This diagram illustrates the Space Shuttle mission sequence. The Space Shuttle was approved as a national program in 1972 and developed through the 1970s. Part spacecraft and part aircraft, the Space Shuttle orbiter, the brain and the heart of the Space Transportation System (STS), required several technological advances, including thousands of insulating tiles able to stand the heat of reentry over the course of many missions, as well as sophisticated engines that could be used again and again without being thrown away. The airplane-like orbiter has three main engines, that burn liquid hydrogen and oxygen stored in the large external tank, the single largest structure in the Shuttle. Attached to the tank are two solid rocket boosters that provide the vehecile with most of the thrust needed for liftoff. Two minutes into the flight, the spent solids drop into the ocean to be recovered and refurbished for reuse, while the orbiter engines continue burning until approximately 8 minutes into the flight. After the mission is completed, the orbiter lands on a runway like an airplane.