Complex sentence: The astronaut sat with his back to the heat shield, in a seat which was molded from his body for maximum support and as a safety procedure he was wearing a pressure suit with its own oxygen supply, which would also cool him.
Split into simpler sentences: The astronaut sat with his back to the heat shield, in a seat which was molded from his body for maximum support. As a safety measure he wore a pressure suit with its own oxygen supply, which would also cool him.