In late 1960, there had been a growing number of concerns about the standards of the Redstone launch vehicle; the MR-2 test flight, "manned" by a chimpanzee, had had technical problems during the launch leading to the spacecraft flying too high, too far and too fast [REF].
The initial launch attempt, on May 2, was canceled due to weather problems two hours and 20 minutes before the launch time, with Shepard waiting in a hangar already suited and prepared.