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It would be irresponsible to assume that any 1 technology is the only practical 1 for space launch without investigating alternatives.
A scramjet powered aircraft, whether manned or unmanned, will almost certainly use liquid fuel, although that's not limited to the hydrogen that is used in the X-51A. A scramjet powered cruise missile might use either liquid or solid fuel. Either way, a new tech cruise missile project would be more likely to be by DARPA than by NASA.