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The extremely cool NASA JPL video Triumph at Saturn (Part I) is really worth a watch and/or listen.

At about 34:32 it covers the period just before SOI (Saturn Orbit Insertion, July 1, 2004, 02:48 UTC) when the fear of the engine not starting or burning long enough was maximum.

Earl Maize, Cassini-Huygens Deputy Program Manager (2003-2013) says

first images started coming in of Saturn's rings. In a JPL press conference later the same day, Carolyn Porco, Cassini Imaging Team Lead says:

Something we've been preparing for four or five years, testing and testing, awfulizing all of the possible things that could go wrong. And this was one of those moments when you're either in orbit, or you're a billion dollar flyby.

During this voiceover, there is a cut to a projection of a slide showing "Cassini Main Engine Operating Box; Wide-Open PR1 (initial dm/dt = 3.34 g/s) w/ Optimal OP-2 Threshold ".

It shows a plot of oxidizer tank pressure versus fuel tank pressure. There are nested six-sided "boxes" (labeled "qual operating box" and "flight operating box") and a lot fo annotations.

Question: How to read this Cassini Main Engine Operating Box (oxidizer vs fuel tank pressures)?

What do the limits of each box mean, why are they six-sided, and (if possible) what do some of the other annotations indicate?

screenshot from the extremely cool NASA JPL video "Triumph at Saturn (Part I)"

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    $\begingroup$ First, thanks for posting this photo. I found this thread by googling for title after I paused on this same shot in the documentary and wanted to know more about this. More googling leads to this excellent article by veteran space reporter Bill Harwood (he was already a veteran in 2004 :) ): spaceflightnow.com/cassini/040612soi.html It talks about how the two engines (REAs) are a redundant pair and the logic that goes into making sure that SOMETHING hits the brakes on the spacecraft. A close reading of this will probably make the chart clearer. And there's this, which should also i $\endgroup$
    – Chris C.
    Oct 29 '21 at 4:50
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    $\begingroup$ (missing part of Chris C. comment, which was converted from an answer: And there's this, which should also illuminate, but should also put your stomach into knots: nasa.gov/pdf/…) $\endgroup$
    – gerrit
    Oct 29 '21 at 13:58
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Partial answer covering only

what do some of the other annotations indicate?

  • LV10 = Primary high-pressure latch valve: opened to provide nominal path for propellant tank pressurization - see yellow arrow in 2nd drawing
  • PR1 = prime pressure regulator for propellant tank pressurization - see yellow arrow in 2nd drawing
  • REA-A & B = Rocket Engine Assembly A & B, the main engines - bottom of first drawing
  • SOI = Saturn Orbit Insertion

annotated exterior schematic of Cassini showing the rocket engines

propulsion system schematic

Source: Final Cassini Propulsion System In-Flight Characterization

The paper also contains a more detailed drawing of the "flight operating box" from the slide in the question. I suspect the full answer to the question can be found in this paper and in Initial Cassini Propulsion System In-Flight Characterization, but I have to go to bed.

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Thanks to @Organic Marble's first "archeological clearing and digging", in particular for providing links to the two papers by Todd Barber, we can now try to get to the global picture.

Let’s rewatch part of the video related to this sequence. At 30:40 in the video, Charles Elaghi recounted how the mishaps of another probe, Mars Observer, were on everybody’s mind. The cause of that failure was identified as a valve malfunction during the pressurization of the bipropellant tanks. Then we were told by the Deputy Program Manager, Earl Maize , how they did numerous tests, « awfulizing all the possible things that could go wrong ». Just after that, we have the screenshot showing the diagram of interest.

It is logical to deduce that its purpose is to illustrate the « awfulizing » methodology. We can interpret the polygons (« boxes ») as the safe areas, one for the purpose of qualification (« the QUAL OPERATING BOX »), which is naturally less conservative than the inner « FLIGHT OPERATING BOX ». Note that the « SOI start » and « SOI end » operating points are inside this smaller box. The other operating points seem to be a hypothetical « out-of-spec » scenario and the corresponding sequence for automatic fault protection and recovery actions by Cassini. That is, if by bad luck they had a malfunction of the type encountered by Mars Observer, they would have a solution for Cassini to recover from such.

Note the « OP-2 trip @324 psia ». « OP » is for Over-Pressure. We know this thanks to a sentence in the Critical Events Readiness Review, pointed to in the comments by @Chis C and @gerrit. On page 5 of the said document, Bullet 3 reads:

A waiver may be required if the overpressure threshold OP-2 is raised above the qualification level of the propulsion system.

So they had put in a safety « fuze » to safeguard against tanks overpressure. It seems plausible that there is a sequence of valves opening/closing as well as engines firing/shutting down to bring the pressure points to within the prescribed safety inner box, and not miss the narrow SOI time window.

Somebody with in-depth knowledge of how bipropellant rockets work may complement the interpretations above.

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    $\begingroup$ I'm pretty sure you are right that the chart in the question shows an off-nominal situation. I was unable to confirm that OP stood for overpressure from the papers, although I suspect that it does. The second paper I linked goes into some detail on the "flight operating box" and the first paper shows the history of the probe's burns plotted on the same box. $\endgroup$ Oct 30 '21 at 17:14
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    $\begingroup$ @Organic Marble, and also, the caption in red, on the upper left corner, seems to identify the boundary of the particular off-nominal scenario, among a very large number of off-nominal scenarios for SOI they probably have investigated (and devised accordingly the autonomous fault-detection and recovery sequence for remedy). $\endgroup$
    – Ng Ph
    Oct 30 '21 at 21:05
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    $\begingroup$ @Organic Marble, have found where I had read that "OP" stands for overpressure. Made the Edit accordingly. $\endgroup$
    – Ng Ph
    Oct 31 '21 at 9:16

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