“It sounds very fancy, but it's really the HVAC of the spacecraft — controlling humidity, temperature and the whole atmosphere onboard.”

— Manuel Retana, NASA Project Manager, ECLSS Artemis II  |  University of Nevada, Reno, April 2026

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A NASA engineer who spent five years building the life support system on the Artemis II spacecraft name-dropped the HVAC industry in an interview about his job. I did some digging and realized that it was a lot more than just a simple comparison. His work has a lot in common with ours!

The Environmental Control and Life Support System (ECLSS) is what kept four astronauts breathing, warm, and alive for ten days aboard the Orion spacecraft as it traveled farther from Earth than any human in history. This system controls temperature, manages humidity, scrubs carbon dioxide from recirculated air, and maintains cabin pressure. Sounds an awful lot like HVAC systems in commercial buildings, doesn’t it? (Well, minus the CO2 scrubbers and cabin pressure!) 

The physics at play in a spacecraft’s HVAC system are the same as those you work with on every job. What changes is the environment, and that environment just happens to have the most hostile conditions any HVAC system has ever faced.

The Problem: There Is No Ambient

Every HVAC system on Earth operates against a relatively predictable ambient. Even on the worst August afternoon in Florida, you know roughly what you're working against. You calculate heat gains and losses, size the equipment around the data, and design a system that meets the client’s comfort needs as closely as you can.

In deep space, there is NO predictable ambient temperature. One side of the spacecraft can be in direct sunlight at temperatures exceeding 250°F, while the other side simultaneously faces deep space at close to −270°F. There's no atmosphere. There's no stable sink temperature. The “outdoor conditions” change continuously based on the spacecraft’s positioning relative to the sun, Moon shadow, and the phase of the mission.

The ECLSS design team had to maintain a stable, breathable cabin environment for four people in those unforgiving conditions, with no service calls, no replacement parts, and no margin for a callback.

The Fluid Loops: Primary/Secondary, No Compressor

Before we get into how the thermal system on the Orion spacecraft works, let me tell you about the refrigerants they used to make it possible.

There's one, and you already know what it is: R-718.

Propylene Glycol/Water Loop  =  Chilled Water Primary Loop

R-718 is water, by the way. It’s mixed with propylene glycol—nothing fancy, just the same antifreeze chemistry you find in commercial hydronic systems. It’s just fluid moving through a loop, doing what fluid in a loop has always done: picking up heat in one place and dropping it off somewhere else. Oh, and there's no compressor or condenser coil. The system that kept four astronauts alive in deep space is, at its core, a pumped hydronic loop with no vapor-compression refrigeration cycle.

So how does it actually work? 

Follow the heat, starting with the crew: four people breathing, moving, and giving off heat. Then we add a wall of avionics running continuously with nowhere to vent. A fluid loop circulating propylene glycol and water picks up all that heat through cold plates and the cabin itself. It stays entirely inside the pressurized crew module and never touches the outside of the spacecraft.

Interface Heat Exchangers (x2)  =  Plate-Frame Isolation HX

When that loop warms up, it passes through one of two heat exchangers that hand the heat off to a completely separate external loop. Think of these heat exchangers as a plate-frame isolation heat exchanger in a large chilled water plant. They’re essentially a transfer point between two isolated circuits. Either one is capable of carrying the full load because if one gives out, we can’t exactly wait for a tech to come out and fix it!

HFE-7200 External Loop  =  Secondary Condenser Loop

Now here's where it gets interesting. That external loop can't use propylene glycol. We’ve already established that surface temperatures can exceed 250°F on the sunny side of the spacecraft. On the shadow side, they approach absolute zero. Propylene glycol would freeze solid, so there’s a specialty heat transfer fluid called HFE-7200, similar to the fluid used to immersion-cool data center servers. This fluid stays liquid and stable across the massive temperature swings in space. Here on earth, the weather NEVER gets that cold.

Radiator Panels  =  Condenser / Heat Rejection Surface

The external loop carries the heat out to radiator panels mounted on the service module. That heat goes into deep space, and it’s gone. There’s no fan or airflow of any kind, just radiant heat doing what it always does: moving from a hot surface to a colder one.

With all that said and done, we’ve got a 47°F supply temperature in the internal loop. The cabin dew point target is 45°F to keep that uninsulated loop running through the cabin from condensating.

On Earth, we'd insulate the pipe and move on. It’s a piece of cake for us, but up in space, every pound of insulation is a pound that had to launch. Instead, they manage the dew point margin, which is just two degrees. 

Of course, the building science people have had the dew point conversation many times before. This is all second nature to them at this point.

The Load Calculation: Square Footage Per Ton Does Not Apply in Space

Every experienced technician knows that square footage per ton is a rule of thumb and just doesn’t fly for designing systems. Space makes that clearer than Manual J ever could.

The Numbers:

• Heat load: 9,017 BTU/hr (2644 W); that’s just under 0.75 tons of total cooling capacity
• Total habitable volume: 330 ft³  (“roughly the size of two minivans”)
• Heat sources: 4 occupants… plus a full avionics suite running at continuous load

As you can see, we have a 330-cubic-foot capsule that requires less than one ton of cooling. By the “400 square feet per ton” rule of thumb, the Orion crew module would qualify for a window unit and not much else.  (Though maybe if we left it up to the white-shirt techs, Orion would've had a 5-ton mega-SEER unit with a sketchy air purifier.)

On Earth, heat gains come from human occupants, appliances and electronics, infiltration, and solar gains through windows. Orion just has the bodies and waste heat from electronics that can’t be turned off. Square footage doesn’t mean squat in space; we have to design to the actual loads.

(Note: The 2,644W figure comes from the constant heat load (Q_in) in a NASA-published thermal modeling study for a possible mission where the vehicle stays in lunar orbit while the crew is on the lunar surface; therefore, there is no occupant heat, just electronics. It is a modeled steady-state design load for that specific mission profile, not an official nameplate specification. Peak loads also vary by mission phase. This particular PCM Trade Study is cited here and in the references because it is the most specific publicly available figure from NASA's own engineering analysis.)

The Backup: A Water Sublimator

When Orion’s radiator capacity isn't enough, such as during peak load phases or when the solar angle interferes with the heat rejection surface, the system has a water sublimator as a backup system.

In a water sublimator, water is metered through a porous metal plate, which is exposed directly to the vacuum of space. In a near-perfect vacuum, the water goes straight from liquid to vapor, absorbing a LOT of latent heat in the process. It’s pretty much the same concept as evaporative cooling: phase change absorbs energy. But instead of dry air driving the reaction, we have a hard vacuum.

It’s got one major drawback, though: it’s not a closed loop, so it consumes water every time it runs. As a backup system, much like heat strips in a heat pump, it’s available and effective when we need it, but it’s not something that can run continuously (just for heat rejection instead of adding heat). The supply is finite, and there's no way to replenish it when you’re 250,000 miles from home.

The Air Side: CO2 Scrubbing as Ventilation

In a completely sealed cabin with four people and no fresh air, CO2 can make things go very wrong very quickly for the crew.

The system used on Orion is called CAMRAS: CO2 And Moisture Removal Amine Swing-bed. It’s got two beds filled with SA9T, a sorbent material made of plastic beads coated with an amine compound, that alternate continuously. One bed adsorbs CO2 and water vapor from recirculated cabin air, while the other is isolated and exposed to the vacuum of space. That vacuum strips the captured gases off and vents them overboard. The two beds are thermally linked to each other, so the regeneration cycle doesn’t need any extra heating or cooling energy to operate.

SA9T is attracted to water vapor, so the same system that scrubs CO2 also handles a major share of cabin dehumidification. The remaining water vapor condenses on the 47°F piping; it’s basically the same as moisture removal on an evaporator coil, just without a dedicated coil surface. The condensate is collected and stored as wastewater. 

So it’s like the equivalent of a ventilating dehumidifier in terms of maintaining safe air and dehumidifying it all at once.

Capacity Control: The Spacecraft Is the Outdoor Unit

This is the part of the system that doesn’t directly match anything in our earthly HVAC.

Now, some systems can control capacity in a few ways. For example, a VRF system modulates compressor speed to match the load. On a chilled water plant, we stage equipment and vary the flow. On Orion, that variation happens by pointing the spacecraft differently.

A radiator panel in direct sunlight is fighting incoming solar radiation and can’t reject much heat. But a panel in shadow has a heat sink temperature near absolute zero and can reject a LOT of heat. Since the panels are fixed to the service module body and can’t move on their own, the orientation of the entire spacecraft determines how much heat the system can reject at any given moment. Modulating capacity just like a VRF system, right?

But there's also a passive technique used on deep space missions called the barbecue roll (yes, that’s really what it’s called). A BBQ roll is a slow, deliberate rotation of the spacecraft along its long axis to evenly distribute solar heat load across all exterior surfaces (no extra hardware required!). The name is exactly what it sounds like: the spacecraft turns like a rotisserie, continuously and slowly, to keep the thermal load balanced across the entire vehicle.

All of that’s to say that managing heat rejection, mechanical design, and planning the operation are IMPOSSIBLE to separate.

The Physics Is the Same, but the Environment Is Not

Heat moves the same way 250,000 miles from Earth as it does on a job site in central Florida. Fluid loops work the same way. A 47°F supply temperature is a 47°F supply temperature. Latent heat is latent heat. The engineers who built Orion's thermal system called it HVAC because, well, that's exactly what it is.

It’s managing heat, air, and humidity. The main thing that changes is the environment the system operates against. There’s no stable ambient whatsoever, and the load doesn’t come from all the typical line items on a Manual J load calc. That load comes from four people, a full electronics suite that can NEVER turn off, and a sun that swings the system from peak load to deep freeze in seconds, depending on how the spacecraft is pointed.

Spacecraft designs don’t use a different set of physics. We just see a different application of the same ones: redundant loops, isolated circuits, a carefully managed supply temperature with a controlled dew point margin, a backup rejection mode, and passive rotation to balance the load. We know what those concepts are! They’re just applied to conditions we don't normally work in.

That's worth understanding. Not because you're going to work on a spacecraft, but because the same rigor that kept four people alive in deep space is available to you on every job you touch.

Oh, and the next time someone tells you 400 square feet per ton is close enough, ask them how that calculation holds up on the Moon. You heard the new rule of thumb here first on HVAC School: 76 square feet per ton. The building science nerds’ brains are going to explode.

—Roman Baugh

References:

NASA Technical Reports Server — Orion ATCS Thermal Performance Study and PCM Trade Study

NASA ECLSS documentation; Lockheed Martin Orion ECLSS overview (April 2026)

Airbus ESM press release (March 2026)

NASA “Life Encapsulated” crew module interior feature; University of Nevada, Reno — Manuel Retana interview (April 2026)

NASA OIG Artemis II Readiness Report (2024)