Cassini's main engines' burns “can be blow-down or pressurized (with Helium)” - why?

Question

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.

https://saturn.jpl.nasa.gov/engine/

(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.

... and...

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.

Answer 1

According to Sutton, both blow-down and conventional pressurization use a pressurant (like helium in this case).

Conventional systems have a pressure regulator between a pressurant tank and fuel tank, so they yield constant pressure and constant thrust until out of pressurant. In blow-down, per Sutton:

Here the gas is stored under pressure inside the propellant tank together with the propellants. As the gas expands during operation, the tank pressure, the thrust, and chamber pressure decrease steadily down to perhaps 25 to 45% of the initial values. This system is simpler (no separate gas tank and no regulator or gas valve), but the mixture ratio can change slightly during the operation, the total volume occupied by the feed system is larger, and the average specific impulse is somewhat lower.

The thruster itself can operate in either mode simply by virtue of handling a wide range of inlet pressures.

Cassini seems to have been designed to switch over to blowdown pressurization in the event of helium regulator problems; the regulator began leaking in 1997 but redundant valves allowed the system to remain sealed between engine firings. When it could no longer maintain regulated pressure, Cassini switched to blow-down mode:

As this [periapsis raise maneuver of August 2004] was to be the last fully pressurised burn of the primary mission (subsequent firings would be made in 'blowdown' mode, using the residual pressure in the propellant tanks) the latch on the helium flow was left open for 33 minutes after the burn in order to build up the pressure in the propellant tanks, then closed to isolate the system from the leaking outflow regulator.