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.