The Cryogenics & Fluids Branch has developed a practical application the fountain effect, a superfluid helium pump. After testing on the ground, the pump flew in the space shuttle as the Superfluid Helium On Orbit Transfer (SHOOT) project. The thermomechanical pump is basically a fountain effect demostration, except that the helium flow, rather than spouting out through a small hole, goes instead through a tube that leads into another storage dewar. SHOOT flew on the space shuttle mission STS-57 in June, 1993.
The porous plugs used in SHOOT were manufactured by Coors, a company which has been making porous filters for many years. Lately, they have branched out as a manufacturer of dilute ethanol solutions, which they filter by pumping them through their own porous filters.
The thermomechanical pump has advantages over a traditional mechanical pump for pumping superfluid helium in space. A mechanical pump, with moving parts, has a risk of mechanical wear, and thus of breakdown. The risk of breakdown is more of a problem in space, where repairs may be more difficult and costly than on earth.
A mechanical pump also may mix bubbles of helium vapor in with the liquid. On earth, there is not a risk of mixing helium vapor into the liquid because the pump can be set at the bottom of the helium tank, where it will be completely covered by liquid. In zero gravity, however, there is no up or down. Thus, the liquid helium does not pool at the bottom of the tank, and a mechanical pump would have nowhere to sit where it would see only liquid. A thermomechanical pump, since it can pump only superfluid helium, does not mix bubbles of vapor in with the liquid.
There is another system used to pump fluids in space which would not work with superfluid helium. In this system, the liquid is held inside a flexible membrane inside a tank. Pressurized gas is used to compress the flexible membrane, forcing the liquid out. There are at least two problems with using this for superfluid helium. Since helium has the lowest boiling point of any gas, there would be no gas that could be used for this. Also, it would be difficult (if not impossible) to find a material for the membrane that would remain flexible at liquid helium temperature.
SHOOT used two dewars, connected by a tube. There was a thermomechanical pump at each end of the tube, that is inside each liquid helium dewar. This setup is shown below in schematic form.D In the diagram, I've drawn the valves as simple lines pivoted on an axle running through the center of the line. Of course, the real valves were more complicated. Also, I've shown one valve as open and the other as closed, just to illustrate the way I'm indicating closed and open valves in these diagrams.
To pump superfluid helium from the left-hand dewar to the right-hand dewar, the heater of the pump in the left-hand dewar was turned on.D
To pump superfluid helium from the right-hand dewar to the left-hand dewar, the heater of the pump in the right-hand dewar was turned on.D We also have an animated version of the SHOOT schematic diagrams.
For more information (including technical references) check our
The story of liquid helium in space continues in
helilum in space, part 2.