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?