Update 10-11-13: See also 4-17-10 "Space Recommendations".
The focus here is the so-called mechanical counterpressure (MCP) space suit. This was a design approach explored (among other things) on the PBS program “Nova Science Now”, aired Wednesday, January 19, 2011, in Texas. This approach has its genesis in the “partial pressure suits” used by USAF in the late 1940’s, and during the 1950’s. It was tested as a possible space suit design with the elastic fabrics of the 1960’s, and in the last decade has come back under consideration again, as a possible way to vastly improve astronaut mobility and dexterity.
General Considerations:
Here on Earth, our atmosphere contains the oxygen we need to support life, and it also exerts its pressure upon our bodies. This pressure has two effects: (1) to concentrate the oxygen enough to diffuse effectively into our blood across the alveoli structures in our lungs, and (2) to keep the water in our blood and tissues from boiling away at body temperature. The minimum partial pressure of oxygen necessary for effective respiration is an indistinct limit, but it is substantially higher than the value of external pressure necessary to keep our blood and tissue moisture from boiling away. This boil-off level is pretty close to the equilibrium vapor pressure of water at a body temperature of 98.6 F (37.0 C), which is 0.06192 of a standard atmosphere.
In space, there is no oxygen and there is no pressure. Unprotected persons die quickly, first losing consciousness in seconds to a couple of minutes, due to anoxia, then suffering anoxic brain death within a very few more minutes (around 10), which event is generally irreversible. Some several minutes after the onset of brain death, the heart stops and blood pressure falls below the moisture boil-off level. The water in the blood and tissues begins to boil away into space, breaking open cell membranes and splitting open tissue structures. Boil-off of this moisture draws heat from the surrounding tissue structures, chilling the body rapidly toward freezing as it partially desiccates.
Thus, a protective enclosure is necessary for us to survive in space, one which provides oxygen at a suitable pressure inside the lungs and breathing passages. This breathing gas pressure must be balanced by the same fluid pressure within the body, in turn produced by an equal pressure applied to the external surface of the body. An important fact: this external pressure can be supplied in two ways: (1) by gas or fluid pressure within a sealed garment that is essentially a balloon, or (2) mechanical pressure applied directly to (and distributed over) the skin.
Historical Specifics:
By about 1930, high-flying pilots had to be protected from lack of oxygen. One way was a modified deep-sea diver’s dress, functioning as a pressurized balloon. The atmosphere inside the sealed garment could be pure oxygen instead of air, and its pressure need only be enough to concentrate the oxygen in the lungs sufficient to support respiration. Such pressures are typically well above the moisture boil-off level. This balance of breathing gas vs internal tissue fluid pressures, with a distributed external pressure creating those internal fluid pressures, is illustrated in the right hand portion of figure 1. The creation of those internal tissue fluid pressures is very much like squeezing a water balloon all over, raising its internal pressure, as in the center portion. This works because the cells of the body are essentially tiny water balloons. Thus, in the aggregate, the body responds like the water balloon.
Figure 1 – Pressure Balance Physics with Fluid-Filled Objects and Aggregates of Same
Military pilots flying “high-gee” maneuvers tend to faint, because the blood supplying oxygen to their brains is pulled by the high accelerations toward their lower extremities (pooling in their legs). Hypoxia thus induced soon leads to blackout, often within a very few seconds. The solution is to drive that blood supply upward against the acceleration by means of the “gee suit”. These garments provide a mechanical squeezing action on the lower body and legs, similar to squeezing just part of a water balloon. This uneven compression drives the fluid within toward the uncompressed portion, instead of generally increased internal pressure. This action is illustrated in the left portion of figure 1. In the water balloon, the unsqueezed portion expands with the extra water driven there. In the body, blood that would have pooled in the legs is driven upward against the acceleration back to the brain.
In the late 1940’s, this “gee-suit” action was proposed as a temporary protection garment for test and fighter pilots having to bail out or deal with loss of cabin pressure at extreme altitudes. By this time, it was already known that an ordinary oxygen mask could not supply a sufficient concentration of oxygen, because the atmospheric external pressure governing that concentration was too low. (The critical altitude for that is a little “fuzzy”, but generally we use 45,000 feet.) Pressure breathing gear was required for flying higher, and this required a counterbalancing internal tissue fluid pressure within the body (as illustrated in the right-hand portion of figure 1). The balloon-type pressure suits of that time were simply too restrictive of movement, too bulky, and too heavy, to serve in this application.
An extension of the gee suit mechanical compression approach provided the answer used in the late 1940’s and 1950’s: the so-called “partial pressure suit”. The mechanical compression was extended over most of the body, providing internal fluid pressures more or less sufficient to balance breathing gas pressures in a helmet, in turn adequate to support life long enough to bail out from around 100,000-foot altitudes. It wasn’t perfect: hands and feet remained uncompressed, and the actual compression achieved over the torso and limbs was very uneven. Blood pooling within inadequately compressed limbs would lead to the pilot blacking out, in about 10 minutes or so, and serious swelling from edema within about an hour. But, it worked well enough to serve for the few minutes of a bailout or emergency descent. Compression was achieved by tensioning the non-elastic fabric across the skin by inflatable “capstans” (tubes). These suits were far less restrictive of movement, far less bulky, and far lighter than the gas balloon-type pressure suits of that time.
By the end of the 1950’s, the gas balloon suits had been sufficiently improved to be competitive with the partial pressure suit in terms of bulk, weight, and movement restrictions. These newer gas balloon suits (“full pressure suits”) had no restriction on protection time, as an even distribution of compression was inherently achieved on all body parts. This eliminates blood pooling problems. Thus, these were chosen as the space suits of the 1960’s, and have been the standard ever since. As evolved for in-space and lunar use since then, these have become very bulky, restrictive garments. Cooling systems are required inside the hermetically-sealed suit. Due to the stiffness and bulk of the pressurized gloves, manual dexterity is very limited. Typically, the suit pressure used is 1/3 of an atmosphere of pure oxygen, near 253 mm Hg.
In the late 1960’s, the partial pressure suit problems of uneven compression distributions and limited coverage were addressed fairly successfully by the use of elastic fabrics. By mechanically compressing the hands and feet as well as the limbs and torso, time-unlimited protection could be had. Such garments need not be one piece, as they were not gas-tight balloons, merely the equivalent of tight panty hose or shrink-wrap. Tailoring the distribution and arrangement of layers of elastic fabric in the various garment sections was the hard part. No cooling system was required: the wearer could sweat right through the porous garment into vacuum. Such a space suit was demonstrated as a prototype in the vacuum tank successfully, under the direction of Dr. Paul Webb, and partly funded by NASA. Including the oxygen supply and helmet, it weighed 85 pounds, with little movement restriction, and marvelous dexterity. Compare that with the 200+ pound full pressure suits used on the moon! The breathing pressure was in the neighborhood of 170-190 mm Hg. These efforts did not lead to application then.
The mechanical compression idea has resurfaced in the last decade, including efforts for NASA. The modern ability to tailor elastic fabrics is even better than that available in the late 1960’s. Up to a certain level of mechanical compression, these techniques now work fine. The level currently achievable is not 1/3 of an atmosphere, however. But, is that level really necessary, considering that the experiments of the late 1960’s were successful with much lower compression? This is important, because if one specifies a compression level higher than can be reached with the technology, this MCP technique could be deemed infeasible, when in fact it is feasible, and offers some very significant advantages.
Design Criteria for Breathing Pressures:
A good startpoint is the concentration of oxygen available to people at sea level. The standard oxygen content of dry air is generally thought to be 20.946 volume percent (v%). One standard atmosphere’s pressure is defined as 14.696 psia, 1013.25 mbar, 29.92 inch Hg (mercury manometer column height), or 760 mm Hg. The gas laws indicate that partial pressure percentages are the same as volume percent composition. See figure 2. At sea level, the partial pressure of oxygen in dry air is then 159.2 mm Hg.
Inside the moist lungs the air is no longer dry. A fairly realistic assumption is that the water vapor is saturated. Water vapor pressure is determined at equilibrium by the temperature of the liquid phase in contact with it, in this case, body temperature of a human (98.6 F or 37.0 C). From the standard steam tables, this vapor pressure is 47.1 mm Hg. That vapor displaces some of the dry air, so that the partial pressures of the dry air and the water vapor now add to the imposed atmospheric pressure, in this case sea level (760 mm Hg). The oxygen partial pressure in the wetted air freshly inhaled into the lungs is then 20.946% of the dry air partial pressure (712.9 mm Hg), or about 149.3 in Hg.
Between inhale and exhale, some of this oxygen is diffused across the alveoli into the blood, and some carbon dioxide diffuses from the blood back into the air in the lungs. Compared to the water vapor displacement effect, these transient effects are small, and are ignored here.
What drives diffusion of oxygen into the blood is the partial pressure of oxygen in the wet air inside the lungs. This must be larger than the dissolved oxygen pressure in the blood for diffusion to occur at a useful rate. Therefore, there is a lower limit to the in-lung partial pressure of oxygen, but it is a little “fuzzy”, and must be determined empirically. The calculation of in-lung oxygen concentrations from atmospheric air at altitude pressure, and their relationship to pure oxygen breathing pressures in a suit, is illustrated in figure 2. It should be noted that the water vapor pressure is a constant set by body temperature, having a larger percentage effect at high-altitude lower atmosphere pressures, and at lower suit oxygen pressures.
Figure 2 – In-Lung Oxygen Estimates from Air at Altitude and In-Suit Oxygen
One estimate of the lower limit for in-lung oxygen concentration comes from in-flight oxygen rules for pilots. Civilian pilots are not required to use oxygen below 10,000 feet altitude under US FAA rules (the USN uses a different figure: 5000 feet). Most other agencies use something like the 10,000 foot rule. Using the procedure outlined in figure 2 at sea level and at 10,000 feet atmosphere pressures produces the results bordered in green in figure 3. For people adapted to more-or-less sea level air, in-lung oxygen partial pressures in the range 149.3 down to 99.6 mm Hg seem quite adequate. These correspond to altitudes in air from sea level up to 10,000 feet. They also correspond to oxygen breathing gas and suit compression pressure levels of 196.4 down to 146.7 mm Hg. These are 25.8 down to 19.3% of a standard atmosphere as the suit compression levels required.
Using the current full pressure suit standard of 1/3 atmosphere pure oxygen (253.3 mm Hg), one has a wet in-lung oxygen of partial pressure of 206.3 mm Hg, substantially more than in sea level air. These are the data bordered in blue in figure 3. They do not correspond to a calculated altitude, which would be far below sea level. Clearly, 33% of a standard atmosphere is an over-stringent pressurization requirement for the MCP suit. Something closer to 25% of an atmosphere is equivalent to sea level air, and serves well as an upper limit for MCP compression design.
Figure 3 – Suit Compression Levels Corresponding to In-Lung Oxygen at Various Altitudes
There are populations of humans who live at very high altitudes. Substantial numbers of people live near 15,000 feet , corresponding to in-lung oxygen near 80 mm Hg, and further corresponding to a suit compression level of 127 mm Hg (16.7% of an atmosphere). However these people are acclimatized to these conditions. It takes substantial time for the body to so acclimatize. Without that acclimatization, altitude sickness and fainting are high risks. There are a few people living near 20,000 feet in the high Andes or Himalayas. That altitude corresponds to in-lung oxygen 63.4 mm Hg, and suit compression 110 mm Hg (14.5% of an atmosphere). The 15,000 and 20,000 foot calculated data are included in the figure 3 table bordered yellow. “Flatlanders” in space suits at those pressures would survive, but would not be functional. Therefore, the 10,000 foot data make a pretty good empirical lower limit for MCP suit compression design.
Further, consider the nominal 45,000 foot requirement for something better than a simple oxygen mask. Those data are included as one of two entries in the red-bordered portion of figure 3. At that altitude with pure oxygen in a simple mask, in-lung oxygen is 64 mm Hg, and external compression is 111 mm Hg (14.6% of an atmosphere). Those levels correspond very closely with the in-lung oxygen levels of long-acclimatized mountain folks living in the open air near 20,000 feet. That is a good rough estimate of an extreme lower compression limit to stave off certain slow death by hypoxia.
The other entry in the red-bordered portion of figure 3 is the moisture boil-off point (where the blood starts to boil, and tissues outgas water vapor as they begin to desiccate and freeze). There is no oxygen at all in the lungs at this pressure; it is all water vapor at 47 mm Hg, which has to be the external compression pressure. Those figures correspond to 64,000 feet altitude and 6.2% of an atmosphere. About 60,000 feet has long been thought to be the short-exposure “deathpoint” for risk of blood boiling.
Conclusions:
The minimum design compression for an MCP-type space suit is very likely near 19 or 20% of a standard atmosphere (147 mm Hg), corresponding very closely with the in-lung wet oxygen partial pressure of 99.6 mm Hg experienced in the open air at 10,000 feet. Compressing less risks dysfunction.
The maximum necessary design compression for an MCP-type space suit is near 25 or 26% of a standard atmosphere (196 mm Hg), corresponding very closely with the in-lung wet oxygen partial pressure of 149 mm Hg experienced in the open air at sea level. It does not hurt to compress more.
The “typical” space suit design standard of 33% of an atmosphere seems to be an unnecessarily stringent design requirement, especially if fabric technology cannot quite achieve it right now.
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