In January 1991, support structures that were designed to hold solar arrays were installed on Kvant-1. In July 1991, the crew constructed the Sofora girder during four EVAs. The Sofora girder was designed to test new construction techniques, mount a propulsion unit, and act as a place to hold experiments outside the station. In September, 1992, the crew installed the VDU propulsion unit on the end of the Sofora girder. It was delivered earlier by Progress M-14. The VDU was designed to increase the station's attitude control capability. The then six-year-old VDU propulsion unit was finally replaced in April 1998 by a new one that was delivered by Progress M-38.