Roberto Destefanis

Ballistic limit evaluation of advanced shielding using numerical simulations

Abstract The advanced shielding concept employed for the Columbus module of the International Space Station consists of an aluminum bumper and an intermediate shield of Nextel and Kevlar-epoxy. Until recently, the lack of adequate material models for the Nextel cloth and Kevlar-epoxy has precluded the practical usage of hydrocodes in evaluating the response of these shields to hypervelocity impact threats. Recently hydrocode material models for these materials have been proposed [1,2] and the further development and completion of this model development is reported in this paper. The resulting models, now implemented in AUTODYN-2D and AUTODYN-3D, enables the coupling of orthotropic constitutive behavior with a non-linear (shock) equation of state. The model has been compared with light gas gun tests for aluminum spheres on the advanced shield at impact velocities between 3.0 and 6.5km/s [3]. Reasonable correspondence has been obtained at these impact velocities and thus the models have been used to perform preliminary assessment of predicted ballistic limits at velocities from 7 to 11km/s. The predicted ballistic limits are compared with ballistic limit curves derived on the basis that damage is proportional to projectile momentum

Accelerator-based tests of radiation shielding properties of materials used in human space infrastructures.

Shielding is the only practical countermeasure for the exposure to cosmic radiation during space travel. It is well known that light, hydrogenated materials, such as water and polyethylene, provide the best shielding against space radiation. Kevlar and Nextel are two materials of great interest for spacecraft shielding because of their known ability to protect human space infrastructures from meteoroids and debris. We measured the response to simulated heavy-ion cosmic radiation of these shielding materials and compared it to polyethylene, Lucite (PMMA), and aluminum. As proxy to galactic nuclei we used 1 GeV n−1 iron or titanium ions. Both physics and biology tests were performed. The results show that Kevlar, which is rich in carbon atoms (about 50% in number), is an excellent space radiation shielding material. Physics tests show that its effectiveness is close (80–90%) to that of polyethylene, and biology data suggest that it can reduce the chromosomal damage more efficiently than PMMA. Nextel is less efficient as a radiation shield, and the expected reduction on dose is roughly half that provided by the same mass of polyethylene. Both Kevlar and Nextel are more effective than aluminum in the attenuation of heavy-ion dose.

SPH simulations of debris impacts using two different computer codes

Abstract This paper reports the results of Smoothed Particle Hydrodynamics (SPH) simulations of debris impact on an all aluminium triple wall system, performed by Alenia Aerospazio using the PAM-SHOCK 3D computer code, and by the University of Rome “La Sapienza” using the AUTODYN 2D hydrocode. The point of view of a user interested in debris shielding design and not that of a software developer or an SPH algorithm expert has been adopted. Comparisons between the results of the two codes at early, mid and late stages of impact are shown. Theoretical predictions are used to evaluate the numerical simulations at early stages of impact. The predictions of the damage induced on the plates are compared with the Light Gas Gun experimental data. X-ray pictures are used to evaluate the simulations of the debris cloud expansion and spalled material detached from the intermediate bumper. Traditional Finite Volume (FV) simulation results are reported and compared with SPH results. The authors do not go into any SPH algorithm details, but using the potentiality of the respective codes show what is the current capability of the hydrocodes to assess the ballistic performance of high resistance debris shielding used to protect space structures.

Testing of advanced materials for high resistance debris shielding

Abstract Innovative 3-wall systems are developed to cope with the severe threat posed by on-orbit impacts of man-made debris. High resistance advanced shielding is based on an aluminum Whipple shield and an intermediate layer of advanced materials protecting an aluminum rear wall against hypervelocity impacts. Several intermediate bumpers have been tested as part of a hypervelocity test campaign using a Light Gas Gun firing aluminum spheres with a mass up to 6.1 g and a velocity up to 7 km/s. The use of low density, high strength cloth and composites (Kevlar™ and Kevlar™-Epoxy) greatly reduces the energy and the mass of the cloud hitting the rear wall. The introduction of ceramic fiber (Nextel™) is necessary to avoid a secondary effect (uprange blast) due to the rebound of projectile and intermediate bumper material against the external Whipple bumper, which causes large plastic deformations. The experiments showed excellent ballistic performance against aluminum projectiles at low velocity (3 km/s), high velocity (6–7 km/s), normal and 30-degree impacts. X-ray photographs are used to monitor the impact process, to study the debris cloud formation and evolution and to investigate the damage mechanism.

Integral model for the description of the debris cloud structure and impact

Abstract The purpose of the present paper is to introduce a new integral model capable to describe the evolution of the debris clouds originated after normal-impacts of orbital debris over a Whipple shield. This work had been developed at Alenia Spazio in the context of a degree thesis. Several numerical SPH simulations of debris impacts on a Whipple shield configuration were performed to determine the ballistic limit and to compare it with semi-empirical damage equations. In the present paper, the numerical simulations were used to investigate the typical behaviour of experimental debris clouds ([6] and [9]) and to support the development of the integral model. With respect to previous papers ([1], [2], [3], [10]) in which a spherical shell-wise debris cloud was considered, here we try to introduce more realistic assumptions. We approximate the clouds shape also introducing ejecta veil effects, which produce a multiplication of the deposited momentum upon the underneath wall. In the present model, the most peculiar hypothesis is a cinematic self-similar behaviour that is, whatever the shape is, the debris cloud evolves keeping unchanged its shape. Then, the material is opportunely distributed inside a volume and the choice of that distribution is described taking into account the results of the numerical simulations. Knowing the spatial material distribution and treating the cloud as a fluid, we can estimate the load time history and the drag-unitary force induced by the cloud impacting upon the rear wall. Of course, such a method uncouples the dynamic response of the rear wall from the evolution of the debris cloud. The balances of mass, momentum and energy allow three global and unknown parameters to be determined. The one-dimensional theory of impact ([10]) is used to take into account the conversion of part of the initial kinetic energy into internal thermal energy. No integration of differential equations is performed since complex propagation phenomena are taken into account through the effects they globally produce. The model still presents some free parameters related to the integral formulation. These parameters cannot be calculated through any balance condition, but they must be imposed to get a good, global reproduction of the debris cloud. The choice of these parameters is still the weak aspect of the method, and it depends on the consideration of the results obtained with more sophisticated tools, as, for instance, SPH simulations. The spatially defined load time history obtained with the debris cloud integral model can be used for further analysis on the back up plate.

Whipple shield ballistic limit at impact velocities higher than 7 km/s

Abstract The Whipple bumper shield was the first system developed to protect space structures against Meteoroids and Orbital Debris (M/OD), and it is still extensively adopted. In particular, Whipple shields are used to protect several elements of the International Space Station, although the most exposed areas to the M/OD environment are shielded by innovative low weigh and high resistance systems. Hydrocode simulations were used to predict the ballistic limit of a typical aluminium Whipple shield configuration for space applications in the impact velocity range not accessible by the available experimental techniques. The simulations were carried out using the AUTODYN-2D and the PAMSHOCK-3D codes, allowing to couple the gridless Smoothed Particles Hydrodynamics with the Lagrange grid-based techniques. The global damage of the structure after the impact was determined with particular attention to the back wall penetration, and the results obtained with the two hydrocodes were compared with those given by semi-empirical damage equations. A few hypervelocity Light Gas Gun impact experiments, performed on the same shield configuration at velocities up to 7.2 km/s, were previously simulated in order to assess the capability and limitations of the two hydrocodes in reproducing the experimental results available in the lower velocity regime. The influence of material models on the numerical predictions is discussed.

Impact of Spacecraft-Shell Composition on 1 GeV/Nucleon

Through the use of experimental data and Monte Carlo simulations we investigate the shielding properties of spacecraft-shell compositions exposed to 1 GeV/nucleon 56Fe ions, representative of the worst part of the Galactic Cosmic Ray (GCR) spectrum. Through the use of the Geant4 Radiation Analysis for Space (GRAS) tool, the dose reduction and the 56Fe-fragmentation induced by those structures currently used to protect part of the International Space Station (ISS) or designed for future inflatable habitats, are analyzed. The possible effects on spacecraft electronics are discussed.

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International space exploration plans and scenarios are driven by the long-term goal to enable human exploration of Moon and Mars. To prepare for these ambitious goals, robotic and human space missions are required to gradually develop and demonstrate the enabling technologies and capabilities. In the f rame of such scenario, the research program STEPS aims at developing technologies needed for the future exploration missions of the solar system. The perva sive presence of lunar dust could jeopardize the re turn of the man to the Moon, seriously affecting the hea lth of future crews. It is therefore essential to d esign dust abatement systems that are reliable, reusable, and efficient. The present paper describes the desi gn and manufacturing of a dust removal system developed as part of the Environmental Control and Life Support System of a manned rover for the exploration of the lunar surface near the South Pole. The Dus t Removal System proposed has been designed to abate the amount of airborne dust present inside the pressurized volume of the rover due to the External Vehicular Activities of the crew. The proposed syste m consists of three dust-abatement stages arranged in succession in order to increase both the collectio n efficiency and the reliability of the whole system. The first stage, a cyclonic separator coupled with a magnetic collector, has been designed to remove the greater part of airborne particulates without the use of filters, through cyclonic and magnetic separatio n. Taking advantages of the peculiar electro-magneti c properties of the lunar regolith, due to abundant n ano-phase metallic Fe, an electro-active filter sec ond stage has been designed that ionizes and removes smaller-grained particulates. The last stage is a High Efficiency Particulate Air filter able to remove at least 99.97% of airborne particles larger than 0.3 micrometers in diameter. To test (on ground) the Dust Removal System, testing processes and procedures with the use of the enhanced lunar dust simulant ar e under investigation.