Spacecraft Fire Experiment-II (Saffire-II) - 01.16.19

Overview | Description | Applications | Operations | Results | Publications | Imagery

ISS Science for Everyone

Science Objectives for Everyone
Fires are difficult to study in space because the risk to crew members is too great, but understanding flammability in microgravity is crucial for the safety of future missions. The Spacecraft Fire Experiment-II (Saffire-II) investigation quantifies the flammability of several materials in microgravity, and compares them to flammability limits in normal Earth gravity. Nine experimental samples of varying materials burn inside an empty Cygnus resupply vehicle after it leaves the International Space Station (ISS) and before it re-enters Earth’s atmosphere.
Science Results for Everyone
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The following content was provided by David L. Urban, Ph.D., and is maintained in a database by the ISS Program Science Office.
Experiment Details

OpNom: Saffire-II

Principal Investigator(s)
David L. Urban, Ph.D., Glenn Research Center, Cleveland, OH, United States

Gary A. Ruff, Glenn Research Center, Cleveland, OH, United States

NASA Glenn Research Center, Cleveland, OH, United States

Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)

Sponsoring Organization
Technology Demonstration Office (TDO)

Research Benefits
Earth Benefits, Scientific Discovery, Space Exploration

ISS Expedition Duration
September 2016 - April 2017

Expeditions Assigned

Previous Missions
Saffire-II is the second of a set of three experiments conducted on three consecutive flights of the Cygnus vehicle. Saffire-I is scheduled to fly on OA-6. Saffire-I investigates the development and spread of a large-scale low-gravity fire using one large sample (approx. 0.4 m wide by 0.94 m tall). Nine smaller samples will be burned on Saffire-II each having a dimension of 5 cm wide x 25 cm long.

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Experiment Description

Research Overview

  • 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 experiments cannot be conducted in an inhabited spacecraft.
  • Data obtained from Spacecraft Fire Experiment-II (Saffire-II) is 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 evaluate NASA’s normal-gravity material flammability screening test for low-gravity conditions. These tests 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 is proposed to be conducted in an expendable spacecraft, enabling such an experiment to be conducted without risk to crew or crewed spacecraft.
In October 2011, the NASA Advanced Exploration Systems program funded 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 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 necessitate 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. As a result, the ATV was the initial vehicle for which an experiment concept was developed. Dr. Olivier Minster, Senior Physical Scientist in the Directorate of Human Spaceflight for the European Space Agency formed an international topical team chaired by Professor Grunde Jomaas (Technical University of Denmark) and Professor Jose L. Torero (BRE Trust/RAEng Chair in Fire Safety Engineering in the School of Engineering at the University of Edinburgh). This 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., the schedule and resources were eventually the most significant. With the planned ATV flights ending with ATV-5 in March 2014, it became unlikely that an experiment could be developed and integrated with the vehicle within that schedule. Since Orbital’s eight Cygnus flights were planned to begin in 2013 and extend through 2016, Cygnus was a more promising vehicle 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 currently planned for July 2015.
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 secured with the standard stowage straps. This duct enables a more uniform flow across the samples, maintain a clear flow path within the experiment module, and prevent burning debris from interacting with the rest of the cargo.
The experiment package has a range of diagnostics to monitor the test conditions. The ambient temperature and the oxygen and CO2 concentrations are measured at the intake of the flow duct with temperature measurements also made just upstream of the fans. A pressure transducer also 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 will be 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 first and third tests (Saffire-I and -III) investigate 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 will propagate over a panel of thin material approximately 0.4 m wide by 1.0 m long. The ignition method is a hot wire along the upstream edge. This material will be 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. The objective of the second set of tests (Saffire-II) is to investigate the low-gravity Maximum Oxygen Concentration (MOC) flammability limits in long-term low gravity. The configuration for these experiments consists of nine samples of varying materials (denoted flammability samples) each having dimensions of approximately 5 cm wide by 30 cm long installed on the same panel in place of the single sample. These samples emulate the configuration used in NASA-STD-6001 Test 1. Each sample is ignited at the bottom using a hot wire. The oxygen concentration in the vehicle will be nearly 21% by volume—the same as in the ISS when the hatch was closed. The materials would be selected to be near their normal-gravity or hypothesized low-gravity maximum oxygen concentration in 21% O2. This complicates the selection of sample materials because most materials relevant for spacecraft do not have normal-gravity flammability limits near 21% oxygen by volume. Camera images would be the primary diagnostics for these tests as the intended result is primarily to determine whether the flame propagates or self-extinguishes.

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Space Applications
Despite decades of experience in manned spaceflight, NASA has limited ability to study fires in microgravity because it is too dangerous. Studying fire in an empty cargo vehicle is a unique, safe way to obtain microgravity data on flammability and flame growth. Previous research investigated how flames develop and spread on one large sample. The Saffire-II investigation studies nine smaller samples, each about 2 inches (5 cm) wide by 10 inches (25 cm) long.

Earth Applications
Studying combustion in an enclosed environment benefits fire safety efforts in controlled environments on Earth, from submarines to underground mines. The Safffire-II investigation is targeted toward spacecraft fire safety, but results improve general understanding of fire phenomena, benefiting people on Earth.

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Operational Requirements and Protocols

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 investigates 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. Nine smaller samples are burned on Saffire-II each having a dimension of 5 cm wide x 25 cm long. These are burned sequentially with the camera recording images only from the sample being burned. Once started, these experiments run automatically. Because of limitations in time available for downlinking, a maximum of 20 gigabits of data can be downlinked.
The Saffire-II 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-II mission operations begin when Cygnus unberths from the ISS. The Cygnus Flight Operations Team (FOT) establishes the vehicle in a lower, circular 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 one day.
The experiment operations are conducted in two phases. The first phase consists of turning on power to the experiment avionics, checking 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. The 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 is deorbited.
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

Information Pending

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Results/More Information

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Results Publications

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Ground Based Results Publications

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ISS Patents

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Related Publications

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Related Websites

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The Saffire Logo. The Cygnus vehicle is shown. The three bright stars represent the three Saffire units. The streak above the Earth represents Cygnus destructive re-entry and the fate of the Saffire hardware.

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image Saffire Experiment Module (top cover removed for clarity). Hardware consists of a flow duct containing the sample card and an avionics bay. All power, computer, and data acquisition modules are contained in the bay. Dimensions are approximately 53- by 90- by 133-cm.
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image Saffire experiment module with foam packing and straps as it will be mounted in Cygnus.
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