Development of normal oblique angled composite shielding (NOACoS) system for spacecraft protection

Abstract

Hypervelocity impacts of space debris on spacecraft happen to be in the velocity range of 7-20 km/s which have enough energy to destroy the spacecraft completely or some of its subsystems. According to NASA, only 6% spacecraft in the LEO region are operational while the rest are a potential danger for them. This danger becomes even more critical when human factor is involved like in the case of the International Space Station. Secondly, the evidence from the experimental study done by NASA on space debris, showed that normal impacts on the spacecraft are only 10~20% while the rest are at oblique angles, which led structural engineers to design more functional shielding. Different shielding concepts especially the Whipple Shield, has already been extensively studied analytically and experimentally. However, due to the recent advancement in the field of composites and their superiority in terms of high strength and stiffness, composite materials are better candidates for spacecraft shielding. In this research, an effort is made to shift the spacecraft shielding system from conventional metallic alloys to carbon/epoxy composites by incorporating Nextel and Kevlar fabrics with the inclusion of the obliquity effect to encounter the majority of space debris impacts. In the first part of this study, impact experiments were conducted for oblique incidence angles. Three different kinds of specimens namely Al6061-T6, carbon/epoxy non-aged and aged under a simulated low Earth orbit environment (CLEO), were manufactured, tested, and validated. These composites were manufactured with 16 layers of CU125NS prepreg with the stacking sequence of $[0/\pm 45/90]_{2s}$ and was cured through an autoclave. Afterwards, the specimens were exposed to LEO space environment with UV radiations, atomic oxygen, high vacuum, and thermal cycling. The specimens were then impacted by Al2017-T4 spherical projectiles of 5.56 mm diameter and 0.25 g in weight within the velocity range of 1000 $\pm$ 100 m/s. Due to the LEO environment exposure, on average a total mass loss of 0.42% was found in the composites along with degradation in other mechanical properties because of the synergistic effects of UV and AO. The average specific energy absorption of the composites was found to be 7% greater than that of Al6061-T6. Additionally, with the increase in incidence angle, the energy absorption by the composites also increased exponentially. For the angle obliquity from $0^\circ$ to $30^\circ$, $45^\circ$, and $60^\circ$, the energy absorption increased 15%, 35%, and 50%, respectively, more than that of normal impact on CLEO. C-SCAN study was also done to evaluate the impact surface and its failure in terms of delamination. Afterwards, the effect of velocity variation towards composite damage behavior was studied. It was found that with the increase in velocity from 500 m/s to 2200 m/s, the delamination contribution towards damage increased, while the fiber and matrix fracture were found almost the same. In the next part, the superiority of angle obliquity for composite bumpers towards energy absorption was investigated. The experiments were conducted in the velocity range of 1500 $\pm$ 500 m/s for a double bumper configuration with front or rear bumper obliquity of either $30^\circ$ or $45^\circ$. It was found that double bumpers with $45^\circ$ obliquity absorbed 14% more specific energy in comparison to that of $30^\circ$ obliquity. Additionally, for double bumper configurations of first bumper obliquity, less damage was found in the rear bumper. Further research was done using triple bumpers with intermediate bumper obliquity of $30^\circ$ or $45^\circ$. Triple bumper configurations with $45^\circ$ intermediate bumper absorbed 10% more specific energy in comparison to that of $30^\circ$ intermediate bumper configurations in the velocity range of 2000 $\pm$ 200 m/s. The final triple bumper configurations with Kevlar and Nextel stuffing was experimented in two configurations namely, $0^\circ$ $-30^\circ$ $-0^\circ$ and $0^\circ$ $-45^\circ$ $-0^\circ$ having average areal density of $1.73 g/cm^2$ and $1.861 g/cm^2$, respectively with 200mm standoff distance. Nextel was used to shock the projectile while Kevlar was used for slowing down the debris cloud. These two configurations safely stopped the projectiles from penetrating the shielding within the velocity range of 2000 $\pm$ 200 m/s. The fabric pull out mechanism along with reduced rear side damage of the first and intermediate composite bumpers were observed. For the $0^\circ$ $-30^\circ$ $-0^\circ$ configurations, last bumper bulged while for $0^\circ$ $-45^\circ$ $-0^\circ$, the last bumper stayed clean, showing enhanced protection capability. Finally, validation was done using the commercially available software LS-DYNA and its SPH dedicated module. Half scale modeling was adopted for the simulation to make it computationally inexpensive. LS-DYNA simulation results of the experiments showed energy absorption difference of 20~30 %, which was because of composite modeling by laminate philosophy, unavailability of strain rate inclusion parameters for the composite case, and lack of supercomputing facility. In conclusion, for the spacecraft shielding system, the proposed normal oblique angled composite shielding system was lighter in areal density, superior in strength and stiffness, and worked with the severity and probability of space debris impact. The high risk of space debris impact on a specific side of spacecraft can be tackled by using an oblique angled shielding which provides additional protection as proven through the present research.