Interplanetary dust particle shielding capability of blanketed spacecraft honeycomb structure

To assure mission success of the Solar Probe Plus (SPP) spacecraft, defined by achieving its final mission orbit with a perihelion distance of less than 10 solar radii, it is necessary to define the dust hypervelocity impact (HVI) protection levels provided by its Multi-Layer Insulation (MLI)/thermal blankets with a reliability that is on par with that available for metallic Whipple shields. Recently, we presented an experimentally validated approach being developed at the Johns Hopkins University Applied Physics Laboratory (JHU/APL) for designing and analyzing MLI to meet this challenge. This paper extends the results to Whipple shield configurations consisting of an MLI bumper, 0–1″ standoff, and an Aluminum honeycomb rear wall. With 0″ MLI-honeycomb standoff, 0.05 g/cm2 MLI layered in a manner similar to that found in actual blankets and an Aluminum honeycomb consisting of 8 mil-thk. facesheets, 0.125″-dia. cells and 1%″-thk. core is adopted for 2D and 3D analyses. Bonding of the core to the facesheets was included in the modeling to account for channeling of the debris cloud within the core. The passage of the dust through the center of the core's symmetry axis is considered to be the most conservative dust-H/C interaction as the other scenarios — normal dust entry closer to a core wall or oblique dust entry — are expected to be less damaging overall owing to interactions with the core material and disruption of channeling. For this reason, dust impact along a core symmetry axis is evaluated. The failure criterion chosen is complete perforation of the rear facesheet and extending to the entire cell area. The average critical particle diameter is indicated to be in the 300–900 μm range. It is noteworthy that the pass-fail transition is not sharp and is indicated to occur over a ±100 μm particle size range for the 0” standoff configuration considered. For comparison purposes, the average critical particle diameter is indicated to be in the 140–750 μm range for the bare honeycomb (no MLI). With 1″ MLI-honeycomb standoff, direct modeling of the passage of the vapor/debris cloud through the honeycomb was found to be computationally prohibitive. Test data shows that a honeycomb offers better HVI shielding than an equivalent monolithic wall with the same areal density. So for this set of analyses, the honeycomb was (conservatively) represented as a monolithic 16 mil-thk. wall, obtained by combining the two facesheets and neglecting the core. The average critical particle diameter is indicated to be in the 425–1300 μm range for this configuration. The size range over which the pass-fail transition occur is ±200 μm at 7 km/s but only ±25 μm at 150 km/s. In addition to presenting these new ballistic limit equations (BLEs) for honeycomb structure shielded by MLI in the 7–150 km/s range, an empirical method for predicting the critical particle size at 7 km/s is provided, developed from data obtained for honeycomb configurations used in the New Horizons, Rosetta, METOP and ATV spacecraft. Insights gained from sensitivity analyses using a 12 mil-thk. facesheet and elimination of the core are also discussed.