SPICE-BASS: Burning and Suppression of Solids (BASS)

A Rapid Turnaround Proposal Utilizing SPICE Hardware

Background

Flame spread behavior, flammability, and suppression data are of interest to NASA for application to fire safety efforts.  The reduced buoyant flows, coupled with low-speed air circulation currents, create an environment quite different than normal Earth gravity.  In addition, there are several oxidizer atmospheres proposed for crew-manned vehicles and habitats [NASA CxP-70024], which further complicates the problem.  Understanding flammability and suppression in a variety of environments is especially important given NASA’s current exploration initiatives.

Suppression systems in the past have included a water-based foam (e.g. Russian Mir Space Station) and Halon (US space shuttle) extinguishers [Friedman and Dietrich, 1991].  Fortunately, there has only been one significant case when a fire extinguisher needed to be discharged—to fight the oxygen generator fire on the Mir Space Station in 1997.  In that case, the crew discharged several water-based foam extinguishers to fight the fire.  But given its unusual nature, the fire only went out when the canister ran out of oxygen.  The water-based foam may have helped to prevent the fire from spreading to nearby surfaces.

Currently, the ISS relies on CO2 extinguishers [Friedman, 1999].  This inert-gas based system relies on using two different nozzles to fight the fires depending on their nature.  One nozzle allows for CO2 injection into ports on the rack assemblies if there are indications of fires burning behind the panels.  The other nozzle is planned for use in open areas where the fire is readily accessible.

The spacecraft fire safety approach f NASA emphasizes fire prevention through material screening [Friedman, 1998].  Normal gravity test methods have been used to select materials for spacecraft cabin construction.  The NASA Standard Flammability Test #1, shown in Figure 1, is a normal gravity material flammability test which assesses a specific material, geometry, and atmosphere based on whether an upward flame spreads less than six inches [NASA-STD-6001, 1998].  The assumption has been that this normal-gravity test method provides a conservative prediction of flammability for spacecraft.  Given the huge number of materials which must be screened, and the relative difficulty of performing extensive microgravity tests, this is a reasonable approach.  However, it is recognized that at least for some fuels, this test may not be a conservative estimate at all [Olson et al., 2008], as shown in Table 1 (Upward Limiting Oxygen Index, or ULOI, is a modified limiting oxygen index for flammability).

NASA grants waivers for some materials (astronaut clothing, computer printouts, etc.) which otherwise would not be allowed to fly.  Other controls for these materials (e.g. stowage restrictions, limiting quantity) may be put in place to reduce the likelihood of fire.  Justifiably, waivers are issued only rarely.

Once a fire breaks out, it must be extinguished.  To determine the effectiveness of a gaseous fire-extinguishing agent, typically used in a total flooding fire suppression system, the cup burner method [NFPA 2001] has been most widely used for terrestrial applications to determine the minimum extinguishing concentration (MEC) of agent that would extinguish the flame.  Takahashi et al. [2007, 2008] conducted microgravity research on the physical and chemical extinguishment mechanisms of cup-burner flames of gaseous fuels using reduced-gravity aircraft.  Transient computations with full methane chemistry and gray-gas radiation model revealed the detailed flame structure and extinguishment phenomena.  However, fundamental understanding of the flame processes leading to solid materials flammability or extinguishment is still limited.  Little is known concerning how materials flammability relates to gaseous flame phenomena and whether the extinguishment occurs due to global flame extinction or destabilization of the edge diffusion flame.  Obviously, the understanding of materials flammability and fire suppression in microgravity would be greatly advanced if the effect of thermal agents (e.g., nitrogen and carbon dioxide) in solid-fuel flames was investigated in the ISS from a fundamental perspective.

Efforts are underway to improve the application of NASA-STD-6001 Test#1 data by performing tests in ground-based reduced gravity facilities (e.g. as shown in Figure 2), and in experiments which attempt to mitigate the effects of buoyancy.  These have been relatively few in number. 

Buoyancy can be mitigated to some degree in normal Earth gravity, though there are limitations and non-idealities.  The obvious benefit is that long-duration tests can be conducted.  Olson and T’ien [2000] examined the combustion of solid materials at low-stretch by varying the radius of curvature of the fuel sample.  In another work, a narrow-height flow tunnel [Olson et al., 2009] was used to confine the buoyant flow in the flame zone, thus making the application of a forced flow the controlling parameter.  A different approach follows some earlier fire scale model efforts to examine how varying pressure in a fire can be equivalent to changing gravity level with the conclusion being that keeping the parameter P2g constant yields approximately similar flame spreading rates [Kleinhenz et al., 2008].

There have been a modest number of research efforts examining combustion of solids using ground-based microgravity facilities [e.g., Foutch, 1987; Olson et al., 1988; Sacksteder and T’ien, 1994; Goldmeer, 1996; Armstrong et al. 2006, Olson et al., 2008].  All of these efforts were limited by the short-duration of time available (up to 20 seconds on the airplane) and therefore could only look at the initial burning behavior of small samples.  There have been very few experiments examining solid combustion in a space-based experiment [e.g. Kimzey, 1974; Ivanov et al. 1999; Olson et al. 2001].  The first two of these examined practical flammability aspects.  Figure 3 shows a typical flame image for a flame burning on a plastic rod in a low speed flow on the Mir space station.

Finally, modeling efforts can provide a powerful tool for predicting flame spread, flammability, and suppression, as shown in Figures 4 and 5 [Yang and T’ien, 1998; Hsu, 2009].  However, even the best models require validation at some point, so actual experiments need to be conducted.  Careful experiments and related modeling efforts tend to bolster the development of each other.

Fuel Assembly for Flat Sample