Materials for Structural and Thermal Preservation of Sample Return Payload During Earth Entry and Landing, Phase II

To maximize reliability, Earth entry/landing vehicles for robotic sample return missions will comprise an aeroshell, a crushable layer that will absorb the energy of the ballistic impact landing, and a sample container inside the crushable layer; parachutes will not be used. Lightweight cellular solids are being considered for the crushable layer, but many other engineered foams with different strengths and energy absorption capacities are available. By using foams of different materials with different mechanical properties and different relative densities, the crush behavior of the layer can be tailored. In addition to brittle crushing of carbon foams or ductile collapse of metallic foams, other energy-absorbing mechanisms are available, some of which have previously been tested at high strain rates for use as underbody armor on military vehicles to mitigate blast effects from improvised explosive devices. In this project, high strain rate compression test data for Ultramets engineered foams will be used to mature the technology for impact absorption applications, with both brittle and ductile foams as a key element. The candidate impact absorption material database will be expanded via additional split Hopkinson bar testing, more detailed characterization of foam behavior will be performed, and an engineering model will be developed to quickly and easily determine the optimal foam architecture for a given set of mission (e.g. spacecraft and Earth impact) parameters. The results will be used to design and fabricate subscale and full-scale prototypes that will incorporate a minimum-mass, low thermal conductivity crushable layer that can be used for sample return missions with high impact velocities. The subscale unit and one full-scale unit will undergo drop testing to verify performance. To maximize reliability, Earth entry/landing vehicles for sample return missions will comprise an aeroshell, a crushable layer to absorb the impact energy of landing, and a sample container. Carbon foam is being considered for the crushable layer, but engineered foams can be much more mass-efficient. In addition to brittle crushing of carbon foams or ductile collapse of metallic foams, other energy-absorbing mechanisms are available, some of which have been tested at high strain rates for use as blast armor for the military. Phase I data showed that brittle foams using a low-density ligament material yield the lowest-mass solution for absorbing impact energy. Phase II will investigate other high-strength, low-density ligament materials, taking advantage of the ability to easily tailor the crush strength of foams by varying the coating thickness on the ligaments. A streamlined workflow will be developed for optimizing the foam architecture, and three test articles will be fabricated, a subscale unit and two full-size units for drop testing and one as a deliverable hardware item. Task 1 will focus on reviewing requirements for various sample return missions, identifying commonalities, and determining minimum material properties for the crushable foam layer and any fillers in the foam to enhance performance. A specific NASA mission will be selected as the baseline for the design to be developed in Task 4. In Task 2, new foam materials suggested by the Phase I data will be fabricated and undergo split Hopkinson bar (SHB) testing, and digital image correlation will be used to measure local strain during testing. Foams of multiple densities will be used to validate models that will be used to parameterize the foam’s stress-strain behavior. Task 3 will focus on developing a workflow to enable the optimal foam architecture and density to be quickly and easily determined for a given mission. A generic vehicle architecture will also be parameterized so that the mass of each component can be calculated based on the optimized thickness of the crushable layer. Task 4 will use the new workflow to design a mass-optimized crushable foam layer for the reference mission identified in Task 1, and three foam shells will be fabricated: one subscale and one full-scale for testing and one that will be a deliverable. In Task 5, the two test articles will undergo drop testing at NASA LaRC at a relevant velocity to verify performance.

The primary NASA application will be sample return missions from solar system bodies including planets, planetary moons, dwarf planets, asteroids, and comets. Because the energy absorption characteristics of the material system can be tailored, it also has the potential to be used for landing payloads on these bodies. Likewise, for a mission to divert an asteroid from collision with Earth, this type of system could be used to transfer momentum to the asteroid over a tailorable time frame to minimize fracture/fragmentation of the target body. Commercial applications for lightweight energy-absorbing structures include backing structures for automobile bumpers, crash barriers on highway exit ramps, and underbody armor for military vehicles to mitigate blast effects from mines and improvised explosive devices. Temporary structures in war zones could also be protected against blast effects with this technology.

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