Shielding the International Space Station
The ISS program plans to shield many ISS elements to protect the station from meteoroids and orbital debris. The meteoroid and debris AIT has developed numerous potential shield designs and tested their performance against hypervelocity impacts. Such shielding will be necessary because meteoroids and debris will impact the ISS at velocities sufficient to cause a wide range of damaging effects.
The pressurized modules, for example, have multiple vulnerabilities to impact. Large enough meteoroids or debris striking a pressurized module at typical velocities will cause the module wall to spall or break off chips, sending pieces of the wall into the module, even if no air pressure is lost. If the wall is perforated, then particles of the impactor will also enter the module. While spallation particles are much slower than the originating impactor, they can be much larger. Both types of particles can cause injury or loss of life to the crew, as well as damage to internal components in the path of the projectile. Aluminum spallation particles (the ISS is primarily constructed of aluminum) can also burn quite actively, creating a fire hazard inside the module.
In addition, the perforation of a module will be accompanied by a strong acoustic shock wave and an intense light flash that could temporarily incapacitate crew members in the module. Such perforations typically are accompanied by rapid temperature changes and a decrease in air pressure, which can cause an internal fog. In a module wall, perforations can lead to crack growth and, for very large perforations, rapid crack growth that can cause the module to “unzip” or break apart.
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
conformal and nonconformal passive shield designs. Active armor, which uses an internal energy source to deflect incoming objects, is not being considered for use on the ISS.
More than 100 different shields have been designed to protect the various critical components of the ISS, although all of the designs are modifications of three ISS primary shielding configurations: the Whipple bumper, the multishock (or stuffed Whipple) shield, and the mesh double-bumper shield.
The Whipple bumper, the simplest shield configuration, consists of a single plate of material (typically aluminum), called the bumper, spaced some distance from the underlying module wall (often called a catcher). The role of the bumper is to break up, melt, or vaporize a high-velocity object on impact. The smaller, slower remnants of the object then travel between the bumper and the catcher and spread the remaining energy of the impact over a larger area on the catcher. This configuration has been studied experimentally for over half a century (Cour-Palais and Crews, 1990; Christiansen et al., 1995b; Hortz et al., 1995). Figure 4-1 shows how a Whipple bumper shield works.
The Whipple bumper is most effective at high impact velocities, where the disruption and dispersion of the impacting projectile can be maximized (Christiansen et al., 1996). At lower velocities, the collision with the bumper may not break up or liquefy the impactor; thus it may still be intact when it strikes the catcher (Christiansen et al., 1995a). Whipple bumpers and their derivative designs are vulnerable to low-velocity (a few km/s) impacts, oblique impacts, and the impacts of objects whose sizes, densities, and shapes differ from the threat the shield was designed to counter. Figure 4-2 shows how the maximum object size that various Whipple bumper derivatives can stop changes with impact velocity.
Whipple bumper shields and their derivatives can be optimized against a specific threat by modifying the bumper materials, bumper thickness, bumper spacing, catcher material, and catcher thickness. Derivatives of the Whipple bumper concept (such as the stuffed Whipple bumper) have been developed by NASA since the mid-1980s (Cour-Palais and Crews, 1990).
The stuffed Whipple bumper consists of an outer bumper, a catcher, and one or more underlying layers of materials spaced between the bumper and the catcher to further disrupt and disperse the impactor. The advantages of this design are its improved performance over the standard Whipple design and, with some bumper materials (e.g., Nextel), its reduced production of secondary ejecta. In current ISS stuffed Whipple designs, the outer bumper is made of aluminum, and the shield is normally stuffed with a single intermediate blanket consisting of six layers of Nextel and six layers of Kevlar. The module wall serves as the catcher. This design performs significantly better than the Whipple bumper, but it does not significantly reduce the production of secondary ejecta.
The mesh double bumper is the newest NASA derivative of the Whipple bumper concept. Developed in the early 1990s, this shield has a metallic mesh disrupter in front of each of two bumpers (Hortz et al., 1995). This design
also provides significantly improved performance over the Whipple bumper. Figure 4-3 shows some of the many shielding concepts planned for use on the ISS.
The ISS shield designs have been extensively tested against the design threat (a 1-cm aluminum sphere), and some tests have been performed with different-sized impactors. As a result, substantial experimental data exist to support the performance claims for these designs against the design threat. Experimental results have been compared with, and extended by, hypervelocity impact simulations using hydrodynamic codes (hydrocodes), such as CTH (developed at Sandia National Laboratories) and SPHINX (developed at Los Alamos National Laboratories) (Kerr et al., 1996; Hertel et al., 1994; Wingate and Stellingwerf,
1994). Data from the impact tests also have been used to develop semi-empirical shield scaling laws. These scaling laws can be used both for designing and optimizing shields, as well as for determining which sizes of aluminum spheres are capable of penetrating specific shield configurations. These scaling laws, along with the meteoroid and debris environment models, have been built into the BUMPER code to allow the rapid evaluation of PNPs resulting from various shield configurations.
For the foreseeable future, the non-Russian ISS crew will use the extra-vehicular activity mobility unit (EMU) currently used by space shuttle astronauts. This space suit is protected by multiple layers of material and a single bladder that, together, provide a pressure vessel and a degree of protection from the thermal extremes of space and from meteoroids and debris (Cour-Palais, 1996). A secondary oxygen pack will provide at least 30 minutes of supplementary oxygen, should a hole up to 4 mm in diameter develop in the suit. There are multiple failure recovery modes from other impact-induced failures. Analysis shows that more than 75 percent of the hazard will result from penetrations of soft parts of the space suit (i.e., arms, gloves, and legs). The ISS program is considering augmenting the arms and legs with removable gauntlets and “chaps” (McCann, 1996) to reduce this hazard.
ANALYSIS AND FINDINGS
The effort to shield the ISS from meteoroid and debris impacts appears to be extensive and thorough. No major aspects of the problem appear to have been overlooked. Critical research is under way to investigate areas where the physics and mechanics are not fully understood. Good engineering is addressing problems where a need has been recognized. The findings and recommendations below identify issues where additional effort or a shift in focus would enhance existing activities to the overall benefit of the ISS.
Shielding for the Actual Environment
As described in Chapter 3, significant strides in defining the orbital debris environment have been made in recent years. As a result of new debris orbit characteristics, the 1996 environment model identifies mean velocity and velocity distribution properties markedly different from those in the 1991 model. Hypotheses about the type of debris most likely to impact the ISS have also changed recently. As described above, limited experimental data or modeling capabilities are available to evaluate how well the ISS will survive these threat excursions.
The 1991 model is still used for ISS design issues, and the ISS meteoroid and debris AIT seems to believe that the newer models indicate a reduced threat (because of the significantly lower flux of objects larger than 1 cm in diameter). This assessment has not yet been evaluated, and it may not be accurate for several reasons. First, the 1991 model did not include debris in elliptical orbits; thus, no debris impacts were predicted to occur on ISS trailing surfaces. Because only high-velocity meteoroids were predicted to strike trailing surfaces, many of these surfaces use a simple Whipple bumper design. The 1996 model, however, includes debris in elliptical orbits, which can strike the ISS trailing surfaces at lower velocities (averaging about 4.5 km/s). Fortunately, most debris known to be in elliptical orbits are small and in lower-inclination orbits. Therefore, the threat of serious damage to ISS trailing surfaces is currently believed to be very small. ISS designers, however, should be aware that debris can strike ISS components from a wider variety of directions than was predicted by the 1991 model.
The altered orbit characteristics in the recent models have also resulted in a reduction of mean relative impact velocity, from about 10.5 km/s in the 1991 model to less than 9 km/s in the 1996 model. This significant statistical reduction in impact velocities does not, however, necessarily represent a reduction in the threat to the ISS. The newer models predict approximately double the object flux
within the 2 to 6 km/s impact velocity range—a regime in which the Whipple effect, the basis for ISS shielding, is less effective (Christiansen et al., 1995a).
Debris resulting from explosions, breakups, degradation, or rocket firings probably will not be spherical, and the densities of meteoroids and debris may vary greatly. However, minimal data have been gathered on the performance of the proposed ISS shields against threats other than the nominal 1-cm aluminum sphere. The performance of shields against impactors with different characteristics (density, impact angle, velocity, etc.) typically have been extrapolated using scaling laws or hypervelocity impact simulations calculated by hydrocodes.
The interaction between threat and shielding in a high-velocity impact event, however, is a complex, nonlinear set of processes involving a host of poorly understood mechanical and physical effects. Dynamic strength, multiphase equations of state, and the fragmentation of both threat and shield materials play interrelated roles in determining the outcome of an impact and potential penetration. For these reasons, scaling laws for projectile shape, projectile density, or impact obliquity should not be applied to impact threats for which data were not collected or included in the development of the scaling law.
Therefore, extensive impact testing is critical to the development of improved ISS shielding and to the characterization and validation of the engineering codes (such as BUMPER) used to assess ISS vulnerability to meteoroid and orbital debris impacts. Testing over a wide range of threat materials, densities, and shapes is needed to acquire a fuller understanding of shield performance in more typical environments and to gain confidence in assessing the vulnerability of ISS components. In particular, it is necessary to emphasize testing in the velocity regimes of 2 to 5 km/s and greater than 10 km/s. In these regimes, current ballistic limit curves indicate the highest susceptibility to shield failure and pressure wall penetration. The lower-velocity regime is particularly complex because material strength effects inhibit the full development of the Whipple effect on which the spaced-armor concept relies. The upper velocity range is critical because of the extreme on-target energy produced by the impact.
Finding 4. ISS shields have been designed to resist a particular threat. However, the actual threat may be quite different from the threat the shields were designed to counter. Extensive testing against a variety of different impactor sizes, shapes, velocities, and compositions is needed to ensure that the shields provide adequate protection.
The ISS has a substantial exposed area of trusses and other noncritical components. The effects of impacts on these components have not yet been evaluated
in detail. Of particular concern is the threat to critical components from secondary ejecta created by impacts into other critical or noncritical components. Ejecta that does not immediately strike the ISS may remain in orbit, posing a potential long-term collision hazard. However, evaluations of ISS vulnerability to date have not included assessments of the threat of secondary ejecta; nor are secondary ejecta included in current orbital debris environment models. Ejecta production is not well understood, and ejecta characteristics (e.g., mass, shape, and velocity) are not well characterized. Tests over a wide range of conditions would be needed to acquire the fuller understanding of ejecta production and characteristics needed to assess the vulnerability of the ISS to this threat.
Computational simulations of shield performance during orbital debris impact, using state-of-the-art continuum and structural codes, offer a unique capability for improving ISS shield performance (Kerr et al., 1996). Recent strides in large-scale parallel computing have brought computational simulation of large-scale, three-dimensional impact events into the realm of practical engineering applications. Concerted efforts toward this end are currently a priority at some Department of Energy and DoD facilities. The effective application of computational tools to ISS shielding problems could provide methods for optimizing shield design and expanding the investigation of threat/shield interactions outside of the range of practical testing limits. NASA has recently begun using hydrodynamic codes to evaluate shield performance in the velocity regimes where testing is not currently feasible. However, NASA has limited capabilities and facilities for these simulations.
Advanced Shield Materials
The properties of materials used in shield construction are crucial to armor performance. The bumper plate should be made of the highest shock-impedance material possible, consistent with other design requirements. High shock impedance will lead to shattering and the lateral momentum dispersion of the impacting object at the lowest possible velocity. Secondary plates should lead to the further diffusion of the threat momentum, while the catcher plate material should have a high transverse acoustic velocity to distribute the impulse of the threat debris. In recent years, a wide range of metal-ceramic and ceramic-ceramic composites, with densities comparable to aluminum but with significantly higher shock impedance, have been developed outside NASA. Also, a number of glass-reinforced polymeric composites have been developed that have served well in other armor applications. In the future, these advanced shielding materials could be used in lighter and more effective shields for the ISS.
In-Orbit Shield Augmentation
Extensive testing and analysis shows that the U.S. segment meets or exceeds its required PNP levels for the design threat. However overall PNP compliance will not be achieved in the early stages of ISS assembly because of the inability of the Russian participants to achieve all of their PNP requirements within the time frame of the launch schedule. NASA and the Russian Space Agency (RSA) have agreed that PNP compliance will be achieved by augmenting the shielding of some Russian modules in orbit.
Augmenting ISS shielding in orbit could be very effective in protecting against meteoroid and debris threats. Both conformal and nonconformal methods provide a range of shielding options. Conformal shielding techniques offer high shielding potential if the shields can be installed without an unacceptable level of extravehicular activity (EVA). The ISS team is also considering nonconformal shield designs, which are easier to attach in orbit and offer the attraction of an extended dispersion path for secondary ejecta. Nonconformal shields, however, are very directional and would leave the shielded item exposed if the station were to operate in a different attitude (as it is projected to do for a period of time during assembly) or if more debris were found in elliptical orbits.
Because of the relatively small surface area of a space suit, the limited exposure times involved, and the light shielding offered by the suits, the primary threat to astronauts performing EVAs comes from particles in the 1-mm size range. NASA has evaluated the EVA suits experimentally, and the meteoroid and debris AIT has a good understanding of the sizes of particles able to perforate the suits for the design threat (Cour-Palais, 1996). These evaluations showed that the EVA suits exceeded their required PNPs and that no component of the EMU will catastrophically release energy upon penetration. The expected probability of ever experiencing a penetration resulting in the consumption of the oxygen reserve in less than 30 minutes was calculated to be less than one percent through the year 2012. This assessment showed that the risk from meteoroids and debris to astronauts performing EVAs was significantly less than the risk posed by other hazards to spacewalking astronauts. However, the assessment was made with a 1989 environment model. Newer environment models indicate an increase by a factor of 2 to 3 in particle flux in the size range of concern for EVA suits.
Recommendation 6. All groups developing shielding for the International Space Station should incorporate new environment models into their design considerations, as soon as official model acceptance is achieved. Shield designers should
recognize that the environment is likely to continue to change and that future shields will have to be designed to resist a broader spectrum of threats. For example, designers should be very wary of nonconformal shield concepts that only block objects coming from one direction.
Recommendation 7. The International Space Station program should initiate an accelerated shield-testing program aimed at acquiring a fuller understanding of shielding performance against a wider range of impactor characteristics. A series of tests, with emphasis on the lower velocity regimes, should be performed to determine how the various shields on the International Space Station perform against the expected threat and to develop a scaling technique for converting nominal test performance to performance when shields are exposed to the actual threat.
Recommendation 8. In conjunction with any impact testing, the International Space Station program should initiate a laboratory-based data collection and test instrumentation program with emphasis on acquiring a fuller understanding of secondary ejecta phenomena and threat characteristics.
Recommendation 9. NASA should evaluate current hydrocode support for International Space Station meteoroid and debris shield development and consider upgrading current capabilities through greater NASA emphasis or through cooperation with other national facilities.
Recommendation 10. The meteoroid and debris analysis integration team should contemplate using advanced shielding materials in upgrades to existing International Space Station shielding and future shield augmentations. The analysis integration team should consider holding a workshop to bring in shielding experts from outside NASA to discuss advanced shielding concepts.
Recommendation 11. The International Space Station program should reassess extravehicular activity suit survivability with respect to the 1996 meteoroid and debris environment model.
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