For BigInch, I decided to start a new thread to discuss propellant tanks in zero G. More than he wanted to know, probably:
Propellant management for spacecraft, or the art of keeping gas bubbles out of the engine intakes, is done by a variety of methods. See
1. Pulse a small thruster to generate accelerations, driving bubbles "uphill" and away from fluid line ports. This technique was used for the Apollo spacecraft during restarts in Earth and lunar orbits, and during transits to and from. Pretty much the technique to use for very large propellant tanks where the other methods don't work as well. There are potential problems with this approach, as fluid "slosh" and bubble entrainment potential increase as the liquid level is depleted.
2. Spin the spacecraft to generate centripetal accelerations that keep the bubbles "uphill". Spin-stabilized spacecraft were designed and built for many years by Hughes, but these end up being out-competed by lighter weight, 3-axis stabilized spacecraft (less structure required for 3-axis s/c, especially when higher power levels for more comm channels is required - imagine huge 30m solar arrays whipping around at 5-7 rpm).
3. Keep the fluid and gas seperated by a bladder or rolling diaphragm, or other mechanical means. Used on many spacecraft, but has the disadvantage that fluid and gas can still permeate the diaphragm over longer mission durations. Also, the diaphragm material (elastomers) can degrade over time, contaminating the fuel. Finally, these devices typically weight more than surface-tension tanks of similar volume/pressure capacity.
4. Place mechanical surface tension devices (screens, baffles, vanes) near the tank outlet, to hold the fluid by capillary forces up against the outlet. Varying degrees of success have occurred with this approach. At least one spacecraft I know of had trouble, wherein a very large volume of pressurant gas (helium) was mixed intimately with the fuel in the propellant lines during loading. The resulting two phase flow through the monopropellant thruster catalyst beds (NOT an original design requirement) required a hasty requalification of the thrusters.
5. There is, somewhere in the vast paperspace, a series of papers written on thermal gradient control of propellant tanks. The idea is that surface tension varies slightly with thermal gradients, and that you can move (cause to "swim" even) a bubble of liquid by differentially heating it while suspended in a gas. I think there were studies done to see if simple tank wall gradients could be used for bubble management in a manner similar to #4, above.
Description of many surface tension "tricks", see last page for description of the thermal effect in #5:
Propellant management for spacecraft, or the art of keeping gas bubbles out of the engine intakes, is done by a variety of methods. See
1. Pulse a small thruster to generate accelerations, driving bubbles "uphill" and away from fluid line ports. This technique was used for the Apollo spacecraft during restarts in Earth and lunar orbits, and during transits to and from. Pretty much the technique to use for very large propellant tanks where the other methods don't work as well. There are potential problems with this approach, as fluid "slosh" and bubble entrainment potential increase as the liquid level is depleted.
2. Spin the spacecraft to generate centripetal accelerations that keep the bubbles "uphill". Spin-stabilized spacecraft were designed and built for many years by Hughes, but these end up being out-competed by lighter weight, 3-axis stabilized spacecraft (less structure required for 3-axis s/c, especially when higher power levels for more comm channels is required - imagine huge 30m solar arrays whipping around at 5-7 rpm).
3. Keep the fluid and gas seperated by a bladder or rolling diaphragm, or other mechanical means. Used on many spacecraft, but has the disadvantage that fluid and gas can still permeate the diaphragm over longer mission durations. Also, the diaphragm material (elastomers) can degrade over time, contaminating the fuel. Finally, these devices typically weight more than surface-tension tanks of similar volume/pressure capacity.
4. Place mechanical surface tension devices (screens, baffles, vanes) near the tank outlet, to hold the fluid by capillary forces up against the outlet. Varying degrees of success have occurred with this approach. At least one spacecraft I know of had trouble, wherein a very large volume of pressurant gas (helium) was mixed intimately with the fuel in the propellant lines during loading. The resulting two phase flow through the monopropellant thruster catalyst beds (NOT an original design requirement) required a hasty requalification of the thrusters.
5. There is, somewhere in the vast paperspace, a series of papers written on thermal gradient control of propellant tanks. The idea is that surface tension varies slightly with thermal gradients, and that you can move (cause to "swim" even) a bubble of liquid by differentially heating it while suspended in a gas. I think there were studies done to see if simple tank wall gradients could be used for bubble management in a manner similar to #4, above.
Description of many surface tension "tricks", see last page for description of the thermal effect in #5:
