Background/Overview

Smoke is a general term that encompasses aerosol materials produced by a number of processes.  In particular it can include unburned, recondensed, original polymer or pyrolysis products that can be liquid, solid, carbonaceous soot, condensed water vapor, or ash particles.  Soot particles dominate the smoke particulate in established flaming fires while unburned pyrolysis products and recondensed polymer fragments are produced by smoldering and pyrolysis in the early stage of fire growth.  Given the constrained space on any spacecraft, the target for the fire detection system is necessarily the early phase and not established flaming fires; consequently, the primary target for detection is the pyrolysis products and not the soot.

Schematic of the SAME hardware

Prior spacecraft systems are summarized in more detail in papers by Friedman and Urban. In the Mercury, Gemini and Apollo missions, the crew quarters were limited and mission durations were short. Consequently it was considered reasonable that the astronauts would rapidly detect any fire. The Skylab module, however, included approximately 30 UV-sensing fire detectors.1 These devices were limited to line-of-sight and were reported to have difficulties with false alarms.  The Space Shuttle (Space Transportation System (STS)) detectors were based upon ionization fire detector technology, the most advanced technology available at the time and used an inertial separator designed to eliminate particles larger than 1-2 micrometers. The International Space Station (ISS) smoke detectors use near-IR forward scattering, rendering them most sensitive to particles larger than a micrometer, outside of the range of sensitivity of the shuttle detector.

More details of the ISS and STS detector requirements are presented by Steisslinger et al. 3 however the basic details are summarized below.  The STS detector, as built, was designed to alarm at 2 mg/m3 (based on 1 micrometer particles) or 0.022 mg/m3/s rise in concentration for 20 seconds.  The ISS detector was designed to alarm at obscuration of 1% per foot using an Underwriters Laboratory (UL) smoke box and a white light extinction meter.  This was implemented using a transfer standard detector and a 0.5 micrometer polystyrene latex-bead aerosol system that was used to set the amplifiers on each unit.  The transfer standard was calibrated in the smoke box and then used to set the levels with the aerosol system.

 As described by Friedman there have been six overheat and failed component failures in the NASA Orbiter fleet in addition to several similar incidents that have occurred on the ISS. None of these events spread into a real fire but as mission durations increase, the likelihood of failures increases. The experience on Mir in 1997 has shown that failure of oxygen generation systems can have significant consequences. As a result, improved understanding of spacecraft fire detection is critically needed.

Previous work on smoke particles from low-gravity sources by Urban et al. found that the particulate produced by low-gravity flames (soot or unburned fuel particles) tends to have larger size particles than in normal gravity.  Results from the CSD (Comparative Soot Diagnostics) Experiment which studied smoke properties in low-gravity from several spacecraft materials suggested that liquid smoke particles could achieve sizes larger than 1 µm while solid particulate remained in the sub-micrometer range.  However, the CSD experiment did not produce sufficient data concerning the size of the liquid smoke particles to guide detector design. The combined impact of these limited results and theoretical predictions is that, as opposed to extrapolation from 1-g data, direct knowledge of low-g combustion particulate is needed for more confident design of smoke detectors for spacecraft. 

Contacts at NASA Glenn Research Center
Project Manager: William Sheredy
William.A.Sheredy@nasa.gov
216-433-3685
Project Scientist: Dr. Gary Ruff
Gary.A.Ruff@nasa.gov
216-433-5697
Principal Investigator: Dr. David Urban. NASA GRC
David.L.Urban@nasa.gov
216-433-2835