During the launch of InSight today one thing sprung out to me: at T+408s the voiceover announces:

Centaur slightly fuel rich.

And again at T+495s:

[Centaur] requesting a fuel rich condition at this point of the burn.

  • What are the remedies of a fuel rich mixture and why would one not always use a perfect mixture?
  • Why does it happen at this point in the launch?
  • Why does it happen at this launch, I don't recall ever hearing that in another Atlas/Centaur launches.
  • Why is this important enough to mention on the webcast - there are thousands of conditions and things changing with the rocket, why is this one important enough to mention.
  • 1
    $\begingroup$ for the first bullet, this is related: space.stackexchange.com/questions/22122/… $\endgroup$
    – Hobbes
    May 6, 2018 at 14:35
  • $\begingroup$ I am thinking that this call relates to an abnormally fuel rich state, not a normally fuel rich state as explained in that link. Otherwise, there would have been no need for the call. $\endgroup$ May 6, 2018 at 18:57

1 Answer 1


Centaur upper stages have a Propellant Utilization (PU) system. While this document is old, the fundamentals apply.

To realize optimum performance in a liquid-fueled bipropellant space vehicle, it is necessary to control both propellants so as to deplete them simultaneously. Such a simultaneous depletion both minimizes vehicle burnout weight (by not allowing any unusable amounts of one propellant or the other to remain in the tanks) and maximizes the mission total impulse (by using all available propellant mass in engine reaction).

Two major factors influence simultaneous propellant depletion. The first is accurate calibration of engine mixture ratios, flow rates, and total thrust under flight conditions. The second is the inability to predict the relative propellant masses to be loaded at lift-off. Even if such a prediction were possible, uncertainties in determining what has actually been loaded onboard provide the second large error source. As an example, for the Centaur two- burn vehicle, these errors would result in a maximum error in mass ratio of approximately 350 lb at burnout, resulting in a loss of 350 lb of payload capability from a mission requiring propellant depletion. Clearly then, one way to improve total payload capability is to provide some sort of system for propellant management.

For Centaur the first function of such a system for proper propellant utilization (PU) is to measure accurately the ratio of propellants in the vehicle tanks during the entire powered flight portion of the mission.....

The second basic function of the PU system is to control the flow of propellant through the engines to adapt the ratio of hydrogen and oxygen to the amounts remaining in the tank.

The calls you quote are most likely reports of a slightly* anomalous condition in the PU system. The flights where it wasn't mentioned, likely didn't have this anomaly.

*I say "slightly" because the launch was successful.


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