# Hybrid Shielding for Hypervelocity Impact of Orbital Debris on Unmanned Spacecraft
## 1 Introduction  
Research into the passive shielding of spacecraft structures has heightened over recent decades owing to an increase in demand for lightweight, cost-effective technology, which is required to counter the damaging effects of micro-meteoroid orbital debris (MMOD) [1]. The amount of MMOD in space has been \"steadily rising since the beginning of the space age\", as stated in the European Space Agency's Annual Space Environment Report in 2020 [2]. In 2007 alone, a further 32% increase in MMOD was observed as a result of major low-earth-orbit (LEO) collisions [3]. The rise in MMOD significantly increased the risk associated with space exploration and, as a result, the Inter-Agency Space Debris Coordination Committee (IDAC) was established in 1993 to mitigate the damage caused by micro-meteoroid orbital debris. The design of the International Space Station (ISS) in the 1990s prompted the further development of protective shielding methods. Hypervelocity impact (HVI) research, along with the hydrocode simulations conducted by NASA and other research facilities andgroups, led to the development of numerous shield configurations, such as the Whipple, Stuffed Whipple, and metallic foam sandwich Whipple shields [4]. These protective structures are designed with the main aim of mitigating the effects of hypervelocity impacts that, due to their extremely high energy, have the potential to perforate shields in space structures. The conventional Whipple shield design consists of two plates, usually made of aluminium, separated by a stand-off distance to allow for the fragmentation and dispersion of debris from the impact on the first plate. A schematic illustration of this design principle is shown in Figure 1.  
The Stuffed Whipple, as shown in Figure 2a, incorporates an additional layer, commonly a combination of Nextel or Kevlar/Epoxy, to improve overall shield performance and energy dissipation [6]. The incorporation of metal foams into the Whipple shield design has also been studied in 2017 by [PERSON] and [PERSON] [6], as shown schematically in Figure 2b. [PERSON] and [PERSON] [5] also demonstrated the potential of such design approaches for space applications, owing to ability to significantly increase the absorption of impact energy compared to more conventional shielding structures. In these studies, it was clearly demonstrated that the two main design characteristics affecting the dissipation of kinetic energy and debris fragmentation are the choice of material(s) (and corresponding material properties) and the geometry of the shield design [1]. In the late 1990s, [PERSON] et al. [7] proposed the use of lightweight, non-metallic materials, such as Nextel ceramic cloth and Kevlar, for the Whipple shield. These materials  
Figure 1: Conventional Whipple shield design: (**left**) Whipple shield configuration, (**centre**) post-impact debris cloud formation and (**right**) effects of impact on rear plate (adapted from [5]). Figure 2: Schematics of alternative Whipple shield designs: (**a**) Stuffed Whipple and (**b**) aluminium foam sandwich (adapted from [6]). were configured in a number of different flexible multi-layer designs. [PERSON] et al. tested the penetration resistance in terms of hypervelocity impact and successfully demonstrated the potential for this lightweight shielding, going as far as deriving a set of ballistic limit equations for the proposed shields for orthogonal and non-orthogonal low (\(v<2700\) [m/s]), intermediate (\(2700<v<6500\) [m/s]) and high velocity impacts (\(v>6500\) [m/s]). [PERSON] et al. [8] conducted HVI experiments using a two-stage light gas gun. A 3-mm aluminium projectile was fired with a velocity of 4119 m/s at a Whipple shield consisting of an aluminium target plate and a witness plate positioned 30 mm apart. The experimental observations were compared to numerical simulation results in LS-DYNA.