ISS Science for Everyone
Science Objectives for Everyone
Fire is extremely hazardous in the enclosed environments inside spacecraft, which makes it difficult to perform controlled flame growth and prevention experiments on the International Space Station (ISS). But understanding how fires spread is vital for designing flame-resistant materials and preventing fires in space. Spacecraft Fire Experiment-III (Saffire-III) is the third flame investigation to use empty Cygnus resupply vehicles after they leave the ISS and re-enter Earth’s atmosphere, providing a unique environment for studying fires in microgravity.
Science Results for Everyone
The following content was provided by David L. Urban, Ph.D., and is maintained in a database by the ISS Program Science Office.
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David L. Urban, Ph.D., Glenn Research Center, Cleveland, OH, United States
NASA Glenn Research Center, Cleveland, OH, United States
Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)
Technology Demonstration Office (TDO)
ISS Expedition Duration
- September 2017
Saffire-III is the third of a set of three experiments conducted on three consecutive flights of the Cygnus vehicle. Saffire-I is scheduled to fly on Orb-5, Saffire-II on Orb-6.
Despite years of experience in manned spaceflight, NASA has a limited degree of verification of the current approach for spacecraft fire protection.
This is a direct result of the inability to study fires of practical size in low-gravity.
Ensuring reliability of the fire safety of future spacecraft requires experiments that provide a realistic examination of the risk.
These Saffire-III experiments cannot be conducted in an inhabited spacecraft.
Data obtained from the experiment are used to validate modeling of spacecraft fire response scenarios.
The data provides the first dataset that can be used to validate spacecraft fire growth and spread models.
The tests evaluates NASA’s normal-gravity material flammability screening test for low-gravity conditions. These test have a long history but their representation of the low-gravity fire risk has not been validated.
This testing provides the first practical scale low-gravity fire testing.
This increases the reliability of the fire safety on all future spacecraft.
Despite decades of research into combustion and fire processes in reduced gravity, there have been very few experiments directly studying spacecraft fire safety under low-gravity conditions. Furthermore, none of these experiments have studied sample and environment sizes typical of those expected in a spacecraft fire. Prior experiments have been limited to samples no larger than 10 cm in length and width. This stands in stark contrast to the full-scale fire safety testing that has been conducted in habitable structures on earth including mines, buildings, airplanes, ships, etc. The large differences between fire behavior in normal and reduced gravity results in a lack of experimental data that forces spacecraft designers to base their designs on terrestrial fires and fire standards. While this approach has been successful thus far, there is inherent risk due to the level of uncertainty. Despite their obvious importance, full scale spacecraft fire experiments have not been possible because of the inherent hazards involved in conducting a large fire test in a manned spacecraft. To address this knowledge gap, an experiment was proposed to use an expendable spacecraft, enabling such an experiment to be conducted without risk to crew or crewed spacecraft.
The NASA Advanced Exploration Systems program began a project to develop and demonstrate spacecraft fire safety technologies in relevant environments. The keystone of these demonstrations is a large-scale fire safety experiment conducted on an International Space Station (ISS) re-supply vehicle after it has undocked from the ISS and before it enters the atmosphere. The project team from NASA John H. Glenn Research Center (GRC) is augmented by an international topical team assembled by the European Space Agency (ESA). Each member of this team brings expertise and funding from their respective space and research agencies for their activities. This participation of members from other countries and space agencies not only brings additional skills to the science team, but also facilitates international cooperation in the development of an approach to spacecraft fire prevention and response for future exploration vehicles. No single experiment can address the range of issues that need to be resolved to fully understand the spacecraft fire risk and to ensure the safety of future flights. The goal of the topical team is to leverage the international capabilities of the team to develop a suite of ground-based and space flight spacecraft fire safety experiments to expand the impact of the flight experiments. The current experiment has been designed to address two objectives. The first objective (Saffire-I and Saffire-III) is to understand the flame spread and growth of a fire over an amount of flammable material consistent with what is likely to be in a spacecraft cabin through the development of an experiment for a sample material approximately 1 meter long. This is at least an order of magnitude larger than any prior low-g flame spread experiment. The second objective (Saffire-II) is to examine the flammability limits of materials in low gravity to determine if NASA’s material selection methods are a reasonable predictor of low-gravity flammability. Supported by the ground-based research by the topical team, the experiment addresses both of these objectives. Their individual contributions are discussed in subsequent sections.
The unique objectives of this experiment necessitated the use of an ISS expendable resupply vehicle such as ESA’s ATV, JAXA’s HTV, or Orbital Sciences Corporation’s (Orbital’s) Cygnus vehicles. Early in the development of the project, the European Space Agency (ESA) became interested in this experiment. The Fire Safety in Space International Topical Team consists of 14 researchers from the European, Japanese, Russian, and U.S. spacecraft fire safety communities and is tasked to define research that would be possible from such a low-gravity fire safety experiment. The group has developed the initial science and technology requirements for this experiment as well as ground-based experiments and modeling efforts that support this experimental campaign.
While many factors could go into the selection of a vehicle such as available volume, power availability, communication, etc., schedule and resources eventually became the most significant. The Cygnus vehicle is the most promising for the successful completion of this experiment. Programmatic requirements later drove the project to plan for three experiments to be performed on three consecutive flights of Cygnus. The first experiment would take place on the 5th Cygnus flight.
The concept for this experiment focuses on conducting two types of material combustion tests that are performed on different flights using the flow duct design. The experiment package consists of a flow duct and an adjacent avionics bay. The avionics bay is connected to the side of the flow duct. The top and bottom structures on the experiment module are the fan unit on the top and the flow straightener unit on the bottom. The airflow is from the bottom to the top of the experiment module. The flow duct/avionics bay assembly is a rigid structure and is secured with the standard stowage straps. This duct enables a more uniform flow across the samples, maintains a clear flow path within the experiment module, and prevents burning debris from interacting with the rest of the cargo.
The Saffire-III experiment package has a range of diagnostics to monitor the test conditions. The ambient temperature and the oxygen and carbon dioxide (CO2) concentrations are measured at the intake of the flow duct with temperature measurements also made just upstream of the fans. A pressure transducer delivers the pressure time-history. Flow anemometers are placed at selected locations in the inlet flow and thereby quantify the oxidizer flux in the duct. Two video cameras provide top views of the entire sample. The sample is periodically illuminated by a LED source to allow the measurement of the pyrolysis length.
For the flame spread sample, the flame stand-off distance is measured using several thermocouples placed at varying heights above the sample surface. These are woven into the sample and then bent so they are perpendicular to the surface. Finally, a calibrated radiometer measures the broadband radiative emission from the sample to provide an estimate of the radiative flux from the burning zone towards the surroundings.
The test investigates flame spread and growth in low-gravity to determine if there is a limiting flame size and to quantify the size and growth rate of flames over large surfaces. The flame propagates over a panel of thin material approximately 0.4 m wide by 1.0 m long. The oxygen concentration in the vehicle is nearly 21% by volume—the same as in the ISS when the hatch was closed. The ignition method is a hot wire along the upstream edge. This material is expected to burn at the anticipated cabin atmosphere. The objective of this test is to quantify the flame development over a large sample in low-gravity.
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After more than five decades of manned spaceflight, scientists still have limited data about the risks involved with spacecraft fires, because it is too risky to conduct experiments of this scale inside an occupied space station. The Saffire-III experiments study the development and growth of a fire in a delivery vehicle without exposing humans to any danger. Instruments in an empty cargo supply vehicle measure air flow, temperature, heat, oxygen, carbon dioxide, and other characteristics while video cameras capture views of a growing flame. Results from this investigation help NASA choose flame-resistant materials for use in space, and provide crucial data for scientists to understand how fires can spread in spacecraft like the ISS.
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Studying combustion in an enclosed, oxygen-rich environment benefits fire safety efforts in similar environments on Earth, from submarines to mines. Results improve general understanding of fire phenomena, which benefits people on Earth.
Operational Requirements and Protocols
Saffire-III flight system is loaded into the Cygnus PCM and be strapped into place. Power, data, and relay connections are then made. Saffire remains unpowered during launch, berthing, and unberthing. Following unberthing, the Cygnus Flight Operations Team (FOT) establishes the vehicle in a separate orbit. The Cygnus FOT powers on the experiment avionics and conducts a check of all experiment components. The FOT places the Cygnus vehicle into a free-drift mode and initiate the experiment. The Saffire-III experiment records and compresses all data obtained during the experiment.A subset of the experiment data is downlinked through passes at multiple ground sites and checked for quality. When the selected data files have been successfully retrieved, the experiment avionics power is turned off and the Cygnus FOT deorbits the Cygnus vehicle.
The Saffire experiments are conducted in three consecutive flights of the Cygnus vehicle. Since Cygnus undergoes a destructive re-entry, three Saffire experiment units (Saffire-I, -II, and –III) are constructed and all data must be downlinked. Even with these three flight opportunities, the experiment is very limited in the amount of data and test conditions that can be investigated. In Saffire-I and –III, the sample material is a single large sample (approx. 0.4 m wide by 0.94 m tall) and demonstrates the development and spread of a large-scale low-gravity fire. Once started, the entire burn of each of these samples is recorded, the data compressed, and downlinked. Because of limitations in time available for downlinking, a maximum of 20 gigabits of data can be downlinked. The Saffire-III experiment begins only after Cygnus is unberthed from the ISS. Prior to unberthing, the crew must check that the inlet and outlet ends of the flow duct be clear of any stowage bags being deorbited.
Saffire-III mission operations begin when Cygnus unberths from the ISS. The Cygnus FOT establishes the vehicle in a separate orbit. Based on this new orbit, ground station contact times are calculated and the experiment start is confirmed and coordinated with the Saffire FOT. From Cygnus undock until experiment start takes about 1/2 day. The experiment operations are conducted in two phases. The first phase consists of turning on power to the experiment avionics, checking that the experiment is initialized successfully, starting the experiment run, and recording and compressing the resulting data. The second phase consists of downloading all experiment data via downlink passes at various ground sites. Data are examined for data quality and files are retransmitted if necessary. When a complete set of data files has been successfully retrieved, or the timeline has reached a predetermined maximum duration, the experiment avionics power is switched off and the Cygnus vehicle deorbits. Cygnus burns up on re-entry into the Earth's atmosphere. Consequently, the flight unit is permanently disposed of during this process. No specific disposal processes are expected if the experiment hardware is burned up upon re-entry.
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Decadal Survey Recommendations
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