The Start-1 launch vehicle is derived from the RT-2PM Topol ICBM. The first three stages of the Topol missile are used as the first three stages of the Start-1 rocket and are essentially unmodified for their new purpose. A fourth stage, specially developed by the Moscow Institute of Thermal Technology, and the payload shroud take the place of the nuclear warhead and are the main differences between the Topol ICBM and the Start-1 rocket. Additionally, a Post-Boost Propulsion System (PBPS) may be added between the fourth stage and the payload to circularize the orbit of the payload if necessary. The PBPS provides propulsion using a solid-propellant nitrogen gas generator. The gas goes through three pairs of nozzles that can swivel to maneuver the payload.