47 SYSTEMS: STRUCTURE AND MECHANISMS—THE INTERNATIONAL SPACE STATION’S SKELETON CHAPTER 3 Really androgynous? Most mechanisms that involve joining two items together have a side that is active and a side that is passive. The active side has all the hardware that moves and all the computers needed to command and control that moving hardware. That moveable hardware (e.g., latches or bolts) interfaces with non-moving (passive) hardware on the other side of the interface. Examples of passive hardware include nuts, non-moving hooks, latch capture plates, etc. In some mechanisms, both halves have the same moveable hardware. In these cases, one side of the mechanism is designated as active, and its hardware is made to move while the hardware on the other side remains stationary and passive. These roles could be reversed on a subsequent use. This setup for a mechanism is termed an “androgynous” configuration. See Figure 14. If the APAS were truly androgynous, the system on the ISS PMA or on the orbiter could serve as the active half of the docking system. Although both halves indeed had the same hooks and latches, the active hardware (i.e., motors, controlling computers, pyrotechnic bolts, etc.) was removed from the ISS halves prior to launching the PMAs. That means the orbiter side was always active. The ISS half of the docking mechanism hooks could not be driven, and the explosive bolt pyrotechnics for releasing those hooks were not installed. Should the hooks on the orbiter side have failed to release the ISS, the orbiter side could have pyrotechnically separated its hooks (and, thus, left that docking port permanently unusable). And, if for some reason the pyrotechnics did not work either, a spacewalking astronaut could manually separate the two docking system halves by removing 96 bolts around the perimeter of the docking mechanism. Thankfully, that task was never required. Figure 14. The two androgynous docking system alignment guides are about to overlap, as seen out the orbiter’s aft flight deck window just prior to docking on STS-100/ISS-6A (2001). The orbital debris strategy must, however, also deal with thousands of smaller objects that cannot be tracked and thus cannot be directly avoided. The ISS modules—US Segment, Russian Segment, and all temporary crew and cargo vehicles— are designed to protect against the impact of very small (1 cm [0.4 in.] diameter or smaller) debris. This protection comes via another secondary structure component, debris shielding, which is described in more detail in this section. Debris too small to be tracked but still too big to be assuredly stopped by debris shielding could penetrate the ISS shields and pressure shell. The ISS crews are trained extensively on how to respond to rapid cabin depressurizations due to midsize orbital debris penetrations, should one ever occur. In these scenarios, crew members first remove themselves from the immediate area of impact, ensure their rescue vehicles are not leaking, and work to isolate the module with the leak by closing various module hatches. In the event a piece of debris penetrates the pressure shell of the ISS, on-board tools and repair kits help the crew pinpoint the leak/ penetration point (which could be a very small hole, numerous small holes, or a larger gash) and attempt to repair the damage. Current on-board repair kits should allow the crew to repair holes up to 1.25 cm (0.5 in.) in diameter, assuming enough reserve time is available to find and repair the leak. Reserve time is the calculated time remaining before the cabin pressure drops below 490 mm Hg (9.5 psi). Once the pressure drops that low, crew members must isolate and seal off the leaking compartment (if the location is known) or isolate