The Sublimator: Cooling by Boiling Ice in the Void
How Apollo engineers solved the thermal problem of spaceflight with a device that froze water and boiled it away into vacuum—no moving parts required
Space wants to kill you in a dozen ways, but the one most people forget is heat. Not the heat of reentry or solar radiation—the heat your own body and your own machines generate, with nowhere for it to go. On Earth, you sweat and the air carries the warmth away. In a spacecraft, there is no air outside the hull, no convection, no breeze. Every watt of electricity your systems consume, every calorie your metabolism burns, becomes trapped thermal energy building toward a slow catastrophe. The Apollo engineers needed a way to reject heat into the void, and they found one of the most elegant solutions in the history of thermal engineering: the sublimator.
The Thermal Problem in Vacuum
Heat moves through three mechanisms: conduction, convection, and radiation. In the vacuum of space, convection—the dominant cooling mechanism on Earth—simply doesn’t exist. There is no fluid medium to carry heat away. Conduction only works where materials touch, which means you can move heat around inside a spacecraft but not get rid of it. Radiation works, but slowly: you need large surface areas and significant temperature differentials to dump meaningful amounts of heat through infrared emission alone.
The Command/Service Module addressed this problem primarily with space radiators—large panels on the Service Module that radiated waste heat into the darkness. But the Lunar Module had a different problem. It was designed to operate for only a few days, it needed to function on the lunar surface where one side faced the brutal heat of the sun, and it simply didn’t have the surface area for effective radiative cooling. The LM needed something else entirely.
The problem was compounded by the sheer thermal load. The LM’s electronics generated significant heat. The Environmental Control System had to maintain a cabin temperature between 70°F and 80°F. The astronauts themselves each contributed about 500 BTU per hour of metabolic heat. All of this had to go somewhere, and it had to go there reliably.
How a Sublimator Works
The sublimator’s operating principle borders on the poetic. Take a porous plate made of sintered nickel—metal powder compressed and heated until the particles fuse together, leaving microscopic interconnected pores throughout the material. Feed water to one side of this plate. On the other side: the vacuum of space.
The water seeps into the pores and immediately encounters the vacuum. At the pressures found in space—effectively zero—water’s boiling point drops below its freezing point. The water freezes in the pores, forming a thin layer of ice across the vacuum-facing surface of the plate. This ice layer is self-sealing: it plugs the pores and prevents more water from escaping.
Now route a warm coolant fluid across the other side of the plate. The heat from this fluid conducts through the nickel and reaches the ice. The ice doesn’t melt—it sublimates, transitioning directly from solid to vapor without ever passing through a liquid phase. The water molecules carry thermal energy away as they launch themselves into space as gas, never to return.
As ice sublimates away, fresh water seeps into the newly opened pores and freezes again. The process is entirely self-regulating. More heat means faster sublimation, which means the ice layer thins, which means more water flows in and freezes. Less heat means slower sublimation, the ice layer thickens, and water flow decreases. No valves, no thermostats, no control circuits, no moving parts whatsoever. The physics does all the work.
The sublimator rejected approximately 5,500 BTU per hour at peak capacity—enough to handle the full thermal load of the Lunar Module’s avionics, cabin environment, and crew metabolic heat. It consumed about 1.3 pounds of water per hour at maximum capacity, which became the practical limit on mission duration rather than any mechanical failure mode.
The LM Environmental Control System
The Lunar Module’s Environmental Control System, built by Hamilton Standard (now Collins Aerospace), used two coolant loops to manage heat. The primary loop circulated a water-glycol solution through cold plates mounted beneath heat-generating electronics, through the cabin heat exchanger, and then through the sublimator where the accumulated heat was finally rejected to space.
A secondary coolant loop provided redundancy. If the primary loop failed, the secondary could maintain adequate cooling for the crew and critical avionics. Both loops fed into their own sublimator sections, ensuring that a single failure couldn’t eliminate the spacecraft’s ability to reject heat.
The sublimator was mounted on the LM’s upper structure, with its porous plate facing the vacuum through a dedicated vent. During pressurized operations inside the LM, the crew cabin was sealed from the sublimator’s vacuum side—the device only worked when exposed to the hard vacuum of space. This meant the sublimator couldn’t operate during ground testing in any realistic way, making qualification a particular challenge for Hamilton Standard’s engineers.
The choice of the sublimator over radiators for the LM was driven by the mission profile. Radiators need time to reach thermal equilibrium and work best in consistent orientations relative to the sun. The LM changed attitudes frequently during descent and ascent, spent time on the lunar surface where ground temperatures could exceed 250°F in direct sunlight, and operated for periods too short to justify the weight and complexity of deployable radiator panels. The sublimator was lighter, simpler, orientation-independent, and perfectly suited to a vehicle with a finite mission duration and a consumable water supply.
Sublimation in the PLSS Backpack
The same principle that cooled the Lunar Module also cooled the astronauts during moonwalks. The Portable Life Support System—the backpack worn during EVAs—contained a miniature sublimator that was functionally identical to its larger sibling in the LM.
Inside the PLSS, a water loop circulated through the Liquid Cooling Garment worn by the astronaut against their skin. This garment was laced with thin plastic tubes carrying cool water, absorbing metabolic heat directly from the body. The warmed water then flowed through the PLSS sublimator, which dumped the heat to vacuum through the same freeze-and-sublimate cycle.
The PLSS carried approximately 11.5 pounds of feedwater for the sublimator. At typical metabolic rates during lunar surface work—which ran significantly higher than restful cabin activities, often reaching 1,000 to 1,600 BTU per hour during vigorous geological traverses—this water supply became the primary constraint on EVA duration. When the water ran low, the moonwalk was over, regardless of how much oxygen remained.
This is why Apollo EVA timelines were so precisely managed. Every minute of lunar surface activity consumed sublimator water. Mission planners had to balance the scientific return of longer EVAs against the thermal consumable budget. The later Apollo missions (15, 16, 17), with their extended surface stays and lunar rover traverses, carried upgraded PLSS units with additional feedwater to support EVAs exceeding seven hours.
Elegant Engineering in the Void
The sublimator represents a design philosophy that recurs throughout Apollo but is rarely stated explicitly: use the environment as part of the solution. The vacuum of space was the problem—it prevented convective cooling. The sublimator turned that same vacuum into the cooling mechanism itself. The near-zero pressure that made heat rejection so difficult was precisely what enabled sublimation to work so effectively.
No moving parts meant nothing to wear out, jam, or fail mechanically. No control electronics meant no software bugs, no sensor drift, no circuit failures. The sublimator’s only failure mode of consequence was contamination of the porous plate, which could clog the pores and prevent water from reaching the vacuum side. Hamilton Standard addressed this with careful material selection, filtered water supplies, and ground handling procedures that bordered on the obsessive.
Across all crewed Apollo missions—from Apollo 7 through Apollo 17—the sublimator never failed. Not once. In a program where nearly every other system experienced some anomaly on some mission, the sublimator performed with perfect reliability. It worked on the LM. It worked in the PLSS backpacks. It worked in the quiet of lunar orbit and on the broiling surface of the Moon.
The sublimator’s legacy extends beyond Apollo. Variations of the technology appeared in Space Shuttle EVA suits and continue to inform thermal management research for future lunar and Mars missions. The fundamental insight—that a self-regulating phase-change device with no moving parts can reliably reject heat in vacuum—remains as valid today as it was in 1969.
In an era of increasingly complex engineering solutions, the sublimator stands as a reminder that sometimes the most reliable machine is the one with the fewest parts. Feed it water, point it at the void, and let the physics handle the rest.