 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 . 
