Gemini

The Gemini crew capsule (referred to as the Reentry Module) was essentially an enlarged version of the Mercury capsule. Unlike Mercury, the retrorockets, electrical power, propulsion systems, oxygen, and water were located in a detachable Adapter Module behind the Reentry Module. A major design improvement in Gemini was to locate all internal spacecraft systems in modular components, which could be independently tested and replaced when necessary, without removing or disturbing other already tested components.