Combustion in Space
Combustion in space refers to the process of burning fuel in the unique conditions of a microgravity environment, such as aboard the International Space Station (ISS). Unlike on Earth, where hot gases rise and draw in additional oxygen to sustain combustion, flames in space behave differently due to the lack of gravity and limited oxygen supply. This altered combustion process is studied using the Combustion Integrated Rack (CIR), a specialized chamber on the ISS that enables scientists to conduct experiments on how flames react in microgravity. Research has revealed that flames burn more slowly and at lower temperatures in space, prompting investigations into fire safety, flame behavior, and combustion efficiency.
Key experiments, such as the Flame Extinguishment Experiment (FLEX) and the Italian Combustion Experiment for Green Air (ICE-GA), have provided insights into the complexities of combustion, including the effectiveness of fire suppressants and the properties of various fuels. The outcomes of these studies are significant not only for ensuring astronaut safety but also for advancing fuel technology and fire control methods both in space and on Earth. As researchers continue to explore combustion in space, their findings hold the potential to improve combustion systems across various applications, from spacecraft to everyday engines.
Combustion in Space
FIELDS OF STUDY: Aerospace Engineering; Spacecraft Propulsion; Space Technology
ABSTRACT: Combustion, or the burning of fuel, is essential for generating the thrust needed to move rockets through space. However, it creates risk of explosion and fire. Scientists have learned that combustion in space is different from combustion on Earth. Studying combustion in space helps researchers learn how to make technology safer and more efficient.
Importance of Combustion
Airplanes, jets, rockets, and many cars burn fuel to create the power that allows them to move. This fuel-burning process is called combustion. During the combustion process, a fuel source is combined with an oxidizer, producing a chemical reaction that creates heat. It also generates a substance called exhaust. Exhaust is often in gaseous form, but some combustion products, such as the soot produced by burning wood or candles, can be solid. Because the exhaust is hot, it provides the heat necessary to keep the combustion process going.
Some propulsion systems use staged combustion, where the heat of the first combustion is forced into a second chamber containing more fuel. The exhaust heat from the first combustion ignites this fuel, completing the staged combustion process.
While combustion is crucial to space flight, it also poses the danger of explosion and fire. To find ways to protect astronauts and the vehicles used for space exploration, scientists study combustion both on Earth and in space.
Combustion Studies
Fire burns differently in space than on Earth. This is due to the vacuum of space and the extremely low gravity, referred to as microgravity. On Earth, the hot exhaust gases from combustion rise, pulling in more oxygen to aid the burning process. In space, these gases do not rise, and there is no oxygen to feed the fire. Flames burn more slowly and at lower temperatures.
To study these differences in detail, researchers have equipped the International Space Station (ISS) with the Combustion Integrated Rack (CIR). This hundred-liter combustion chamber contains the hardware and diagnostic equipment needed to perform experiments in space. With it, scientists have learned much about combustion in space, including how flames behave in microgravity, how combustion produces power and exhaust, and what happens to the waste products.
The CIR includes a fluid-oxidizer system to condition the gases and a chromatograph to measure them. Lights, cameras, and other diagnostic equipment are used for recording and analysis. The system is designed for remote operation, either from within the Payload Operations Center on the ISS or from Earth.
The chamber offers a variety of components for different experiments. Scientists can control the gas level, temperature, and position of the chamber. Eight removable optics windows allow them to position the diagnostic equipment for the best results for each experiment. An environmental system removes heat generated during the tests. The CIR also includes a component called the Multi-User Droplet Combustion Apparatus (MDCA), which allows researchers to conduct experiments with single droplets of fuel.
CIR Experiments
Scientists can customize the environment in the chamber for each experiment. One key experiment that used the MDCA insert in the CIR was the Flame Extinguishment Experiment (FLEX). From 2009 to 2013, tests were conducted to determine the effectiveness of various gas-based fire suppressants in space. FLEX researchers studied how fuel vaporizes, how heat is created and dispelled in microgravity, and how different flammable chemicals interact in space. The results of these tests will help researchers develop new ways to control fires in future spacecraft.
The CIR was also the site of the 2013–2014 Italian Combustion Experiment for Green Air (ICE-GA). This experiment focused on the efficiency and rate of evaporation of several types of biofuel. In 2014–2015, the Burning and Suppression of Solids II (BASS-II) experiments used the CIR to test the combustion of plastics, fabrics, and other solid matter.
Additional experiments in the CIR include a second round of FLEX experiments designed to study the behavior of flame when fuel is sprayed (2015) and an MDCA/Cool Flames experiment to look for ways to decrease exhaust emissions in the second stage of staged combustion (2016). In 2017, astronauts installed a new system, Advanced Combustion via Microgravity Experiments (ACME), that has allowed for studies aimed at developing high-efficiency, low-emission fuels. These fuels will affect space travel as well as all devices powered by combustion. A secondary priority is fire prevention, notably in spacecraft.
Applications of CIR Combustion Experiments
The results of the CIR experiments have far-reaching implications not just in space but also on Earth. Scientists believe that the microgravity of space enables them to study properties of fire that may be obscured by the effects of gravity, such as buoyancy. For instance, on Earth, hot gases rise, causing flames to flicker and present an elongated appearance. In space, the minimal gravitational effects and altered atmosphere makes flames burn in a rounder shape, with less flicker and less interference from buoyancy.
Through the FLEX studies, scientists have discovered that even after a flame has been extinguished, small droplets of a relatively unstable, highly flammable, colorless liquid hydrocarbon called heptane sometimes continue to burn. The cause of this phenomenon, which has since been seen with n-octane and decane fuels, remains under investigation. Further research could provide insight into why some fires are more difficult to put out.
The results of the many CIR combustion tests provide insights that will lead to safer spacecraft, improved space suits for astronauts, and new ways to control fire during in-space emergencies. They also have applications on Earth, allowing scientists to develop better ways to prevent, detect, and extinguish fires. Researchers also hope to develop new and greener fuels as well as more efficient combustion systems for everything from jet engines to cars to home furnaces.
PRINCIPAL TERMS
- Combustion Integrated Rack (CIR): a combustion chamber on the International Space Station (ISS) that allows scientists to remotely conduct combustion experiments in the extreme low-gravity conditions of space.
- Flame Extinguishment Experiments (FLEX): a series of experiments conducted in the CIR from 2009 to 2013 that studied the burning of fuel droplets in the microgravity conditions of space and their response to various gases intended to suppress burning.
- gravity: a fundamental force that attracts two or more bodies to each other.
- staged combustion: a chemical process in which fuel is ignited, reacts with oxygen, and begins burning; the resulting heat is forced into a second stage chamber, where it ignites additional fuel and completes the combustion process.
- vacuum: an empty space devoid of air or any other gas; also sometimes defined as the absence of matter, although scientists now believe that even the vacuum of space, where no atomic matter is found, is occupied by dark energy and dark matter.
Bibliography
Benson, Tom, ed. "Combustion." Glenn Research Center. NASA, 12 June 2014. Web. 10 Apr. 2015.
"CIR." Space Flight Systems. NASA, 29 Dec. 2014. Web. 10 Apr. 2015.
Giannone, Mike. "Combustion Continues to Draw Researchers to Space Station." International Space Station. NASA, 17 Jan. 2014. Web. 10 Apr. 2015.
Giannone, Mike. "FLEX-ible Insight into Flame Behavior." International Space Station. NASA, 28 Nov. 2011. Web. 10 Apr. 2015.
Nave, Carl R. "Gravity." HyperPhysics. Georgia State U, n.d. Web. 10 Apr. 2015.
Reckart, Timothy, editor. "ACME." Glenn Research Center, 1 June 2022, www1.grc.nasa.gov/space/iss-research/iss-fcf/cir/acme/. Accessed 14 June 2022.
Stiennon, Patrick J. G., and David M. Hoerr. The Rocket Company. Reston: AIAA. 2005. Print.
"Vacuum." Cosmos: The SAO Encyclopedia of Astronomy. Swinburne U of Technology, n.d. Web. 10 April 2015.
Williams, Forman A. "Flame Extinguishment Experiment (FLEX)." International Space Station. NASA, 21 Oct. 2014. Web. 10 Apr. 2015.