A research project relating fire safety in space, FLARE (Flammability Limits at Reduced Gravity Experiment), is briefly introduced in the article. The project is aiming at proposal of new fire safety standard for screening material intended to use in spacecraft, which is promoted by JAXA as a high-priority project utilizing ISS (International Space Station). Major efforts in the project are made for building up a formula to allow quantitative estimation of solid material flammability limit under microgravity (MLOC) based on Limiting Oxygen Index (LOI) according to ISO 4589-2. The formula will be verified by ISS flight experiments at the end of the project period and, then, is expected to be included as a key element in the new fire safety standard. Further, some potential advantages of the new standard such as shift from Pass/Fail test to index method, allowing use of available material flammability data as LOI values, and potential reduction of cost for material screening, are emphasized under comparison with NASA-STD-6001B, which is often used for screening material for space use. The project team consists of members from JAXA, NASA, ESA and eight (8) universities from all over the world and the role of individual member to reach the goal are summarized.
Considering the recent increasing human activities in space development, the reduction of fire risk in a microgravity environment and performing convenient flammability tests for materials used in space will become important research issues. However, there is currently no reliable flammability test based on scientific background for space environments. In the FLARE project, it is one of the main objectives to derive a conversion equation which can predict the minimum limiting oxygen concentration (MLOC) of flat materials in microgravity from a ground-based flammability test, such as JIS K 7201. In the present article, a simplified model for predicting MLOC of a thin flat material with an opposed flow using ground-based flammability data is introduced.
An Experimental setup was designed to conduct ignition experiments for mixtures of the polymethyl methacrylate (PMMA) pyrolyzed gas and air. Suitable ignition methods for microgravity experiments were reviewed, and the laser induced spark ignition was employed to investigate the ignition limit of the mixture. The pyrolyzed gas of PMMA was produced under an emission of the halogen lamp in a vessel filled with nitrogen and air. The gaseous components were sampled and analyzed by a gas chromatograph. Most pyrolyzed component was methyl methacrylate (MMA). Therefore, ignition process of MMA/air mixtures was experimentally investigated under a microgravity condition available from parabolic flights. Ignition tests were performed at 0.1MPa under microgravity and normal gravity conditions. The results indicated that the minimum ignition energy in microgravity decreased compared with that in normal gravity. Initial flame kernel formation and the growth had the preferable direction due to flow fields induced by gravity, resulting in the local quenching phenomena.
This paper describes the key strategy in FLARE project to estimate/find the flammability limit of the material subjected to various (reduced) gravity fields using the currently-available limiting oxygen index (LOI) obtained through ISO4589-2 protocol. Assuming that the sample thickness and the gravity are independent factors, “two-stage approach” has been proposed to obtain the flammability limit under reduced gravity field of any thickness sample by using LOI. Several attempts which we have been made so far in order to formulate the effect of sample thickness on the limit are presented, then the further challenges and remained tasks to achieve the ultimate goal are briefly summarized.
This paper summarizes the potential problems for currently-applied safety verification test at JAXA and NASA; NASA-STD-6001B. There are two issues to be become serious problems; one is based on the fact that the test is not capable to evaluate the material flammability under various gravity environment, and the other is based on the fact that the igniter used in the test is not capable to heat up the test specimen with high thermal inertia. NASA-STD-6001/TEST1/TEST4 is based on the upward flame propagation testing so that “worst-case” must be identical to “most flammable under normal gravity”. However, it turned out that this definition is not always right; it has been reported that the flammability limit becomes lower under microgravity environment because the sufficient heat is not washed away via buoyancy-induced flow. Therefore, gravity effect must be counted to the test in order to apply not only ISS but also universal space environment. Moreover, the chemical igniter used in NASA-STD-6001B can generate certain limiting heat so that it could be insufficient to ignite the test specimen. In FLARE project led by JAXA would propose other safety verification concept which is improved these potential problem and to be applicable to the wide range of future space mission.
This paper describes overview of the “Solid Combustion” experiment to be performed in Kibo on the ISS. In this experiment, three types of solid material (polyethylene insulated wires, thin PMMA sheets and thin filter papers) are selected as test samples. Flammability of these materials are quantitatively determined in microgravity by evaluating the limits of two fundamental processes of solid material combustion, which are (1) ignition limit of the solid material, and (2) flame spread limit (extinction limit) over the solid material. Results from “Solid Combustion” will play an important basis of another experiment called “FLARE”. Overview of the Solid Combustion Experiment Module, to be developed for executing the “Solid Combustion” and “FLARE” experiment, is also introduced in this paper.
The measurement of interfacial tension between molten slag and molten iron under microgravity in the International Space Station (ISS) is planned, where an oscillating drop technique with an electrostatically-levitated compound droplet will be used. In this work, numerical simulations for oscillation behaviors of a compound droplet composed of molten iron core and molten slag shell phases were performed using OpenFOAM, to determine the operational conditions for measuring the interfacial tension under microgravity. Here, the effects of viscosity and radius ratios of shell to core phases on the oscillation frequency of the compound droplet, i.e., on the interfacial tension evaluated from the frequency, were investigated. As a result, it was found that the oscillating drop technique is available to measure the interfacial tension if the appropriate condition of the radius ratio is selected.
Dust particles of micro-meter size are levitated around a sheath in discharges. Gravity pushes the dust particles from a bulk of plasma to the sheath on the ground. Microgravity conditions brought by sounding rockets, parabolic flights of aircract and the International Space Station allow the dust particles to suspend in the bulk of plasma. Many researches have required phenomena of the dust particles under microgravity to be understood with connected to plasma parameters. Here several examples estimating the plasma parameters of microgravity experiments were shown, and described as manners to elucidate the phenomena in dusty plasmas with the plasma parameters. A rough estimation of ion density was obtained in observing wave propagation, and spatial distribution of the dust particles changed by a discharge control was understood in measuring electron density.
The electrostatic levitation furnace (ELF) for the International Space Station (ISS) has been developed and sent to the ISS. The main target of the furnace are oxide samples, which are difficult to levitate in 1-G. Thermophysical properties such as density, surface tension, and viscosity of molten oxides at high temperature will be measured using microgravity condition in the ISS. In this paper, the ISS-ELF and the first ISS experiment (thermophysical property measurement of some oxide materials) are introduced.