Keeping Rocket Engine Fuel Lines Bubble Free in Space
Science image from CCF camera during Experiment #1, or EU#1, square groove geometry operations. The free surface, or gas/liquid interface, assumes a curved shape under subcritical flow conditions as its mean curvature adjusts to the pressure drop in the channel.
(Joerg Klatte, Center of Applied Space Technology and Microgravity, or ZARM)
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Astronaut Scott Kelley installing the Capillary Channel Flow, or CCF, in the Microgravity Science Glovebox, or MSG, on board the International Space Station. (NASA)
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You are in space...your spacecraft is tumbling out of control, you need to fire your control rockets, the fuel is sloshing all around the inside of the tank...where is your liquid fuel? Without gravity in the space environment, how do you keep the fuel contained so it can be transported to where it is needed? How do you keep gas bubbles out of the fuel lines?
Being able to use all of the fuel in a spacecraft tank has been an ongoing challenge in spacecraft design for the past 50 years, but great advances on the problem are being made using the International Space Station as a laboratory. In the microgravity of space, the "bottom" of the tank is NOT apparent.
When a spacecraft tank is nearly full, the fuel tends to "cling" to all sides of the tank leaving a small gas bubble, or ullage, near the center of the tank. Once the tank has emptied to the point where there is not enough liquid to cover the walls of the tank, it is not clear where the remaining fluid is "positioned." Here on Earth this is not an issue. For example, in the gasoline tank in your car, gravity always positions the remaining fluid at the bottom of the tank, allowing the car's fuel pump to draw the last bit of fuel from the tank.
"Presently, the low risk solution to this problem is to size the fuel tank larger than what is needed for the mission, but this adds extra launch mass and volume to the spacecraft," states Robert Green at NASA's Glenn Research Center. Another method is to add special channel-like structures, called vanes, inside the tank to purposely "wick" the remaining fuel to the exit. A key area of study is how different shapes of channels work and whether they remove any gas bubbles that can get captured in the flow.
Scientists from Germany and the U.S. have been studying these processes as part of an investigation called Capillary Channel Flow, or CCF. The CCF study looks at several capillary channel geometries that mimic the shape and physical characteristics of vanes in fuel tanks.
One set of capillary channel geometries was developed by Michael E. Dreyer at the Center of Applied Space Technology and Microgravity, or ZARM, at the University of Bremen in Bremen, Germany, and sponsored by the German Aerospace Center, or DLR. The geometries included parallel plates and square-grooves. This part of the investigation was completed in March 2011, after 78 days of nearly continuous ground-controlled operation.
The second set of channel geometries was designed by Mark M. Weislogel at Portland State University in Portland, Ore. Sponsored by NASA, it will begin operation this month. The geometry is a wedge-shaped channel with only one side exposed to the interior of the tank. Weislogel is studying the fluid behavior in the interior corner where the two plates meet. This area forms a wedge-shaped channel geometry, which forces gas bubbles to rise and burst past the liquid surface. This new shape enables the passive separation of gas from liquid.
Watch a video from the CCF camera during Experiment #1, or EU#1.
Every space system that includes a fluid, from drinking water, to radiators, to toilets, can have problems with transport and bubbles. So using the geometry of the channel to remove bubbles can be a real advantage, as Weislogel explained when discussing the importance of studying the wedge shape. "In a spacecraft tank application, if gas bubbles get to the engine, the engine can sputter or stall. If the fuel lines have these wedge-shaped sections, they can expel the gas en route, and the wedge-shaped section takes care of the separation for you," said Weislogel.
The CCF investigation was installed in the Microgravity Science Glovebox, or MSG, a research facility aboard the space station. The MSG facility is designed to accommodate small science and technology experiments in a workbench type environment. The experiment can be controlled from NASA's Glenn Research Center, from Germany, or at Portland State.
"Technologies utilizing capillary flow can be used in applications on Earth," explained Green. "CCF results may potentially be applied for improving fluid flow in miniaturized biological devices used for health screening and analysis -- referred to as lab-on-a-chip."