Rocket planes are aircraft that use rocket engines for propulsion instead of traditional jet engines. These engines burn fuel and oxidizer to produce thrust, allowing rocket planes to reach very high speeds and altitudes. They are often used for research and testing purposes, especially in the fields of aerodynamics and space exploration. One famous example of a rocket plane is the X-15, which was used by NASA and the U.S. Air Force in the 1960s. The X-15 reached altitudes above the Kármán line, the boundary of space, and provided valuable data for the development of future spacecraft and technologies.