A ballistic limit equation for hypervelocity impacts on composite honeycomb sandwich panel satellite structures

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Abstract

During a recent experimental test campaign performed in the framework of ESA Contract 16721, the ballistic performance of multiple satellite-representative Carbon Fibre Reinforced Plastic (CFRP)/Aluminium honeycomb sandwich panel structural configurations (GOCE, Radarsat-2, Herschel/Planck, BeppoSax) was investigated using the two-stage light-gas guns at EMI. The experimental results were used to develop and validate a new empirical Ballistic Limit Equation (BLE), which was derived from an existing Whipple-shield BLE. This new BLE provided a good level of accuracy in predicting the ballistic performance of stand-alone sandwich panel structures. Additionally, the equation is capable of predicting the ballistic limit of a thin Al plate located at a standoff behind the sandwich panel structure. This thin plate is the representative of internal satellite systems, e.g. an Al electronic box cover, a wall of a metallic vessel, etc. Good agreement was achieved with both the experimental test campaign results and additional test data from the literature for the vast majority of set-ups investigated. For some experiments, the ballistic limit was conservatively predicted, a result attributed to shortcomings in correctly accounting for the presence of high surface density multi-layer insulation on the outer facesheet. Four existing BLEs commonly applied for application with stand-alone sandwich panels were reviewed using the new impact test data. It was found that a number of these common approaches provided non-conservative predictions for sandwich panels with CFRP facesheets.

Introduction

Composite sandwich panels consisting of Carbon Fibre Reinforced Plastic facesheets bonded to Aluminium honeycomb cores (CFRP/Al HC SP) are amongst the most commonly used structures for satellites due to their relative low mass and high thermal and mechanical stability. However, the protective performance of these structure types with regards to hypervelocity space debris and micrometeoroid impacts is poor compared to that of more traditional structural wall types (e.g. an Al Whipple shield). With the growing number of debris particles in Earth orbit, the need to characterise the risk of these particles on the successful fulfilment of mission objectives becomes increasingly critical.

To assess the threat of micrometeoroid/orbital debris (M/OD) on a satellite mission, equations that define the limits of structural perforation in terms of impactor mass, velocity and angle are required. This type of equation is referred to as a ballistic limit equation (BLE). For risk assessment of satellite structures ballistic equations derived for application with metallic Whipple shields are commonly applied. However, it is recognised that, unlike for manned spacecraft, a perforation event is not necessarily critical for satellite structures. It is only when internal systems essential to the satellite function become inoperable that the impact event is, by definition, critical. Thus, for more accurate risk assessment of satellite structures, equations that allow for these more specific failure mechanisms to be predicted are required. Currently, no validated BLEs existing for application in the risk assessment of CFRP/Al HC satellite structures are available.

Section snippets

Overview of the test program

In the framework of ESA Contract 16721/02/NL/CK (Schaefer and Ryan, 2005) an extensive experimental campaign was performed to investigate the ballistic limit of various CFRP/Al HC SP structures and thin Al plates located behind the structure walls at a standoff. A total of 55 impact tests were performed at a range of impact velocities and angles on six representative satellite structures. Given that perforation does not necessarily represent critical failure for a satellite mission, 15 of the

State-of-the-art

Schaefer et al. (2004) identified four different approaches for predicting the ballistic limit of CFRP/Al HC sandwich panel structures, all of which are based on the Christiansen/modified Cour-Palais Whipple-shield equation (Christiansen, 1993). The Whipple-shield equation is reproduced from Christiansen (1993) here. It should be noted that the nomenclature has been modified from the original publication.

Ballistic regime (vn  3 km/s):dc=tb·(σ/40)0.5+tob0.6·(cosθ)5/3·ρp0.5·v2/318/19

Comparison with current BLEs for CFRP/Al HC sandwich panels

Schaefer et al. (2004) identified four different approaches for predicting the ballistic limit of CFRP/Al HC sandwich panel structures (the Whipple shield, Taylor, Frost, and MET approaches as defined previously). Predictions of the impact test experimental results calculated using these approaches was compared to assess the improvement in the BL prediction capability of the new equation.

In Fig. 9, Fig. 10, BLCs of a selection of the structures described in Table 1, Table 8 are shown, along

Summary and conclusions

In this paper, a new ballistic limit equation for CFRP/Al HC satellite structure walls has been presented. This equation is applicable for ballistic limit prediction of stand-alone sandwich panels, and for thin Al plates located at a standoff behind these sandwich panels. The thin Al-plates are representative of internal systems, e.g. lid of an electronic box, metallic vessel, etc. Ballistic limit impact tests have been performed on six representative CFRP/Al HC sandwich panels, and the results

Acknowledgement

This study was performed in the framework of ESA Contract 16721/02/NL/CK “Composite Materials Impact Damage Analysis”.

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