Lunar Module systems are listed from top to bottom and include EPS (Electrical Power Subsystem), ED (Explosive Devices), ECS (Environmental Control Subsystem), Comm (Communications), G&C (Guidance and Control), DPS (Descent Propulsion Section), APS (Ascent Propulsion Section), and RCS (Reaction Control Subsystem). From left to right along the top the five key milestones of a lunar landing – DOI (Descent Orbit Insertion, which the DPS lowers the LM lunar orbit down to 50,000 feet), PDI (Powered Descent Initiation, which is a long burn of the DPS to break out of lunar orbit and descent to the surface), PDI → 5 M (the period starting at the PDI burn and continuing through the first 5 minutes), 5M → LO GATE (5 minutes after the start of the PDI burn until LO GATE, the LM being 2000 feet down range and 500 feet in altitude above the landing site), LO GATE → TD (from the LO GATE altitude point until TouchDown).
Each of the eight subsystems have at least one or up to 14 subcomponents listed, each with a long arrow moving to the right and stopping at one of the five landing phase headings along the top. These arrows indicate when the crew must abort the landing if a failure occurs in the landing sequence and not to continue on to the lunar landing.
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