Flerf Whisperer: Water-glycol cooling: what it cooled, the radiators, and the oxygen warmer

2026-10-13T23:25:54Z

Water-glycol cooling: what it cooled, the radiators, and the oxygen warmer

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{{series apollo}}

'''Apollo command module cooling''' was two loops of ethylene glycol and water. The pumps, the cold plates, and the water boiler sat in the command module. The radiators that threw the heat away sat on the service module. The same hot glycol line warmed the oxygen as it came out of the tanks. That oxygen path is on [[Apollo command module oxygen]].

== What it cooled ==
The Apollo 14 command-module news reference states the job of the coolant. The primary loop is pumped "to cool operating electric and electronic equipment and the suit and cabin heat exchangers." The glycol is there "to carry heat to the space radiator from cabin, space suits, electronic equipment, and the potable water chiller."<ref name="csm14">[https://apollojournals.org/alsj/CSM14_Environmental_Control_Subsystem_pp117-136.pdf Environmental control subsystem], Apollo command and service module news reference, Apollo 14.</ref>

A 1972 experience report, written from the flights, follows the same loop in more detail. After the coolant has been brought down to temperature, 35 pounds an hour goes to the inertial measurement unit of the guidance equipment. The other 165 pounds an hour goes through the drinking-water chiller and then the suit heat exchanger, which is also the humidity control for the cabin. From there it enters the cabin heat exchanger and takes heat from the lighting, from electronic equipment that is not mounted on cold plates, from the environmental loads, and from the crew when they are in shirtsleeves. The two streams join again and pass through 22 cold plates, "which absorb the major portion of the thermal load." Heat from those plates can be sent back through the cabin heat exchanger when the cabin needs warming.<ref name="ecs">Frank H. Samonski Jr. and Elton M. Tucker, [https://ntrs.nasa.gov/citations/19720012252 ''Apollo Experience Report: Command and Service Module Environmental Control System''], NASA TN D-6718 (March 1972).</ref>

== The loop ==
The news reference calls the fluid a mixture of ethylene glycol and water, 62.5 percent glycol and 37.5 percent water. There are two loops. The primary runs all the time. The secondary is the backup, and it is also used when more cooling is wanted and before entry, to cold-soak the cabin for the plunge into the atmosphere. The secondary loop cools the suit, the cabin, and the electronic equipment. It does not heat the cabin, and it does not cool the guidance equipment.<ref name="csm14" /><ref name="ecs" />

The pump package holds three centrifugal pumps, two for the primary loop and one for the secondary, and only one pump runs at a time. The experience report gives the primary flow as 200 pounds an hour.<ref name="csm14" /><ref name="ecs" />

The temperature is held by mixing, and by boiling water when mixing is not enough. If the coolant coming back from the radiators is colder than 45 degrees Fahrenheit, it is mixed with fluid that bypassed the radiators, and the mix is held at 45 degrees. When the radiators cannot reject the whole load, that bypass stops. The glycol evaporator then cools the 200 pound an hour flow to 41.5 degrees Fahrenheit by evaporating water at about 0.1 [[Pounds per square inch#Absolute pressure|psia]]. The news reference gives the evaporator band as 37 to 45 degrees Fahrenheit.<ref name="ecs" /><ref name="csm14" />

The hardware that does this lives in the command module. The environmental control unit, in the left-hand equipment bay, holds the coolant control panel, the water chiller, and the two glycol evaporators, one for each loop. Each evaporator is a stack of stainless plates and felt-metal wicks, about 8 by 4.7 by 6.6 inches, and weighs 18 pounds.<ref name="csm14" />

== Where the heat went ==
In normal flight the heat left through the service-module radiators. The news reference follows one pass. The glycol picks up heat in the command module, "is routed to the service module, where it passes through radiator tubes on the outside skin," and "radiates its heat to space." The mixture comes back cold and the cycle repeats. Each of the later panels could reject about 4,415 Btu an hour.<ref name="csm14" />

The radiators were not available for the whole mission. During ascent they were heated by aerodynamic friction, so a bypass shut off the service-module part of the loop. From liftoff to 110,000 feet the heat stayed in the coolant and in structure that had been pre-chilled. Above 110,000 feet the excess was rejected by evaporating water in the primary glycol evaporator.<ref name="csm14" />

That steam did not stay in the cabin. The experience report describes "the steam duct leading from the evaporator to space vacuum." The duct that flew was 2 inches in diameter and about 8 feet long, with three right-angle bends, because of where the equipment sat in the crew compartment and because of the spacecraft attitude during entry. A backpressure valve on that duct set the boiling pressure, and that pressure set the coolant temperature.<ref name="ecs" />

The early radiator panels could not carry a 14-day load by themselves. The experience report puts their combined area at 60 square feet and their capability at 3,700 Btu an hour, against a 4,850 Btu an hour requirement in an average earth-orbit environment. The difference was boiled off as water. A supplementary tank of 112 pounds of water was added in the service module so an earth-orbit mission could still last 14 days.<ref name="ecs" />

== The radiators ==
<div style="float:right; width:320px; margin:0.15em 0 0.8em 1.15em; clear:right;">
[[File:Apollo 13 service module radiators AS13-59-8500 small.jpg|link=File:Apollo 13 service module radiators AS13-59-8500 full.jpg]]
<div style="font-size:13px; line-height:1.35; margin-top:6px;">Apollo 13, 17 April 1970, the service module after it was jettisoned. The environmental-control radiators are panels on the lower half of this skin, on opposite sides. The opened bay is sector 4, the oxygen tanks and the fuel cells. NASA AS13-59-8500, [https://images.nasa.gov/details/as13-59-8500 NASA].</div>
</div>
The radiators were not on the command module. They were part of the service-module skin.

A service-module description places them on the lower half, on opposite sides. One radiator is part of the panel covers for sectors 2 and 3. The other is part of the panel covers for sectors 5 and 6. Sectors 2 and 5 are 70 degrees each. Sectors 3 and 6 are 60 degrees each. Each pair is a 130-degree arc. That description gives each radiator about 30 square feet.<ref name="sm">Service module section, Apollo command and service module news reference. Exterior, and sectors 2, 3, 5, and 6.</ref>

The Apollo 14 news reference, written for the lunar spacecraft, says two aluminum panels of about 49 square feet each, "around the outside surface of the service module in a 130-degree arc." Each panel has five tubes for the primary coolant and a secondary tube for the backup loop. As the glycol flows through the tubes, "its heat is rejected through radiation to space."<ref name="csm14" />

The 1972 experience report describes the same lunar redesign as two panels of 50 square feet each, on opposite sides of the service module. One panel can face deep space while the other faces the sun, the earth, or the moon. A flow-proportioning valve sends more coolant to the colder panel. At a 10 degree difference it puts about 95 percent of the flow through the cold one. The panels also stagnate on purpose. Five tubes run in parallel, and the tube with the least flow gets the coldest. As that outlet temperature falls, the flow resistance in that tube rises, which cuts the flow further. In a fully stagnated panel, three tubes have essentially no flow. At a high heat load the cold tubes thaw and the whole panel works. At a low heat load the stagnated panel radiates less, so it does not freeze the loop. An in-line heater turns on if the mixed coolant leaving the radiators falls to 15 degrees below zero Fahrenheit.<ref name="ecs" />

The three documents do not print the same area. The service-module description says about 30 square feet. The news reference says about 49. The experience report says 50 for the lunar panels, and 30 for the earlier ones. They agree on the place: opposite sides of the service module, not the command module. Sectors 2 and 3 add to 130 degrees, and so do sectors 5 and 6. That is the arc the news reference gives for each panel.

A second set of radiators, on the fairing at the top of the service module, belongs to the electrical power system. Eight small panels there dump heat from the fuel cells, on a separate loop. They are not the panels the oxygen capillaries are wound against.<ref name="sm" />

<div style="clear:both;"></div>
<div style="margin:0.6em auto 1.2em; max-width:640px; text-align:center;">
[[File:Apollo coolant radiator schematic small.jpg|link=File:Apollo coolant radiator schematic full.jpg]]
<div style="font-size:13px; line-height:1.35; margin-top:6px;">Figure 12 of the 1972 experience report. The command-module coolant box is at the top. The radiator panels are the skin of the service module, with a primary circuit and a secondary circuit. The drawing is the report figure, not a photograph.</div>
</div>

== The oxygen ==
The oxygen tanks were supercritical, near 900 [[Pounds per square inch|psi]], and cold. Letting that oxygen expand without adding heat would chill it, far enough to arrive as liquid and to freeze the glycol it passed. The command module warmed it on the way in. The 1969 handbook says the capillaries at the inlet "are wound around the hot glycol line" and "heat the oxygen to prevent it from entering the CM in a liquid state." The spacecraft 012 handbook says the restrictors "are coiled around a warm water-glycol line," and it says the line that matters is the one to the space radiators. Before a cabin refill the mixing valve is closed, so warm coolant, rather than coolant just back from the radiators, flows through the coil. The valves and the quoted wording are on [[Apollo command module oxygen]].<ref name="aoh">NASA, [https://ibiblio.org/apollo/ApolloProjectOnline/Documents/SMA2A-03-BLOCK%20II%20Volume%201%2019691015/aoh-v1-2-07-ecs.pdf ''Apollo Operations Handbook, Block II Spacecraft, Volume 1''], SM2A-03, environmental control system, basic date 15 April 1969.</ref><ref name="sc012">NASA, [https://www.ibiblio.org/apollo/Documents/SM2A-03-SC012-ApolloOperationsHandbook-Spacecraft012.pdf ''Apollo Operations Handbook, Spacecraft 012''], SM2A-03-SC012, basic date 12 November 1966.</ref>

== See also ==
* [[Apollo command module oxygen]]
* [[Apollo oxygen]]
* [[Apollo fuel cells]]
* [[Pounds per square inch]]
* [[Moon landings]]

== References ==
<references />

[[Category:Apollo program]]
[[Category:Apollo evidence]]
[[Category:Moon]]
[[Category:Spaceflight]]

Apollo command module cooling - Revision history

Flerf Whisperer: Water-glycol cooling: what it cooled, the radiators, and the oxygen warmer