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Oct 31, 2016

Command Module ECS (Part 12, Apollo Control Systems)

Environmental Control System (ECS)

The Apollo environmental control system (ECS) was designed and qualified to support three crewmen for 14 days and to maintain electronic equipment within operating thermal boundaries. The system maintains the pressure atmosphere of 100 percent oxygen and removes trace contaminants and metabolic carbon dioxide by absorption in charcoal and lithium hydroxide beds. (After the Apollo 1 CM accident the launch atmosphere was changed to 60-percent oxygen and 40-percent nitrogen.)

Apollo CM Environmental Control Unit (ECU), a major part of the ECS

[An Apollo Command Module (Block II) Environmental Control Unit (ECU) a major part of the Environmental Control subsystem (ECS), produced by Garrett Corp.'s AiResearch Division, Los Angeles under subcontract to North American Aviation (NAA), prime for the Apollo Command Service Module (CSM) under NASA Contact NAS 9-150. The Environmental Control Unit was the heart of the environmental control subsystem. It is a compact grouping of equipment about 29 inches long, 16 inches deep, and 33 inches at its widest point. It was mounted in the left-hand equipment bay. The unit contains the coolant control panel, water chiller, two water-glycol evaporators, carbon dioxide-odor absorber canisters, and suit heat exchanger, water separator, and compressors.]

Temperature control is provided by heat rejection from radiators and a water evaporator. Oxygen is supplied by the cryogenic storage system, and water is supplied as a byproduct of the fuel cells. The knowledge gained from extensive ground testing and inflight experiments on the behavior of water in zero gravity led to the incorporation of a wick-type porous-plate condensate separator.

The two hardware items requiring the most extensive development were the water evaporator and the radiator. During the Apollo Program, continuous refinements have been required in the construction, material selection, and quality control of the evaporator and its control system. The wide range of the maximum and minimum heat loads led to the use of a selective stagnation radiator designed to employ the viscosity characteristics of the coolant fluid (ethylene glycol and water). The other major problems experienced were in materials selection to reduce corrosion (particularly in the coolant system), materials selection for fabrication of porous plates and heat exchangers, and material and process refinements to eliminate weld crazing.

The following schematic diagram of the Command Module ECS may be conveniently divided into the

  1.  Oxygen,
  2.  Water,
  3.  Coolant,
  4.  Pressure-Suit, and
  5.  Cabin Circuits.

Apollo Environmental Control System ECS

ECS - Oxygen

The primary oxygen source consists of two supercritical cryogenic tanks located in the Service Module (SM). These tanks also supply the oxygen reqirements of the fuel cells and are generally considered as part of the electrical power system (EPS). The two tanks contain a total of 640 pounds of oxygen, and the design specification allocates 172.6 pounds of this amount to the ECS.

In comparison, the actual oxygen allocation to the ECS for the Apollo 11 mission was 72.4 pounds for planned use, 10.4 pounds for LM support, and 15.6 pounds for contingency use. The reduced consumption during the Apollo 11 mission resulted because the mission duration (196 hours) was less than that of the specification mission (336 hours or 14 days), and the cabin leakage and crew metabolic requirement values were lower than design specification requirements (see table below).

ECS Oxygen Consumption (Design 336 h 172.6 lbs)
Apollo 7 259.7 h 102 lbs
Apollo 8 146.5 h 51 lbs
Apollo 9 240.5 h 99 lbs
Apollo 10 190.0 h 71 lbs
Apollo 11 196.0 h 82 lbs

ECS - Water

The primary source of water for the ECS is the fuel cells, which produce approximately 0.77 lb/kWh as a byproduct of fuel-cell operation. The water storage provisions consist of a 36-pound-capacity potable water tank, and a 56-poundcapacity waste-water tank. Excess moisture in the cabin or suit circuit gas is removed by the water separator in the suit heat exchanger and is transferred by the cyclic accumulator to the waste-water tank for subsequent use as an expendable coolant. The effluent from the fuel cells is directed to the potable-water tank and is used for drinking and food reconstitution.

Periodic injection of chlorine by the crewmembers maintains bacteria control in the potable-water system. When the potable-water tank is full, the water circuit automatically diverts the fuel-cell output to the waste tank by elevating the water-system pressure from 25 to 30 psia. When both tanks are full, the water-system pressure is increased to 40 psia, and the fuel-cell effluent is dumped directly overboard. Excess water may also be dumped manually, and this capability has been used in all missions. This manual operation was chosen to preclude interference with photography, sightings with the guidance and navigation equipment, and trajectory determination.

ECS - Coolant

The coolant system consists of a primary loop, which is operated continuously, and a secondary loop, which serves as a backup system. The primary loop uses a centrifugal pump to circulate 200 lb/hr of coolant (ethylene glycol and water) through the heat-absorption and heat-rejection equipment in the CSM. If the coolant returning from the space radiator is less than 45 degrees F, it is mixed with fluid from the CM thermal load, which has bypassed the radiator, to obtain a mixed-coolant temperature of 45 degrees F.

Under mission conditions when the space radiator cannot reject the total load, no bypass occurs; instead, the glycol evaporator cools the 200-lb/hr flow to 41.5 degrees F by evaporating water at a controlled pressure of approximately 0.1 psia. The coolant flow leaving the evaporator is divided into a 35-lb/hr flow directed to the inertial measurement unit (IMU) of the guidance and navigation equipment, and a 165-lb/hr flow is routed to the suit heat exchanger through the drinking-water chiller.

The suit heat exchanger provides the humidity control for the CM. The coolant leaving the suit heat exchanger enters the cabin heat exchanger and absorbs heat from the CM lighting, the electronic equipment not mounted on coldplates, the environmental loads, and the crewmembers in the shirtsleeve mode. The effluent coolant from the guidance and navigation equipment mixes with that from the cabin heat exchanger, and the 200-lb/hr flow is directed through a series-parallel arrangement of 22 coldplates, which absorb the major portion of the thermal load. The heat from the coldplate network may be diverted to the cabin heat exchanger through the cabin-temperature control valve for heating the cabin, when required. The fluid leaving the cabin-temperature control valve enters the pump, and the flow is directed to the space radiator.

A secondary coolant loop is provided as a backup for the primary loop and may be operated at the discretion of the crewmembers. Both loops provide cooling for the suit and cabin atmospheres and fo,r the electronic equipment. The secondary loop does not have cabin-heating capability, nor does it provide cooling to the guidance and navigation equipment.

ECS - Pressure-Suit Circuit

The pressure-suit circuit controls the levels of carbon dioxide, odor, and humidity and can provide a habitable environment for the crewmembers if cabin pressurization is lost. When the crewmembers are in the pressure-suit mode, they are isolated from the cabin. The ventilating gas flow leaving the pressure suits passes through a debris trap which removes particles larger than 0.04 inch. Suit circuit flow is accomplished by one of two centrifugal-flow compressors which deliver 55 lb/hr of suit-circuit gas (35 cu ft/min) at a pressure rise of 10.0 inches of water with an inlet density of 0.0266 lb/cu ft.

As the ventilation gas passes through two parallel elements of lithium hydroxide and activated carbon, the carbon dioxide and odor control for the CM is accomplished. Each element is sized for 1.5 man-days of operation at the design metabolic loads, and the elements are changed by the crew alternately every 12 hours. Twenty elements are carried for 8- to 10-day missions. The element holder, or canister, incorporates the necessary check valves, diverter valve, and interlock mechanisms which permit the changing of elements in a depressurized cabin. The canister is also designed to preclude inadvertent depressurization of the suit circuit.

The gas leaving the carbon dioxide canister enters the suit-circuit heat exchanger, where suit-circuit heat loads are absorbed by the water and glycol. At the heat exchanger, the moisture is condensed, removed by the wicking, and transferred to the waste-water circuit by pneumatically actuated accumulators which are cycled every 10 minutes by a timing device. The normal gas exit temperature from the heat exchanger is 50" F.

Apollo 11 astronauts in the shirtsleeve mode 1969 inside CM

The cool gas is distributed to the three suit-hose-connector units, which incorporate a flow-control adjustment lever and a flow-limiting Venturi tube. When the crewmembers are in the shirtsleeve mode, their portion of the suit-circuit flow is delivered to the cabin through an orifice in the connector unit which approximates the pressure drop of the suit. This flow is returned to the suit circuit for carbon dioxide and humidity removal by the cabin-air-return valve located upstream of the suit compressors.

During manned ground testing and during launch, the cabin atmosphere is a mixture of 60 percent oxygen and 40 percent nitrogen. This is the minimum oxygen concentration which will provide a viable atmosphere with a reduction to 5.0 psia in the cabin pressure. Subsequent to orbit insertion, a bleed flow overboard establishes a demand on the cabin-pressure regulator and enriches the mixture to sealevel equivalent (an oxygen partial pressure of 3.1 psia). The nitrogen content is reduced further by leakage or LM pressurizations. Technical considerations associated with the selection of the launch environment are presented in the following section.

The crewmembers undergo a period of oxygen prebreathing prior to insertion into the CM suit circuit, which has been purged to an oxygen level greater than 95 percent. This oxygen prebreathing minimizes the possibility of aeroembolism during the boost phase when cabin pressure is reduced from 14.7 to 6 psia. To prevent nitrogen leaking into the suit circuit, a positive pressure relative to the cabin pressure is maintained by a 0.5-lb/hr excess flow.

ECS - Cabin Circuits

The cabin circuit consists of two axial-flow fans. Each fan has a capacity of 86 cu ft/min at 5 psia, which circulates the CM gas through the cabin heat exchanger. A cabin-pressure relief valve relieves the cabin pressure at a differential of 6.0 psi during ascent of the spacecraft and repressurizes the cabin during descent, when the ambient pressure exceeds cabin pressure by approximately 1 psid. After splashdown, a postlanding ventilation system, consisting of an inlet valve and fan and an outlet valve, is activated by the crewmembers to ventilate the cabin until recovery.

In the event of smoke, a toxic gas, or another harmful atmosphere in the cabin during the shirtsleeve environment, three oxygen masks are provided. The mask is a modified commercial full-face-type assembly with headstraps to hold it on. The oxygen is supplied at 100 psi through a flexible hose from the emergency oxygen and repressurization unit. The mask has an integral regulator that supplies oxygen on demand when the crewman inhales.

YouTube - "Command Module Documentary"


/1/ Wikipedia

/2/ Apollo archives, NASA

/3/ Apollo Experience Report - Command and Service Module - ECS

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