David B. Hirsch

Test Method to Determine Flammability of Aerospace Materials

Qualitative correlations between ground upward flammability tests and flammability testing in microgravity indicate that the NASA STD 6001 Test 1 provides conservative results by sustaining flaming combustion in less severe environments than those in which extinguishment occurs in quiescent microgravity environments. The upward flammability test is conducted in the most severe flaming combustion environment expected in the spacecraft. Its pass/fail test logic does not allow a precise quantitative comparison with other ground or microgravity materials flammability test results. Thus, although reasonable from a flammability safety point of view, the test is likely to eliminate materials that may be safe for use on spacecraft. A different test logic that will determine materials self-extinguishment limits is suggested to address these impediments. Data to support this approach are presented, including self-extinguishment limits in concurrent and countercurrent flows and under quiescent conditions. The proposed method will allow continued use of existing NASA flammability data and make possible quantitative correlations between ground testing and microgravity test results. Quantitative correlations between ground test results and microgravity combustion data will improve the aerospace materials selection process and allow realistic estimates of spacecraft fire extinguishment requirements. Theoretical analyses of flaming combustion will be possible, leading to a better understanding of materials combustion. This will benefit not only the aerospace community, but also the combustion community at large.

Pressure Effects on Oxygen Concentration Flammability Thresholds of Polymeric Materials for Aerospace Applications

Spacecraft materials selection is based on an upward flammability test conducted in a quiescent environment at the highest expected oxygen concentration. However, NASA’s advanced space exploration program is anticipating using various habitable environments. Because limited data are available to support current program requirements, a different test logic is suggested to address the expanded atmospheric environments through the determination of materials self-extinguishment limits. This paper provides additional pressure effects data on oxygen concentration and partial pressure self-extinguishment limits under quiescent conditions. For the range of total pressures tested, the oxygen concentration and oxygen partial pressure flammability thresholds show a near linear dependence on total pressure, and appear to increase with increasing oxygen concentration (and oxygen partial pressure) thresholds. For the Constellation Program, the flammability threshold information will allow NASA to identify materials with increased flammability risk from oxygen concentration and total pressure changes, minimize potential impacts, and allow for development of sound requirements for new spacecraft and extraterrestrial landers and habitats.

Microgravity Effects on Combustion of Polymers

A viewgraph presentation describing various microgravity effects on the combustion of polymers is shown. The topics include: 1) Major combustion processes and controlling mechanisms in normal and microgravity environments; 2) Review of some buoyancy effects on combustion: melting of thermoplastics; flame strength, geometry and temperature; smoldering combustion; 3) Video comparing polymeric rods burning in normal and microgravity environments; and 4) Relation to spacecraft fire safety of current knowledge of polymers microgravity combustion.

Evaluation of polyimide foam as a fire barrier for spacecraft cushion materials

Polyimide foam is an intrinsically flame resistant foam material. This study uses the NASA upward flame propagation test and the cone calorimeter test to evaluate the application of polyimide foam as a fire barrier for spacecraft cushion materials. The flame propagation test results demonstrate that a thin-layer of polyimide foam (3-mm thick) could totally stop the flame spread on the underneath polyurethane foam in an environment containing 30% oxygen. The cone calorimeter test results show that polyimide foam increased the minimum heat flux for ignition of a cushion sample from 27 to 48kW/m2. This suggests that application of polyimide foam can significantly reduce the ignition risk. It was also found that polyimide foam significantly reduced the peak heat release rate, mass burning rate, and the generation of carbon monoxide and smoke in the flaming combustion. For polyimide foam-covered polyurethane, smoldering combustion was insignificant at 25 and 35kW/m 2 incident heat fluxes; it became more important when the incident heat flux reached 45 kW/m2. The smoldering combustion of polyurethane foam and polyimide foam-covered polyurethane could not be self-sustaining if the external heat source was removed.

Pressure Effects on the Self-Extinguishment Limits of Aerospace Materials

The Orion Crew Exploration Vehicle Module (CM) is being designed to operate in an atmosphere of up to 30% oxygen at a pressure of 10.2 psia for lunar missions. Spacecraft materials selection is based on an upward flammability test conducted in a closed chamber under the worst expected conditions of pressure and oxygen concentration. Material flammability depends on both oxygen concentration and pressure but, since oxygen concentration is the primary driver, all materials are certified in the 30% oxygen, 10.2 psia environment. Extensive data exist from the Shuttle Program at this condition which used relatively the same test methodology as currently used in the Constellation Program. When the CM returns to Earth, a snorkel device will be activated after splashdown to provide outside air to the crew; however, for operational reasons, it is desirable to maximize the time the crew is able to breathe cabin air before the snorkel device is activated. To maximize this time, it has been proposed to raise the partial pressure of oxygen in the CM immediately before reentry while maintaining the total cabin pressure at 14.7 psia. In addition, it has been proposed to leak-test the Orion CM with ambient air at a maximum pressure of 17.3 psia. No data exist to assess how high the cabin oxygen concentration can be at 14.7 psia or 17.3 psia. One is to re-test a large number of materials at these pressures at a significant cost. However, since the maximum oxygen concentration (MOC) at which a material will self-extinguish has been determined for a variety of spacecraft materials as a function of pressure, a second alternative is to use existing data to estimate the MOC at 14.7 psia and 17.3 psia. This data will be examined in this paper and an analysis presented to determine the oxygen concentrations at the increased pressures that will result in self-extinguishment of a material. This analysis showed that the oxygen concentration for the Orion CM at 14.7 psia cannot be set higher than 25.6% without potentially invalidating the materials flammability certification in 30% oxygen at 10.2 psia for some materials. Materials certified under these conditions would still be self-extinguishing in ambient air at 17.3 psia. alternative

Selected Parametric Effects on Materials Flammability Limits

NASA-STD-(I)-6001B Test 1 is currently used to evaluate the flammability of materials intended for use in habitable environments of U.S. spacecraft. The method is a pass/fail upward flame propagation test conducted in the worst case configuration, which is defined as a combination of a material s thickness, test pressure, oxygen concentration, and temperature that make the material most flammable. Although simple parametric effects may be intuitive (such as increasing oxygen concentrations resulting in increased flammability), combinations of multi-parameter effects could be more complex. In addition, there are a variety of material configurations used in spacecraft. Such configurations could include, for example, exposed free edges where fire propagation may be different when compared to configurations commonly employed in standard testing. Studies involving combined oxygen concentration, pressure, and temperature on flammability limits have been conducted and are summarized in this paper. Additional effects on flammability limits of a material s thickness, mode of ignition, burn-length criteria, and exposed edges are presented. The information obtained will allow proper selection of ground flammability test conditions, support further studies comparing flammability in 1-g with microgravity and reduced gravity environments, and contribute to persuasive scientific cases for rigorous space system fire risk assessments.

Proficiency Testing for Evaluating Aerospace Materials Test Anomalies

ASTM G 86 “Standard Test Method for Determining Ignition Sensitivity of Materials to Mechanical Impact in Ambient Pressure Liquid Oxygen and Pressurized Liquid and Gaseous Oxygen” and ASTM G 74 “Standard Test Method for Ignition Sensitivity of Materials to Gaseous Fluid Impact” are commonly used to evaluate materials susceptibility to ignition in liquid and gaseous oxygen systems. However, the methods have been known for their lack of repeatability. The inherent problems identified with the test logic would either not allow precise identification or the magnitude of problems related to running the tests, such as lack of consistency of systems performance, lack of adherence to procedures, etc. Excessive variability leads to increasing instances of accepting the null hypothesis erroneously, and so to the false logical deduction that problems are nonexistent when they really do exist. This paper attempts to develop and recommend an approach that could lead to increased accuracy in problem diagnostics by using the 50 % reactivity point, which has been shown to be more repeatable. The initial tests conducted indicate that PTFE and Viton® A5 (for pneumatic impact) and Buna S (for mechanical impact) would be good choices for additional testing and consideration for interlaboratory evaluations. The approach presented could also be used to evaluate variable effects with increased confidence and tolerance optimization.

Development of a Standard Test Scenario to Evaluate the Effectiveness of Portable Fire Extinguishers in an Elevated Oxygen Environment

Spacecraft are currently operated in an elevated oxygen environment for various reasons for extended amounts of time. On the International Space Station, the most common and severe fire hazard scenario surrounds airlock operations. Materials in elevated oxygen are more flammable in this environment as a result of the increased availability of oxygen to sustain combustion in the event of a fire. Fires occurring in an elevated oxygen environment are much more energetic than in standard conditions, offering a greater challenge to a fire extinguisher. Carbon dioxide fire extinguishers are currently used aboard international spacecraft as a means of mitigation in the event of a fire. Currently NASA is developing a fine water mist portable fire extinguisher for future use on international spacecraft. As development ensues, a need for the evaluation of various types of fire extinguishers is required to provide an unbiased means of ranking and comparison between units. The elevated oxygen fire scenario discussed is proposed as the standard test in evaluating fire extinguisher performance against elevated oxygen fires in spacecraft.

Development of a Standard Test Scenario to Evaluate the Effectiveness of Portable Fire Extinguishers on Stored Energy Fires: Li-Ion Battery Scenario

Many sources of fuel are present aboard current spacecraft, with one especially hazardous source of stored energy: lithium ion batteries. Lithium ion batteries are a very hazardous form of fuel due to their self-sustaining combustion once ignited, for example, by an external heat source. Batteries can become extremely energetic fire sources due to their high density electrochemical energy content that may, under duress, be violently converted to thermal energy and fire in the form of a thermal runaway. Currently, lithium ion batteries are the preferred types of batteries aboard international spacecraft and therefore are routinely installed, collectively forming a potentially devastating fire threat to a spacecraft and its crew. Currently NASA is developing a fine water mist portable fire extinguisher for future use on international spacecraft. As its development ensues, a need for the standard evaluation of various types of fire extinguishers against this potential threat is required to provide an unbiased means of comparing between fire extinguisher technologies and ranking them based on performance.

Evaluating the Effectiveness of Portable Fire Extinguishers on Stored Energy Fires: International Space Station Self Contained Oxygen Generators

Many sources of fuel are present aboard current spacecraft, with one being an especially hazardous source of stored energy: the self-contained oxygen generator. Self-contained oxygen generators are currently used aboard vessels that are typically isolated from any significant means of resupplying breathable air to passengers in the event of a loss of atmosphere, as is the case in airplanes, submarines, and spacecraft. Self-contained oxygen generators are particularly hazardous forms of fire sources, as they supply the fire with oxygen and as a result are able to sustain combustion despite extinguishment efforts. Carbon dioxide fire extinguishers are currently used aboard international spacecraft as a means of fire mitigation. Currently NASA is developing a fine water mist portable fire extinguisher for future use on international spacecraft. Due to a previous fire incident aboard the International Space Station (ISS) involving oxygen generators, it is of special interest to understand how a fire extinguisher performs against a similar event. Testing was performed simulating the ISS oxygen generator fire incident, and portable fire extinguishers were evaluated.