宇宙航空研究開発機構

Sample return missions play a significant role in planetary science by providing pristine extraterrestrial materials. JAXA's Hayabusa2 and NASA's OSIRIS-REx missions have returned samples from the C-type asteroids Ryugu and Bennu, respectively. The chemical and mineralogical compositions of these samples closely resemble those of CI chondrites, the traditional reference material for solar system abundances. Based on the findings of the Hayabusa2 mission, JAXA launched the Ryugu Reference Project (RRP) to maximize the scientific value of the returned samples and formed the RRP Measurement Definition Team (RRP-MDT) to elucidate the RRP's scientific goal and objectives. The RRP-MDT defined the goal of RRP to reassess the elemental abundances and isotopic compositions of the solar system through comprehensive analyses of the returned asteroid samples and CI chondrites. To this end, the team recommended preparing homogeneously powdered Ryugu reference materials (RRM) using approximately 750 and 400 mg of samples from Chambers A and C, respectively, to address observed compositional heterogeneities. The team proposed to measure the elemental abundances and isotopic compositions of the RRM by analytical techniques involving seven specific measurement groups. Through comprehensive analytical methodologies, interlaboratory calibration, and statistical evaluation, the RRP aims to refine our understanding of solar system formation and evolution.

A fused multi-filament fabrication (FmFF) method for the 3D printing of continuous carbon fiber-reinforced polymer (cCFRP) composites was proposed as an extension of the fused filament fabrication (FFF) method. The FmFF method enables the simultaneous deposition of multiple filaments using a single nozzle. The effect of simultaneous multi-filament deposition on the quality and mechanical properties of 3D-printed cCFRP composites was evaluated. Specifically, the FmFF process was investigated using a simultaneous five-filament deposition process, with the five filaments being integrated into a bundle and printed using a single nozzle. The quality differed depending on the path. In straight sections, the filament bundles were arranged in a regular pattern, whereas in curved sections, they were arranged unevenly owing to filament movement. The tensile and bending moduli of the cCFRP composites fabricated using the FmFF method (59 GPa and 50 GPa using Vf = 0.3 filaments) were almost identical to those obtained using the FFF method (60 GPa and 51 GPa). However, the bending strength was relatively low (FmFF: 451 MPa, FFF: 540 MPa), whereas the tensile strength was higher due to a larger curvature radius in the FmFF nozzle (FmFF: 1090 MPa, FFF: 800 MPa). Moreover, the bending failure mode was partial fragmentation and delamination, indicating relatively low interfacial adhesion between the layers. This study demonstrates the applicability of the FmFF method for high-throughput fabrication of lightweight small structural parts, while simultaneously identifying future challenges for improvement.

Hypervelocity impacts of small objects, such as space debris, pose significant challenges to the structural reliability of glass due to the extreme stresses and crack propagation they induce. While conventional optical techniques can qualitatively observe stress wave propagation, quantitative evaluation of stress fields and their association with fracture mechanisms remains a critical gap. This study aims to develop and validate a novel in situ photoelastic imaging method for quantitatively analyzing stress wave propagation behavior in glass under hypervelocity impact conditions. A new combined experimental and analytical system was developed using a two-stage light-gas gun to conduct hypervelocity impact experiments on silica glass plates (dimensions: 60 × 60 × 15 mm3). Both linear- and circular-polarized photoelastic images were obtained during the experiments using synchronized high-speed cameras. Time-dependent stress fields were calculated via an integrated photoelasticity approach using the onion-peeling method, which was reformulated into an integral representation to accommodate general axisymmetric conditions. The proposed analysis method successfully determined time-dependent stress-fields in impacted glass plates. Longitudinal stress waves exhibited wavelength elongation due to mechanical responses near the elastic limit under hypervelocity conditions. Observations also revealed dynamic interactions between stress wave propagation and crack initiation, highlighting the critical role of back-surface-reflected stress waves in fracture initiation. This study demonstrates the utility of in situ photoelastic observation combined with quantitative analysis for exploring stress wave behavior and subsequent fracture mechanisms in glass. The proposed method has the potential to advance the understanding of dynamic material behavior and improve the design and reliability of glasses in high-energy applications, such as space debris protection.

As part of the FLARE project, long-duration microgravity experiments were conducted on two additional materials-non-charring PMMA and charring NOMEX HT90-40-using the Solid Combustion Experimental Module (SCEM) onboard the ISS/Kibo module. Flammability maps were obtained under opposed-flow velocities, including quiescent conditions, extending previous studies performed with filter paper. For PMMA, both the limiting oxygen concentration (LOC) and the minimum LOC (MLOC) agreed well with predictions from a simplified two-dimensional scaling model, confirming its applicability to thermally thin, non-charring materials. In contrast, NOMEX exhibited robust three-dimensional spherical flames once the two-dimensional thermal balance broke down, even at moderate flow velocities. Under these conditions, the flame radius Rf decreased with decreasing opposed-flow velocity, and extinction occurred when Rf reached a critical value. To quantify this behavior, the preheat-zone length Lg of three-dimensional flames was modeled as a function of Rf and the Reynolds number Re, and incorporated into the thermal balance to derive a limiting oxygen concentration for three-dimensional flames. The resulting expression reproduced the observed relationships among Rf, Vg, and Lg, and correctly predicted the extinction behavior. Applying the same formulation to filter paper and PMMA further demonstrated that the critical flame radius provides a unified criterion for the transition and extinction of threedimensional flames across different material classes. These findings demonstrate that both the two-dimensional and three-dimensional flammability limits of charring and non-charring materials can be predicted within a unified experimental-modeling framework, and they provide essential guidance for advancing microgravity firesafety modeling. Novelty and significance statement: The novelty of this work lies in establishing a unified, physics-based framework for predicting flame-spread limits of both charring and non-charring thermally thin materials in microgravity. First, long-duration ISS experiments demonstrated that the limiting oxygen concentration (LOC) and minimum LOC of PMMA are accurately captured by a simplified two-dimensional model, confirming that extinction is governed by the breakdown of two-dimensional thermal balance. A second and central contribution is the quantitative characterization of three-dimensional spherical flames observed in NOMEX beyond the twodimensional limit. By modeling the preheat-zone length Lg as a function of flame radius Rf and Reynolds number Re and incorporating this into the thermal balance, an explicit LOC criterion for three-dimensional flames was derived. Applying the same formulation to filter paper and PMMA showed that the critical flame radius provides a consistent extinction threshold across materials, offering a unified experimental-modeling basis for predicting flame-spread limits in microgravity.

This study investigates the relationship between low-frequency shock-wave oscillations and buffet cells on the main wing of the NASA common research model. The flow conditions were set at a Mach number of 0.85, Reynolds number of 2.27 x 106, and angle of attack of 4.82 degrees. Buffet cells are cellular patterns with spanwise periodicity that propagate toward the wing tip, with nondimensional frequencies (Strouhal numbers) of 0.2-0.6, higher than those of shock oscillations associated with transonic buffet. However, the physical mechanisms driving buffet cells and their interaction with low-frequency shock motion are not fully understood. This study employs variational mode decomposition-based nonstationary coherent structure (VMD-NCS) analysis applied to unsteady pressure-sensitive paint measurement data. The results reveal that the amplitude of buffet cells is not directly coupled to the low-frequency shock-wave oscillations but instead shows correlation with the boundary layer separation state. Strong buffet cell fluctuations persist in regions of sustained separation (midspan), while their amplitude varies with local separation/reattachment dynamics in regions of intermittent separation (outboard). These observations suggest that shock-waves are important to generate the adverse pressure gradient necessary for boundary layer separation, but their influence on buffet cells depends on the resulting separation behavior. This insight from experimental observations in this study is consistent with previous computational studies suggesting that buffet cell behavior depends on boundary layer separation characteristics. Moreover, this study demonstrates the effectiveness of VMD-NCS analysis for investigating nonstationary fluid phenomena with multiple spatial and temporal patterns.