I was watching a segment on the History channel about experiments in modern flight when this question came to mind: For test pilots like Chuck Yager, they show the test pilot climbing into the experimental aircraft which is tucked under the belly of the mother craft. Why isn’t the pilot already in the test plane prior to take-off of the mother craft? Or, does this practice reduce risk of something potentially happening to the test pilot prior to the start of the experiment? - Jinx
My understanding is that the arrangement made the X-1 easier to jettison if the rockets went off prematurely, or some other emergency occurred. The pilot would have had difficulty bailing out, without an ejection seat, so keeping him aboard the B-29 was a safety measure. Also, if the B-29 had a problem and had to return to base with the X-1 still hanging under it, the landing would be safer for the X-1 pilot if he weren’t in it.
For aircraft launched from the B-52 wing, like the X-15, it was not feasible to do that and the test pilot was indeed aboard the X-plane the entire time.
Because of the way the X-1 protruded from the belly of the B-29 it would have been a particularly dangerous place to be during an aborted landing. By contrast pilots of the X-15 had no choice but to ride the entire flight in the experimental plane. At least the X-15 had an ejection system but I don’t know if it could be used while still attached to the B-52’s wing pylon.