I just read that when Cassini makes it's first close pass of Saturn's F-ring on December 4, it will be the 183rd time the engine fires. I googled Cassini's engine and found this site which has a cool 3D WebGL(?) model of Cassini's engine(s) that you can rotate with your cursor.
(edit: here's another: https://saturn.jpl.nasa.gov/the-journey/the-spacecraft/)
I show a screen shot below. The accompanying text says:
The main engine is used for spacecraft velocity and trajectory correction changes. To be on the safe side, there are two identical main engines: One is in use and the other is a backup. There are also 16 monopropellant hydrazine thrusters of which eight are prime and eight are backups. The thrusters are used for attitude control and also for small velocity-change maneuvers.
Bipropellant system- Nitrogen Tetroxide (NTO)/Monomethylhydrazine (MMH) Main (445 Newton) engine for propulsive maneuvers – Burns can be blow-down or pressurized (with Helium)
Engines are protected from micrometeoroid particles by articulating cover
Redundant main engine (never used in flight)
Question: What does it actually mean that the main engines' "burns can be blow-down or pressurized (with Helium)?" Are both of these "modes" actually used by Cassini for different purposes? If so, how do they differ in performance?
aboveL Screenshot of this 3D interactive model of Cassini's engines.