# Hybrid Shielding for Hypervelocity Impact of Orbital Debris on Unmanned Spacecraft
## 1 Introduction
The proposed numerical model was found to be a sufficiently accurate representation of the experiment, justifying the use of the hydrocode in further analysis and the shielding design for the hypervelocity impact of orbital debris on unmanned spacecraft. Research into the material properties of the Whipple shield was more recently conducted by [PERSON] et al. [9], where the energy absorption efficiency of the combination of a homogeneous aluminium sheet with a Ti-Al-nylon impedance-graded material (IGM) was compared using both laboratory testing and numerical simulations. The experiments were performed using a two-stage light gas gun and focussed on post-impact effects, using 3D scanners to detect physical damage in detail. [PERSON] et al. proposed a smooth particle hydrodynamics (SPH) numerical model, developed in AUTODYN, and performed a detailed analysis of the relevant kinetic energy dissipation, fragmentation, and pressure distribution. Their results revealed a significant improvement in shield performance using the IGM with regards to both fragmentation and energy dissipation. The increased shock pressure that was experienced, achieved through the interaction between travelling shock waves and reflected rarefractions, allowed for an optimised shield design. The increase in debris spread angle was further confirmation that the material properties of the IGM were suited to HVI shield design. Recently, the addition of a honeycomb structure to Whipple shield design has been extensively explored by authors such as [PERSON] and [PERSON] [10,11] and [PERSON] and [PERSON] [12], among others. These researchers adopted an orientation of the honeycomb, where the cells' axis is perpendicular to the front and bumper plates. This allows for a significant reduction in the debris spread angle, but creates a channelling effect, as the honeycomb is orientated parallel to the direction of debris travel, as shown in Figure 3. Although the reduction in the debris spread angle is significant, it creates the adverse effect of concentrating the impact on a smaller area, adversely affecting the energy dissipation of the projectile. Double/multi-honeycomb core configurations were noted as being less prone to channelling effects due to the implementation of a staggered design [10]. More recently, [PERSON] and [PERSON] [13] presented a detailed review of recent advances in the Whipple shield design, noting that debris channelling (also referred to as ejecta-tunnelling effect) should be considered in the design process of Whipple shields, and can be detrimental to the energy dissipation and mechanisms. A solution to the channelling effects, however, would be to change the orientation of the honeycomb cells from parallel to perpendicular to debris travel, with the added advantages that the weight of the structure could remain unchanged. In 2009, [PERSON] et al. [14] conducted a comparative study between the use of a parallel-orientated honeycomb core and metallic open-cell foam for Whipple shield application. These authors found that the foam had several advantages over the honeycomb due to the elimination of channelling. Very few studies have been dedicated to determining the effectiveness of a perpendicular honeycomb cell orientation regarding the energy absorption of Whipple shields. Therefore, the proposed research aims to explore the potential benefits of implementing a perpendicularly orientated honeycomb core in relation to the energy dissipation of an impact projectile at hypervelocity, and exploring the effects of the topology of the honeycomb structure on the energy absorption of the shield as a whole.