Cosmos 638 is the codename for an unmanned flight of the Soviet Soyuz spacecraft. It was common in the Soviet Union to give only successful missions an official name. Failed flights were mostly not announced and test flights only under the general cover name cosmos. It was the 25th flight in the Soviet Sojusprogramm.
This was the first flight in preparation for the joint Apollo -Soyuz project with NASA. The spacecraft had instead of the standard coupling system, a modified system, called APAS -75 to dock onto the connection module APDS ( Androgynous Peripheral Docking System) can. In addition, the spacecraft with two solar panels was provided.