Thursday, November 23, 2017

A Better Version of the MCP Space Suit?

This is a concept proposal for a better version of the mechanical counter-pressure (MCP) space suit.  It combines the best features and eliminates the worst disadvantages of the particular two MCP design approaches upon which it is based.  These are the “partial pressure” suit of the 1950’s and the “elastic space leotard” of Dr. Paul Webb.  The result should be a lightweight,  supple (non-restrictive) suit that with suitable unpressurized outerwear,  can be used on pretty much any planetary surface even if totally airless,  or even in space.  It need not use exotically-tailored materials in its construction.  It should be relatively easy to doff and don.

This article updates earlier articles on this subject.  Those are:

Date           title             

2-15-16     Suits and Atmospheres for Space  (supersedes those following)
1-15-16     Astronaut Facing Drowning Points Out Need for Better Space Suit
11-17-14    Space Suit and Habitat Atmospheres
2-11-14      On-Orbit Repair and Assembly Facility
1-21-11     Fundamental Design Criteria for Alternative Space Suit Approaches


The idea here is to combine the two demonstrated approaches that both apply the fundamental MCP principle:  the body needs pressure applied to its skin to counterbalance the necessary breathing gas pressure.  The body simply does not care whether this counter-pressure is applied as gas pressure inside a gas balloon suit,  or is exerted upon the skin by mechanical means.

The first article cited in the list above (“Suits and Atmospheres for Space” dated 2-15-16) determines that pure oxygen breathing gas pressures from 0.18 atm to 0.25+ atm should be feasible.  How that was calculated is not repeated here.  My preferred range of helmet oxygen pressures is 0.18 to 0.20 atm,  for which wet in-lung oxygen partial pressures range from 0.11 to 0.13 atm,  same as the wet in-lung oxygen partial pressures in Earth’s atmosphere at altitudes between 10,000 and 14,000 feet. 

However,  only 0.26 atm gives you the same wet in-lung oxygen pressure as sea level Earth air.  The 0.33 atm used by NASA is entirely unnecessary,  unless to help overcome the exhaustive efforts necessary to move or perform tasks,  in the extremely stiff and resistive,   heavy,  and bulky “gas balloon” suits they use.

The 1940’s design that operationally met the need for extreme altitude protection for short periods of time was the “partial pressure” suit of Figure 1,  in which compression was achieved with inflated “capstan tubes”.  These suits were widely used into the 1960’s.  The capstans pulled the non-stretchable fabric tight upon the torso and extremities.  This provided the counterpressure necessary for pressure-breathing oxygen during exposures to vacuum or near vacuum,  for durations up to about 10 minutes long.  This was for bailouts from above 70,000 feet,  and would have worked for similar short periods even in hard vacuum.  Hands and feet were left uncompressed,  but for only 10 minutes’ exposure,  these body parts could not begin to swell from vacuum effects. 


The advantages of this design were (1) ease of doff and don,  (2) it was simple enough to be quite reliable,  and (3) it was not very restrictive,  whether the capstan tubes were pressurized or not.  The disadvantages were the achievement of rather-uneven compression,  and leaving the hands and feet completely uncompressed.  This limited the allowable exposure time by (1) uncompressed small body parts begin swelling in about 30 minutes,  and (2) between the uncompressed parts and the uneven compression achieved on the extremities,  blood pooling into the under-compressed parts could lead to fainting within about 10 to 15 minutes. 


Figure 1 – Partial Pressure Suit Design Used From the late 1940’s to the Early 1960’s

In the late 1960’s,  Dr. Paul Webb performed striking experiments with an alternative way to achieve mechanical counterpressure upon the body.  He used multiple layers of elastic fabric (the then-new panty hose material) as a tight-fitting leotard-like garment.  This was not a single-piece garment.  It achieved more-uniform compression on the torso and extremities than did the older partial pressure suit.  Dr. Webb included elastic compression gloves and booties,  so that the entire body was compressed,  removing the time limits.  Breathing difficulties were solved with a tidal volume breathing bag enclosed by an inelastic jacket. 

Breathing gas was pure oxygen at 190 mm Hg pressure (0.25 atm) fed into the helmet from a small backpack with a liquid oxygen Dewar for makeup oxygen.  This type of garment was very unrestrictive of movement,  and was demonstrated quite adequate for near-vacuum exposures equivalent to 87,000 feet,  for durations up to 30 minutes.  It was intended for possible application as an Apollo moon suit,  but could not be made operationally ready in time.  It has been mostly forgotten ever since.

The advantages are very unrestricted movement,  very light weight (85 pounds for suit plus helmet plus oxygen backpack),  and no need for a cooling system:  you just sweat right through the porous garment,  same as ordinary street clothing.   Plus,  the garment’s pieces were quite launderable.  Dr. Webb’s test rig is shown in Figure 2.  6 or 7 layers of the panty hose material provided adequate counter-pressure.


Figure 2 – Dr. Webb’s “Elastic Leotard” MCP Space Suit Prototype as Demonstrated

The disadvantages were essentially just difficult (time-consuming) efforts to don and to doff the garment’s pieces,  precisely because they were inherently very tight-fitting.  For use on a planetary surface or out in space,  one treats the suit as “vacuum-protective underwear”,  and adds insulating or otherwise protective non-pressurized outerwear over it.  So protection from hazards is not a disadvantage at all,  but only if one uses the vacuum-protective underwear notion. 

The main advantage of Dr. Webb’s “elastic space leotard” over the “partial pressure” suit was the more even (and more complete) compression achievable with the elastic fabrics.  The main advantage of the “partial pressure” suit over the “elastic space leotard” was the ease of donning and doffing the garment,  when the capstan tubes were depressurized,  releasing the fabric tension.  Both approaches offer very significant advantages over the “gas balloon” suits in use since the 1960’s as space suits:  lighter,  launderable,  and far,  far more supple and non-restrictive for the wearer. 

That suggests combining both of the successful MCP design approaches (inflated capstans and elastic fabrics) into a single mechanical counterpressure suit design.  The capstans apply and relax the tension in the fabric which provides the counter-pressure on the body,  and the elastic fabric makes the achievable compression far more uniform.  What is required from a development standpoint is experimental determination of the number of layers of elastic fabric required for each piece of the garment,  in order to achieve the desired compression in every piece. 

If done this way,  there is no need for directionally-tailored stiffness properties in specialty fabrics,  the basis of Dr. Dava Newman’s work with mechanical compression suits (see Figure 3).   Ordinary commercial elastic fabrics and ordinary commercial joining techniques can be used.  In other words,  pretty much anyone can build one of these space suits!


Figure 3 – Dr. Dava Newman’s MCP Suit Based on Directionally-Tailored Fabric Properties

So,  the MCP suit proposed here has certain key features (see list below).  It will resemble the old “partial pressure” suits,  except that protective outerwear (insulated coveralls,  etc.) get worn over the compression suit itself,  and the helmet is likely a clear bubble for visibility.  There is an oxygen backpack with a radio.  There is no need for any sort of cooling system.  Everything is easily cleaned or laundered free of dust,  dirt,  sweat,  and similar contamination.

Key features list:

#1. Pressurized capstan tubes pull the elastic fabric tight whenever the helmet oxygen is “on”,  but depressurize and slack the garment tension when helmet oxygen is “off”.  The capstan tubes are just part of the oxygen pressure breathing system.  Slacking the fabric tension makes doff and don far easier.

#2. The multi-piece garment is composed of multiple layers of elastic fabric to provide the desired level of stiffness that will achieve the desired level of compression in each piece of the garment.  This depends upon both the shape of the piece,  and upon how much circumferential shortening is achieved by inflating the capstan.

#3. The pressure garment is vacuum-protective underwear,  over which whatever protective outerwear garments are worn that are appropriate to the task at hand.  For example,  the wearer might need white insulated coveralls and insulated hiking boots,  plus insulated gloves.  One could even add some sort of simple broad-brimmed hat to the helmet if sunlight were intense. 

#4. The clear bubble helmet is attached to the torso garment piece. This torso garment piece also incorporates an inelastic jacket surrounding a tidal volume breathing bag.  Helmet,  breathing bag,  and capstans all pressurize with oxygen from the supply simultaneously,  and are (in fact) connected.  All are activated by one on/off control.

#5.  The oxygen backpack is just that,  no cooling system required.  It probably uses liquid oxygen from a Dewar as make-up oxygen,  has regeneratable carbon dioxide absorption canisters,  and a battery-powered radio.   It might also contain a drinking water feed connected to the helmet.  Attitude and translation thrusters for free flight in space can be a separate chair-like unit,  and this function is entirely unnecessary on a planetary surface.

#6. For concave body surfaces and complex shapes like genitalia,  the pressure suit can incorporate semi-fluid gel packs that surround these body parts,  making the body effectively convex everywhere.

How all this works together is shown conceptually in Figure 4.


Figure 4 – How the Capstans and Elastic Fabric Work Together for an Improved MCP Suit

About the only caveat might be that the breathing gas pressure could be too small to also serve as the capstan inflation pressure.  If that should prove to be true,  then there need to be two final pressure regulators in the oxygen backpack,  instead of just one.  That problem can be easily solved!

2 comments:

  1. Good explanation of the challenges involved in making a lighter suit using MCP. I was surprised that only 0.25atm is required.

    One question though: how do you handle hands and gloves? Do you need pressurized gloves?

    Another thing you mentioned is the use of gel to simplify the complex shapes and cavities of the body. Could this be further expanded? A 'gel-suit', that creates compression by pressurizing the mostly incompressible gel?

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  2. Yes, compression gloves and booties are required, otherwise hands and feet swell into painful uselessness after about 30 minutes. These could be gas-pressurized or mechanical counterpressure (both exist).

    As for the gel, it is not itself compressed in volume, it merely transmits pressure from the compression garment to the body surface. The garment wants to be everywhere convex to the world, while the body surface is not.

    As for the "startlingly-low" oxygen pressure requirement, check out the spreadsheet table of calculations in the 2-15-16 article on this site. Click on any figure and you can see all the figures enlarged. That helps you read the spreadsheet table. The thing is based on the local atmospheric pressure equaling the sum of the water vapor, oxygen, and nitrogen (if any) partial pressures. Water vapor pressure depends only upon body temperature, and displaces a bigger percentage when the atmospheric pressure is lower.

    GW

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