Leading fire signatures of spacecraft materials: Light gases, condensables, and particulates

Abstract

The primary goal of this research is to provide recommendations for the eventual development of more effective and efficient fire sensors to be installed in space vehicles and habitats. An entirely new ground-based testing facility that generated fire signatures was developed to perform the combustion and pyrolysis experiments of eight different practical spacecraft materials. The flaming and smoldering of polymers approved by the National Aeronautics and Space Administration (NASA) generate three types of residues: condensables, light gases, and particulates. The residues were characterized by gas chromatography (GC), Fourier transform infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM). The analysis was interpreted as a function of oxygen concentration, temperature, and flow direction. Key findings are that the combustion of some materials such as Kevlar and cotton can only be identified by light gases, while the combustion of other materials, such as silicone and melamine, is best detected using a particulate-specific sensor. The implications during a fire event, its suppression, astronaut health in post-event cleanup as well as material recommendations are briefly discussed.

Introduction

Polymer degradation is a complex phenomenon, and the nature of the chemistry occurring during thermal decomposition of a fire event can be difficult to determine precisely [1]. The observed products of the degradation can be the result of primary, secondary, or even tertiary decomposition processes. As a consequence, the distribution of products will depend on such factors as sample thickness and melt viscosity, in addition to the more obvious factors such as temperature, atmosphere, and the chemical structure of the polymer itself. The main degradation products are of three types: (1) light gases consisting of volatile small molecules, (2) condensables that are monomers or fragments of the chains, volatile at the degradation temperature but are condensed outside the degradation zone, and (3) particulates that are solid residue formed by either aerosol processes or evolved directly from the solid.

During a fire in the normal gravity, flames tend to taper near the top and emit yellow light because of the constant air circulation. Differences in gas densities (so-called buoyant convection) drive convective flows. Oxygen is entrained by these flows in a normal gravity environment.

In microgravity, there is no buoyant acceleration and, accordingly, an absence of convective flow. In the absence of convective flow, flame shapes are altered, oxygen entrainment is reduced, combustion product removal from the flame environment is inhibited, and the overall flame temperature is reduced. A consequence of these effects is a longer residence time for combustion products near the flame zone [2].

In current spacecraft, automated early warning of fire events is achieved through smoke detectors, using principles of light scattering or ionization-current detection. The space shuttle orbiter has used particle-ionization smoke detectors that will indicate a fire when the particulates are in the 0.4–0.7 μm range [3]. Conversely, the US modules have photoelectric smoke detectors with an infrared (IR) laser beam source, and their sensitivity is greatest for particles larger than 1 μm. However, the effectiveness of these detectors that were initially designed to work in a normal gravity environment has been contested because the potential fire-causing accidents in the space shuttle were identified by the crewmembers, rather than the fire detectors [4].

There is a significant difference in identity and concentration of combustion products formed by fires in a low-gravity environment compared to those formed in a normal gravity environment. For instance, smoke particles produced in a quiescent microgravity environment were observed to be significantly larger than those produced by fires in normal gravity [5]. In this study, Ku et al. [5] evaluated the effect of buoyancy-induced differences in soot morphology of polyethylene (PE) and polypropylene flame soot. The authors reported that the mean aggregate lengths of PE and PP soots generated in 1G were 0.21 and 0.27 μm, respectively. When the soot is generated in 0G, for polypropylene the mean aggregate length increased to 1.53 μm and for polyethylene the mean length increased by approximately a factor of 23. Also, the evolution of carbon monoxide due to combustion is generally observed to be greater in low-buoyancy conditions, an aspect that can present a major safety issue for crewmembers, who function within the confined environment of the spacecraft [6]. It is precisely because of such differences that fire safety has become a critical issue in space exploration research. The overarching goal for the Fire Prevention, Detection, and Suppression (FPDS) project, developed by NASA, is to develop technologies that will ensure crew health and safety by minimizing the risk of a fire, establish fast and accurate fire detectors and maximize fire suppression and cleanup effectiveness on exploration missions [4].

In this study, we have evaluated the fire signatures of eight materials: cotton cord, Kapton film, Kevlar sheet, polyimide foam, melamine foam, silicone rubber, Halar, and polyester wire insulants. The three main signatures chosen for analysis include light gases, condensables, and particulates. Kinetic effects were investigated by temperature, chemical effects by ambient environment, and residence time effects by downward flow. Temperatures of 500 and 1000 °C were chosen to represent smoldering or flaming combustion. Nitrogen ambient was chosen to represent either an extreme vitiated or extinguished environment, while air reflects a normal (pre-event) environment. Downward flow was used as a means to increase residence time near the flame environment.

Since anticipated fires will most commonly arise from burning materials within air environments, our focus for particulates has been upon those produced by combustion of the materials in air. Where applicable, differences in size and/or morphology dependent upon ambient conditions are reported. Pyrolysis conditions under an inert environment are less applicable for early fire detection. An inert environment would arise as extinguishment in response to an already detected fire. Therein, such information is more applicable to the extinguishment activity itself and/or post-fire cleanup.

The objective of this research is to report the identity and concentration of species evolved during a fire-simulated event with a new combustion apparatus, and this information was used to increase the database of fire signature for practical spacecraft material. Practical, in this context, refers to materials that are currently accepted for use in space exploration based on existing National Aeronautics and Space Administration (NASA) standards [7]. With this database, we could propose final recommendation for the eventual development of more effective and efficient sensors to be installed in space vehicles and habitats. In addition, the identification of byproducts produced under different temperatures and environments from the studied materials allowed us to predict the effect of different extinguishment approaches and associated implication for astronaut health during post-fire cleanup.