THERMAX PROTECTION SYSTEM FOR EXTRAVEHICUIAR SPACE SUITS G. B. Whisenhunt1 and R. A . Khezek^
Chance Vought Corporation, D a l l a s , Texas ABSTRACT
A pressure suit system that w i l l provide thermal protection of a worker performing tasks outside a space vehicle as well as provide emergency pressurization within the vehicle i s de- scribed. The system uses an insulated "coverall" garment with a low solar absorptivity outer cover to minimize the effects of the external environment. Calculations indicate that s u f f i - cient heat blockage i s obtained with the coverall garment to allow control of temperature l e v e l s and distribution with a i r circulation from a portable environmental unit. The discussion includes the problems, requirements, methods, and design f o r a cislunar and lunar surface extravehicular space s u i t .
INTRODUCTION
Manned Space Vehicle study programs have indicated that man w i l l be required to perform functions outside the protective enclosure of the sealed spacecraft. These functions may in- clude repairs to the outside of the vehicle, assembly of space station components, exploration of the lunar surface, and transfer between space vehicles ( F i g . 1 ) . Current f u l l - p r e s - sure suits w i l l not adequately protect a space worker from the extreme thermal conditions present in space.
Past studies have shown the d e s i r a b i l i t y of developing a multi-purpose pressure suit system which can be used for emer- gency pressurization within the space vehicle, as well as
Presented at the ARS Lunar Missions Mseting, Cleveland, Ohio, July 17-19, 1962.
1Engineering Specialist, Power and Environment Section, Astronautics Division
^Environment Engineer, Power and Environment Section, Astronautics Division.
G . B. WHISENHUNT A N D R. Α. KNEZEK
protection of the astronaut while performing tasks outside the vehicle. Preliminary analysis indicates that adequate thermal protection for extravehicular operation can be obtained by use of an insulated "coverall" garment worn over the normal vehic- ular s u i t . This approach i s feasible because of the extremely low thermal conductivities exhibited by standard low density insulations under vacuum conditions. These low vacuum conduc- t i v i t i e s provide considerable heat blockage for small insula- tion thicknesses. Preliminary calculations indicate that a
"coverall" garment with approximately l/h inch of insulation and a porous outer fabric with a low absorptivity to solar radiation w i l l reduce heat inputs and heat losses from the suit s u f f i c i e n t l y to allow adequate thermal control of the suit i n - t e r i o r to be accomplished by an a i r circulating and condition- ing system. Since the a i r circulating and conditioning system i s required for pressurization and breathing g^s, no compli- cated equipment must be added to accomplish the extravehicular thermal control function.
This paper presents a discussion of the foregoing thermal control concept. The discussion includes the problems, require- ments, methods, and design for a cislunar and lunar surface extravehicular space s u i t .
REQUIREMENTS
The primary requirement for an extravehicular space suit thermal protection system i s to protect the space worker from the extremes in thermal environment which w i l l be encountered.
In some instances, the worker may be on the sunlit side of a vehicle where he receives heat in the form of radiation from the vehicle, sun, Earth or moon. In other instances, he may be required to work on the shaded side of a vehicle or in the
shade of a planet where he i s subjected to the extreme cold of space and receives only a small amount of external heating.
Operation on the lunar surface also presents a widely varying thermal problem. On the sunlit side of the moon, the surface temperature may reach 250°F, whereas temperatures as low as -250°F can occur during the lunar night. In some instances, one side of the worker may be subjected to heating while the other side is radiating to deep space. In addition, mainte- nance work may be required on cryogenic propellant tankage, or high temperature a u x i l i a r y power units. Without suitable thermal protection, severe thermal gradients can occur in the space suit causing discomfort and/or injury to the astronaut.
Design requirements for a system which w i l l provide thermal protection in these extreme environments include the following:
^Numbers in parentheses indicate References at end of paper.
1) Suit components in contact with the "body should not vary from 75 + 5°F at any point. Pressurization gas average temper- ature shall be maintained within the range of 70 - 80°F at a l l times.
2) The maximum metabolic heat load i s approximately 1000 BTU/
hr. The average metabolic heat load i s approximately 1*00 BTU/hr.
3) The design shall function s a t i s f a c t o r i l y at the following limiting design environments:
a. Steady-state at lunar surface subsolar conditions (extreme hot condition).
b . Steady-state in lunar darkness (extreme cold condi- t i o n ) .
c. Steady-state in deep space with any one side facing the sun and the other side facing deep space. (Maximum thermal gradient)
k) The extravehicular stay time shall be four hours.
5) The design shall function s a t i s f a c t o r i l y with any fixed orientation f o r the f u l l extravehicular stay time ( e . g . , one side faces the sun and the other side facing deep space f o r the f u l l four hours).
6) The maximum heat loss to the space environment shall not exceed 10 BTU/hr-ft . The maximum heat gain shall not exceed 6.5 B T U / h r - f t2.
METHODS
There are a number of methods which could be used f o r thermal control of extravehicular space s u i t s . The thermal gradients which occur due to the difference in the environment on the different sides of the suit could be controlled by a r i g i d metal shell surrounding the worker ( l) . 3 This approach would provide a r i g i d suit and would be heavy. Liquids such as water anti-freeze solutions circulated through passages in the walls of the pressure suit could be used to distribute heat. This approach would also be heavy and would require continuous power to circulate the f l u i d . Both of these methods either depend on transient heating and cooling or other systems to maintain the desired temperature l e v e l .
G . B. WHISENHUNT A N D R. Α . KNEZEK
Considering a l l known methods for thermal control of extra- vehicular space s u i t s , the concept of minimizing the effects of the external environment with heat blockage by insulation and a thermal control coating appears to be the most desirable.
With adequate external heat blockage, the thermal control of the worker may be accomplished in the same manner as with cur- rent vehicular type pressure suit systems. In current suit systems, thermal control i s accomplished by proper conditioning and distribution of the pressurization gas. For extravehicular operation, the gas conditioning can be accomplished by a port- able environmental control unit and the correct distribution can be achieved with proper suit design.
The heat blockage concept for thermal control has the added advantage of providing a multi-purpose pressure suit assembly.
The suit may be used inside the vehicle f o r emergency pressur- ization ( i n the event of a cabin decompression) or outside the vehicle as the primary source of protection. The space worker would don a portable environmental unit and an insulated cover- a l l garment ( i n addition to the basic s u i t ) prior to leaving the space cabin.
The insulated coverall garment w i l l provide the basic heat blockage required f o r adequate thermal control. Other schemes are required for providing additional protection to c r i t i c a l areas of the s u i t . These c r i t i c a l areas include the f l e x i b l e j o i n t s , helmet, feet, and hands. Schemes f o r providing this additional protection include:
Greater thickness of insulation in c r i t i c a l areas Expendable coolants placed at c r i t i c a l areas Increased airflow
Electric heating and/or thermoelectric cooling Fixed heat sinks
"Thermopane" type construction of the face plate
An analysis of the insulation-selective coating approach f o r extravehicular space suit thermal control i s presented in the following section. An analysis of methods f o r handling c r i t i - cal areas i s a l s o included.
ANALYSIS
The two basic functions of a thermal protection system f o r a
man in the space environment are: ( l ) the control of the t o t a l heat load which must be dissipated by the portable environ- mental control system; and(2) the control of l o c a l temperature variations at the inner surface of the pressure s u i t . Environ- mental control systems f o r current space vehicle pressure suits are designed to dissipate the metabolic heat generated by the crewman and a l l equipment within the pressure s h e l l . There i s no requirement f o r a heating system since the metabolic and
equipment heat loads provide adequate heating f o r a l l conditions.
This same approach i s suitable f o r extravehicular space suits when insulation i s used to limit the t o t a l heat loss from the suit to the amount of heat generated. Cooling can be accom- plished by an expendable coolant or radiator system.
When a space worker i s exposed to the radiant energy of the sun and reflected or radiated energy from a vehicle or the hot lunar surface, the exterior of the pressure suit w i l l become hot and add to the environmental control system heat load.
Sufficient a i r f l o w through the suit must be provided to remove this heat without exceeding the required temperature l i m i t s . Comfort of the worker i s affected by a i r velocity, humidity, and temperature. Although the desired combination of these parameters varies widely among individuals, a difference b e - tween i n l e t and outlet suit a i r temperature of 20 to 30°F i s considered to be a practical maximum. F i g . 2 shows the sensi- ble heat removed by the conditioning a i r as a function of flow rate and temperature r i s e . Flow rates normally considered for pressure suits are between 5 and 15 standard cubic feet per minute since excessive flow tends to "dehydrate" the worker.
These considerations indicate that the additional heat load from the environment must be kept to a minimum i f a simple a i r circulation system i s to provide a satisfactory environment within the s u i t .
Even when the environmental heat loads are maintained within acceptable limits, local temperature variations can cause d i s - comfort or i n j u r y . I f the worker touches a hot surface, burn- ing of the skin w i l l occur. Similarly, i f the surface i s allowed to become cold, numbness or f r o s t b i t e may occur. I f the surface becomes colder than the dewpoint of the a i r in the s u i t , condensation w i l l occur and w i l l cause a very disagreeable condition. These conditions are used as limiting factors f o r the analyses that follow.
The low thermal conductivity of insulating materials under high vacuum conditions may be u t i l i z e d since thermal protection is required only during extravehicular operations. Evacuation of the insulation to space i s assured by a porous outer f a b r i c . Considerable heat blockage i s obtained with small thicknesses
G. B. WHISENHUNT AND R. Α. KNEZEK
of common insulating materials. Fig. 3 shows the effect of pressure on the thermal conductivity of a typical glass f i b e r insulation.
The maximum cold condition occurs when the crewman i s working in a shadow of a vehicle in space or when a crewman i s located on the cold side of the lunar surface. Limiting conditions may be described by an environment at absolute zero temperature.
Heat i s l o s t by conduction through the insulating layer and radiation to space. The sensible heat dissipated by the crew- man i s assumed to be kOO BTU/hr. Part of this number must be reserved for areas that are d i f f i c u l t to protect, such as the helmet, hands and heat shorts in the insulating l a y e r . Allow- ing approximately kofi of the available heat loss through these areas leaves 250 BTU/hr as a maximum f o r heat loss through the insulation l a y e r . For a suit with a surface area of 25 f t 2 , the maximum heat loss per unit area i s 10 B T U / h r - f t2.
The heat loss i s a function of surface emissivity in addition to the thickness and thermal conductivity of insulation. F i g . k shows the effect of surface emissivity on the thickness of insulation ( t y p i c a l glass f i b e r ) required to limit the heat loss to 10 BTU/hr-ft^. White nylon parachute f a b r i c i s a s u i t - able outer cover for the insulation and has an emissivity of approximately 0-93 · Fig. h indicates that 0.25 i n . of the glass f i b e r insulation w i l l be required i f this covering mate- r i a l i s used. The effect of insulation thickness on heat loss i s shown by Fig. 5·
When the crewman i s illuminated by the sun, the outer sur- face of the suit w i l l be heated. The amount of heat conducted from the outer cover through the insulation i s small compared with the radiant energy absorbed and re-emitted to space.
Therefore, the surface temperature w i l l approach an adiabatic equilibrium temperature. The surface temperature and the r e - sulting heat absorbed per unit area are shown as a function of the solar absorptivity to emissivity r a t i o in Fig. 6. The white nylon parachute f a b r i c discussed previously has an ab- sorptivity to emissivity r a t i o of approximately 0 . 6 . Using this outer fabric and 0.25 i n . of glass f i b e r insulation, heat i s absorbed by the suit at the rate of approximately 3 BTU/hr- tt*.
When the crewman i s operating on or near the lunar surface, the maximum insulation surface temperature that i s l i k e l y to be encountered i s 250°F. This corresponds to a solar absorp- t i v i t y to emissivity r a t i o of approximately 1 in Fig. 6. A heat absorption rate of 6.5 BTU/hr-ft would be obtained. This condition imposes the maximum cooling load on the environmental
backpack unit since the entire surface would be heated. This maximum condition could s t i l l be handled by a i r f l o w , since a good portion of the metabolic heat load under this condition would be dissipated by evaporation of sweat or latent cooling.
Optimum design of an insulation thermal protection layer must give consideration to the a v a i l a b i l i t y and mechanical properties of insulating materials as well as thermal conduc- t i v i t i e s . Some of the super-insulations look very promising from a thermal standpoint and w i l l v i r t u a l l y eliminate thermal losses i f they prove to be mechanically suitable. Fig. 7 shows the effect of thermal conductivity on thickness of insulation required. Several current insulations are located on this curve to show the benefit that may be obtained from insulation development. This curve shows that with Linde SI-93 super- insulation, the heat loss to space i s p r a c t i c a l l y eliminated.
Another area where new developments are desirable i s the outer protective covering. This i s i l l u s t r a t e d best by Fig.
6, which shows the advantages of a low absorptivity to emis- s i v i t y r a t i o .
Seams, zippers, and other i r r e g u l a r i t i e s in the insulating layer w i l l cause heat leaks through the insulation and i n - crease the rate of heat loss and heat absorbed. The heat transmitted through these heat shorts i s determined b ypt h e i r area and the incident and radiated energy. Up to 1 f t of ex- posed area can be tolerated in the cold environment without exceeding a total heat load equal to the sensible heat d i s s i - pated by the crewman. Special consideration must be given to these areas to prevent discomfort or injury since they may become very cold or very hot. In the maximum heating environ- ment, 1 ft^ of exposed area (heat shorts) would more than double the heat absorbed, giving a t o t a l of approximately
^00 BTU/hr. This i s equal to the heat generated by the crew- man. The heat leaks can be reduced considerably by applica- tion of a low emissivity (and also a low absorptivity) surface f i n i s h to the exposed areas.
Thermal protection of the crewman1s hands presents a partic- ular problem since this must be performed with minimum impair- ment to mobility. This might be accomplished by l o c a l l y i n - creasing the a i r f l o w . A low emissivity surface would be
desirable in reducing the heat load, but i t would be extremely d i f f i c u l t to maintain. Another approach i s to cover the hand with a mitten which allows the fingers to be extended for de- t a i l tasks.
G . B. W H I S E N H U N T A N D R. Α . KNEZEK
Another problem area is the helmet. Insulation inside the helmet would not be as effective as external insulation. Large variations in the helmet temperature would also r e s u l t . A soft type of insulation on the external surface would not be de- sirable from the standpoint of possible damage during donning.
The soft insulation might also interfere with operation of the v i s o r . This problem could be overcome by using a dual-wall construction and possibly adding insulation between the inner and outer s h e l l s . Another p o s s i b i l i t y i s a "hood" for the coverall which encloses the entire helmet except for the v i s o r .
Although the visor covers a small area, i t also presents a problem. I f the v i s o r temperature drops below the dewpoint of the a i r , i t w i l l cloud and impair the vision of the crewman.
Airflows in this region must be kept low because of the sensi- t i v i t y of the eyes. The visor temperature could be adequately controlled by i n s t a l l i n g a shield in front of the v i s o r with a low emissivity outer surface. The low emissivity can be ob- tained by application of a p a r t i a l l y transparent metal film.
A thin gold film would provide a low emissivity and s t i l l allow the crewman to see through. A shield without the low emissivity surface would not be adequate to prevent clouding. An e l e c t r i - c a l l y heated shield might also be used for this purpose.
SUGGESTED METHOD
An a r t i s t ' s conception of an extravehicular space suit sys- tem for operation on the lunar surface or in the cislunar en- vironment i s shown in Pig. 8 and consists of the following major components:(l) an anthropomorphic vehicular pressure suit improved to provide satisfactory mobility; (2) an insulated coverall garment with a porous outer f a b r i c to allow the i n - sulation to outgas quickly (the coverall would be worn over the suit during extravehicular operation); and (3) a portable en- vironmental unit to provide 5-15 standard cubic feet per minute of conditioned a i r to the s u i t .
These basic components can provide suitable thermal protec- tion in the space environment. A possible method of construc- tion for an insulation system i s shown in P i g . 8. I t consists of a layer of insulation quilted between two layers of white nylqn parachute f a b r i c . Estimated insulation, outer f a b r i c , and quilting material weights are presented in Fig. 9 as a function of the thickness of the insulation used. For l/h i n . of insulation, the t o t a l coverall weight would be approximately 6 l b s . This weight can be reduced and thermal protection effectiveness increased by a development program aimed at providing a better outer cover and-super insulations.
CONCLUSIONS
The following i s a l i s t of conclusions based on the analysis presented herein:
1) An insulated coverall garment with a low absorptivity outer fabric w i l l provide sufficient heat blockage to allow adequate thermal control of an extravehicular space suit to be accomplished by the a i r circulation and conditioning system.
2) Special protection i s required f o r c r i t i c a l areas of the suit such as j o i n t s , closures, helmet, f e e t , and hands. Simple passive schemes w i l l provide protection f o r these areas.
3) A development program designed to integrate super-
insulations into the coverall garment and to find a better outer cover i s desirable in order to reduce system weight and increase thermal protection effectiveness.
REFERENCES
1 I r v i n e , T. F . , J r . , and Cramer, K. R . , "Thermal analysis of space suits in o r b i t , " WADD TN 60-1*1-5 (May i 9 6 0 ) .
2 Cramer, K. R . , and I r v i n e , T. F . , J r . , "Analysis of non- uniform suit temperatures f o r space suits in o r b i t , " ASD Report No. MRL-TDR-62-8.
G. Β. WHISENHUNT AND R. Α. KNEZEK
Fig# 1 Manned extravehicular space operations
S E N S I B L E H E A T R E M O V E D BY A I R - B T U / H R .
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3 Thermal conductivity of a typical glass f i b e r insulation
G. B. WHISENHUNT AND R. Α. KNEZEK
U I .L .0 .1 -J Ο . / .Ο .7 ι,ν S U R F A C E E M I S S I V I T Y <2
Fig. k Effect of surface emissivity on insulation thickness required
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Fig. 6 Effect of radiation properties of heat load of suit exposed to sun
A S S U M P T I O N S :
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G. Β. WHISENHUNT AND R. A. KNEZEK
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