Mark Yates charts the development of Apollo’s portable life support system
WHEN this issue of The Chemical Engineer lands on doorsteps, the world will be celebrating the 50th anniversary of a very different touchdown, when the Apollo 11 mission allowed Neil Armstrong and Buzz Aldrin to first set foot on the Moon.
Whilst the story of design and development of the Apollo spacesuit is worthy of an article in its own right, this is the story of the backpack – the portable life support system (PLSS and pronounced “PLISS”) that would keep Armstrong and Aldrin alive when out on the lunar surface.
In October 1962 NASA awarded a contract to aircraft propeller firm Hamilton Standard to develop and produce a number of development and flight articles, including the PLSS. Its design, development and enhancements are an excellent example of good system design which incorporated many of the key elements of sound engineering that we use today. Its designers would have to meet and exceed a series of demanding and changing requirements leading up to the flight of Apollo 11. As with all engineering projects, it would also undergo subsequent modifications and upgrades resulting in eight distinct engineering versions. The last two versions of this backpack would enable multiple moonwalks of increasing duration and complexity over six missions and keep 12 men alive out on the lunar surface of the Moon for a little over 154 hours.
In 1962, President Kennedy’s challenge to the American people was clear – a man on the Moon by the end of the decade. Vast resources were focussed on the rapid development of two new spacecraft and the gigantic Saturn V rocket that would be needed to lift them to the Moon to realise that goal.
Kennedy’s challenge would only be met on 21 July 1969 when man’s first footprints would reach the lunar surface – an event that was witnessed by the public as it unfolded. It is said that more people on Earth witnessed the first Apollo 11 moonwalk than any other spectacle up to that point.
Armstrong’s heartbeat had hit 156 beats per minute when the lunar module, Eagle, reached the surface of the Moon and although they were down safely, no one knew exactly where they had landed, only that they had landed long and overshot the target point. The landing itself had been dramatic – a number of programme alarms had everyone on their toes and there would be less than 60 seconds remaining in the descent engine fuel tanks.
Having just achieved the first manned landing on the Moon, those first historic steps on another world would require Armstrong and Aldrin to fully depressurise and leave the pressurised cocoon of the lunar module and descend a ladder to the lunar surface. A television camera would beam back to earth the historic but ghostly black-and-white images of Armstrong descending the ladder and taking those first steps on another world.
Apollo 11 would have a single extravehicular activity (EVA) and would be cut down to just two hours. A second EVA would have been possible but it was conservatively decided to be reduced to just one.
The lunar explorers would not be able to use the hoses of Eagle to supply cooling and oxygen whilst on the surface; instead, their individual PLSS backpacks would turn each spacesuit into the smallest of the Apollo spacecraft. The bulky PLSS backpacks could be lifted one-handed as they prepared for their moonwalk, a feat only Superman could have achieved back on Earth!
Finally, with suit checks complete and with the reassuring sound of oxygen swishing around their helmets it was time to depressurise the lunar module cabin – from 5 psi (~34.5 kPa)to vacuum, their suits pressurised to 3.75 psi, a compromise between competing requirements of suit mobility and preventing “the bends” (decompression sickness).
Venting the oxygen out of Eagle’s cabin took longer than anticipated because of filters installed on the vent in the hatch. These filters were there to trap any Earth germs that could potentially contaminate the lunar surface.1
Eagle’s rectangular hatch could be opened inwards, allowing Armstrong to back out of the hatch feet first, facing backwards and low enough that his bulky PLSS would clear the top rim of the hatch.
As he descended the ladder he pulled a handle to release the module equipment stowage assembly (MESA), which in turn activated the black-and-white camera which transmitted live images of his descent to the lunar surface and entry into the history books.
On leaving NASA in 1970, Armstrong would teach aeronautical engineering at the University of Cincinnati, including a specific course on the systems design behind the PLSS (some of his hand-drawn slides are used in this article).
Like all good engineering projects, the functional requirements for the Apollo PLSS were identified clearly early on. The job was made easier, as NASA had chosen a single gas oxygen environment for all its spacecraft on the grounds of simplicity and hence enhanced reliability. Such a design would also remove the need for the astronauts to pre-breathe oxygen prior to donning the spacesuit and backpack. However, the selection of pure oxygen would introduce an additional fire hazard, and NASA adopted some very stringent fire safety material standards for equipment items that would be immersed in a pure oxygen environment.
The PLSS functional requirements were:
Out on the hostile lunar surface 240,000 miles from home, the PLSS and spacesuit would simply have to work. Its first use on the lunar surface would be one of unprecedented attention and huge national prestige. On the later missions the astronauts would be travelling out of sight of their lunar lander, where a failure so far from their lunar base could prove fatal.
Time on the lunar surface was as precious as it was finite so the reliability of its systems was key. From the outset the design philosophy was one of simplicity (the key to high reliability) - with a minimum number of total parts overall with very few moving parts and with ample performance margins and redundancy if required. Reliability models would guide the engineers in the selection and specification of components. Then there were the five hoses that connected the backpack to the front of the spacesuit – these connections would need to be connected and disconnected many times so had to be of high integrity with no tolerances for leaks or losses from this closed system. There would be no means to try and fix or repair a faulty backpack during the mission. There was however a contingency procedure prepared in the event that only a single crewman would venture out onto the surface with the other crewman’s suit connected to the lunar module’s systems.
The lunar surface is hostile to life. Lacking oxygen and water, it experiences extremes of temperature in sunlight (1180C) and in the shadows (-1730C). There is no atmosphere to attenuate the Sun’s rays. The heat flux from the Sun was 440 Btu/h/ft2 or a calculated total heat load of 10,000 Btu/h over the whole of the spacesuit’s exposed surface area.2 The design of the spacesuit with its 11 different material layers would limit the heat leak into the suit at roughly 250 Btu/h during the day and 350 Btu/h out of the suit during the lunar night.2 This insulation would prove to be so efficient that it would ‘trap’ the heat generated by the astronaut and the mechanical systems (such as the circulation pumps, fans etc) and this additional heat needed to be disposed of.
The rates of oxygen consumption and generation of carbon dioxide were well known to medics and engineers but other factors had to be carefully controlled. Humidity inside the suit was one such factor – too low, and eye irritation would occur and with it the potential for static charge to build up, leading to an ignition hazard, something that NASA had tragically learnt would be fatal in a pure oxygen environment. If humidity got too high due to exhaled water vapour the environment would become unpleasant and in a zero gravity environment water droplets would form and could be inhaled, with the very real potential for drowning. Excess moisture would also prevent the lithium hydroxide from efficiently ‘scrubbing’ the exhaled poisonous carbon dioxide from the suit’s atmosphere.
The design changed in 1967 when NASA specified that both astronauts would venture out onto the Moon’s surface. Today it seems incredible that one crewman would have been expected to remain inside the lander whilst his colleague explored a whole new world alone
Despite these requirements being clearly laid out, the design changed in 1967 when NASA specified that both astronauts would venture out onto the Moon’s surface. Today it seems incredible that one crewman would have been expected to remain inside the lander whilst his colleague explored a whole new world alone. This change required further modifications – principally in the area of crewman communications and relaying telemetry, which increased the complexity of the integral communication systems in the backpack.
The PLSS also had to be designed so it could be recharged between moonwalks. Consumables such as water, oxygen, lithium hydroxide and battery power needed to be easily replenished or changed out for fresh components. (The lithium hydroxide would be packed into cylindrical cartridges that were discarded once depleted.)
In addition, there were weight limits and physical constraints on how big the PLSS could be, as the backpack and its owner had to fit through the lunar module’s rectangular hatch. He also had to be able to stand upright without the backpack adversely affecting his centre of gravity – learning to walk on the Moon would prove to be tricky enough. Telemetry would include information such as heartrate, body temperature of the astronauts and also critical engineering data such as flow rates, pressures, and temperatures within the backpack that would be relayed to the doctors and engineers in Mission Control in Houston.
The flowrate of oxygen continually circulated around the suit and backpack was relatively low – just sufficient to prevent carbon dioxide buildup in the helmet and to prevent the visor from fogging up, not a welcome occurrence when out on the lunar surface! In addition to removal of carbon dioxide, the oxygen returning to the backpack would contain moisture, waste heat from the astronaut, and various gas contaminants. The outlet oxygen hose from the suit routed these gases to the backpack and through a contaminant removal package. From there it was routed to a heat exchanger and free water removed by means of a static elbow water separator which induces an acceleration to trap the water, and through capillary action it was then routed to a water reservoir for storage. Meanwhile, the cleaned, cooled and dry oxygen was then accelerated by a battery-powered centrifugal fan through a ventilation flow sensor and then a check valve (thereby preventing reverse flow) before exiting the backpack via the oxygen supply hose back into the suit and the whole process was repeated.
Over time, oxygen would be removed from the closed system by suit leakage or oxygen that had been converted into carbon dioxide and subsequently removed from circulation by the lithium hydroxide. The primary oxygen system provided pressurisation via a single-stage in-flow pressure regulator and replaced the oxygen lost in this way.
Lithium hydroxide canisters are used in rebreathers, spacecraft and submarines and removes carbon dioxide from exhaled gas – it is an irreversible reaction that produces lithium carbonate and water. Anhydrous hydroxide is used in spacecraft due to its lower mass and reduced water production. The reaction is exothermic (it generates heat) and therefore contributes to the heat generated within the overall system and had to be included within the heat generated by the backpack itself.
Each spacesuit was individually hand-made for each moonwalker and had a known leak rate - (in the case of Apollo 12 Commander Pete Conrad it was 0.0169 lb/h, or 7.68 g/h). This needed to be allowed for by the oxygen supply system (and also needed to be accounted for in the calculation of metabolic rate from oxygen consumption data).
In spite of the strong emphasis on simplicity and reliability, NASA engineers called for an entirely independent backup system to be added in the event the PLSS failed or was degraded in some way during operation. Again for its design, simplicity was key – it comprised two spherical oxygen tanks (containing 5.8 lbs, or 2.64 kg of oxygen at 5,800 psi), a pressure regulator and a heater supplied by an electrical battery.
The oxygen purge system (OPS) was housed separately on top of the PLSS and designed as a modular unit so that it could be used without the PLSS. Whilst the PLSS would be discarded on the Moon by the departing crew, both OPS units would be retained. The OPS had two further uses – it could be used by the astronauts in the event that they were unable to transfer to the command module from the lunar module through the docking hatch and would have to transfer in their suits by climbing along the outside of the two spacecraft. It was also used as a backup system on the last three missions to the Moon, during the three deep-space EVAs conducted by a single crewman (the primary oxygen was supplied via an umbilical from the spacecraft) whilst heading back to Earth to retrieve experiments and photographic film housed on the outside the spacecraft.
The OPS had two flow rates which could be selected depending upon the PLSS malfunction. A purge valve was located in the spacesuit within easy reach of a gloved hand. When activated, the valve slowly discharged the suit’s oxygen supply into space. The first valve setting was a low flow rate of 4 lb/h, which would remove carbon dioxide and humidity with liquid cooling still provided by the PLSS. In the event of a complete PLSS failure, the second valve setting allowed a flow rate of 8 lb/h and would also provide gas cooling as well, albeit for a shorter duration.
One of the fundamental parameters that influenced design was the predicted metabolic rates of an astronaut out on the lunar surface. Initial assumptions that metabolic rate would be lower under one sixth of gravity on Earth were countered by concerns that mobility would be problematic with traction on the surface being a big unknown. Heat generated would be a combination of solar heat absorbed from the surroundings, heat generated by the operating backpack and the astronaut’s own metabolism.
(Metabolic rate can be defined in British thermal units – an imperial unit of energy, the amount of energy needed to heat or cool one pound of water through 1oF. It is most often used as a measure of power in the steam generation, power, heating and air conditioning industries – its units are Btu/h.)
Pilots in aircraft and astronauts in earlier spacecraft had been cooled using gas circulation alone. However, whilst oxygen would provide some level of cooling as it evaporated sweat from the skin, it was evident from the experience in vacuum chamber tests and later during EVAs during the successful two-man Gemini missions in 1965–66 that gas cooling alone was not likely to be the solution. This was a big surprise – metabolic heat generation was much higher than predicted and this was in a zero-gravity environment. Cooling the astronaut by gas flow alone was not going to be sufficient and the space medics were now predicting metabolic rates possibly as high as 3,000 Btu/h. NASA would modify its requirements by stipulating that the PLSS be designed to handle a peak rate of 2,000 Btu/h and average of 1,600 Btu/h.3
This new requirement meant that liquid cooling would have to be considered. However, it introduced three new engineering problems and it was time for the engineers to get creative and develop some innovative solutions.
The first of these challenges was how to design a liquid cooling garment that could be worn by the astronaut that would be in direct contact with the skin, would not kink and hinder circulation and yet would not restrict movement for the wearer. The second was how to remove this waste heat, and the third and final problem was the requirement for a pump to circulate the cooling water.
Fortunately there was no need to start from scratch as there was some existing research available that had been conducted by the British at the Royal Aircraft Establishment in the 1950s which had looked at both heating and cooling of pilots, firemen and deep-sea divers using hot or cold water circulated in a vest. The engineers were able to draw on this, and like all good ideas, a process of experimentation followed – the first prototype comprised 300 ft of 3/16” vinyl tubing wrapped around the torso, legs and arms of a test subject who was then covered in additional layers of warm clothing and an outer layer of plastic to contain all perspiration. Under medical supervision the volunteer, an HSD engineer called Harlan Brose exercised on a treadmill for tests up to 2 hours in duration.4
Initial results were promising, and later developments led to a single one-piece garment – a pair of long johns with liquid cooling tubes held directly against the skin. It comprised an open mesh that also permitted air circulation which provided further cooling by evaporating perspiration away from the skin. In effect it was a heat exchanger made of plastic tubing.5
For the problem of waste heat disposal the engineers had to get really creative, mindful that reliability and simplicity was the key.
Several engineering solutions were developed and evaluated, including an early design using a plate fin wick-fed boiler with a back-pressure control valve opened to the vacuum of space. This design worked, but was prone to blocking and was very temperamental.
The winning design would prove to be very elegant by nature of its simplicity, as it had no valves, moving parts or power requirements and would not require any instrumentation or diagnostics. It was simply a static porous plate (a surface that has very small spaces – microscopic pores – through which air or liquid may pass) that acted as a radiator. One side of the plate was exposed to the vacuum of space and the other was in contact with the warmed water that has been circulated in the cooling loop. The warmed water is sucked into these pores where it freezes because it experiences a pressure and temperature drop when exposed to the vacuum of space. The ice then sublimes (it evaporates without melting first) and each plug gradually gets smaller due to mass loss. Ultimately, the ice plug in the pore is ejected and the pore refills with warm liquid water and the process continues. The size of these pores would be critical – they had to be large enough to meet the required flow rate of water but small enough so that ice forming in the pores would not expand and damage the surface of the plate. As the rate of sublimation was directly proportional to the heat flow it was an elegant engineering solution with no moving parts.
The third challenge was how to design a water pump that would be both reliable and would not place greater power demand on the battery capacity and increase an already considerable mass and volume contribution to the PLSS overall. Centrifugal pumps were evaluated but found to be very inefficient at the relatively low flow rates and pressures (4 lb/min with a head of 5.65 psi) requiring a power demand of 30 W. A novel diaphragm pump design was needed and the Whittaker Corporation was contracted to deliver a prototype of a pump it had under development. Its novel design had some fascinating features.
Two small diaphragms are located in separate chambers each with inlet and outlet valves at either end of a ‘walking beam’ which is supported by a torsion rod at its centre.
At each end of the walking beam structure is a magnetic armature which can move in either direction in response to an electromagnetic field, so as the armature moves it displaces the diaphragms. The polarity can be reversed by an electronic control at a frequency chosen to nearly coincide with the natural resonant frequency (all objects have a natural frequency and will vibrate strongly when subjected to a vibration on or close to it) of the spring mass system, thereby greatly improving the efficiency of the system. The system’s power demand was 10 W, a third of the competing conventional centrifugal pump
The water circulation system comprised this innovative diaphragm pump that forwarded water from the suit to the sublimator and then through a diverter valve that had three settings that could be selected by the astronaut at will, depending on the level of cooling required. Lastly, a gas separator ensured that only gas-free cooled water was routed through the water supply hose to the liquid cooling garment and the cycle repeated.
Figure 7 shows the measured duration of PLSS consumables as a function of calculated metabolic rate and clearly shows that the PLSS design met or exceeded its performance requirements. In addition, the metabolic rates experienced on the Moon were below the assumed 1,200 Btu/h design criteria for a four-hour EVA (see Table 1). In fact, the initial measurements of metabolic rate on Apollos 11 and 12 gave the engineers and mission planners the confidence to exceed the 4-hour time constraint spent on the surface by the Apollo 14 crew.
Apollo 9 would present the only opportunity to test the backpack in the zero-gravity environment of Earth orbit in March 1969. The test lasted just over an hour but it was significant for two reasons: firstly it would be the first time that any astronaut would float free of their spacecraft and be completely reliant on the suit and backpack; and secondly because the second time the backpack would be worn would be on the first moonwalk. The EVA was a great success and the backpack performed flawlessly. The only practical modification that was needed was the installation of an external camera bracket on the front of the EMU control unit to reduce camera shake. The PLSS was recharged after the flight to demonstrate its replenishment capabilities for later missions.
The engineers were able to monitor live key metrics of the PLSS performance such as temperatures, pressures, flow rates and voltages during operations on the lunar surface. The design philosophy adopted for a simple reliable configuration with minimal moving parts paid off handsomely and despite the OPS emergency system being thoroughly engineered and tested in vacuum chambers on Earth it would be carried to the Moon but would never be needed. The PLSS was considered to meet or exceed all of its design requirements and still retain ample margin of safety. The -7 PLSS would enable the later ‘J’ missions to conduct significantly greater scientific exploration, ironically at a time when the public interest in going to the Moon was waning.
As to the question of actual metabolic rates, this was a key performance metric as it allowed the medics and mission controllers to plan future missions with a good understanding of the limitations of the crew and equipment. However, metabolic rate could not be reported directly whilst on the lunar surface and a number of calculation methods were used at the end of the mission. These calculations were based upon oxygen consumption rate, heart rate, feedwater consumption and thermal balance – each introducing its own sources of error, such as scale reading and so an average value was calculated.
All 12 PLSS backpacks would be unceremoniously jettisoned prior to leaving the Moon, as weight and space was at a premium, and equipment deemed surplus to requirements was carefully manifested and jettisoned from the lunar module’s front hatch
As an example, in the case of the feedwater calculation the amount of cooling water that was sublimed off into space is a direct measure of the heat load on the PLSS during an EVA. Weighing the PLSS using a lunar calibrated spring balance at the end of an EVA gave the engineers some hard data on the amount of water that had been used. The amount of water consumed multiplied by the heat of conversion from a liquid to solid to a vapour yields the total heat removed. From this, total electrical heat loads and heat generated by the reaction with carbon dioxide and lithium hydroxide are subtracted and divided by the duration of the EVA - giving an average metabolic rate for each crew member. The heart rate method was the least accurate method and for that reason was not used but offered advantages in that heat load calculations could be made for specific surface activities in real time, thanks to live bio data being relayed and these calculations did not need to be averaged over the duration of an EVA lasting many hours.
In this way, metabolic rates for each astronaut per EVA were prepared. For comparison, the lowest rate was Armstrong’s at 777 Btu/h, whilst his colleague Aldrin would average the highest (1,118 Btu/h).
The Apollo programme would conduct 15 lunar surface EVAs over six missions without incident. The penultimate moonwalk of Apollo 17 would set a record for the longest EVA, at 7 hours and 37 minutes on 12 December 1972, and is about the limit of time that people can be expected to work intensively and productively anywhere. This record would be unbroken for 20 years, an impressive accomplishment. The crew of Apollo 17 would also traverse 35.7 km on the Moon’s surface. By comparison, the unmanned exploration rover Opportunity on Mars took eight years to cover the same distance.
All 12 PLSS backpacks would be unceremoniously jettisoned prior to leaving the Moon, as weight and space was at a premium, and equipment deemed surplus to requirements was carefully manifested and jettisoned from the lunar module’s front hatch. The backpacks still remain today on the surface, strewn around the lunar module’s descent stage, along with other miscellaneous equipment.
Having safely returned to Earth, the Apollo 11 crew was held in quarantine for three weeks in the event of potential biological contamination. These three weeks also allowed NASA to debrief the crew on all aspects of the flight and specifically, exploration of the lunar surface. On the operation of the PLSS both Armstrong and Aldrin were complimentary, Armstrong saying: “There were a lot of things to do, and we had a hard time getting them finished. We had very little trouble, much less trouble than expected, on the surface. It was a pleasant operation. Temperatures weren’t high. They were very comfortable. The little EMU, the combination of spacesuit and backpack, sustained our life on the surface, operated magnificently. The primary difficulty was just far too little time to do the variety of things we would have liked. We had the problem of the five-year-old boy in a candy store”.7,8
Armstrong’s association with the PLSS would not end there. On leaving NASA in 1970, he taught aeronautical engineering at the University of Cincinnati, US, for nearly a decade, creating courses such as aircraft design and flight test, doing it with the same humility and quiet dedication he always had. One of his lecture courses was on systems design and he chose the design of the PLSS that kept him alive on the Moon as an example for his engineering class. His lectures used a series of hand-drawn overhead projector slides that were used in the classroom (some of these slides survive to this day, and three are used within this article).
It seems apt to let the man who spoke the first words on the lunar surface to have the last words on the future of exploration. When interviewed prior to the flight of Apollo 11, Armstrong said: “I think that we are going to the Moon because it is in the nature of the human being to face challenges. It’s by the nature of his deep inner soul we are required to do these things just as salmon swim upstream.”
All 12 PLSS backpacks will remain on the Moon until one day when they may be recovered and take their rightful place in museums back on Earth and maybe one day, even the Moon!
So next time you look up and see the Moon high in the night sky, spare a thought for the bold explorers that made it happen and those innovative engineers that designed these little marvels of engineering that would help make one of mankind’s greatest achievements possible. All 12 PLSS backpacks will remain on the Moon until one day when they may be recovered and take their rightful place in museums back on Earth and maybe one day, even museums on the Moon!
1. First Man: The life of Neil A Armstrong, the authorised biography by James R Hansen.
2. Space Suit Development Status, Richard S Johnson, James V Corredale and Mathew I Radnofsky, Manned Spacecraft Centre Houston, Texas, February 1966.
3. Apollo PLSS: Environmental Control of the Smallest Manned Space Vehicle, John C Beggs and Fred H Goodwin, Space Systems Department, Hamilton Standard, Windsor Locks, Conn
4. “The Apollo Portable Life Support System”, by Kenneth S Thomas. Apollo Lunar Surface Journal.
5. Apollo Experience Report, Development of the Extravehicular Mobility Unit, Charles C Lutz, Harley L Stuteman, Maurice A Carson, and James W McBarron II, Lyndon B Johnson Space Center Houston, Texas 77058, NASA, November 1975.
6. Apollo Portable Life Support System Performance Report, Maurice A Carson, NASA Manned Spaceflight Centre, 1972.
7. Apollo Expeditions to the Moon, NASA SP-350, page 215.
8. Apollo 11, the NASA Mission Reports, volumes 1 and 2.