FLEXIBLE SPACE SUITS FOR THE EXPLORATION OF MARS
Current gas-pressurised space suits are bulky stiff shells severely limiting astronaut function and capability. The rigidity of these so-called ‘soft’ suits is due to the high pressure differential between the interior of the suit and the exterior environment, causing the air-tight layers to inflate into a taut neutral position. Research into new materials have produced mild improvements over the last 30 years, but the simple motion of clenching a fist in the current suit is still likened by astronauts to squeezing a tennis ball. More recent efforts using external electromechanical actuators reduce required bending torques but present many more problems. As the dilemma of solving rigidity persists, it seems gas-pressurised suits will severely hamper all external field operations on Mars.
An alternative approach to pressurisation using mechanical counter pressure (MCP) may improve the flexibility of extravehicular suits. An MCP suit would exert pressure on the body (except the head) by form-fitting elastic garments. A standard helmet would be worn to provide both pressurisation to the head and oxygen for breathing. MCP suits have been found to offer dramatic improvements in reach, dexterity and tactility due to the replacement of stiff joints and bearings with light, flexible elastics. Recent studies have concluded that prototype MCP gloves compress the hand at similar levels to current gas-pressurised suits and effectively protect the hand physiologically when worn in a vacuum chamber.
MCP suits, though unproven, offer many more improvements over current suits such as safety (as a tear or hole would remain a local defect rather than cause a catastrophic puncture), lower suit costs and vastly reduced weight and volume (including the life support systems), low susceptibility to dust (as bearings are not required) and highly durable materials.
While the life support efficacy of space suits has been demonstrated for more than 30 years, the exploration of Mars provides new challenges in space suit design. While the surface environment of Mars is completely different to any encountered before, the long-term duration and physical nature of proposed field operations are several orders of magnitude more demanding than any and all previous extravehicular activities (EVAs).
A Mars space suit must safely and efficiently accommodate and support an astronaut for hundreds of hours during rigorous activities (such as habitat construction and geologic exploration) on a surface of significant atmosphere, gravity and dust. At the most basic level, therefore, an effective suit must be light, robust and flexible. It is also important for the suits to be cheaper to produce than current suits, and to be packable into a small volume for stowage .
LIMITATIONS OF GAS-PRESSURISED SUITS
All current and previous space suits have been multilayered air-tight balloons pressurised with the breathing gas. The Primary Life Support System (PLSS) has been bulky and complex primarily in order to manage the oxygen for breathing and pressurisation, and water for cooling. Together, the space suit and the PLSS are known as the Extravehicular Mobility Unit (EMU). All previous EMUs were designed for use in vacuous, low- to micro-gravity environments for short duration missions, and as such many aspects and characteristics are unsuited to Martian conditions. Some aspects, of course, are more easily adaptable than others. The atmospheric pressure of Mars, for instance, would prevent the current cooling and insulation systems from functioning as they require a vacuum to operate effectively (if at all) . However, the consistently cold surface and atmosphere of Mars may be utilised itself as a heat sink; further, the use of dense membranes in the suit layers may allow perspiration (but not air) to escape, thereby permitting the body a degree of thermal self-regulation. In terms of insulation, Mars should require less insulation as radiation levels, temperature extremes and micrometeorite strikes are less extreme than on Earth orbit or Lunar environments. Thus, exotic materials may not be required.
As the Mars environment may also harbour current or past life, it is also a consideration that a Mars EMU provide some measure of quarantine against human/bacterial contamination of the surface (and indeed vice-versa). The high volume of oxygen and required flow rates in gas-pressurised suits leads to a high leakage rate: it is predicted that over 50 litres of human borne bacteria and other airborne effluent would escape through suit bearings and joints during each EVA, potentially contaminating soil, fossil and atmospheric samples . Perhaps more critically, this leakage represents a sizable wastage in oxygen supplies. A solution may be to pressurise the body with the Martian atmosphere, leaving only the head pressurised with oxygen.
The most challenging aspect of adapting gas-pressurised suits to Martian conditions, however, is to create a light, durable EMU while at the same time making it flexible. While it is quite easy to produce a light, air-tight anthropomorphic balloon, such a suit would be very stiff as any movement would require bending a taut section of the inflated layers. Current suits increase mobility (but sacrifice weight) by placing bearings at critical movement centres such as the wrists, upper arms and waist. (The bearings also allow for donning/doffing and the ability to swap individual components if damaged or incorrectly sized.)
An Apollo EMU weighed 100 kg on earth, but only 17 kg on the lunar surface. While this is already a considerable burden, the same suit would weigh 38 kg on Mars, and the current EMU would weigh 50 kg [12,13]. While the latter may include heavier equipment for use only on orbital and therefore weightless environments, it should be noted that gas-pressurised suits are inherently heavy due to the multiple layers required to form the air-tight pressure garment, the multiple number of seals and bearings to allow a freedom of movement, and the large and complex Personal Life Support System backpack (PLSS) which includes the oxygen, fans, pumps, contaminant-control cartridges/filters, regulators and valves to manage the oxygenation and pressurisation of the whole suit.
Initial prototypes of Mars gas-pressurised suits were developed in the late 1990’s by ILC Dover (who produce the current EMU) and David Clark (who produce the orange shuttle launch and re-entry suit) to explore new ways of reducing suit mass, cost and bulk while improving flexibility . The ILC Dover I-1 suit modified the existing EMU by primarily replacing the hard upper torso structure with lightweight fabrics. The design saved about 15 kg, to give a total Mars weight of 44 kg [11,15]. The David Clark D-1 suit has modified fabric joints, but were found to give unsatisfactory flexibility. The total Mars weight of this suit was about 40kg [8,15]. Improvements to the life-support systems could further reduce this mass, however huge advancements must be made in order to achieve similar in-situ weights as the Apollo suits on the lunar surface. As this weight was tolerable, but future suits should incorporate better mass distribution over the body, the Mars EMU weight goal is approximately that of the Apollo suit on the moon (17kg).
Perhaps the greatest limitation of gas-pressurised suits is their lack of flexibility. This is due to the high pressure differential between the interior of the suit and the exterior environment, thus causing the air-tight layers to inflate into a taut neutral position . Ever since Apollo, the suits were found to be akin to working in a rigid balloon . The gloves were found to be a particularly painful hindrance to hand function, with Apollo 17 Astronaut Gene Cernan noting that they became as stiff as the cast on a broken arm . The gloves constantly fought finger articulation, causing the knuckles and skin to scrape red raw against the unyielding inner layer. The Apollo EMU gloves also caused intense soreness and fatigue in the forearm muscles after only several hours EVA .
Since Apollo, development has focused on producing more flexible gloves as they were a hindrance to hand functionality and glove dexterity would be essential for orbital operations. However, the gloves of the Space Shuttle EMU are still known to be highly fatiguing and a severe hindrance to normal dexterity . Retired EVA astronaut Story Musgrave describes the current EMU as ‘miserable’ . The force of moving your fingers in the EMU glove is likened to squeezing a tennis ball. Astronauts therefore avoid finger movements as much as possible, preferring to push and touch objects rather than hold them. Power tools, for example, are cradled between the gloves rather than gripped in the usual fashion . Current physiologic symptoms of use include calluses, abrasions, lost fingernails, wrist and forearm muscle stress and nerve damage between the thumb and forefinger .
The immediate development of new EVA glove designs has become a NASA priority . While new materials have offered little increase in glove flexibility, the most recent efforts to try and improve the dexterity of these gloves involves the placement of an electromechanical actuator on the dorsum of the glove that provides power-assistance to the major metacarpophalangeal joint [10,16]. While power-assist increases range of motion and decreases effort, the system has not been successful in producing an effective gas pressurized EVA glove due to the added bulk to the hands and the uncertain eventuality of a system failure when the hand is clasped to an object [16,19]. Furthermore, the system drains critical power from the life support system and does not provide assistance in the articulation of the finger joints.
An alternative solution to achieve better flexibility is the ‘hard’ suit, which does not consist of any fabrics or soft materials at all. The suit is a fixed-volume shell of aluminium alloy or carbon composites joined by many bearings to form a hollow anthropomorphic enclosure. As this suit is not inflated, the freedom and resistance to motion is not related to the interior/exterior pressure difference – the suit therefore functions just as well in an atmosphere as in a vacuum. Movement is restricted, however, by the multitude and alignment of bearings linking the hollow sections together. The latest NASA Ames hard suit, however, weighs 36 kg more than the shuttle EMU and is very susceptible to dust .
As gas-pressurised suits do not appear to be capable of providing the flexibility and weight balance required for planetary exploration, the Advanced EVA Projects Office at NASA JSC believes that the exploration of Mars requires a new generation of suit .
TABLE 2 - Gas-pressurised suit attributes and Mars requirements
MECHANICAL COUNTER PRESSURE
An alternate approach to EVA suit pressurization using mechanical counter pressure (MCP) may provide a solution to EMU glove dexterity. An MCP suit would exert pressure on the body (except the head) by form-fitting, light elastic garments, rather than by the breathing gas. A standard helmet would be worn to provide both pressurisation to the head and oxygen for breathing.
Background of MCP Development
The concept and early experiments of a MCP suit were published by Webb , and the first demonstration that highlighted the many advantages of the MCP approach was described by Annis & Webb . This garment design - called the Space Activity Suit - aimed to overcome the limited mobility and high energy cost of activity in the full-pressure suits already used in the Gemini and Apollo programs (figure 2). The suit was a rubber and elastic leotard without any of the mechanical joints that were necessary for inflexible pressurized suits. The suit was found to be flexible, safe, light, and cost effective, but was cancelled with the demise of the Apollo program. In 1984, W. Clapp of MIT conducted flexibility studies of two forms of MCP glove and compared them with the Apollo glove . He found that the MCP gloves offered a considerable increase in both mobility and dexterity, and concluded that they were especially suitable for astronauts on the surface of Mars.
The success of the original MCP Space Activity Suit, the considerable advances in textile technology for fibers, yarns, textile creation and automated knitting machines, and the continued drawbacks of gas-pressurized EMU suits has prompted new interest in the development of a MCP suit. During the last two years, Honeywell, in collaboration with Dr. Paul Webb, has developed a prototype of a MCP glove. The glove was chosen as the first section to be designed due to the immediate need to improve current EMU gloves, but also because of the ease of chamber testing and the defining challenge of applying MCP to the most complex jointed geometry of the body.
The primary goal of MCP suit design is to engineer a compressive elastic garment which exerts pressure on the body at the same magnitude as the breathing gas. If the MCP of the suit is insufficient on any part of the body, the astronaut might experience symptoms such as pain, blisters, burst vessels or joint pain (decompression effects); excessive compression may cause pain, loss of sensation, impaired/occluded circulation or compartment syndrome. While stretched elastics compress the majority of the body adequately and uniformly, concave or flat areas such as the palm and back of the hand, under the arms, the spinal trough, the groin and the back of the knees experience elastic ‘bridging’. Compression in these areas can be supplemented by small padding inserts, gel packs or inflatable bladders.
Compression studies have been performed on the Honeywell glove, which consists of several individual components - most of which are made of seamlessly knit elastics and other yarns - designed to exert approximately the same pressure on the hand as the shuttle EMU. The prototype also consisted of an inflatable bladder that was placed on the dorsum of the hand under the MCP layers and a non-stretchable girdle. While an insert can be placed on the palm to increase compression (to the possible detriment of hand flexibility), it was found that the palm did not react physiologically to lower pressurisation than other regions of the hand. The study measured the compression on the back of the hand, the middle finger and the palm. It was found that the glove exerted the design compression on the finger and the back of the hand, but the concave region of the palm only received about one-third of the design compression.
Physiological tests have also been performed on the Honeywell MCP glove in a vacuum chamber for the hand. Without the MCP glove, skin microvascular flow and finger girth significantly increased with negative pressure, and the skin temperature decreased compared to the control condition. However, these changes were not observed when the subject wore the MCP glove [17,18]. These results suggest that the glove is effective in suppressing the tested physiological symptoms of low pressure exposure, such as would be encountered on Mars.
The overall viability of the glove was tested at a NASA JSC vacuum chamber. The subject wore the glove at pressures below 0.006 atm for 60 minutes, and found it effectively protected the hand with only a very small amount of swelling at the base of the little finger  (figure 1).
FIGURE 1 - Testing the MCP glove at NASA JSC at pressures below 0.006 atm
ATTRIBUTES OF MCP
Annis and Webb found that MCP garments offered dramatic improvements to gas pressurized suits in dexterity and tactility due to the replacement of stiff joints and bearings with light, flexible elastics. Running, cycling and even crawling was possible in a MCP suit at pressures below 0.02 atm (figure 3). As a measure of full body flexibility, oxygen demand during walking was reduced 66% when compared to the Apollo suit .
Later studies by Clapp compared the Apollo gas-pressurised gloves with MCP ‘skinsuit’ gloves made of spandex and another of elastic rubber fibres. Mobility, dexterity and tactility tests were performed in a chamber to simulate the low pressure of Mars. The mobility tests measured the angle of deflection of the finger, wrist and thumb joints; the flexibility tests involved a pegboard exercise; the tactile tests measured the ability to accurately determine patterns made of dots of 2 and 4 mm protrusion.
Clapp also performed strength and fatigue tests, which revealed that while initial grip strength was similar in all cases, the Apollo gloves caused greater fatigue than the MCP skinsuit glove (figure 4).
FIGURE 4 - Comparison of fatigue with MCP Skinsuit and Apollo A7L-B gloves.
The flexibility of the Honeywell MCP glove was qualitatively explored during the Mars Society of Australia’s Jarntimarra Expedition to the Outback in October/November 2001. The glove was worn by several expeditioners as they handled tools such as geo-picks, small spades, shovels and pens during the course of their work (figures 5,6). They found that the glove presented little to no hindrance at any stage, with one subject even happy to eat dinner with the glove on. The glove was worn without a protective dust/insulation layer (which would be essential on Mars), however such a layer would not greatly inhibit flexibility as it would be unpressurised. The trial also highlighted the need to have good grip between the glove and tool: perhaps rubber pads can be placed at specific finger/palm areas on the outer layer of the glove to achieve this. The need for rugged, comfortable boots was found to be essential for climbing and exploring rocky terrain. Conventional shoes can be worn with a MCP garment as the boot does not need to be pressurised and the MCP garment over the foot is very similar to a sock. Apart from the hand and wrist, it was noted that the greatest areas requiring flexible garments was at the knees and the groin for walking and sampling on one knee.
The Martian atmosphere, though thin, will cause heat loss through conduction, convection, and wind-induced evaporation. The current EMU cools the astronaut via the Liquid Cooling and Ventilation Garment (LCVG), an inner layer containing a network of small tubes that circulates cool water around the body. The LCVG may be redundant in MCP suits due to the ability of the astronaut to sweat through the porous MCP garment. Evaporation cools the skin, body heat is dissipated, and the rate is controlled by the astronaut's normal physiology . This natural cooling is further aided by conduction and convection. The dust/protection overgarment can be left unsealed to allow free circulation of the cold Martian atmosphere, however attention must still be paid to contamination issues. If an effective filter can be placed at the venting point, then most of the contaminants could be collected as the cooling gas is returned to the environment. The gaseous leakage from gas-pressurised suits is due to the high internal pressure and the number of joints and bearings. The leakage from an MCP outer layer would be considerably if not totally reduced as there are far fewer bearings and joints and because the interior gaseous pressure (except for the helmet) is equal to the outside environment – the leakage points are therefore reduced and the force for the gas to escape nullified.
If the cooling allowed by MCP in the Martian environment is too severe, then the filter in the outer layer may be closed to reduce air circulation. In this way, the sealed interior of the suit warms with body heat. Adjustment of the filter valve may effectively serve to regulate the suit internal temperature.
A tear or hole in a gas-pressurised suit would result in a rapid and probably fatal decompression. Tears in a MCP suit would remain a local defect as the elastic weave prevents the tear from propagating. A tear, therefore, would cause symptoms of localized low pressure exposure at the site of the tear (such as bruising and edema), while the rest of the body remains protected. The severity of these symptoms is quite mild and dissipates within hours/days (depending on exposure time), especially when occurring on a small area.
Improved mobility, greater reach, better tactility and improved dexterity all contribute to the effectiveness of performing EVA tasks, reducing EVA duration and therefore risk. The same improvements should also lessen fatigue for accomplishing a given task list, thereby reducing the tendency to make errors that are potentially harmful .
Both the suit and the PLSS should be significantly lighter than a gas-pressurised suit. While both suits might share outer protective layers, the inner air-tight layers and bearings of gas-pressurised suits are replaced instead with the light MCP elastics, saving considerable weight. Comparisons between current EMU and MCP gloves show the latter to conservatively weigh about 1/4 of the former. A MCP suit could therefore weigh about 14 kg or less (based on weight of shuttle suit, boots, gloves etc) .
The PLSS will also provide significant weight savings due to the savings in oxygen supply and the omission of the cooling loop hardware. As the oxygen demand is decreased, the total volume of required pressurised oxygen drastically reduced and the oxygen leakage virtually eliminated, there is an associated scaling down in both the required oxygen and pressurisation/management hardware weight. Further, omitting the coolant loop means saving the weight of the water, the water pump, its battery, the connecting tubes and valves, and all associated control hardware. The MCP PLSS could be less than half the weight and volume of the Apollo EMU, therefore weighing less than 30 kg [1,13]. The total weight of the suit is therefore 44 kg, with a Mars weight of about 16 kg. This figure is almost exactly the design weight of the Apollo suits on the moon.
An MCP EMU would also be comparatively small. With a suitably sized outer dust/protection layer, the suit could be no more bulky than winter clothes. Conceivably, the MCP garment could be stuffed within the helmet itself for stowage/storage.
A full MCP garment will cost far less to make than the current EMU. The main reason is the absence of expensive mechanical joints and the relative simplicity and size of the PLSS. Even though the elastics are durable, there could be many replacement garments available due to their small cost and storage size.
CHALLENGES TO MCP SUIT DESIGN
The previous studies have discovered two main areas of MCP design which pose the most significant problems in producing an effective, practical suit: donning and doffing, and ensuring all areas of the body receive uniform or sufficient compression.
The powerful elastics of the MCP garments are currently designed to exert the same pressure on the skin as the pressure found in the current shuttle EMU (i.e. about 0.3 atm). However, as donning and doffing will be required in the pressurised environment of the spacecraft/habitat, the combined pressure can be painful after only several minutes. The act of donning and doffing such a constrictive garment can also take considerable time and effort. A possible solution may be to incorporate an electroactivated polymer into the weave of the garment, thereby allowing it to relax and compress at will. A Martian astronaut could conceivably don the MCP EMU in the airlock and feel it gradually compress on the skin as the pressure in the airlock decreases. Further, this approach could be used in intravehicular activity (IVA) suits used during launch and re-entry: the suit would be relaxed by default but triggered to compress in the event of a cabin depressurisation.
Uniform compression distribution
Applying uniform compression over the human body is difficult because it is a complex shape in some areas. As Webb discovered, flat or concave sections experiences reduced compression due to elastic ‘bridging’ . Areas such as the back of the hand, the spinal trough, under the arms and the back of the knees all require some form of supplemental device in order to transfer the compression to the skin. In the Honeywell glove, an inflatable bladder was used, but gel packs and foam padding may also suffice in such areas. The Honeywell glove tests also revealed that some areas of the body are more resilient to low pressure (like the palm), and so therefore may not need the same compression as other areas.
Providing extra compression under the arms and behind the knees is more difficult as these areas are required to be free for good mobility, and because the joint undergoes large amounts of articulation. However, the most challenging (and delicate) area to pressurise is the groin. It may be deemed more effective to create a gas-pressurised system for the groin, which would share the PLSS helmet pressurisation hardware. This would require seals above (abdomen) and below the groin (thigh) in addition to that around the base of the neck (for the helmet).
MARS MCP SUIT CONCEPT
With development, mechanical counter pressure may provide the most appropriate form of space suit for Mars exploration. The results of past research has shown that a MCP suit is light, flexible, cheap, small and effective in physiologically protecting wearers for up to one hour. Table 4 summarises a Mars MCP concept and compares it to the Mars EVA suit requirements from Table 2. Figure 7 shows an artists conception of a Honeywell MCP suit on Mars.
TABLE 4 - MCP EMU concept attributes and Mars requirements
FIGURE 7 - Mars MCP suit (Honeywell)
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