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The recent discussions about the SpaceX SuperDraco thrusters got me to thinking:

When you restart a turbopump rocket engine in zero-g, you have to perform a "settling" or "ullage" burn to get the propellant that's floating around in the tanks to settle down near the pump inlets, or the pumps ingest the tank pressurization gas and bad things happen. Typically, you do this by firing your RCS thrusters just enough to put some force on the tank, causing liquid to go to the "bottom" of the tank and the pressurization gas to be buoyed to the "top".

But here's where I got confused: Why don't the RCS thrusters themselves need settling? If they're cold gas thrusters, there's no problem, but why don't hypergolic thrusters have the same problem as turbopumped engines? How does this problem get solved?

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    $\begingroup$ See space.stackexchange.com/questions/2446/… $\endgroup$ – Hobbes May 3 '16 at 19:09
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    $\begingroup$ On the US space shuttle orbiters, there were devices in the RCS tanks to trap a certain amount of propellant at the tank outlet, enough to get you going. $\endgroup$ – Organic Marble May 3 '16 at 23:04
  • $\begingroup$ That makes sense. It especially makes sense since you have to have multi-axis settling if you're clustering the thrusters in groups of four. Thanks. $\endgroup$ – TheRadicalModerate May 4 '16 at 4:45
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The question is spot on, the same problem of getting liquid propellant to the exit feed of a propellant tank applies to all liquid propulsion systems. This applies equally to RCS thrusters for settling for a main launch vehicle stage engine and any type of satellite borne propulsion, large or small.

Design solutions that I am aware of:

  1. An elastomeric diaphragm separating the liquid propellant and gaseous pressurant. A common choice has been silicon rubber, which works at least for N2H4, hydrazine, though there were concerns in the 1980's of the silicon leeching into the propellant and poisoning the catalysts of mono-propellant thrusters. As far as I am aware it is not compatible with N2O4 or MON.
  2. Surface tension devices in the tank that provide a continual presence of liquid covering the exit. There are lots of interesting designs including channels that run from the top of the tank to the bottom and a "sponge" or other trap to guarantee propellant supply even under adverse accelerations. The nature of the surface tension devices have to be very carefully thought out in conjunction with the mass of the satellite and the thrusters and expected manoeuvre types.
  3. I understand that the Soyuz spacecraft may use a metal diaphragm. I imagine there is quite a weight penalty compared to the rubber type and also that the expulsion efficiency could be rather poorer. On the plus side with careful materials choice it could be long term compatible with N204/MON and N2H4/MMH would seem to grant much more freedom for arbitrary manoeuvres than a surface tension device.
  4. I believe some satellites continue to use settling thrusters, I've heard of ion thrusters being used in this way to initiate a good propellant supply for a larger chemical thruster.

All these designs carry a mass penalty, use-ability limitations for manoeuvres or expulsion efficiency limits and so there has to be a design trade-off for each mission.

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