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Home > What's Next > Outline Proposals for Developing the Capstan Suit as an Anti-G Suit:

OUTLINE PROPOSALS FOR DEVELOPING THE CAPSTAN SUIT AS AN ANTI-G SUIT

by Thomas R. Sharp

Introduction and Description

The capstan suit was originally developed by Dr. James Henry of the University of Southern California from 1946 onwards on behalf of the U.S. Air Force as a garment to be worn by aircrews for protection against loss of cabin pressure at high altitudes. Eventually several other Air Forces researched and/or adopted this garment for their own use. It was in regular service from about 1952 but was withdrawn some years ago.

The suit consisted of a flight coverall of non-stretch nylon material, extending from the ankles, along the lower extremities, torso and upper extremities down to the wrists. A pressurized helmet and, where deemed necessary, pressurized gloves completed the assembly. The suit contained gussets, let into each side, which extended the whole outside length of the suit from the ankles to the wrists. Within both of these gussets were positioned ‘capstans’ or capstan levers, one for each side of the suit, which were inflatable rubber tubes covered by nylon restraint sleeves. The restraint sleeves’ diameters varied along their whole length in proportion to body sizing, being approximately one-fifth (20%) of a standard body size. The capstan levers were attached to the suit by a series of tapes running along the whole of their length. One end of each tape was joined to the suit on one side of the gusset, was wound once around the capstan lever, and then joined to the suit again at the other side of the gusset. The tapes were referred to as ‘interdigitating’ in Air Force nomenclature. When the capstan levers were inflated, the tapes would pull the gussets towards the capstan levers, thereby tightening the suit around the wearer. The suit was adjusted for different wearer’s girths by vertical lacing adjustments, much like a corset. Indeed, corset manufacturers helped in the original design of the suit.

Pressure in the suit was triggered when an aircraft’s cabin lost pressure at high altitude. With a pressure of 15 p.s.i. applied to the capstan levers, the suit would provide counter-pressure to a wearer’s body at 3 p.s.i. (15 x 20%), and most wearers were able to tolerate the suit under this pressure for up to 30 minutes. This gave aircrews valuable time to seek a lower altitude.

The suit had the following disadvantages:

  1. The suit’s nylon fabric was non-stretch. This rendered the suit relatively inflexible, and so movement at the joints was restricted;
  2. Since the interdigitating tapes had to be spaced at intervals, the leverage on the suit was variable when the suit was pressurized.
  3. The suit’s nylon fabric was close-weave and heavy in order to counteract the problem above; underwear was worn under the suit. Both features made the suit hot to wear, and therefore uncomfortable;
  4. Pressure marks could form on the body surfaces caused by the adjustment lacings pressing on body surfaces;
  5. Depending on the suit’s adjustment and the body size of the wearer, gaps would remain between the interdigitating tapes, and these areas would remain unpressurized, again giving rise to body markings;
  6. Discomfort could be caused during pressurization by underwear puckering and creasing;
  7. Suits were manufactured in 12 sizes, but even so, for a small proportion of wearers problems remained;
  8. Since the restraint sleeve/capstan configuration was preset at the manufacturing stage, there was no guarantee that the suit would provide equal counter-pressure along its length. Neither would the lacing adjustments ensure this.
  9. There was some difficulty in donning and doffing the suit. Wearers had to wriggle in and out of the tight-fitting suit, negotiating it over the undergarments.

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Further Development

The U.S. Air Force continued some research with the suit during the early 1980’s, but this time solely for purposes of providing anti-g protection for its fighter crews. In this capacity, the research achieved reasonable results and research by other Air Forces also proved successful.

The author first began considering proposals for further developing the suit in the late 1970’s. The main thrust of the development was to utilize stretch fabric in place of the non-stretch nylon fabric hitherto employed in the suit. By the 1970’s power stretch fabric was being marketed for the first time, and this was composed of a mixture of either cotton or nylon fibers, and elastane, in varying proportions. Elastane is known in the U.S.A. as ‘Spandex’ and in the U.K. as ‘Lycra’. Fabrics incorporating ‘Spandex’ have the ability to stretch and then retract immediately to their original size when the stretching force is withdrawn. Furthermore, power stretch fabrics can be woven in such a way that stretch can be applied in one direction only. For example, the fabric could stretch up to 200% of its length in the direction of the weft, but not at all in the direction of the warp. Such fabrics are known in the U.S.A. as ‘two-way stretch’ and in the U.K. as ‘one-way stretch’.

How can this characteristic be utilized in the capstan suit? By tailoring the suit so that the non-stretch direction of the fabric lies horizontally across the suit and the stretch direction of the fabric lies along the length of suit. In these circumstances, inflating the capstan levers, thereby pulling the interdigitating tapes, still tightens the suit around the wearer, because the suit fabric retains its horizontal dimensions. However, when a limb, say the knee-joint, is flexed, the suit stretches appropriately and retracts immediately the knee is straightened again. This renders the suit much more flexible and movement is enhanced.

The second feature of this development stage followed on from above. A further problem with the military capstan suit resulted from the dual role it had to play, both as a pressure garment and a military flying suit. Power stretch fabrics can be relatively lightweight, and employing these, rather than heavy non-stretch fabrics, would enable better ventilation over the wearer’s body surfaces. In such circumstances it makes sense to turn the capstan suit into an undergarment, leotard, or activity suit. A flight coverall would then be worn over the capstan suit, obviating the obvious problem that one garment was attempting unsuccessfully to perform two tasks.

Additional advantages arise by employing stretch fabrics in this way. Firstly, within certain parameters, the suit can be stretched lengthwise so as to accommodate different sizes of wearer. It is estimated that, for a lower garment covering the abdomen and lower extremities, a range of three different lengths of suit would be sufficient. If one accepts that a minimum stretch of, say, 15% would be needed in any case to eliminate wrinkles and creases, and taking leg lengths, for example, from 27 to 36 inches, a suit would require to be stretched for a maximum 25% overall, well within the capabilities of most power stretch fabrics. The restraint sleeve, contrariwise, must be constructed of non-stretch or virtually non-stretch material and would therefore need to be manufactured in a larger number of sizes. Secondly, the suit can be donned much more easily without the necessity of undergarments. With lightweight fabrics the suit can adopt the characteristics almost of a stocking which can be rolled over the limbs. Thirdly, ventilation to the wearer’s body surfaces is also much enhanced compared with traditional bladder-type anti-g garments.

In all, the incorporation of power stretch fabrics and the switch to an undergarment would greatly alleviate or solve several of the problems posted above, namely items 1, 3, 6, 7 and 9. There were further features in this early development, including the replacement of adjustment lacings by tapes, but these have since been abandoned.

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Recent Improvements

In recent years, the author has worked upon several further ideas to enhance the capstan suit, bearing in mind the unsolved problems remaining. There are a number of points to be borne in mind. For example, the capstan lever/tape configuration should be capable of tightening the suit evenly over the whole length of the suit, whereas in the existing configuration the interdigitating tapes are spaced at intervals. As a result, a rippling effect occurs along the gusset edges when the suit is pressurized. The tapes would have to be placed adjacent to each other so that they are conjoined at the edges of the suit. This would solve problem 2, and for the most part problem 5 above.

The second problem concerns the vertical adjustment lacings and the zip fasteners; incorporating the latter is necessitated when the suit is pulled over the ankles (and wrists in a full suit) when donning and doffing. Using lightweight stretch fabrics, these features would be an impediment to both the stretch capability of the suit and also the attainment of smooth counter-pressure over body surfaces when the suit is pressurized. Both such aims must be satisfied in order to make it worthwhile continuing further development on the lines already indicated. By eliminating both the lacings and zip fasteners over the main body of the suit, problem 4 would be solved.

How is this to be done? At this stage, further developments, which the author has worked upon to remedy the remaining problems, are not yet in the public domain and must therefore remain undivulged, since there may be implications for any future patent applications, not necessarily just for the author, but for any prospective manufacturer. However, it can be stated that a trial anti-g capstan suit covering the abdomen and lower extremities has indeed been worked up from these ideas and the results are acceptable. It is easily donned and doffed and weighs approximately 26 ounces (730 grams) to include capstan levers, shoulder straps and quick-release fasteners. With a capstan lever-to-body ratio of 40% and an air pressure of 15 p.s.i. in the capstan levers, a potential maximum suit pressure of 6 p.s.i. can be obtained (15 x 0.4). There are no lacings, zip fasteners or any other solid fittings attached to the suit over body surfaces where pressure is applied.

One problem remains, that of securing equalised counter-pressure over body surfaces when the suit is pressurized. In cases where the suit is employed medically for the purposes of ameliorating orthostatic intolerance, the author understands that the suit’s counter-pressure need be as low as 1 p.s.i., and any differences in pressure from one area to another would be small. In such instances, the size of the capstan levers would be reduced accordingly and/or a lesser pressure maintained in the capstan levers. For use as an anti-g suit for aircrews, on the other hand, graded adjustments by trial and error methods, either by some method of lacing or other means, may partially provide a solution. However, it may well be that ultrasonic or other techniques could be applied to test the uniformity of counter-pressure over body surfaces. The author is not competent in this field to discuss this further.

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A Development Strategy

The author appreciates that the proposed developments, both those clarified above and those so far undisclosed, represent an untried area of anti-g suit design. Whilst the sample suit that has been manufactured shows that the author’s overall ideas are practical, there will undoubtedly be a necessity to approach any design strategy one step at a time to conserve resources. It may, for example, be in the military’s interest to fabricate a whole-body suit, such a garment then serving both anti-g and high altitude protection. This would be unwise as an initial step. A sample suit on lines similar to the author’s own sample, mentioned above, should be developed, one, moreover which employs a stretch fabric already available on the commercial market. So far as the author can ascertain, appropriate fabrics can be readily purchased from Far Eastern companies, but not apparently from any source within either Europe or North America. These fabrics generally comprise a nylon/elastane (‘Spandex’) mix and contain about 8-10% elastane. The extensibility should be 100% or more in the direction of stretch and have zero extensibility in the other direction. The other components, tapes, inflatable tubing, restraint sleeve fabric, fittings and so on can either be purchased commercially or are easily produced ‘in-house’. Once the viability of the design has been established, it may well be that a bespoke fabric should be commissioned, for example, a fabric with a mix of silk for comfort, nylon for durability and elastane for stretch, and a fabric light enough to withstand wear – particularly the wear and tear associated with donning and doffing – but which will provide maximum air ventilation over body surfaces.

Thomas R. Sharp
sharp4091@tesco.net
3 April 2008

Copyright © 2008, Thomas R. Sharp

Posted with permission from the author.

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