Apollo’s PLSS And The Science Of Keeping Humans Alive In Space

Ever since humans came up with the bright idea to explore parts of the Earth which were significantly less hospitable to human life than the plains of Africa where humankind evolved, there’s been a constant pressure to better protect ourselves against the elements to keep our bodies comfortable. Those first tests of a new frontier required little more than a warm set of clothes. Over the course of millennia, challenging those frontiers became more and more difficult. In the modern age we set our sights on altitude and space, where a warm set of clothes won’t do much to protect you.

With the launch of Sputnik in 1957 and the heating up of the space race between the US and USSR, many firsts had to be accomplished with minimal time for testing and refinement. From developing 1945’s then state-of-the-art V-2 sounding rockets into something capable of launching people to the moon and beyond, to finding out what would be required to keep people alive in Earth orbit and on the Moon. Let’s take a look at what was required to make this technological marvel happen, and develop the Portable Life Support System — an essential component of those space suits that kept astronauts so comfortable they were able to crack jokes while standing on the surface of the Moon.

Setting the scene

V-2 sounding rocket at White Sands Missile Range in 1946

It was 1945: the end of the second World War in Europe. The Allied and Soviet forces had finished fighting over the scraps of the Nazi rocket program which had already developed the most advanced rocket technology at the time, the V-2 liquid-fueled rocket.

Wernher von Braun had come to the US with his team of German rocket scientists to improve upon the V-2 design at Redstone Arsenal (RSA) in Alabama. The race was on against the Soviets who had obtained the V-2 manufacturing facilities and another team of German rocket scientists.

Much about space was still unknown at the time. From experiments using balloons and high-altitude airplanes we had a pretty good idea of the Earth’s atmospheric properties and the issues that altitude posed. Yet it was not known whether a human could stay alive at all after leaving the Earth’s protective atmosphere. There were many fears related to the effects of weightlessness on the human body, radiation in space, and other unseen and unknown dangers.

This was why the first human in space, Yuri Gagarin in Vostok 1, didn’t actually fly the Vostok-3KA spacecraft when he made history on April 12, 1961. Instead had on-board automatic systems and ground control doing the piloting. Though he had been provided with a code to unlock manual controls, they feared that after launch Yuri might black out from the weightlessness and be unable to control the spacecraft.

Discovering the Need for Robust Life Support Systems


The first US satellite, Explorer 1, was launched on January 31, 1958 from LC-26A at Cape Canaveral. It carried with it a number of instruments, one of which was a Geiger-Müller tube, capable of detecting radiation in the form of protons and electrons (β-radiation) and transmitting the reading back to Earth. The device was saturated during most of the experiment.

This led to the discovery of what we now know as the Van Allen radiation belts around the Earth: areas where the Earth’s magnetic field traps charged particles (mostly from the Sun’s solar wind). Theorized years before its discovery, it was seen as a major obstacle in any journey to the Moon, potentially exposing a spacecraft’s crew to lethal doses of radiation.

Schematic overview of the Van Allen belts.

The Soviets launched Sputnik 2 on November 3, 1957, as the first biological mission. This satellite carried Laika, the first dog and also first animal in space. An oxygen generator with a CO2 absorbing unit and sufficient water and food for a number of days were supposed to keep Laika alive for a while. Unfortunately, due to technical issues with the separation from the launch vehicle, the thermal insulation got damaged and the thermal regulation system malfunctioned. Temperatures in the capsule quickly rose to 40 °C. After about 5-6 hours into the flight, no signs of life were received from Laika, suggesting she died from overheating.

Laika, while still on Earth.

After these satellites, the US launched Vanguard 1 and the Soviets Sputnik 3, which mostly focused on examining the Earth’s atmosphere, as well as adding to research done by the opposing side (mapping the Van Allen belts by Sputnik 3).

All That Human Bodies Crave

Our bodies are fairly fragile biological systems. The human body operates within a fairly small temperature range, requires regular access to water and food and above all oxygen. It has a low tolerance for pollutants like carbon dioxide (CO2) which is a natural byproduct of its metabolism. Rising CO2 levels in the air one breathes eventually leads to hypercapnia (CO2 poisoning).

When it comes to keeping a human alive in a low-oxygen, low-temperature environment such as a mountain climber would experience, we usually reach for warm clothes and thermal underwear to retain as much body heat as possible. For oxygen one would use an oxygen tank and a mask — essentially breathing pure oxygen, or environmental air mixed with extra oxygen. We had the most to learn from deep sea diving which comes closest to the experience of breathing in space. Divers use the open-circuit system, or an advanced ‘rebreather‘ system that scrubs CO2 from the air breathed out by the diver.

For Yuri Gagarin, his SK-1 (Скафандр Космический #1: “Skafandr Kosmicheskiy” or “Space Diving Suit”) spacesuit was pressurized, supplied with fresh oxygen and scrubbed of excess CO2 by the systems built into the spacecraft, with the SK-1 suit connected up to them using hoses. Even then, descending to Earth in a Vostok spacecraft required the cosmonaut to eject at 7 km altitude and parachute down to the surface, as the landing of the spacecraft’s descent module was not deemed survivable due to the extreme deceleration.

The Vostok 1 capsule when it was on display at the RKK Energiya museum.

In the end, Yuri managed to stay conscious throughout the entire mission, making it safely back onto the Earth’s surface. This showed that manned space missions in Earth orbit are definitely survivable.

Developing the Portable Life Support System (PLSS) for the Moon

Following a number of firsts achieved in Yuri’s historical mission, both the Soviets and the Americans gained experience at keeping their cosmonauts and astronauts alive and comfortable in Earth orbit. In addition to umbilical cord systems to the spacecraft’s oxygen generating, CO2 -scrubbing and temperature regulating facilities, the Soviets developed for Voskhod 2 an open-circuit oxygen-providing backpack good for about 45 minutes. The next big challenge was to take the spacecraft’s complete life support system and turn it into a portable system.

Apollo portable life support system, as used with the A7L suit.

EVAs (extra-vehicular activities) at the time involved staying quite near the spacecraft, but these were all for short flights in orbit. There was no question that using an open-circuit system on the Moon would be exceedingly wasteful and impractical. Using an umbilical cord to the landing craft would also be impractical for many fairly obvious reasons. The picked solution would become the Apollo Portable Life Support System (PLSS).

The PLSS as used with the Apollo Moon missions had a range of functions:

  • Regulate spacesuit pressure.
  • Provide fresh oxygen to replenish oxygen levels in circulation.
  • Remove CO2, humidity (water), odors and other contaminants from the oxygen.
  • Cool the oxygen as it circulates.
  • Cool and circulate cooling water through tubing in the suit.
  • Provide two-way voice communication.
  • Receive and transmit telemetry of sensors indicating the wearer’s vital signs.

This document (PDF) from Hamilton Standard covers the PLSS system in great detail. The cooling of the circulating oxygen and water is accomplished by leading it past a sublimator: a device consisting out of porous plates through which water is being forced. Upon contact with the vacuum of space, the water freezes into ice, after which it sublimates from this solid phase into gas. This endothermic process causes the sintered nickel plates of the sublimator to become very cold, cooling any oxygen and water that is led over it.

The sublimating process is self-regulating, in that the rate of vapor formation depends on the amount of heat that is applied to the device. The pressure that forces the feed water into the sublimator’s plates is provided by the squeezing of the feed water bladder that’s placed between the PLLS and the user’s back. Though fairly compact, this sublimator can dissipate over 2 MJ (2,000 BTU) peak, making it the equivalent of an air conditioning unit sized for a bedroom. This allows a human in the full heat of a Moon day to stay nice and cool.

Schematic overview of the Apollo PLSS, provided by Hamilton Standard.

The primary oxygen system has the circulation loop, which injects fresh, filtered oxygen into the helmet of the astronaut. This also pressurizes the suit to a comfortable 3.7 PSI (25.5 kPa) or thereabouts using its battery-powered fan. Air is removed from the suit at the extremities, then it enters lithium hydroxide canisters which absorb the CO2 . A subsequent bed of activated charcoal removes trace contaminants, including body odors.

The filtered oxygen flow is cooled by the sublimator, excess water (mostly from respiration and perspiration) is removed by a water separator and stored in a waste water bladder. This waste water is dumped into the waste water tank of the Lunar Module (LM), which also carried spare lithium hydroxide canisters and allowed the astronauts to refill their oxygen and water supplies.

A water cooling loop powered by a single pump circulated cool water through a special garment containing tubes, drawing heat away from the user’s skin, before the warmed up water was cooled again by the sublimator.

With the PLSS backpack attached using umbilical hoses to the suit’s waist, the user would be able to freely move around for a maximum of about seven hours, depending on their metabolic rate, before having to return to the LM for a refill. As the Apollo program progressed, each subsequent moon landing’s crew would feature more advanced PLSS designs that increased the total EVA time.

Houston, We Have a Problem

With each part of the PLSS being essentially a single point of failure, a backup system was provided in the form of the Oxygen Purge System (OPS), another oxygen providing system mounted on the top of the PLSS pack. This could be triggered in case the primary oxygen supply failed, providing oxygen and cooling (from the oxygen depressurizing outside the OPS oxygen tank) for 30-90 minutes, depending on flowrate. This is an open-circuit system, venting out into space.

Another backup is the ‘Buddy secondary life support system’, whereby another astronaut can use their functioning cooling system to cool a second suit if its PLSS’ cooling system fails. With external cooling, the OPS’ operational time can be extended from 30 minutes to 60-90 minutes.

Naturally, each PLSS is also equipped with a transceiver system, allowing for bidirectional communications not only with their fellow astronauts, but also with the control center at Earth. This would prove to be invaluable during each mission, while navigating the terrain and setting up experiments. The psychological comfort of constant contact also shouldn’t be underestimated.

To Space and Beyond

The Apollo PLSS was of course not the end of the line for this kind of system. As further moon missions were canceled and the focus shifted towards low Earth orbit, the PLSS would find use in EVAs, including untethered ones. The Extra-vehicular Mobility Unit (EMUs) as used with the Space Shuttle since the 1980s and now with the International Space Station (ISS) employs a PLSS that is essentially the evolution of the system that was pioneered by engineers during the 1960s.

Colony in space, with a clear view of nearby planets.

Now that humanity is on the verge again of returning to the Moon, and likely beyond it for the first time in its history, we can only hope that before long humans can once again enjoy the freedom that a PLSS provides as they frolic around on the moon, setting up experiments, cracking jokes with Earth control and being awed by the wonders of this amazing world that exists right outside the reaches of Earth’s atmosphere.

13 thoughts on “Apollo’s PLSS And The Science Of Keeping Humans Alive In Space

  1. ” The psychological comfort of constant contact also shouldn’t be underestimated.”

    Many cellphone owners know this.

    “Though fairly compact, this sublimator can dissipate over 2 MJ (2,000 BTU) peak, making it the equivalent of an air conditioning unit sized for a bedroom. This allows a human in the full heat of a Moon day to stay nice and cool.”

    I could see this applied to more than astronauts. Space, a cool idea.

    1. With each summer getting steadily hotter I’m currently thinking about ordering a mini fridge compressor online, a small liquid-to-liquid heat exchanger, some surgical tubing, a condenser coil, a heavy computer fan, and building my own little air-conditioned stillsuit. Instead of chilling air it can chill water and circulate it around. Maybe even collect condensate from ambient humidity, who knows. Looks like I’ll need one eventually. I think I read that parts of southeast Asia are already nearing 35 Celsius wet-bulb temperature, at which point normal human temperature regulation breaks down and perfectly healthy people start to croak. Sweat doesn’t carry any more heat away at that point.

      Would be a nice addition when riding my motorbike to work. Could have a little 12v pigtail at the waist or whatever, plug it into the engine’s alternator. Batteries probably won’t keep it going for terribly long. No hard vacuum around here to make a sublimator do that work for me.

  2. I love the sublimator cooler. A fairly simple, self regulating, likely foolproof, and highly functional device. This could be a good example of what I would call beatiful design. You don’t always need program code to make things work the way you want…

    1. Yes! If something can be made stable with regular old analog mechanics, one should seriously consider just doing that. I often look at a bicycle’s steering and rake and the system of balance that entails and wonder if we’d ever come up with such a thing if we had access to microcontrollers and PID control loops back then. Probably would’ve bodged something else up long before the first person learned to balance a bike.

  3. I hired on to a company in 1970 that had acquired a small meter manufacturing company some years before. They made miniature, highly rugged, mil spec analog meters. The meters were all hand made. This company supplied the “Remaining Oxygen” indicators used on the PLSS. The astronauts made it look easy, but can you imagine what the people that made those meters went through whenever the astronauts were skipping and jumping around on the moon?

    Of course now that bit of information would just be another number on a heads up digital display. It is good to remember how crude the technology was that got us to the moon and back.

  4. I’ve spoken with NASA engineers who are/were/are-again working on the next generation of EVA suits for the moon (and maybe Mars). The biggest problem was that the old suits leaked “air” like a sieve. The zipper on the back being the biggest offender.

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