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Artifact: Environmental Control Unit (ECU), Apollo Command Module, Block II

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Identifier: A20120002
Artifact Category: Spacecraft Hardware
Period of Employment: October 11 1968 to July 24 1975
Manufacturer: AiResearch Manufacturing Corp
NASA Contract Number: NAS9-150
Spacecraft/Launch Systems: Apollo Command Module

Aluminum, Stainless Steel

Dimensions: (H) 28 x (W) 36 x (D) 20 inches
Weight: 140 Pounds (estimated)
Flight Vehicle System or Component: Environmental Control (ECS)
Program: Apollo
Flown Status: Undetermined

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.


The environmental control subsystem provided a controlled environment for three astronauts for up to 14 days. For normal conditions, this environment includes a pressurized cabin (5 pounds per square inch), a 100- percent oxygen atmosphere, and a cabin temperature of 70 to 75 degrees. For use during critical mission phases and for emergencies, the subsystem provides a pressurized suit circuit. The subsystem provided oxygen and hot and cold water, removed carbon dioxide and odors from the CM cabin, provided for venting of waste, and dissipated excessive heat from the cabin and from operating electronic equipment. It is designed so that a minimum amount of crew time was required for its normal operation.

A key function of the ECU included the pressure-suit circuit, which controlled the levels of carbon dioxide, odor, and humidity and was capable of providing a habitable environment for the crewmembers if cabin pressurization is lost. When the crewmembers were in the pressure-suit mode, they were isolated from the cabin (this was normally the case during launch and reentry, spacecraft thrusting, critical maneuvers, during EVA or in an emergency). The ventilating gas flow leaving the pressure suits passed through a debris trap (visible at the left side of the ECU) which removed 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 (seen at the right side of the ECU), the carbon dioxide and odor control for the Apollo Command Module was accomplished. Each element is sized for 1.5 man-days of operation at the design metabolic loads, and the elements were changed by the crew alternately every 12 hours. Twenty elements were 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 entered the suit-circuit heat exchanger, where suit-circuit heat loads were 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 was 50 degrees F.

The cool gas was distributed to the three suit-hose-connector units, (located above the ECU) which incorporated a flow-control adjustment lever and a flow-limiting Venturi tube. When the crewmembers were in the shirtsleeve mode, their portion of the suit-circuit flow was 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 was a mixture of 60 percent oxygen and 40 percent nitrogen. This was the minimum oxygen concentration required to provide a viable atmosphere with a reduction to 5.0 psia in the cabin pressure. Subsequent to orbit insertion, a bleed flow overboard established a demand on the cabin-pressure regulator and enriches the mixture to sea level equivalent (an oxygen partial pressure of 3.1 psia).

Key subcomponents integrated into the ECU:

Evaporators: Two evaporators, one for the primary and the other for the secondary coolant system, are made of special corrosion-resistant stainless steel plate and fin passages for the water-glycol arranged in a series of stacks alternated with sinntered Feltmetal wicks. Each wick pad was fed water through a plate which has tiny holes (5/1000 of an inch in diameter). Each evaporator is 8 by 4.7 by 6.62 inches and weighs 18 pounds. The wicks are vented to the very low space pressure and water boils at 35 to 40 degrees F. Its evaporation cooled the plates, through which the water-glycol passed, thus cooling the water-glycol to between 37 and 45 degrees F. The water-glycol flow was about 24 gallons an hour. About 8000 Btu per hour could be removed.

Lithium Hydroxide Canister: There are two canisters in aluminum housings of 8-1/2 by 20 by 7-1/2 inches. The canisters, a diverter valve, and inlet and outlet ducts weigh 19.7 pounds. The canisters have removable lithium hydroxide elements. The elements are alternately changed, one every 12 hours. The elements absorb carbon dioxide and also contain activated charcoal, which absorbs odors.

Suit Compressors: (AiResearch) Two centrifugal blowers made of aluminum are conical with a diameter of 6-1/2 inches and a length of 7/8 inches. One was used at a time. It circulates gasses through the suit circuit at a rate of 30 cubic feet per minute during normal operation. Each weighs 10.8 pounds. They operated on 3-phase, 110-volt, 400-Hertz power. Power consumption was 85 watts during normal operation.

Suit Heat Exchanger: The suit heat exchanger is made of two separate stacks of stainless steel fins and plates. One set is connected to the primary coolant system and the other is connected to the secondary coolant system. The unit is 15 by 1 1 by 5.2 inches. It cools suit gas to 50 to 55 degrees F and controls humidity by removing excess water. The water is collected by metal wicks and transported to the waste water storage tank.

Accumulators: Two reciprocating water pumps on the suit heat exchangers collect condensate from the suit circuit and pump it into the waste water tank. One accumulator is operated at a time; the other is standby. On automatic mode, a pump goes through a cycle every 10 minutes.

Oxygen Demand Regulator: An 02 Demand Regulator supplies oxygen to the suit circuit whenever the suit circuit is isolated from the Command Module cabin and during depressurized operations. The assembly contains redundant regulators, a single relief valve for venting excess suit pressure, an inlet selector valve for selecting either or both regulators, and a suit test valve for performing suit integrity tests.

Water Chiller: Consists of stainless steel coil tubing with a 1/4-inch water inlet and outlet, and 5/8-inch water-glycol inlet/outlet. The tubing holds about a tenth of a gallon of water. The water-glycol flows around the tubing, which contains the water, at 20 gallons an hour at about 45 degrees F to cool the water. The cooled water is used for drinking.

Glycol: Ethylene glycol, one of a large class of dibydroxy alcohols, was mixed with water (62.5 percent glycol to 37.5 percent water) to carry heat to the space radiator from cabin, and space suits (as well as onboard electronic equipment, and a potable water chiller) .

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