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 ( 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 caused by the intense heat of the fire.