Mobility, Mobility, Mobility!
(Methods of Cross-Section Area Reduction
in Space Suit Joints)

Mobility, Mobility, Mobility!

There's no point in having a space suit if you can't do what you need to do. I already talked about making space suits mobile in the page in The "Secret" to Space Suit Design Page. But a space suit will never be mobile enough, and there will always be ways to improve the design.

When looking for a project, a co-worker suggested an idea he had had for some time but never done any proof of. See, the diameter of the fabric cylinder (of an arm, leg or waist) is limited by the size of the largest body part that has to pass through it to don (put on) the suit (the wrist, foot or shoulders). If the joint was pulled tighter after the suit is donned but before it is pressurized, the plug loads (loads elongating the joint, see here) would be smaller, your arms (no longer as thick as tree trunks) would take up less of your work envelope, the total volume of air would be less, and it would be easier to maintain constant volume. All of these attribute to a greater mobility inside the space suit.

Because NASA never starts expensive research without expensive studies beforehand, my job was to prove that this theory had no fatal flaws and merited further investigation. Math is never a final test, so this proof meant I had to design and build space suit joint based on this principal. At first I planned on modifying an existing design and using data from that as a control to compare with. However, I ran into proprietary information problems as well as the fact it is much harder to build a real space suit arm than a simple, basic joint. So I designed a control joint, very basic with a diameter of 6 1/4 in. and adapted that to a joint with a diameter of 4 in. when an extra sizing panel was laced shut and 6 1/4 in. when it was open. These diameters are the measured diameters of the upper arm bearing (6 1/4 in.) and the wrist bearing (4 in.) on actual space suits.

I designed several different methods to close the panel quickly and securely, but to avoid extra unknowns I put tried and true loop tape (already used on the EMU for resizing) even though it is far too labor intensive to ever really use for this purpose.

When both arms were fabricated I was to compare them to see if the modified one had less, more or about the same force required to bend it. This doesn't sound very scientific, and there are machines to measure the exact torque values to "activate" a joint, but most of space suit design is, as one the men who has worked on space suits since before Apollo said, "...more of an art than science." The other measures of increased mobility is the decrease in the diameter of the arm, and the decrease in the plug load. The decrease in the diameter is easy to measure (if it isn't smaller by 36% I screwed up, because those were the factors I meant to have control over) and the plug load is easy to calculate, but very hard to measure directly. The only other factor is the decrease in volume of the joint, allowing less of an airflow and therefore making it easier to miniaturize the life support systems. I can calculate how much smaller the volume is, and even the decrease in the amount of air to get the same pressure. (I will do this calculation with a base pressure of 3.75 psi, 4.3 psi, and 8.3 psi, 3.75 psi is the Apollo suit pressure and it is being considered for future space suits, 4.3 psi is the pressure of the EMU, and 8.3 psi is the pressure for a zero-prebreath suit.)

Sadly, time constraints limited the arms I was going to build to only the modified arm. I will still do all the calculations I had planned for the comparison, and rate the joint I made on the Cooper-Harper Rating Scale, a scale used to determine how effective aeronautical modifications (including space suit modifications) are. This data should prove the arm worthwhile of further improvements or a dead end path.

The Future of Space Suits

The ultimate goal of this project is to design a new, mobile space suit. The question is how will this concept be applied to achieve this goal? The answer to that question merits its own section.

Multiple panels would be needed along with reliable split ring bearings (unless you want to design a VERY clever fabric bearing), and a way to seal all this mess up quickly. It is a little easier than it sounds because only one layer of the suit needs these closures. The suit is made (from the outside in) of a Thermal and Micrometeoroid protection Garment (TMG), which consists of 9 layers of 3 different fabrics, the restraint layer, the bladder layer, and the Liquid Cooling and Ventilation Garment (LCVG-3 layers, all different materials). Of these, only the restraint needs to be modified, those layers under the restraint will be held by it, and the outer layers aren't pressurized.

This suit could be used with the same PLSS used today and have a greater safety margin or a new PLSS could be designed to take advantage of a smaller airflow. It may even be possible to dam the neck so the head and body are in their own air pockets and recycle all the air from the head, but either fill the body with air at the beginning and let it sit or even flow waste air into the body part.

Meanwhile your astronaut is enjoying a smaller suit that allows movement with even less force than the current EMU requires for movement. All this allows him to work for his eight hour EVA with less fatigue, staying more alert in a very dangerous environment while working on sensitive, technical jobs. After all, that is the primary purpose of any space suit.

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