The docking module originated in the 1992 design version of the cancelled "Mir"-2 space station, which featured a combined docking compartment and airlock to facilitate docking missions during the Soviet "Buran" space shuttle programme (this module, SO-1, was eventually incorporated into the Russian Orbital Segment of the International Space Station as "Pirs"). When the Shuttle-"Mir" programme began, engineers realised that in order to enable US space shuttles to dock to "Mir", the "Kristall" module would have to be relocated to the forward port of the core module and back to its own lateral port each time a shuttle docked, a process which was not only time consuming but would also be entirely reliant on "Kristall"'s Lyappa arm, which, should it fail, would prevent any further shuttle missions to the station. Adding a small extension to "Kristall", however, would provide the shuttles the clearance they needed to dock without necessitating the relocation of the module on each occasion, and it was decided to base the design of the new module loosely on that of the "Mir"-2 docking compartment.