Kosmos 638 was an uncrewed test of the 1975 Apollo–Soyuz Test Project Soyuz. It carried an APAS-75 androgynous docking system. This was followed by another uncrewed test of this spacecraft type, Kosmos 672. It was a Soyuz 7K-TM spacecraft. When the air was released from the orbital module (which is ejected before re-entry of the capsule) it caused unexpected motions with the spacecraft. This led to the next test also being uncrewed.

Mission parameters