Fire can be a catastrophic hazard for manned spacecraft. NASA mitigates the risk of fire with the implementation of NASA-STD-6001, which establishes program requirements for evaluation, testing, and selection of materials to preclude unsafe conditions related to flammability, odor, offgassing, and fluid compatibility. NASA-STD-6001 impelements a 1-g flame propagation test for proposed space flight materials which some researchers believe is not conservative in low-g.
In 2009, NASA selected 5 investigations that would help further our understanding of solid surface combustion and material flammability. Each investigation addresses different elements fo material flammability.
To study and characterize ignition and flammability of solid spacecraft materials in practical geometries and realistic atmospheric conditions.
Improve EVA suit design
Determine safer selection of cabin materials and validate NASA materials flammability selection 1-g test protocols for low-gravity fires
Improve understanding of early fire growth behavior
Validate material flammability numerical models
Determine optimal suppression techniques for burning materials by diluents, flow reduction, and venting
Residence Time Driven Flame Spread (RTDFS)
Prof. Subrata Bhattacharjee, San Diego State Univ
The RTDFS experiment will investigate steady and unsteady flame propagation over solid fuels in a microgravity environment. Theoretical modeling predicts that steady flame spread over solid fuels cannot be sustained, even in a high oxygen environment if the fuel exceeds a critical thickness. Flame spread rate over thin fuels in a quiescent microgravity environment decreases with fuel thickness not only because of increased thermal mass but also because the relative importance of radiative losses, as quantified by a non-dimensional radiation number, increases in direct proportion of fuel thickness.
The experiment will collect data of flame propagation across a solid fuel in quiescent and mild opposing airflows to correct and validate solid combustion models. A thoroughly validated model then can be used for predicting presence or absence of steadily spreading flame over different fuels under different ambient conditions with implications of developing computational tools to predict fire safety of different materials in a microgravity environment.
Although the focus of the experiment is to get data for quiescent environment, a mild opposing flow for a certain initial period (TBD) will serve three purposes. First, the extra energy released by the ignition system will be convected away by the flow leaving the flame-spread zone free of any after effect of ignition energy. Second, for thicker fuels presence of a flow will help establish a steady flame, absent which eventual flame extinguishment may be attributable to inadequate amount of ignition energy. Finally, the effect of a prescribed flow on flame propagation and extinguishment will provide additional data for expanding the scope of our analysis and further validating the computational results.
In order to test the proposed theory and refine the computational model, data from long duration microgravity experiments is necessary. The specific objectives of the flight experiments are: (1) Establish that steady flame spread over solid fuel in a quiescent microgravity environment is possible only if the fuel thickness is less than a critical value; (2) Investigate the propagation of the products and temperature field to uncover the mechanism of flame extinguishment; and (3) Characterize how the ambient pressure and presence of a mild opposing flow affect the flame spread rate and extinction behavior.
Narrow Channel Validation (NCV)
Prof. Fletcher Miller, San Diego State University
The experimental approach is to measure flame spread across planar samples of PMMA in a forced opposed flow configuration. The atmosphere will be a mix of oxygen and nitrogen, with the oxygen percentage ranging from atmospheric up to 85%. The pressure will be varied accordingly to stay on or near the normoxic curve. Samples will be tested at variety of flow velocities to obtain spread rate and extinction limits.
The sample will be held flush in a non-combustible sample holder that will be of small thermal conductivity to minimize heat loss and will prevent the fuel edges from burning. The slab samples will be burned in an opposed flow. The upper velocity is chosen to match the highest spacecraft ventilation velocity expected and to provide a transition to data obtained in normal gravity. The lower velocity is chosen to determine the extinction limits. The velocity will be started at the high velocity and reduced stair-step fashion to the lower limit or until the flame extinguishes. This will allow the collection of spread rate data or several opposed-flow velocities using only one sample.
The airflow over the sample must be laminar. The inlet (upstream) temperature is not critical, but needs to be measured for modeling purposes. The upstream oxygen mole fraction needs to be measured, since that is an experimental variable. It is understood that the oxygen level in the chamber may change during a burn, so only the initial oxygen concentration can be precisely specified and controlled. Ideally, the oxygen level would be controlled throughout the burn, but that may not be possible. The pressure is also a variable in this experiment, and thus needs to be both controlled and measured. As with oxygen, it is understood that the pressure may change once the material is burning, though ideally it would be controlled during a burn.
Growth and Extinction Limit (GEL)
Prof. James T’ien, CWRU
This proposed experiment will concentrate on the flame growth, decay and extinction over the surface of a non-flat thick solid in microgravity. In particular, a solid sphere of substantial size (i.e. 4 to 5 cm diameter) is chosen as a representative of non-flat samples. In addition to the parameters influencing the flammability in thin solids, the degree of interior heat-up is an important parameter on the solid burning characteristics of thick specimen. In spherical samples, the degree of interior heating is always changing. The problem is therefore unsteady in nature. In addition, flow around a sphere is different from that around a flat surface. The existence of a forward stagnation point, shoulder and wake regions result in different local flow pattern, hence a different flame-solid interaction. These can affect the burning and extinction characteristics.
In the proposed experiment, cast Polymethylmethacrylate (PMMA) spheres will be instrumented with several imbedded thermocouples to record the interior temperatures during the preheating and the combustion processes. The project objectives are (1) Experimentally determine the flame growth characteristics (growth rate, flame shape and dimensions) over thick 3 solid fuel as a function of flow velocity, oxygen percentage, pressure and the degree of internal heating; (2) Experimentally determine the flame extinction characteristics (quenching and blow-off limits) over thick solid fuel as a function of flow velocity, oxygen percentage, pressure and the degree of internal heating; and (3) Establish a high-fidelity numerical model that can be compared with the microgravity results and to serve as a tool connecting normal gravity and microgravity performance.
Material Ignition and Suppression Test (MIST)
Prof. Carlos Fernandez-Pello, UC Berkeley
The MIST experimental apparatus would consist of a small-scale combustion wind tunnel, a cylindrical fuel sample, radiant heaters, an igniter, and supporting instrumentation. The flow duct will provide a consistent and uniform flow of the oxidizer gas to the test section where the fuel sample will be located. The test section will have most of the area of its sidewalls available for optical access. Each cylindrical fuel sample will be supported in a holder and placed along the central axis of the flow duct, such that the oxidizer will flow smoothly over the sample surface. An electrical igniter consisting of coiled Kanthal wire will be located around the fuel sample on the downstream end. All tests will be conducted in an opposed-flow configuration.
The fuel sample will be irradiated approximately uniformly over its surface by a prescribed heat flux distribution. After the sample has been exposed to the radiant heat for sufficient time, the fuel sample will begin to pyrolyze. The pyrolyzed fuel will mix with the oxidizer flow, and the resulting combustible mixture will eventually react when it reaches the igniter location. The time from the instant that the fuel sample is exposed to the radiant flux, until the time at which ignition is observed via flame flashing determines the piloted “flash ignition time” (analogous to the flash point condition with liquid fuels). The time when persistent burning of the fuel is observed determines the “fire ignition time” (analogous to the fire point), at the particular test conditions of radiant flux, flow velocity and oxygen concentration (and pressure if varied). The variation of the fire ignition time with the external radiant flux yields the “ignition” branch of the fuel “Flammability Diagram” for a given set of environmental conditions. An equivalent flammability diagram can be obtained with the flash ignition time. For a given fuel and set of environmental conditions, the minimum radiant flux for ignition will be referred to as the critical radiant flux for ignition in micro-gravity.
Spacecraft Materials Microgravity Research on Flammability (SMuRF)
Dr. Sandra Olson, GRC
The SMµRF experiment will correlate normal gravity flammability test data with data under ventilated microgravity conditions. Rigorous correlations between ground test flammability data in ventilated spacecraft environment would allow selection of materials with increased fire resistance, and thus decrease the fire risk in space systems. The operational flexibility will also increase because the safety factors will be assessable.
SmµRF will obtain the lower portion of the concurrent microgravity flammability map for select materials as a function of ventilation flow, ambient oxygen concentration, and pressure, to find the minimum oxygen concentration (MOC) on the map over the range of interest for spacecraft exploration atmospheres. SMµRF will further obtain the lower portion of the opposed microgravity flammability map for selected materials as a function of ventilation flow and ambient oxygen concentration to determine if opposed or concurrent flow has the lower MOC in microgravity. And finally, SMµRF will compare the worst-case materials flammability limits in microgravity to modified NASA-STD-6001 Test 1 MOC limits to evaluate the oxygen margins of safety for the materials.