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Burning and Suppression of Solids (BASS)

 
BASS Status

January 12, 2014

The Burning and Suppression of Solids - II (BASS-II) is a consortium of five separate investigations with different research teams that will all utilize the same small flow duct hardware used in the BASS experiment within the Microgravity Science Glovebox (MSG) for observations of burning solid materials on board the ISS. BASS-II will operate from February through August, with breaks for vehicle traffic at ISS.

The precursor BASS experiment demonstrated the capability to vitiate the MSG working volume late in their test matrix, so the new investigations plan to take advantage of that added variable. While each investigation has its own goal (flammability, flame spread, extinguishment, etc.), they share the same objective: a better understanding flame behavior in space and on Earth. The main variables to be tested in BASS-II are the effects of ambient oxygen concentration, ventilation flow velocity, and fuel type, thickness, and geometry.

BASS-II consists of 100 fuel samples and associated igniter wires, which were delivered to the ISS on January 12, 2014 via the Cygnus cargo vehicle. There are three categories of samples: flat samples, rod samples, and a section of a large solid sphere. Thin flat samples (10 cm long by 1 and 2 cm wide) yield concurrent or opposed-flow spread rates, limiting flame lengths, and extinction limits. The flat sample materials will include acrylic films and sheets of different thicknesses, and a cotton-fiberglass fabric blend Solid Inflammability Boundary at Low-Speeds (SIBAL) fuel which was previously tested in BASS. The rod samples are made of black or clear acrylic, and will provide solid fuel regression rates and extinction limits for both opposed and concurrent flow. The large solid spherical section, also made of acrylic, will be used to study ignition of thick materials and flame growth over the thick material.

Many of the tests will focus on finding a minimum oxygen concentration or flow velocity where a material will burn in space, to compare with the Earth-based flammability limits. The crewmember is directly involved with the investigator team throughout the experiment to load fuel samples, seal the working volume and adjust gaseous nitrogen vitiation of the working volume atmosphere, ignite the fuel, monitor the test, adjust flow rates, take still photographs, and provide additional observations about the test.

BASS experiment
The BASS experiment is conducted within the SPICE Experiment Assembly.
 

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BASS Overview Chart


  • October 2013
    – The BASS combustion experiment operations concluded on May 30, 2013.
    For the cotton-fiberglass blend fabric samples, the effects of air flow speed on flame appearance, flame growth, and spread rates were determined in both the opposed and concurrent-flow configuration. For the opposed flow configuration, the flame quickly reached steady spread for each flow speed, and the spread rate was fastest at an intermediate value of flow speed. These tests show the enhanced flammability in microgravity for this geometry, since, in normal gravity air, a flame self-extinguishes in the opposed flow geometry (downward flame spread). In the concurrent-flow configuration, flame size grew with time during the tests. A limiting length and steady spread rate were obtained only in low flow speeds (≤ 10 cm/s) for the short-length samples that fit in the small wind tunnel. For these conditions, flame spread rate increased linearly with increasing flow. This is the first time that detailed transient flame growth data was obtained in purely forced flows in microgravity. In addition, by decreasing flow speed to a very low value (around 1 cm/s), quenching extinction was observed. The valuable results from these long-duration experiments validate a number of theoretical predictions and also provide the data for a transient flame growth model under development.


    June 2013 –
    The BASS combustion experiment finished the second session of operations on board the ISS in the Microgravity Science Glovebox. Many of the second session solid fuel test points were conducted at reduced oxygen levels. The BASS second session of operations were conducted by ISS crew member Chris Cassidy.


    June 2012 – The Burning And Suppression of Solids (BASS) is currently being operated by Don Pettit on the International Space Station. On June 11, 2012 four tests were conducted, burning a new 2-cm diameter sphere of PMMA (poly methyl meth acrylate) in the normal configuration. Based on earlier tests, the team decided to move the sample 2.5 cm closer to the nitrogen nozzle to facilitate extinguishment. A flame was established at a flow speed of 10 cm/s, and then the flow was reduced to a minimal value (around 1 cm/s). The flame remained very dim blue and spread quite slowly around the sphere. The nitrogen was turned on after a few minutes, and the flame extinguished in the stagnation region but remained stabilized on the sides even with maximum nitrogen flow. After a minute, the nitrogen was turned off and the flame slowly spread back toward the stagnation region. Finally, the flow was turned off and the flame extinguished. In the second through fourth tests, Don adjusted the sphere location to improve the framing of the image by moving it 1 cm downstream. Ignition was achieved at 10 cm/s air flow, then the air flow was again gradually reduced to minimal a value (around 1 cm/s). A very stable, dim blue flame was obtained for these tests. The reduction of soot in the flame is desirable for comparisons to the model, and the team appreciates Don’s efforts to carefully adjust the air flow to a very low value. After experiment operations, a fan calibration was performed.

    The Burning And Suppression of Solids (BASS) combustion experiment was operated on June 5, 2012. Don Pettit installed a new small sphere (1-cm diameter PMMA) in the wake configuration. All four tests planned for the day were successfully completed. Don was able to spend a good portion of the session training Joe Acaba on BASS operations and we appreciate the effort--it will ensure a smooth handover. For all tests, the air flow was set to 5 cm/s for ignition and then changed to the desired value after the flame was established. The fourth test (Test 33) was particularly interesting as Don was able to set the air flow to a very small value yielding a large, stable, mostly-blue flame which persisted until all the fuel was consumed. In fig. 1, the last portion of the burn is shown for this test. After the sphere tests were complete, Don offered to use the remaining time to perform some additional operations. The BASS ground team at NASA Glenn Research Center instructed him to load the previously-used Nomex sample after making some adjustments to the igniter. However, we were still unable to get the sample to burn in two additional attempts.


    May 2012 – The BASS experiment burned 3 flat samples on May 10, 2012. The first test was 2 cm wide SIBAL fabric (50% cotton-fiberglass blend) which was burned at a constant flow speed of 20 cm/s. The ignition and flame spread were nominal yielding a long yellow flame lasting approximately 30 seconds. The second test was a 1 cm wide SIBAL fabric which was burned at a constant flow speed of 10 cm/s. The flame was very short and dim blue and burned for about 50 seconds. Don Pettit was able to finish early and agreed to burn one additional sample, a Nomex-blend fabric. We expected ignition would be difficult to achieve, and indeed we were only able to observe a brief ignition flash and the flame could not be established.


    April 2012 – The ISS crewmember Don Pettit operated BASS on April 9, 2012. The first test was with a previously burned 1 cm diameter PMMA (poly methyl meth acrylate) sphere. Ignition was achieved at 5 cm/s air flow speed and the flow speed was held constant. A blue flame appeared and transitioned to a sooty envelope flame. The air flow was turned off to extinguish the flame, and extinction dynamics were observed. In the second test, the flame was again ignited at 5 cm/s and then flow was reduced to 2 cm/s resulting in a very large standoff distance. The flame was extinguished by turning off the air flow. The third test the flame was ignited at a low flow speed of 2 cm/s, and then the flow was turned down. The flame maintained an almost spherical shape in the early stages. After several seconds, the air flow was turned up to maximum (around 25 cm/s), and the flame became long and quite sooty. In about 20 seconds the fuel was completely consumed and the flame went out.


    March 2012 – The BASS combustion experiment started operations on the International Space Station on March 30, 2012 with ISS crewmember Don Pettit. The first BASS samples ignited were a SIBAL flat sample, a 2 cm PMMA sphere and 1 cm PMMA sphere. The BASS experiment hardware was installed in the Microgravity Science Glovebox on March 26, 2012.


    November 2011 – The Burning And Suppression of Solids (BASS) experiment is currently on board the International Space Station and is scheduled to begin operations in the Microgravity Science Glovebox (MSG) in February 2012. The BASS operations will follow the Structure and Liftoff In Combustion Experiment (SLICE). The astronaut Don Pettit is scheduled to operate the BASS experiment during Increment 30-31.

    The BASS experiment launched on Shuttle STS-133/Flight ULF-5 on February 24, 2011.



    Description

    The Burning and Suppression of Solids (BASS) investigation examines the burning and extinction characteristics of a wide variety of fuel samples in microgravity.  The BASS experiment will guide strategies for extinguishing accidental fires in microgravity.  BASS results contribute to the combustion computational models used in the design of fire detection and suppression systems in microgravity and on Earth.



    Research Summary:

    • Burning and Suppression of Solids (BASS) tests the hypothesis that materials in microgravity, with adequate ventilation, burn as well if not better than the same material in normal gravity with other conditions being identical (pressure, oxygen concentration, temperature, etc.).
     

    • There are important differences in the suppression of fires in space compared to on Earth.  On Earth it is understood that the best results are generally obtained when the extinguisher “attacks” the base of the flame, which is both the stabilization point and the point where fresh air first enters the flame.
     

    • For a fire burning in microgravity, the best point of application of suppressant may not be immediately apparent, especially for a partially obstructed flame or a wake-stabilized flame.  Depending on the geometry of the flame and the characteristics of the extinguisher (distance from flame, dispersion angle) it is possible that the suppressant stream will be ineffective or might actually make the flame worse through the entrainment of oxygen.  Using nitrogen as a flame suppressant in microgravity provides a direct link to current and planned extinguishment techniques.


    Detailed Research Description:


    Burning and Suppression of Solids (BASS) utilizes slightly modified Smoke Point In Co-flow Experiment (SPICE) hardware within the Microgravity Science Glovebox (MSG) for observations of burning solid materials on board the ISS.
     
    BASS consists of 41 fuel samples. There are three categories of samples:  flat, solid spheres, and candles within tubes. Thin flat samples (12 cm long by 1 and 2 cm wide) yield concurrent-flow spread rate and limiting flame length. The cotton-fiberglass fabric blend Solid Inflammability Boundary at Low-Speeds (SIBAL) fuel is our principal thin material, and it was specially developed just for this purpose.  Other thin materials are burned including Nomex® and Ultem®. Thick flat samples (5 cm long by 1 cm wide by 1 and 2 mm thick) of Polymethylmethacrylate (PMMA) and wax-saturated fiberglass fabric yield thickness effects on flame spread and extinguishment. Solid spheres of PMMA (1 and 2 cm in diameter) have the advantage of an axisymmetric geometry and permit multiple tests as the flame is extinguished and reignited. Ignition of either the front or back portion of the spheres is achieved. Finally, candles within a thin ceramic tube (6 mm in diameter by 25 mm long) are examined. Two types of wax are used, common paraffin and “Japan wax,” which has a very low soot point. For many of these tests, the nitrogen suppressant system is engaged at a gradually increasing level until extinction is reached.

    The important experimental observations from BASS with respect to the burning process include flame shape and appearance as a function of flow speed, flame spread rate (how fast the flame develops), and flame dynamics (pulsations, oscillations, etc.).  With respect to extinction, the critical observations and data are the time to extinction as a function of fuel geometry, the nitrogen flow rate, and the flame distance from the nozzle.  The dynamics of the flame before extinction are also important for comparison to the modeling work.


    The modeling effort includes:

    • Modeling flame spread over flat samples: For flat samples, the steady spread characteristics can be examined using the three-dimensional model currently available.  Alterations are the new tunnel and sample geometry and the upstream boundary condition.  For the flame growing phase, a transient model is currently being developed.

    • Suppression by nitrogen injection:  This can also be modeled readily using the current model.

    • Modeling burning and extinction of PMMA spheres:  Similar problem on modeling two-dimensional circular PMMA cylinder in cross flow has been performed.  Some changes are needed for the sphere and the duct flow.


Zero-g facility test burning 2-cm-diameter PMMA sphere in 30 cm/s airflow. Left: 1 g; Middle: 0 g, 1 s after drop; right: 0 g, 4 s after application of nitrogen extinguishing agent.
PMMA Sphere


Drop tower results and modeling of a 2-cm-diameter PMMA sphere burning in 17% oxygen, 1 atm. pressure, at 2 cm/s forced convective flow. Experiment images are overlaid with computer simulations, which can capture the flame response as it transitions from 1-g to 0-g. Left-side contours show reaction rate (kgmol/m3/s); right side contours show temperature (K).

PMMA Sphere Results

Brief Research Operations:   

• The crewmember unstows and assembles the BASS hardware on the base plate within the MSG working volume and connects electrical and data harnesses.

• Before each test point, the crew installs and positions fuel samples and igniters.  Tunnel flow speed, nitrogen flow rate, and flame ignition are controlled.

• After ignition and some period of burning, the crew turns on the nitrogen suppressant at an increasing level until the flame goes out.

• The crew controls video and still camera functions.  Some photos are downlinked for near-real time coordination with the investigator. The crewmember uninstalls experiment from the MSG and stows the hardware once BASS is completed.

Operational Requirements:BASS is conducted inside the sealed MSG work volume. The crewmember is involved throughout the experiment to load fuel samples, initiate tests, ignite the fuel, adjust suppression, monitor and record data, exchange fuel samples, and replace the igniter. Forty-one test samples will be burned in a variety of flow conditions for a total of 89 test points.
Data is downlinked via video during or immediately after each flame test. Digital photos are downlinked after selected flame tests for ground confirmation before proceeding. BASS testing session must be conducted during periods when no major reboost or docking procedures are underway on the International Space Station (ISS).

Operational Protocols:
The crewmember installs the BASS hardware in the MSG work volume.  The BASS hardware consists of a small flow duct with an igniter and a small nozzle along with exchangeable fuel samples.  During BASS opeations a fan produces a co-flow of air through the duct.  An anemometer is used to measure the actual flow rate.  The crewmember adjusts the airflow from 5 to 50 cm/s.  The flame is ignited and allowed to burn for about a minute.   A nitrogen suppressant is then supplied via a mass flow controller, from 0 to 500 cc/min.  A radiometer measures flame output. The crewmember conducts each test.  They install the correct fuel assembly and set the air flow rate through the duct before igniting the flame.  When the flame is ignited, the crewmember allows some time for the flame to stabilize then adjusts the flow of nitrogen suppressant through the nozzle until the flame goes out.  After the test, the crewmember turns off the nitrogen flow and prepares for the next test.  The science team on the ground monitors the video downlink to assist the crewmember in determining any peculiar flame behaviors and reviews the sensor data overlaid on the video image.   Upon completion of the tests the crewmember stows the hardware and the stored images and data are returned to Earth for analysis.

Space Applications:
The current NASA spacecraft materials selection is based on a standard test method (NASA–STD–6001 Test 1) that segregates material based on 1-g behavior without consideration of low gravity effects. A critical element of this understanding is the radiative heat emission from the flame. These results are used in first order models and predictions of heat release in spacecraft fires and as a means to extend heat release data from tests like the NASA cone calorimeter test (NASA–STD–6001 Test 2) to a performance-based material selection process. Using nitrogen as a flame suppressant in microgravity provides a direct link to current and planned extinguishment techniques.

Earth Applications:
BASS results provide essential guidance to ground-based microgravity combustion research efforts.  Detailed combustion models are validated using the simpler flow environment afforded by tests in microgravity.  Once validated, they can be used to build more complex combustion models needed to capture the important details of flames burning in normal gravity.  These models have wide applicability to the general understanding of many terrestrial combustion problems.


Project Management:

Principal Investigator: Dr. Paul Ferkul, NCSER GRC

Project Scientist/Co-Investigator: Dr. Sandra Olson, NASA GRC

Co-Investigator: Prof. James T’ien, CWRU

Co-Investigator: Dr. Fumiaki Takahashi, NCSER

Project Manager: Robert Hawersaat, NASA GRC

Engineering Team: ZIN Technologies, Inc.