VAULT // 1965
Space Suit for the Moon
The basic design of the Apollo space suit has been adapted to include in it personal liquidcooling, an approach developed in England.
Written by William C. Kincaide
An illustration from the 1965 article showing the lunar space suit’s various components.
EXPLORERS OF THE MOON will be exposed to one of the harshest environments yet encountered by man. Lunar-surface temperatures will vary from approximately -250 °F to +250 °F as the sun rises and sets against the infinite heat sink of deep space. The almost complete lack of atmosphere will permit the full solar flux to reach the moon’s crust. Indeed, the total atmospheric pressure will be less than 10-1” torr, with a gravity of only one-sixth of the Earth to retain any gases which may evolve from the surface. Cosmic radiation and meteoroid particles will pelt the surface continuously, and both radiation and meteoroid storms will occur intermittently. In addition, the physical characteristics of the surface itself will vary from a crusty plain to crater-packed areas covered with several centimeters of dust.
To isolate the Apollo astronauts from these extremes and allow them to perform useful scientific excursions on the lunar surface, personal protective systems have been under development for the past several years by the Hamilton Standard Division of the United Aircraft Corporation, under contract to the National Aeronautics and Space Administration.
Basically, these systems include a space suit with helmet, boots, and gloves, a thermal insulative overgarment, a micrometeoroid protective garment, and a back-mounted portable life-support system (PLSS), which also contains the communications and telemetry package.
The primary purpose of the portable life-support system is to condition and replenish the atmosphere inside the space suit. The PLSS maintains spacesuit oxygen pressure at 3.7 ± 0.2 psia and controls temperature, carbon dioxide, odor, and moisture levels inside the suit for average metabolic rates of 1,200 to 1,600 Btu/hr and short-term peaks to 2,000 Btu/hr.
Each of the two PLSS units carried to the lunar surface is rechargeable from spacecraft supplies to allow multiple excursions. The system is designed to operate for periods of up to 4 hours without recharging, with 3 hours for nominal excursion and 1 hour reserved for contingency operations.
Apollo 11 Moon Landing. Image: NASA
DEVELOPMENT BACKGROUND
The original concept selected for the Apollo PLSS was basically similar to the Project Mercury Environmental Control System. This system provided cooling and ventilation by circulating oxygen through the space suit at flow rates of 15 to 17 actual cubic feet per minute (acfm). At the high metabolic heat rates produced by the lunar explorer, the gas ventilation system relied almost entirely upon evaporative cooling and thus produced high rates of perspiration by the astronaut. These high perspiration rates, although tolerable, imposed serious thermal discomfort, reducing performance efficiency at a time when mental and physical capacities are most needed. In addition, dehydration may result if the lost water is not replaced and the soaked skin becomes susceptible to maceration and bacterial infection. Also, physical discomforts such as thirst and perspiration in the eyes may occur.
Within the past year, the Apollo PLSS has undergone a major design improvement. The system has been adapted to include a personal, liquid-cooled approach pioneered at the British Royal Aircraft Establishment in Farnborough, England. This unique concept relies upon the circulation of cool water through a network of tubes built into the space suit undergarment in such a way that the tubing comes into contact with the skin. The skin is cooled by direct conduction, and the mean skin temperature is lowered to a level where little, if any, perspiration occurs. This, of course, almost entirely eliminates the problems associated with the gaseous system previously discussed.
Diagrams of the original and present Apollo portable life-support systems from the 1965 article that ran in Mechanical Engineering.
PRESENT PORTABLE LIFE-SUPPORT SYSTEM
The adoption of the liquid-cooling concept has had a dramatic impact upon the performance of the PLSS yet the actual design changes are not as extensive as one might imagine. System cooling capacity has been increased by over 42 percent with only a 6-pound weight growth. At the same time, the basic gas loop has been resized for the liquid PLSS, utilizing, in most cases, the same component concepts.
Comparing the original gas PLSS schematic with the ventilation loop of the liquid-cooled PLSS, the similarity is apparent. Gas circulation is provided by a brushless DC motor centrifugal-fan unit, which produces a constant flow of 6 acfm at 4 inches of water. Since the primary function is now ventilation, gas flow rate has been reduced from 17 to 6 acfm. This reduction in flow rate results in a much more efficient unit, with a savings of over 110 watt-hours (whr) of battery power. The silver-zinc battery has been reduced in size from 324 whr of capacity to only 200 whr.
The contaminant control canister, which contains approximately 22 pounds of lithium hydroxide and 5 ounces of activated charcoal, maintains acceptable carbon dioxide and odor levels. This canister is a radial-flow device in which gas is introduced to the center of a cylindrical cartridge, flows through the lithium hydroxide, and is collected at the periphery. This design effects a substantial reduction in pressure drop over axial-flow canisters because the area of absolute filter material is eight times that of a plain circular axial filter, reducing pressure drop proportionately. The quantity of lithium hydroxide has been increased over the gas PLSS for greater metabolic capacity.
Apollo 11 Moon Landing. Photo: NASA
The high-pressure oxygen subsystem consists of a recharge fitting and check valve, an absolute filter, a primary oxygen pressure vessel, and a primary regulator assembly. The primary oxygen tank is a cylindrical stainless-steel vessel, operating at a nominal pressure of 900 psia. The tank contains approximately 1.1 lb of oxygen for metabolic consumption and a nominal leakage of 200 standard cc/m from the spacesuit assemblage. The primary regulator is a single stage device maintaining suit pressure at 3.7 ± 0.2 psia. The quantity of oxygen also has been increased over that of the gas PLSS to accommodate the increase in metabolic capacity.
The wick-type heat exchanger previously selected for the gaseous PLSS has been replaced with a porousplate sublimator. The sublimator is selfregulated in response to heat loads, and does not require either feedwater or back-pressure control. The coolant, in this case water, is supplied to the sublimator at 3.7 psia from a feedwater reservoir. The water enters the sublimator between the surfaces to be cooled and porous nickel plates. The other sides of the plates are exposed to ambient vacuum conditions. As the water begins to flow through the pores of the plates, it is affected by the vacuum and freezes when the vapor pressure approaches the triple point. In each pore, the ice sublimates directly to space; and at low heat loads, a layer of ice may form on the inside surface of the plate. The thickness of this layer varies with heat load. At high loads; no ice will be present with direct evaporation taking place. An overall coefficient of heat transfer at 160 Btu/hr-°F-sq ft has been obtained in subsystem-development tests. This value is time dependent, owing to degradation of the plate as pores clog.
Moisture collected by the ventilation gas stream from respiration is condensed on the gas side of the heat exchanger. As the condensate leaves the boiler, it passes through an elbow, which throws the droplets against the duct walls. As the water condensate flows along the duct wall, it is trapped in a wick-type water separator and transported to the heat-exchanger feedwater reservoir for storage.
The incorporation of the water-cooling concept in the PLSS requires the addition of four major components, namely, a water pump, a water accumulator, a feedwater reservoir, and a liquid-cooling garment. The feedwater reservoir is not actually a new requirement, since in the wick heat exchanger, water was stored within the wick itself. It is new, however, in that an active bladder expulsion system, rather than the passive-wicking approach, is required. Pressure for the bladder is supplied directly from the ventilation loop through the water separator, which passes both condensate water and gas. As stated previously, the condensate water is stowed on the back side of the reservoir and is disposed of between missions. This waste water cannot be used in the sublimator for two reasons: (a) The differential pressure is in the wrong direction, and (b) the contaminants in the water would tend to restrict the sublimator pores and, over a period of time, degrade its performance. The weight penalty incurred in recycling this water is much greater than the 0.25 to 1.0 pounds of water that would be recovered.
The prime mover for the liquid-coolant loop is a brushless DC motor-centrifugal pump unit which circulates a constant water flow of 4 pounds per minute. Comfort control is accomplished with a heat exchanger by a pass loop and manual diverter valve. The diverter valve allows the astronaut to select one of three inlet water temperatures, 45 °F, 65 °F, or 77 °F, according to the amount of effort he is expending. Liquid-loop pressure and level are controlled by a spring-loaded accumulator. The spring maintains water pressure at from 11 to 20 psia. The accumulator holds approximately ½ pound of water to make up leakage and relieve pressure surges caused by the motion of the man. This loop is normally not recharged; however, it can be topped off to make up leakage.
LIQUID-GARMENT DESIGN
The liquid garment is designed with sufficient cooling capacity to remove a metabolic heat load of up to 2,000 Btu/hr, without relying upon perspiration cooling. The garment is tailored to conform to the body, covering all areas except the hands, feet and head. Fine plastic tubing is attached to the inside surface of the garment in such a way that it will contact the skin of the wearer. Tube-length distribution is proportional to body-muscle distribution, in order to provide local cooling approximately equivalent to heat-generation potentials. Since comfort is generally observed when the extremities are maintained cooler than the trunk, coolant flow from the PLSS is supplied at the wrists and ankles and returned to the PLSS at the waist. Blood circulation is sufficient to cool the hands and feet, since these are not large heat producers.
An open-mesh fabric forms the basic material for the garment itself. The primary reason for this choice is to be compatible with gas-ventilation systems which are used in the Apollo spacecraft. Regular space: suit undergarments are designed to wick the moisture from the skin. In this case, the coolant tubes of the liquid garment would significantly reduce the probable contact area, thereby interrupting the wicking action. The open weave allows direct gas-flow impingement on the skin, promoting sweat evaporation. Although the final tube configuration is still being developed, it appears that a total tube length of about 300 feet will be sufficient. This length will be divided into 40 parallel flow paths to reduce pressure drop. To accommodate 2,000 Btu/hr, with inlet conditions of 47 °F and 4 pounds per minute, the temperature differential will be 9 °F.
CONCLUSION
The decision to implement the water-cooling concept included many trade-offs other than the obvious considerations of weight, volume, cost, and schedule. Since safety of life is a prime objective throughout the manned spacecraft effort, reliability was a major concern. The portable life-support system has bee apportioned a reliability goal of 0.999 for mission success and 0.9999 for crew safety. At first, it would appear that the addition of several new component would hinder the attainment of these reliability go and from a purely mathematical standpoint this is probably true. However, the incalculable reliability advantage gained by providing the astronaut with significantly less stressful environment undoubtedly far outweighs this mathematical penalty. In addition, failure of the liquid loop will not leave the astronaut completely without cooling. The ventilation loop, although only 6 acfm, is still capable of absorbing approximately 850 Btu/hr by perspiration cooling, which, with a tolerable heat storage of 450 Btu by the man, will allow him a safe return to the spacecraft.
A scan of the original article shows clockwise from left a concept undergarment, contaminent control canister, a porous plate sublimator, and feedwater reservoir.
William C. Kincaide was manager of the Apollo Portable Life Support Systems Office in the Crew Systems Division at NASA’s Manned Spacecraft Center in Houston.
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