“The main trouble with the Mercury capsule was that most system components were in the pilot’s cabin; and often, to pack them in this very confined space, they had to be stacked like a layer cake and components of one system had to be scattered about the craft to use all available space. This arrangement generated a maze of interconnecting wires, tubing, and mechanical linkages. To replace one malfunctioning system, other systems had to be disturbed; and then, after the trouble had been corrected, the systems that had been disturbed as well as the malfunctioning system had to be checked out again.” James Chamberlin
This brings us to Project Gemini. Gemini started after Apollo had begun, in part to answer a crucial question for Apollo. Was rendezvous and docking in orbit a feasible basis for a manned lunar landing mission?