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Drinking water in the lunar module originated from four water tanks: two in the ascent stage and two in the descent stage. (Unlike the CSM, there were no fuel cells to make more water.) Section 7.2.4 of the Apollo 13 Mission Report states for the lunar module

At launch, the total loaded water available for inflight use was 338 pounds. At the time of undocking, approximately 50 pounds of water remained

This seems like more than enough water for drinking, and yet they made the Apollo 13 astronauts ration their drinking water! Why did they need to ration water?

This question is part of a series honoring the 50th anniversary of Apollo 13, "NASA's finest hour".

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Water was needed for cooling the electronics of the Lunar Module. To remove excess heat, water was evaporated by sublimator plates into space. Without cooling water, the electronics mounted on cold plates may overheat and damaged.

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SUBLIMATORS The porous-plate-type sublimators (one in the primary loop and another in the secondary loop) are identical, except that the primary sublimator has a larger capacity. Each sublimator has a coolant inlet and outlet, a water inlet, and a steam outlet. Water makes one pass through the unit; coolant makes six passes through the primary sublimator and four passes through the secondary sublimator. For proper sublimator operation, water pressure must exceed 4.0 psia, but be less than 6.5 psia. The water pressure must also be less than the suit circuit static pressure plus the head pressure from the water separators to the sublimator.

The unit rejects heat to space by sublimation of ice. Water from the WMS flows through the water passages, into the porous plates, and is exposed to space environment. The vacuum pressure is below the triple point of water; this causes an ice layer to develop within the pores and on the inner surface of the plates. As the hot coolant flows through the sublimator passages, heat transfers from the coolant to the water and to the ice layer. The ice sublimates from the solid state to steam without passing through a liquid state, rejecting its heat load overboard through a duct. The thickness of the ice layer varies with the heat load imposed on the sublimator, resulting in a regulated output temperature over a range of input temperatures.

COLD PLATES AND RAILS Electronic equipment that requires active temperature control is cooled by cold plates and cold rails. Most flat cold plates are installed between electronic equipment and the LM structure in a manner that minimizes heat transfer from the structure to the coolant, to avoid a reduction of the coolant cooling capacity. The surrounding structure and equipment may have a temperature range of 0 F to +160 F. The remaining flat cold plates are installed directly on the electronic equipment without making contact with the LM structure. Cold rails are also structural members and are used in the aft equipment bay in the descent stage for the DSEA. The IMU has an integral cooling circuit. Cold plates and cold rails for equipment essential for mission abort have two independent coolant passages, one for the primary loop and one for the secondary loop.

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Graphics and block quote from LM intro PDF and ECS description.

ECS = Environmental Control Subsystem

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