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Topic Name: New Rocket Engine Simulator and Imaging Techniques that can Explain Rocket Mystery Developed at Georgia Institute

Category: Geo sciences & technology

Research persons: Ben Zinn, the David S. Lewis Jr. Chair and Regents

Location: Georgia Institute of Technology, United States

Details

There’s a strange wave phenomenon that’s plagued rocket scientists for years, a lurking threat with the power to destroy an engine at almost any time. For decades, scientists have had a limited understanding of how or why it happens because they could not replicate or investigate the problem under controlled laboratory conditions.

Scientists generally believe that these powerful and unstable sound waves, created by energy supplied by the combustion process, were the cause of rocket failures in several U.S. and Russian rockets. Scientists have also observed these mysterious oscillations in other propulsion and power-generating systems such as missiles and gas turbines.

Now, researchers at the Georgia Institute of Technology have developed a liquid rocket engine simulator and imaging techniques that can help demystify the cause of these explosive sound waves and bring scientists a little closer to being able to understand and prevent them. The Georgia Tech research team was able to clearly demonstrate that the phenomenon manifests itself in the form of spinning acoustic waves that gain destructive power as they rotate around the rocket’s combustion chamber.

“This is a very troublesome phenomenon in rockets,” said Ben Zinn, the David S. Lewis Jr. Chair and Regents’ Professor in the Guggenheim School of Aerospace Engineering at Georgia Tech. “These spinning acoustic oscillations destroy engines without anyone fully understanding how these waves are formed. Visualizing this phenomenon brings us a step closer to understanding it.”

The research was presented at the 2008 American Institute of Aeronautics and Astronautics (AIAA) Aerospace Sciences Meeting in Reno, Nevada, and funded by the Air Force Office of Scientific Research.

During past investigations into this damaging instability, scientists were able to observe initial stages of the problem but were forced to shut down engines before the waves could fully develop and cause serious damage to the engine. Researchers were also hindered by their inability to clearly observe the complex processes inside the investigated rocket engines.

But with a great deal of help from Dr. Oleksandr Bibik, a visiting physicist and research scientist from Ukraine, the Georgia Tech research team developed an experimental setup and imaging technique that provides detailed information on how these waves form and behave — without blowing up an engine or endangering lives.

First, the researchers developed a low-pressure combustor that serves as a true simulator of larger rocket engines. Bibik then used a very-high-speed camera in combination with series of fiber optic probes that together allowed researchers to clearly observe the formation and behavior of excited spinning sound waves within the engine. Additionally, Bibik’s new imaging method enabled researchers to determine the conditions under which these waves are excited and how they can be controlled.

Bibik’s method uses a high-speed camera to view the reaction zone via a system of filters that allow only specific light radiation generated in the combustion zone to reach the camera’s lens. This strategy eliminates all background light interference and provides clear images of combustion (and sound) waves spinning around the engine’s periphery. Simultaneously, strategically placed fiber optic probes collect information on the reaction process oscillations in various locations in the combustor.

Using these new techniques, the research team discovered that the destructive waves gained energy as they spun around the engine’s periphery at a rate of 5,000 revolutions per second.

The capability to simulate and observe these destructive oscillations in a controlled laboratory environment could help researchers develop techniques to prevent their occurrence in real engines.

“Better understanding this phenomenon could very likely lead to safer tactical and space missions and save billions of dollars for technologies that use combustors,” Zinn said.

Note for Optical Fiber
An optical fiber (or fibre) is a glass or plastic fiber designed to guide light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers. Optical fibers are widely used in fiber-optic communication, which permits transmission over longer distances and at higher data rates than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less loss, and they are immune to electromagnetic interference. Optical fibers are also used to form sensors, and in a variety of other applications.

Light is kept in the "core" of the optical fiber by total internal reflection. This causes the fiber to act as a waveguide. Fibers which support many propagation paths or transverse modes are called multimode fibers (MMF). Fibers which support only a single mode are called singlemode fibers (SMF). Multimode fibers generally have a large-diameter core, and are used for short-distance communication links or for applications where high power must be transmitted. Singlemode fibers are used for most communication links longer than 200 meters.

Joining lengths of optical fiber is more complex than joining electrical wire or cable. The ends of the fibers must be carefully cleaved, and then spliced together either mechanically or by fusing them together with an electric arc. Special connectors are used to make removable connections.

Optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. It is especially advantageous for long-distance communications, because light propagates through the fiber with little attenuation compared to electrical cables. This allows long distances to be spanned with few repeaters. Additionally, the light signals propagating in the fiber can be modulated at rates as high as 40 Gb/s, and each fiber can carry many independent channels, each by a different wavelength of light (wavelength-division multiplexing). Over short distances, such as networking within a building, fiber saves space in cable ducts because a single fiber can carry much more data than a single electrical cable. Fiber is also immune to electrical interference, which prevents cross-talk between signals in different cables and pickup of environmental noise. Also, wiretapping is more difficult compared to electrical connections, and there are concentric dual core fibers that are said to be tap-proof. Because they are non-electrical, fiber cables can bridge very high electrical potential differences and can be used in environments where explosive fumes are present, without danger of ignition.

Although fibers can be made out of transparent plastic, glass, or a combination of the two, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical attenuation. Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances (up to 500 m), and single-mode fiber used for longer distance links. Because of the tighter tolerances required to couple light into and between single-mode fibers (core diameter about 10 micrometers), single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.

In figure 1, An image of destructive acoustic waves building inside a small, simulated rocket combustor.

In figure 2, A video of a small combustor subjected to the acoustic oscillations that have destroyed rockets and other types of engines.

In figure 3, A video of very high speed acoustic waves building inside a small, simulated rocket combustor.