrey G. Lewis, an arms control specialist at the New America Foundation, a research group in Washington, said in an interview. “But they don’t test enough to develop confidence that they’re getting over the problems.”Dr. Lewis added that an influential 1998 report by Donald H. Rumsfeld, before he became secretary of defense in the Bush administration, argued that the North Korean rockets might be good enough to pose a threat to the United States, even without flight testing. “But given that both versions of the Taepodong-2 have failed now,” he said, “we have very little confidence in the reliability of the system.”