Material Properties That Control Ignition and Spread of a Fire in Micro-gravity Environments

A study of the different mechanisms controlling the initial stages of a fire in a micro-gravity environment is presented. Three different processes are deemed important for evaluation of material flammability, piloted ignition, co-current and counter-current flame spread. The three processes are evaluated in terms of thermal theory and the different material properties controlling these combustion processes are extracted. Experimental results obtained from ground testing, drop towers, parabolic flights and sounding rocket experiments serve to validate the present approach. INTRODUCTION The necessary flammability requirements for all materials to be used in space vehicles (NASA specifications) are given by the “Flammability, Odor, Offgassing, and Compatibility Requirements and Test Procedures for Materials in Environments that Support Combustion” document [1]. This document specifies two test that need to be performed before a material is qualified to be used in a space vehicle, the “Upward Flame Propagation Test” (Test 1) and the “Heat and Visible Smoke Release Rates Test” (Test 2). These two tests are expected to properly assess the flammability of a material in micro-gravity conditions. The basic principle behind these two test methods is an attempt to provide a worst case scenario (Test 1) and a measure of the heat release (Test 2), and consequently, the “damage potential” of a fire. A detailed description of these test methods is provided in NASA-NHB 8060.1 [1] and an extensive list of the materials that have been tested is provided in the “Materials Selection List for Space Hardware Systems” [2]. Background information for Test 2 is summarized in reference [3]. A general overview of fire safety practices is provided by Friedman [4] and future fire safety requirements and research needs for space exploration are described in reference [5]. Few studies have addressed the issue of material flammability for spacecraft applications. The relevance of Test 1 to material flammability for micro-gravity applications was explored by Ohlemiller and Villa [6]. Ohlemiller and Villa conducted a series of tests following the protocol of Test 1, modify the test to include pre-heating by external radiation and compared the results with tests conducted with the cone calorimeter and the L.I.F.T. (ASTM-E-1321). This work will be described in more detail, in following sections, since it provides significant insight to the issues addressed in this work. Following the recommendations of Ohlemiller and Villa [6], Cordova et al. [7] and Long et al. [8] presented some preliminary results on the adaptation of the L.I.F.T. apparatus (ASTM-E-1321) to asses the performance of materials for micro-gravity applications. In their work they suggest the use of a forced flow version of the L.I.F.T. to determine the potential of a material to ignition and lateral (opposed) flame spread. Recent experiments conducted in the Skorost combustion tunnel apparatus on board of the Orbital Station MIR [9] have shown that there a limiting velocities below which a diffusion flame established over PMMA, Delrin and high-density polyethylene ceases to exist. Flow shutoff was therefore deemed to be an appropriate methodology to follow the detection of a fire where flames were observed to extinguish in less than twenty seconds after the suppression of the flow. Criticism towards the use of the current methodology is common in the micro-gravity combustion literature [10] where studies have challenged the concept of co-current spread as being a worst case scenario [11-13] and even reverse flammability rankings have been presented [14]. The imminent construction of the Space Station and the projected Human Mission to Mars, due to the extent of the missions, high voltage of the on-board power supplies and greater volume of high temperature scientific instrumentation will result in enhanced hazard and more frequent fire initiation events [9]. Therefore,