Fire is deadly and unpredictable on Earth; in an enclosed space vehicle in orbit, its presence takes on even more serious implications. To prevent the threat of smoke and fire on missions, NASA conducted the Smoke Aerosol Measurement Experiment (SAME), designed to create devices that will adequately detect combustion in a zero-gravity environment. Heading SAME is NASA researcher Bill Sheredy.
NASA Tech Briefs:Describe SAME and its purpose.
Bill Sheredy: It's an on-going microgravity investigation, which is designed and built for operation aboard the International Space Station. It was designed and developed by NASA at Glenn Research Center, along with ZIN Technologies, the National Center for Space Exploration Research, and the National Institute for Standards and Technology. Right now, SAME is manifested to launch aboard STS 118, or shuttle flight 13A.1, and it will be assembled and operated in the Microgravity Science Box, or MSG. It is a follow-on to the Comparative Soot Diagnostics (CSD) experiment, which flew on STS 75; its objective is to improve the reliability of future spacecraft smoke detectors by making measurements of the smoke particulate size distributions of common spacecraft materials when they burn.
The result from this experiment will enable the rational design of smoke detectors for NASA's future exploration missions. To that end, when the SAME experiment generates smoke particulates, three diagnostic measurements are taken to determine the particle size distribution for each of these smokes. The instruments used to obtain these measurements are industry standard for monitoring atmospheric particulates, but they have been modified for use in microgravity. Response data from the ISS and shuttle smoke detectors is also gathered for these sample smokes. The results of SAME provide statistics of the smoke particulate size distributions for a range of conditions and measurement of a readily modeled reference for validation of smoke growth models. The experiment design and practical applications of the data is enhanced by the development of a numerical code to predict the smoke droplet growth as a function of the fuel pyrolysis rate - the chemical change brought about by the action of heat - the thermodynamic properties of pyrolysis vapor, and the flow environment. SAME also has the capability to evaluate other fire detection/particulate devices for the test materials at NASA's request.
NTB: Why is SAME being conducted?
Sheredy: The CSD experiment found a significant difference in the smoke detector response for overheated spacecraft materials in one-g versus microgravity. The SAME experiment will enhance the CSD with additional diagnostics to better quantify the characteristics of the low-gravity smoke. The particulate pre-fire signatures obtained from this experiment will be used to design fire detectors for future space exploration systems - a pre-fire state, for clarification, is one in which a material becomes overheated and it begins to produce smoke, but it hasn't reached the state of steady burning. The smoke itself could consist of liquid droplets or solid particulates depending on the source material; to the layperson, these two smokes look essentially the same.
NTB: How does microgravity fire and smoke differ from terrestrial?
Sheredy: When smoke is generated under microgravity conditions, because of the absence of buoyant acceleration and the resulting high concentration of smoke in the generation region, it's logical that low-gravity smoke particulate size distributions could be significantly different from smoke produced in one-g.
In the absence of low-gravity data, the current spacecraft smoke detectors that are used are based on one-g experience. The most important finding of the CSD experiment was that for liquid aerosol smokes, such as those that are produced when paper, silicon, or rubber smolder, the microgravity performance of the space shuttle smoke detector was substantially reduced than that in normal gravity. It is hypothesized that this performance difference was due to extended growth of the liquid smoke particulate in low gravity due to the enhanced residence time in high smoke concentration regions.
NTB: How would the tools developed in SAME be different from terrestrial smoke detectors?
Sheredy: Most of the diagnostics we use for SAME are not smoke detectors per se, but rather, they are instruments that measure specific characteristics of the smoke that we need to determine the particle size distributions. Although flying instruments that provide complete measurements of the particle size distributions would be desirable, and such instruments are available, this would require either extensive up- and down-mass or instruments that are too large to be implemented in a space experiment such as this.
Our approach is to make integrated measurements of the particle distributions using simpler diagnostics systems. Two of the three instruments we're flying are commercial, off-the-shelf (COTS) devices that have been repackaged and modified to operate properly in microgravity. They provide the number and mass concentration measurements of the smoke particulate. The third diagnostic utilized the sensor from a residential smoke detector, the same type that most of us have in our home, and it provided the additional measurement that is required. We also have developed and will be using a custom device called the Thermal Precipitator, and this device will capture physical samples of the smokes on a small grid, and they can be analyzed using a transition electron microscope once these devices are returned to the ground after the experiment is done. This will allow the science team to study the shape and morphology of the smoke particulates that are generated from our samples.
Essentially, you can divide the diagnostics for SAME into three groups. Three of the instruments, the P-Trak, the DustTrak, and the modified "First Alert" smoke detector, are the instruments that will gather the moment measurement data we need to determine the particle size distributions. That's one class of instruments. The second, the Thermal Precipitator that we are also flying, will actually collect physical samples of the smoke that the science team can then use to look at their size and morphologies, and then the third class of instruments would be the ISS and shuttle smoke detectors. We'll be characterizing their response under microgravity conditions. In all, we're talking about six different diagnostic instruments that are being flown as a part of SAME.
NTB: What kind of fire and smoke detection systems are currently used in the shuttles and ISS?
Sheredy: Nothing from SAME is in use right now. The shuttle smoke detector is an ionization detector, and the ISS detector in the US segment is an optical device. They have particle sensitivities that are almost mutually exclusive. Their designs were based on the best available data at the time, but that came from one-g data and experience. It's worth noting that SAME does include both a shuttle detector and an ISS smoke detector, along with its other diagnostics. Conducting tests with these smoke detectors under such controlled conditions will provide direct evidence of the "detectability" of microgravity smoke particulates, and it will be helpful in interpreting the signals from the ISS and STS systems during the remainder of their operational lives.
NTB: How could SAME-derived technology be applied on Earth?
Sheredy: SAME itself is designed for developing detectors for zero-g, space-based applications and NASA's exploration missions, so there is no direct terrestrial application. I would like to mention, however, that as part of NASA's Fire Prevention, Detection, and Suppression Program, NASA is developing smoke sensors for terrestrial applications. They could be based on either particle detection technologies or chemical detection technologies, but they would be a personal smoke detector per se. They are small in size, they will be robust, and they can be used by first-responders in emergency situations.
Two of the key diagnostics for SAME, the P-Trak and the DustTrak, are already commercially available aerosol measurement devices that are normally used on the ground in a one-g environment, such as for determining indoor air quality. They have been modified and repackaged for use with SAME and in the case of the P-Trak, one of the modifications was meant to improve its microgravity performance. Another key diagnostic, the ionization detector, is essentially a modified and repackaged "First Alert" smoke detector, literally the same device the you have in your home, but modified, so obviously it is normally meant for one-g applications. This device should not be confused with the space shuttle smoke detector that we are flying, which is also an ionization type detector.
But, to get to the heart of the question as I understand it, the anticipated results from SAME and the technologies that we have developed and modified for this experiment do not really have a direct or indirect terrestrial application. I would hope that someone who reads this will understand that the results from SAME will lead to the development of better, more advanced smoke detectors for use in NASA's exploration mission. This will result in increased safety of the astronaut crew and a higher probability of success for future microgravity missions.
NTB: What was the Dust and Aerosol Measurement Feasibility Test (DAFT) done as a part of SAME?
Sheredy: One of the devices we are flying as part of SAME is a commercial device called the P-Trak. It's made by a company called TSI, and it can count very small, individual particles in an aerosol stream. Early on, it was the only device we identified that would meet our needs for the experiment. But we had some concerns about its potential operation in microgravity because it relies upon the internal recirculation and flow is isopropyl alcohol, and as it's designed to rely upon gravity for its proper operation. After some consultation, with some of our fluid physics experts here at Glenn Research Center, we made some modification for the design of the device. We developed and flew DAFT as a risk mitigation experiment to demonstrate that the P-Trak would work properly in microgravity when we used it for SAME.
We designed and built the DAFT experiment, it went up to the ISS on two launches, the Russian Progress flight 16-P and on STS 121 this past July, it was operated by astronaut Leroy Chiao and Jeff Williams, during two successive periods of performance, and the results from DAFT demonstrated that, indeed, the P-Trak does operate normally in microgravity. Now that we know that, we'll feel more comfortable using it as one of our primary diagnostics for SAME.