The test vehicle, which was used as an atmospheric drop test glider, had no propulsion system. Instead of an operational vehicle's payload bay doors it had an enclosed and reinforced upper fuselage structure to allow it to be mated with a mothership. Most of the thermal protection tiles were 'fake', made of inexpensive foam rather than ceramic; a smaller number of the X-37's tiles were actual TPS tiles, and TPS blankets were used in areas where heating would not have been severe enough to require tiles.