 Three minutes after the count was resumed, the hatch installation was started. The hatch consisted of three parts: a removable inner hatch, which stayed inside the cabin; a hinged outer hatch, which was part of the spacecraft's heat shield; and an outer hatch cover, which was part of the boost protective cover enveloping the entire Command Module to protect it from aerodynamic heating during launch, and from launch escape rocket exhaust in the event of a launch abort. The boost hatch cover was partially, but not fully, latched in place because the flexible boost protective cover was slightly distorted by some cabling run under it to provide the simulated internal power. (The spacecraft's fuel cell reactants were not loaded for this test.) After the hatches were sealed, the air in the cabin was replaced with pure oxygen at 16.7 psi (115 kPa), 2 psi (14 kPa) higher than atmospheric pressure.
