 The inner hatch cover used a plug door design , sealed by higher pressure inside the cabin than outside . The normal pressure level used for launch ( 2 psi ( 14 kPa ) above ambient ) created sufficient force to prevent removing the cover until the excess pressure was vented . Emergency procedure called for Grissom to open the cabin vent valve first , allowing White to remove the cover , but Grissom was prevented from doing this because the valve was located to the left , behind the initial wall of flames . Also , while the system could easily vent the normal pressure , its flow capacity was utterly incapable of handling the rapid increase to 29 psi ( 200 kPa ) absolute caused by the intense heat of the fire . 
