Navaho G-26 / X-15 - Navaho G-26 / X-15 Spaceplane Credit: Tom Johnson. 11,029 bytes. 123 x 362 pixels.

North American proposed several methods of taking the X-15 spaceplane to higher velocities and altitudes. One of these involved the use of one to three Navaho booster rockets, which could even place the X-15 into orbit. This incremental approach to manned spaceflight was not pursued - the Mercury and X-20 Dynasoar programs were favored instead.