Pressurized storage tanks can be subject to the same spallation, perforation, shocks, temperature and pressure changes, and crack growth phenomena when they are struck by high-velocity debris. If a tank containing liquid is hit, then the perforation can also result in a hydrodynamic ram effect (depending on the size of the storage tank and the density of the liquid) that can lead to increased crack growth and catastrophic tank failure. Impacts may also result in the release of stored energy in the tanks, perhaps in the form of chemical reactions or explosions. In addition, the venting of pressurized gases from modules or tanks could result in strong torques to the ISS structure.
The ISS attitude stabilization gyrodynes are another source of stored energy that could damage the ISS if they are penetrated. Stored rotational energy in the gyrodynes could be released in the event of an impact-induced fragmentation of the rotating components of a gyrodyne. In addition to causing the loss of ISS stabilization, fragments created in the breakup could damage other ISS components or adjacent gyrodynes.
Impact damage or perforations of noncritical ISS components can also lead to performance degradation and the failure of ISS systems. Impacts into thermal control radiators, for example, could result in loss of the working fluids, and impacts into solar arrays could result in arcing and short circuits. Secondary ejecta expelled from impact sites can damage other components or even penetrate a critical module. Depending on the angle and the velocity of impact, these ejecta can range from low-speed (a few km/s) particles to highly energetic jets, either of which can be more lethal than the original impactor.
As discussed in Chapter 2, the ISS program requires that both the Russian and the non-Russian (U.S./European/Japanese) segments have an overall PNP of 0.90 or better (a maximum 10 percent probability of penetration) over 10 years. To achieve this goal, the ISS will employ different shield designs to protect various critical components. In general, the approach aims to prevent internal damage from the nominal threat of an aluminum sphere approximately 1 cm in diameter, over the predicted velocity range. Other more effective shields are placed in forward-facing areas where most impacts are expected. Less capable shields are located in aft and nadir-facing areas that are expected to be hit less frequently.
In addition to being capable of preventing penetration by the nominal threat, shields for the ISS must be lightweight, low in volume (to fit in the space shuttle payload bay), and durable in the space environment. These constraints have led the ISS program to use passive conformal (i.e., conforming to the shape of the module being protected) armor for the initial ISS configuration. The ISS, however, is designed to allow for future shield augmentations if the threat increases or if the life of the station is extended. NASA is currently evaluating various augmentation concepts and shield design modifications for future use, including both