The influence of environmental conditions on the flammability of combustible solids is of importance to spacecraft fire safety. In a manned spacecraft, the environment is maintained at a normoxic condition, which is the combination of ambient pressure and oxygen concentration that results in a partial pressure of oxygen equal to that of normal atmosphere at sea level. Future spacecraft will have atmospheres with reduced pressures and increased oxygen concentrations at normoxic conditions (Space Exploration Atmospheres - SEA), designed to reduce preparation time for extravehicular activities. This paper presents experimental results on upward spread of flames over a flat thin cotton fabric under normoxic conditions. Experiments are conducted with ambient pressures ranging between 100 and 60 kPa and oxygen concentrations between 21% and 35% by volume. Additional experiments are carried out with a fixed oxygen concentration of 21% (ambient air) and a pressure ranging between 100 and 30 kPa and varying oxygen concentration between 17% and 30% at a constant pressure (100 kPa) to isolate the roles of pressure and oxygen variations. Results show that moving to normoxic environments with reduced pressure and increased oxygen concentration increases the flammability of the fabric and the flame spread rate. Normoxic conditions also showed an increase in distance between the outer flame and the fuel bed at lower pressures. During the experiments, O2, CO, and CO2 were measured in the combustion products, and were used to calculate the heat release rate at different pressures and in normoxic conditions. The results show that as the cabin environment transitions to normoxic conditions with higher O2 concentrations, the heat released by the flame also increases. The data presented here provide information about the flammability of spacecraft materials in the future SEA in support of NASA research, yielding insight for future designs when considering fire safety in spacecrafts.

The Combustion Product Monitor (CPM) instrument was operated as part of the Saffire-VI Spacecraft Fire Safety Demonstration on the Cygnus NG-19 International Space Station resupply vehicle from December 22 to 24, 2023. CPM is a multi-channel laser spectrometer designed and built at NASA's Jet Propulsion Laboratory as a technology demonstration instrument for spacecraft atmospheric monitoring and low-gravity combustion science. This paper presents infrared absorption spectroscopy data collected by the CPM instrument during Saffire-VI operations, including time-resolved concentration measurements of oxygen, carbon dioxide, carbon monoxide, hydrogen chloride, and hydrogen fluoride inside the spacecraft during various combustion experiments. These data provide insight into the production, diffusion, and decay of combustion-related species in microgravity.

Concurrent flame spread data for thermally-thin charring solid fuels are presented from Saffire and BASS experiments performed in habitable spacecraft for three duct sizes, five sample sizes, two materials, and two atmospheres. The flame spread rates and flame lengths were strongly affected by duct size even for the relatively large ducts (> 30 cm tall). A transient excess pyrolysis length (i.e., flame length overshoot) was observed for the cotton fabric that burned away, which indicates that the transient excess pyrolysis length phenomenon is caused by more than just the flame moving into the developing boundary layer thickness as was the case with the SIBAL sample. A burnout time, defined as the pyrolysis length divided by the flame spread rate, normalized the pyrolysis length histories into a single curve with a steady burnout time of 22 s for the SIBAL fabric. The transient excess pyrolysis length is hypothesized to be a post-ignition flame growth transient for the essentially two-dimensional flames where the burnout time becomes very long until the preheat and pyrolysis lengths develop. The three-dimensional flames over narrow samples have lateral thermal expansion and lateral oxygen diffusion which allows them to transition to a steady state length without the transient excess pyrolysis length. Surface temperature profiles, nondimensionalized by the pyrolysis length, indicate that the temperature profiles exhibit the same shape across the pyrolysis zone. A surface energy balance calculation in the preheat region revealed that the heat flux increased rapidly at the pyrolysis front to near the critical heat flux for ignition. An estimate of the acceleration of the inviscid core flow in the duct due to thermal expansion and developing boundary layers on the duct walls and the SIBAL sample surface seems to explain the observed spread rate trends across three duct sizes and multiple sample sizes. (c) 2022 Published by Elsevier Inc. on behalf of The Combustion Institute.

Fire is a very serious threat in any confined volume but particularly so when a crew is confined in a spacecraft far from Earth, where there is little or no possibility of receiving aid or being rescued. Therefore, every measure must be taken to prevent a fire from happening. Even with material controls and other fire prevention measures, fire detection, suppression, and clean-up systems must be provided on spacecraft to allow the crew to respond to a fire. Unlike flames in normal gravity, fires in low gravity are not supported by the strong pumping of oxidizer and fuel into the flame zone and the subsequent pumping of the reactants out of the reaction zone. The lack of buoyancy allows a flame to propagate at lower air velocities than on Earth that changes the transport of oxygen into the flame, heat transfer to the fuel surface, and transport of heat out of the reaction zone. Under these conditions, the flame spread can occur at conditions and with characteristics that are not duplicated readily in normal gravity because of the large buoyant flows generated by the flame. Differences in flame characteristics also impact requirements for fire detection and suppression of spacecraft fires. Of course, any fire response protocol must be compatible with the other spacecraft systems such as the Environmental Control and Life Support System (ECLSS).

Fire spread inside a spacecraft is a constant concern in space travel. Understanding how the fire grows and spreads, and how it can potentially be extinguished is critical for planning future missions. The conditions in-side a spacecraft can greatly vary from those encountered on earth, including microgravity, low velocity flows, reduced ambient pressure and high oxygen, and thus affecting the combustion processes. In microgravity, the contributions of thermal radiation from gaseous species and soot can play a critical role in the spread of a flame and the problem has not been fully understood yet. The overall objective of this work is to address this by studying the soot temperature of microgravity flames spreading over a thin solid in microgravity. The ex-periments presented here were performed as part of the NASA project Saffire IV, conducted in orbit on board the Cygnus resupply vehicle before it re-entered the Earth's atmosphere. The fuel considered is a thin fabric made of cotton and fiberglass (Sibal) exposed to a forced flow of 20 cm/s in a concurrent flow configuration. Reconstruction of the flame temperature fields is extracted from two color broadband emission pyrometry (B2CP) as the flame propagates over the solid fuel. A methodology, relevant assumptions and its applicability to other microgravity experiments are discussed here. The data obtained shows that the technique provides an acceptable average temperature around similar to 1300 K, which remains relatively constant during the spread with an error value smaller than 117 K. The data presented in this work provides a methodology that could be applied to other microgravity experiments to be performed by NASA. It is expected that the results will provide insight for what is to be expected in different conditions relevant for fire safety in future space facilities. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Fire is a very serious threat in any confined volume but particularly is so when a crew is confined in a spacecraft far from Earth, where there is little or no possibility to receive aid or be rescued. Therefore, every measure must be taken to prevent a fire from happening. Unlike flames in normal gravity, fires in low gravity are not supported by the strong pumping of oxidizer and fuel into the flame zone and the subsequent pumping of the reactants out of the reaction zone. For this reason, low gravity provides the only opportunity to observe combustion under truly quiescent conditions, that is, with no flow other than that caused by the processes of thermal expansion, production of gaseous species, and fuel surface evaporation or blowing. Under these conditions, molecular diffusion is usually the dominant mode of transport for the oxidizer to the fuel. In many cases, the diffusive transport of oxidizer is less than the flame spread rate, and flame spread then becomes the dominant oxidizer transport mode, creating a virtual wind as the flame moves into fresh fuel and oxidizer. Under these conditions, flame spread can occur in conditions that are not duplicated readily in normal gravity because of the large buoyant flows generated by the flame.

The variation of ambient pressure is a potential tool for studying the driving parameters of fire dynamics and heat release in low-pressure environments such as high-altitude locations, aircraft, and spacecraft. The study of upward flame spread over a solid fuel has direct implications on material flammability and fire development, and low pressure environments have recently gained more attention for the possible comparison with the reduced gravity conditions encountered during space missions. In this work, we consider upward spreading flames over thin acrylic sheets in ambient pressures between 30 and 100 kPa. A forced flow velocity of 20 cm/s is added to the naturally-driven buoyant flow, creating a mixed flow field (natural and forced) that varies with pressure. Flame characteristics such as spread rate and standoff distance are measured from the video analysis of the experiments. It is observed that the former decreases with pressure while the latter increases. The larger flame stand-off distance at low pressures partially explains the decrease of the flame spread rate since the convective heat flux from the flame to the solid decreases. Additionally, volumetric concentrations of the combustion products are measured during the experiments. The results show lower O-2 consumption and CO2 production rates at lower pressures. Based on these rates, we could calculate the heat release rate from upward spreading flames at low pressure, providing fundamental information for better understanding pressure-gravity correlations. According to the results, the volumetric heat release rate is proportional to pressure, which is consistent with previous studies on pressure modeling of fires. This suggests that chemical kinetics is not a constraint for the conditions tested in this study, which could help make future flammability tests comparable to low gravity ones. (C) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The environment inside a spacecraft, or a space habitat, can greatly differ from those on earth, affecting the burning behavior of solid fuels. Because in a gravity field there is a flame-induced buoyancy, it is very difficult to reproduce on Earth the environmental conditions of a spacecraft, or a habitat in the Moon or Mars, thus making fire testing and burning predictions harder. A potential alternative approach to overcome this problem is to reduce buoyancy effects by using reduced ambient pressure in normal gravity. The objective of this work is to explore the possibility of simulating the effect of gravity, and in turn buoyancy, through changes in ambient pressure, on upward/concurrent flame spread over a thin combustible solid. Comparisons with available data at different gravity levels are used to determine the extent to which low-pressure can be used to replicate flame spread characteristics at different gravities. Experiments of the upward/concurrent flame spread over thin Kimwipe paper sheets were conducted in normal gravity inside a pressure chamber with ambient pressures ranging between 100 and 30 kPa and a forced flow velocity of 10 cm/s. Results show that reductions of pressure slow down the flame spread over the paper surface, and also reduces flame intensity. Comparison with published partial gravity data shows that as the pressure is reduced, the normal gravity flame spread rate approaches that observed at different gravity levels. The data presented is correlated in terms of a mixed convection non-dimensional number that describes the convective heat transferred from the flame to the solid, and that also describes the primary mechanism controlling the spread of the flame. The correlation provides information about the similitudes of the flame spread process in variable pressure, flow velocity and gravity environments, providing guidance for potential ground-based testing for fire safety design in spacecraft. (c) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Spacecraft fire safety is an important consideration when designing future exploration missions. Concurrent flow flame spread experiments were carried out aboard the Cygnus spacecraft while in orbit to address current knowledge gaps related to solid fuel combustion in microgravity. The experiments used PMMA (polymethylmethacrylate) samples that were 50 mm wide and 290 mm long with two variations-one sample was a 10 mm thick flat slab, while the other was a 10 mm thick flat sample that had a 4 mm thick grooved center section. The thickness variation had a major impact on the flame shape, and the grooved sample developed a deep inverted-V shaped notch as the thin center section burned through. This notch enhanced heat transfer from the flame to the solid through an effectively wider flame base, which is the part of the flame with the highest temperature. Unlike in normal gravity (and also likely in partial gravity), where buoyant flow promotes acceleratory upward flame growth, the microgravity flames reached a steady size (limiting length) for a fixed forced convective flow in agreement with theory (i.e. there is a zero net heat flux at the flame tip). The limiting length implies that the spread rate of the flame will be controlled by the regression rate (burnout rate) of the material because the flames remains anchored to the upstream end of the fuel samples. This is a significant finding for spacecraft fire safety, and makes the probability of flashover in a spacecraft unlikely as the flame size will be small for low convective ventilation flow environments typical in spacecraft. On the other hand, long-burning flames in small vehicles will generate significant quantities of fuel vapor that may reach the lean flammability limit and cause a backdraft. The rapid extinction of the flame when the flow was turned off also supports the existing fire mitigation strategy on the ISS to deactivate the ventilation system in the event of fire alarm. These findings can be applied to improve the safety of future space exploration missions.

An accidental fire involving the Lithium-Ion (Li-ion) battery in a laptop computer is one of the most likely fire scenarios on-board a spacecraft. These fires can occur from a defect in the battery that worsens with time, over-charging the battery and leading to failure or accidental damage caused by thermal runaway. While this is a relatively likely fire scenario, very little is known about the how a laptop computer fire would impact a sealed spacecraft. The heat release would likely cause a pressure rise, possibly exceeding the pressure limit of the vehicle and causing a relief valve to open. The combustion products from the fire could pose a short-term and long-term health hazard to the crew and the fire itself could cause injury to the crew and damage to the spacecraft. Despite the hazard posed by a laptop fire, there is little quantitative data on the fire size, heat release and toxic product formation. This paper presents the results of initial attempts to quantify the fire resulting from a failed laptop fire tested at the NASA White Sands Test Facility (WSTF). The data from the testing is useful to attempt to determine the fire size and characteristics such as maximum heat release rate, total heat release, maximum temperatures and fire duration are determined. Using existing models and correlations for fires, the measured fire characteristics are extrapolated to laptop fires on a vehicle the approximate size of the Orion spacecraft.

A fire on-board the International Space Station (ISS) resulting from a commercial Surface Pro tablet Lithium-Ion (Li-ion) battery can be detrimental to the spacercraft and the astronauts. The Spacecraft Fire Safety Demonstration (Saffire) program is focused on identifying and quantifying the risks that potentially arise inside a space vehicle by conducting large fire experiments inside the Cygnus cargo vehicle upon re-entry to Earth. The potential candidate that will be flown on the ISS is a 4- cell Li-ion battery pack Surface Pro tablet (42 Wh). The tablets were tested at the White Sands Test Facility (WSTF) using a localized heating method to emulate the failing mechanism that leads the unit into thermal runaway. Measurements inside the test chamber were performed on aerosol mass concentrations and for specific toxic products (i.e. from CO, HCN, HCl and CO2). These toxic products depend on the size of the fire and energy content of the tablet. Comparisons will be made with the Dell XPS (97 Wh). The concentrations are then used to extrapolate to laptop fires on a vehicle at the approximate volumetric size of the Orion spacecraft. The presentation provides an analysis on the detection capability and the response time to trigger fire alarms aboard a vehicle by using the mass concentration levels. These results will consider the rate at which the life support system is able to filter the atmosphere in order to provide a hazardous free environment.

Fire-resistant materials are used in multiple applications where protection from fire is needed. Their fire-resistant capacity is often tested under specific conditions that might not represent the situation in an actual fire. Particularly relevant for this work is the application for astronaut spacesuit, since a spacecraft environment may be different than earth atmospheres. There, a material is exposed to low velocity flows, microgravity, reduced pressure, and enriched oxygen concentration. Under these conditions, material flammability can be altered. In addition, flammability tests are based primarily on the exposure of the material to an external radiant flux to simulate an adjacent fire, but not a real flame. In this work, an experimental study was performed to investigate the effect of ambient pressure and oxygen concentration on the concurrent/upward flame spread over a fire-resistant fabric (Nomex HT90-40) exposed to two different external heat sources. One is the radiation from infrared lamps, and the other is the flame from a burning polymethyl methacrylate (PMMA) sheet placed below the fabric. The experimental results show that an external heat flux extends the limiting oxygen concentration (24% LOC) of Nomex. This effect is more pronounced when the PMMA flame provides the heat flux (17% LOC). For oxygen concentrations larger than the Nomex LOC, the flame spread rate decreases as the ambient pressure is decreased, indicating that reducing buoyancy reduces the flame spread rate. A simple analysis of concurrent flame spread that incorporates mixed flow heat transfer correlates well with the experimental data. This suggest that flame spread in microgravity can be predicted in terms of a mixed flow velocity that includes the Reynolds and Grashof numbers. The results of this work provide further information about the effect of the type of external heating on material flammability. They may also guide future fire safety design in space exploration.

The flammability of combustible materials in a spacecraft is important for fire safety applications because the conditions in spacecraft environments differ from those on earth. Experimental testing in space is difficult and expensive. However, reducing buoyancy by decreasing ambient pressure is a possible approach to simulate on-earth the burning behavior inside spacecraft environments. The objective of this work is to determine that possibility by studying the effect of pressure on concurrent flame spread, and by comparison with microgravity data, observe up to what point low-pressure can be used to replicate flame spread characteristics observed in microgravity. Specifically, this work studies the effect of pressure and microgravity on upward/concurrent flame spread over 10 mm thick polymethyl methacrylate (PMMA) slabs. Experiments in normal gravity were conducted over pressures ranging between 100 and 40 kPa and a forced flow velocity of 200 mm/s. Microgravity experiments were conducted during NASA's Spacecraft Fire Experiment (Saffire II), on board the Cygnus spacecraft at 100 kPa with an air flow velocity of 200 mm/s. Results show that reductions of pressure slow down the flame spread over the PMMA surface approaching that in microgravity. The data is correlated in terms of a non-dimensional mixed convection analysis that describes the convective heat transferred from the flame to the solid, and the primary mechanism controlling the spread of the flame. The extrapolation of the correlation to low pressures predicts well the flame spread rate obtained in microgravity in the Saffire II experiments. Similar results were obtained by the authors with similar experiments with a thin composite cotton/fiberglass fabric (published elsewhere). Both results suggest that reduced pressure can be used to approximately replicate flame behavior of untested gravity conditions for the burning of thick and thin solids. This work could provide guidance for potential ground-based testing for fire safety design in spacecraft and space habitats.

The flammability of combustible materials in spacecraft environments is of importance for fire safety applications because the environmental conditions can greatly differ from those on earth, and a fire in a spacecraft could be catastrophic. Moreover, experimental testing in spacecraft environments can be difficult and expensive, so using ground-based tests to inform microgravity tests is vital. Reducing buoyancy effects by decreasing ambient pressure is a possible approach to simulate a spacecraft environment on earth. The objective of this work is to study the effect of pressure on material flammability, and by comparison with microgravity data, determine the extent to which reducing pressure can be used to simulate reduced gravity. Specifically, this work studies the effect of pressure and microgravity on upward/concurrent flame spread rates and flame appearance of a burning thin composite fabric made of 75% cotton and 25% fiberglass (Sibal). Experiments in normal gravity were conducted using pressures ranging between 100 and 30 kPa and a forced flow velocity of 20 cm/s. Microgravity experiments were conducted during NASA's Spacecraft Fire Experiment (Saffire), on board of the Orbital Corporation Cygnus spacecraft at 100 kPa and an air flow velocity of 20 cm/s. Results show that reductions of ambient pressure slow the flame spread over the fabric. As pressure is reduced, flame intensity is also reduced. Comparison with the concurrent flame spread rates in microgravity show that similar flame spread rates are obtained at around 30 kPa. The normal gravity and microgravity data is correlated in terms of a mixed convection non-dimensional parameter that describes the heat transferred from the flame to the solid surface. The correlation provides information about the similitudes of the flame spread process in variable pressure and reduced gravity environments, providing guidance for potential on-earth testing for fire safety design in spacecraft and space habitats.