The PLSS: A Life Support System You Wore on Your Back
How Hamilton Standard packed oxygen, cooling water, CO2 scrubbing, communications, and telemetry into an 84-pound backpack that kept astronauts alive on the lunar surface
When an Apollo astronaut stepped off the LM’s footpad onto the lunar surface, he was wearing his own spacecraft. The Portable Life Support System—the PLSS, pronounced “pliss”—was the rectangular backpack that kept him alive outside the Lunar Module. It supplied oxygen at 3.7 psi for breathing and suit pressurization. It circulated cooling water through a network of tubes in his undergarment to carry away metabolic heat. It scrubbed carbon dioxide from his breathing loop. It provided two-way voice communications and biomedical telemetry. And it did all of this for up to 7 hours, running on batteries and consumable supplies, while the astronaut hiked across a landscape that would kill an unprotected human in approximately 15 seconds.
Hamilton Standard Division of United Aircraft Corporation designed and built the PLSS. It was, by any measure, the most compact and capable life support system ever built for individual human use. The unit weighed approximately 84 pounds on Earth (about 14 pounds in lunar gravity) and measured roughly 26 by 18 by 10 inches. Everything a human needed to survive in a vacuum—atmosphere, thermal regulation, CO2 removal, humidity control, and communications—was packed into a box slightly larger than a carry-on suitcase.
The Oxygen System: Breathing in a Vacuum
The PLSS carried oxygen in a single high-pressure tank charged to approximately 1,020 psi. A two-stage regulator reduced this to the suit operating pressure of 3.7 psi (lower than the LM cabin’s 4.8 psi, to reduce the suit’s stiffness and improve mobility). The oxygen flowed into the suit at the helmet area, creating a downward flow pattern that swept past the astronaut’s face, through the torso, and out at the extremities, where it was collected and returned to the PLSS through a return hose.
The flow rate was approximately 6 cubic feet per minute—enough to provide breathing oxygen, maintain suit pressure against the small leakage inherent in the suit’s joints and seals, and flush carbon dioxide away from the astronaut’s face. The downward flow pattern was critical: without it, exhaled CO2 would accumulate in a bubble around the astronaut’s face inside the helmet. In zero gravity or low gravity, there’s no convective mixing—warm, CO2-rich exhaled air doesn’t rise. It stays put. The forced flow from the PLSS prevented CO2 buildup by continuously washing fresh oxygen past the face.
The oxygen supply was finite—enough for the planned EVA duration plus a contingency reserve. On the later J-missions (Apollo 15, 16, 17), where EVAs extended to 7+ hours, the oxygen budget was tight. The astronauts were briefed on the expected consumption rate and could monitor their suit pressure as a rough indicator of system health. A suit pressure drop could indicate a leak—a scenario that demanded immediate return to the LM.
CO2 Removal: Lithium Hydroxide in the Loop
The PLSS used a lithium hydroxide canister—the same chemistry as the LM’s cabin ECS—to scrub carbon dioxide from the circulating oxygen. The canister was a cylindrical cartridge packed with LiOH granules, sized for the expected CO2 production during a single EVA. Used canisters were replaced between EVAs.
The CO2 removal capacity was the primary duration-limiting factor for the PLSS. An astronaut performing moderate to heavy physical work (walking, carrying equipment, drilling core samples) produced CO2 at a higher rate than one performing light work (standing, observing, photographing). The LiOH canister had a fixed capacity, and harder work consumed that capacity faster. The flight surgeons and EVA planners built the EVA timeline with metabolic rate estimates for each activity, and the CO2 budget was tracked against the canister’s rated life.
On later missions, as EVA durations increased and surface activities became more strenuous (the Apollo 15-17 crews performed extensive geological traverses with the Lunar Roving Vehicle), the CO2 margin became a serious planning constraint. The PLSS canisters were redesigned for the J-missions with slightly larger LiOH charges, and the EVA timelines were carefully planned to balance high-exertion activities (driving, sample collection, drilling) with lower-exertion rest periods.
The Liquid Cooling Garment: 300 Feet of Tubing Against Your Skin
A human performing physical work in a sealed suit generates metabolic heat that, without active removal, would raise body temperature to dangerous levels within an hour. The suit itself is an insulator—multiple layers of fabric, thermal protection, and micrometeorite shielding trap body heat effectively. On the lunar surface, in direct sunlight, the suit’s outer surface temperature could exceed 250°F while the astronaut inside needed to maintain a core temperature near 98.6°F.
The PLSS’s primary thermal control mechanism was the Liquid Cooling Garment (LCG)—a lightweight, form-fitting undergarment with approximately 300 feet of thin polyvinyl chloride tubing woven through it. Cool water from the PLSS circulated through this tubing network, picking up body heat through conduction and carrying it back to the PLSS for rejection.
The water entered the LCG at approximately 45°F and exited at roughly 70-80°F, depending on the astronaut’s metabolic rate. The PLSS’s water pump circulated the coolant at about 4 pounds per minute—a flow rate balanced between adequate cooling capacity and the pump’s power consumption. The astronaut could adjust the cooling level by turning a valve on the suit’s chest-mounted remote control unit, diverting more or less water through the LCG. Higher flow meant more cooling; lower flow meant less.
The heat picked up by the LCG water was rejected through a porous-plate sublimator in the PLSS—the same type of device used in the LM’s ECS. Feedwater was supplied to a porous metal plate exposed to the lunar vacuum. The water seeped through, froze, and sublimated, carrying the heat away as water vapor. The sublimator consumed feedwater at a rate proportional to the heat load—about 1 to 1.8 pounds per hour during moderate activity.
The feedwater supply was the second duration-limiting consumable after the LiOH canister. The PLSS carried approximately 8.5 pounds of feedwater (on J-mission units), and the consumption rate during strenuous activity could deplete this supply before the oxygen or CO2 capacity was exhausted. EVA planners monitored the predicted feedwater consumption alongside the CO2 budget to ensure neither limit was exceeded.
Communications: Talking Through the Backpack
The PLSS included a VHF transceiver that provided two-way voice communication between the astronaut and the LM’s relay system, which retransmitted the signal to Earth via the S-band antenna. The voice link was the astronaut’s connection to his crewmate, to the LM, and to Mission Control.
The transceiver operated in the VHF band (about 259 MHz) with a transmit power of approximately 0.2 watts. The antenna was a small whip mounted on the top of the PLSS. The range was limited to a few miles—sufficient for all planned surface activities, but the signal quality degraded with distance and terrain obstruction. Astronauts who ventured behind hills or into craters sometimes experienced brief communication dropouts as the line of sight to the LM was obstructed.
The communications system also transmitted biomedical telemetry—the astronaut’s heart rate and respiration rate, measured by sensors in the suit’s biomedical harness. This data was received at the LM, relayed to Earth, and monitored by the flight surgeons in Mission Control. The surgeons used the heart rate data to assess the astronaut’s workload and fatigue level, and could recommend rest periods if an astronaut’s heart rate exceeded sustained limits.
The biomedical monitoring was not just precautionary. During Apollo 15’s third EVA, Dave Scott’s heart rate showed irregularities (bigeminal rhythm—a pattern of premature ventricular contractions) that the flight surgeons monitored closely. The irregularities were attributed to potassium depletion from the crew’s inadequate fluid and food intake during the demanding surface activities. The incident led to changes in crew dietary protocols for subsequent missions.
The Oxygen Purge System: The Emergency Backup
Mounted on top of the PLSS was a smaller unit called the Oxygen Purge System (OPS)—an emergency backup that provided approximately 30 to 75 minutes of breathing oxygen (depending on the suit leak rate) in case the PLSS failed. The OPS was a simple, self-contained unit with a high-pressure oxygen bottle, a regulator, and a feed line that connected directly to the suit.
If the PLSS failed—loss of oxygen flow, loss of cooling, fan failure—the astronaut activated the OPS by pulling a red handle on the unit. The OPS opened its oxygen valve and provided a continuous flow of oxygen into the suit. The oxygen was not scrubbed—there was no LiOH canister in the OPS—so the astronaut would be breathing a mixture of fresh oxygen and his own exhaled CO2. The CO2 concentration would rise gradually, and the 30-75 minute supply was based on how long the astronaut could function before CO2 levels became incapacitating.
The OPS also provided no cooling. With the PLSS offline, the LCG water pump would stop, and the astronaut would begin overheating within 15-20 minutes during moderate activity. The OPS was not designed for continued EVA operations—it was designed to give the astronaut enough time to return to the LM, get inside, close the hatch, and repressurize the cabin, where the LM’s ECS would take over life support.
The walkback constraint—the maximum distance from the LM that an astronaut was allowed to travel—was calculated in part based on the OPS supply. If the PLSS failed at the maximum distance, the astronaut had to be able to walk back to the LM within the OPS’s endurance. On the J-missions, with the Lunar Roving Vehicle enabling traverses of several miles from the LM, the walkback constraint was calculated assuming the rover was also inoperable (since a PLSS failure and a rover failure might have the same cause). This limited the traverse distance to what the astronaut could walk on foot within the OPS supply—roughly 1 to 2 miles, depending on terrain.
Donning and Doffing: Getting Into the Backpack
The process of donning the PLSS—attaching it to the suit, connecting the oxygen hoses, cooling water lines, and electrical connections, and verifying all systems before depressurizing the cabin—took approximately 45 minutes. The crew helped each other, since many of the connections were on the back of the suit and couldn’t be reached by the wearer.
The connections between the PLSS and the suit were multiple quick-disconnect fittings:
- Oxygen supply hose (from PLSS to suit inlet)
- Oxygen return hose (from suit to PLSS for CO2 scrubbing)
- Cooling water supply line (from PLSS to LCG inlet)
- Cooling water return line (from LCG to PLSS)
- Electrical connector (for communications, telemetry, and the suit’s remote control unit)
Each connection had to be verified for secure engagement and leak-free sealing. An oxygen connection leak would reduce the PLSS’s endurance by wasting oxygen to the cabin (or to vacuum, after depressurization). A water connection leak would contaminate the suit interior with free water—a serious nuisance in low gravity, where water globules would float and could obstruct the astronaut’s vision or interfere with equipment.
After the EVA, the crew returned to the LM, repressurized the cabin, and doffed the PLSS units. The units were inspected, spent LiOH canisters were replaced, feedwater was replenished from the LM’s supply, and batteries were swapped—each step following a checklist procedure that ensured the PLSS was ready for the next EVA. On missions with multiple EVAs (Apollo 15, 16, 17 each had three), the between-EVA refurbishment procedure was a critical timeline item.
84 Pounds of Everything
The PLSS was, in miniature, everything a spacecraft was—an atmosphere, a thermal control system, a communications system, a power supply, and a safety margin, all packaged to be carried by a single human being. Hamilton Standard’s engineers faced the same design challenges as Grumman’s LM designers: weight constraints, power budgets, consumable limits, redundancy trade-offs, and the unforgiving requirement that every system work perfectly in an environment where failure meant death.
The PLSS worked on every EVA of every Apollo mission. No astronaut ever activated the OPS in flight. No PLSS failed during a surface excursion. No EVA was cut short by a PLSS consumable running out ahead of schedule. The system supported 14 EVAs across six landing missions, totaling roughly 80 hours of surface exploration—80 hours during which the only thing between each astronaut and the vacuum was a suit pressurized by a backpack running on batteries and feedwater.
The moonwalkers moved across the lunar surface carrying their own life support, their own communications, their own cooling. They carried it all on their backs, 84 pounds on Earth reduced to 14 in lunar gravity, and the system that kept them alive was quiet enough that they forgot about it—which, for a life support system, is the highest possible compliment.