X-15 was a rocket in the possession of UNIT. In 2010, it was located at UNIT Base 5. It was build by the Groske. According to Tia Karim, the rocket was going to take the Eleventh Doctor's dead body into space. However, it turned out that the Doctor wasn't dead and it was all made up by Tia Karim and the Shansheeth to gain access to the TARDIS. (TV: Death of the Doctor)
