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The "Secret" to Space Suit Design
A space suit is composed of two main parts, an anthropomorphic balloon and a climate-in-a-can. These are known better as the Space Suit Assembly (SSA) and the Primary Life Support System (PLSS) together they make the Extra-vehicular Mobility Unit or EMU. The SSA is the actual space suit while the PLSS is the backpack. My project involved working on the SSA, so that's what I will describe here.
| The simpler the better, so lets make our space suit out of fabric cylinders. As the astronaut bends a suit joint, the fabric cylinder will develop folds on the inner side of the bend. The outer side will remain the same length as it started out. this will cause the volume of the joint to decrease. The work to operate this joint will be the force required to it multiplied by the distance (d) through which this force acts. The work can also be viewed as the work required to decrease the volume plus the work required to bend the fabric (which is a insignificant force). |
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From the laws of thermodynamics, for a constant pressure process (the space suit pressure is regulated at a constant 4.3 psi) the work required to change the gas volume is
The initial volume of the joint is the area of the cylinder cross section multiplied by the joint length.
Assuming the cross section remains circular and the inner and outer edges can be approximated as circles, the final volume of the joint can be calculated as the area of the cross section multiplied by the centerline length of the deformed joint.
Returning to the equation for work leads to
The joint activation force can be calculated from this expression if an approximation can be had for the length through which the activation force operates. A reasonably good approximation for this distance leads to
Using this formula the forces for the various joints in a space suit are calculated and tabulated below.
| Joint | D (in) | L (in) | F (lb) | Human
Capability F (lb) |
| Finger | 1 | 2 | 1.7 | 7 to 16 |
| Wrist | 4 | 5 | 43.2 | No data |
| Elbow | 5 | 8 | 52.8 | 30 to 40 |
| Knee | 6 | 5 | 91.2 | No data |
| Waist | 18 | 12 | 1641.3 | 200 |
This data shows joint operation forces which are a large part of
maximum capability. A fabric cylinder space suit would tire an
astronaut out very quickly. Some motions such as waist bending
would be impossible. That's why the Gemini space suit had a block
and tackle arrangement to allow waist bending. From this it is
easy to see why the "SECRET" of space suit design is to
maintain constant volume.
Actual force measurements lead to the values for the operating forces given below:
| Joint | Calculated F (lb) |
Max Spec. F (lb) |
| Elbow | 52.8 | 1.5 |
| Knee | 91.2 | 1.5 |
| Waist | 1641.3 | 4.0 |
| Obviously the space suit designers are good at keeping constant volume in the space suit designs. There are several ways to do this. The space shuttle suit uses fabric cylinders for its arms and legs and still approximates constant volume. This is accomplished by constructing the joint so that as volume is decreased in the inside (closing part of the joint) it is increased a like amount in the outside of the joint. This is done by holding the centerline of the joint at a constant length instead of the outside of the joint. An elbow joint designed on this principal looks like this: |  |
The axial restraint lines, located across the diagonal of the fabric cylinder, take the pressure load that tries to elongate the joint. This prevents the fabric cylinder part of the joint from carrying that load and allows placement of excess fabric on the outside of the joint. Without the axial restraint line the pressure would cause the joint to elongate until all of the excess fabric was placed in tension. As the joint is bent in use the inside of the joint folds up just as it did in our fabric cylinder. As that happens the outside of the joint where the excess fabric has been placed expands to compensate for the lost volume. The flexed joint looks like this:
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If the picture to the left is considered as a free body diagram of a space suit joint, astute observers will note that the forces are not balanced. In practice, a space suit joint does not bend so that the centerline is in the shape of a circle as shown here. The centerline shifts slightly to the outside to compensate the otherwise unbalanced forces. Also, the designer does not place the axial restraint lines exactly at the joint's centerline but slightly off set so that the joint will be balanced and stable. If the placement of the axial restraint line is not done carefully the joint will be very difficult to bend or will actually bend itself over when it is pressurized. Well designed joints are stable and remain in the position they are placed with little restraining or spring back force. In addition, all mobility joint elements are designed for various man-induced loads imposed on the space suit by the wearer. |
This information adapted from a handout prepared by Michael Rouen, Space Suit Systems Specialist at JSC
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