I've just read that when Cassini makes its first close pass of Saturn's F-ring on December 4, it will be the 183rd engine burn which sounds like a lot for orbital maneuvers using a spacecraft's main engine.

This is not only excellent engine reliability, it's great from an orbital maneuver perspective as well. Each maneuver requires careful measurements, calculation, simulation, generation of instruction sets, execution, and follow-up measurements. That's a lot of work for the humans!

So has Cassini's long series of orbit maneuvers using repeated restarts of the main engine some kind of record for the engine?

In the New York Times article How Cassini Will Begin Its Date With Death on Saturn there is a cool 3D rotating GIF of the orbit.

Below is an unattractive 3D plot of the Horizons data, the NYT article and associated GIF are easier on the eyes, but too big to include here.

enter image description here

  • $\begingroup$ I recall hearing that they fire each engine for at least a small burst periodically to ensure it is working properly. I'm sure the backup engine has been used several times in such a capacity. $\endgroup$
    – PearsonArtPhoto
    Commented Nov 29, 2016 at 13:10
  • $\begingroup$ @PearsonArtPhoto So they say here "Redundant main engine (never used in flight)" as I've quoted in this question. Are all sources correct but "used" actually means "tested but not used" in your context, or is there conflicting information here? $\endgroup$
    – uhoh
    Commented Nov 29, 2016 at 13:15
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    $\begingroup$ More like maintained, but not used for major operations. I'm sure they fire it for a few seconds every year or so at least to make sure everything is still in proper working order. I can't find a source on that, but I remember seeing such claims in the past... $\endgroup$
    – PearsonArtPhoto
    Commented Nov 29, 2016 at 13:28
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    $\begingroup$ The smaller the engine, the more starts it's qualified for. I recently came across a datasheet for a 1N engine designed for 1 million starts. Any record would have to be limited to e.g. 'this type of engine' or 'engines in this thrust class'. $\endgroup$
    – Hobbes
    Commented Sep 14, 2017 at 16:22

1 Answer 1


Hard to say.

Cassini's "main engine" is a modernized version of the attitude thrusters used on the Apollo spacecraft: MMH/NTO bipropellant thrusters delivering about 440N thrust. Every maneuver made by the Apollo CSM or LM would have used several of these thrusters in short bursts. It would not surprise me if that amounted to several hundred fires per chamber. The modern version of these thrusters are rated for ~20000 pulses or 12000 seconds continuous fire.

  • $\begingroup$ While a "typical burn" might be a lot longer than a "typical pulse" they are both engine starts. It sounds like if this were a record, it would have to be narrowly defined and so not a remarkable one - except that anything that works so well in space is pretty remarkable! $\endgroup$
    – uhoh
    Commented Nov 29, 2016 at 0:34
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    $\begingroup$ But in this case a "start" just needs a valve to open on command; electrically operated valves doing their job hundreds or thousands of times aren't remarkable. $\endgroup$ Commented Nov 29, 2016 at 0:43
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    $\begingroup$ Unless you are a helium valve on Juno. $\endgroup$
    – uhoh
    Commented Nov 29, 2016 at 0:49
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    $\begingroup$ Proves the point: it's remarkable because it's not working. $\endgroup$ Commented Nov 29, 2016 at 2:11
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    $\begingroup$ In your current edit, the emphasis in the first paragraphs is on the engine, and only the latter part on the burn role, so you might consider reworking it to be clear from the start -- some people barely read past the title, let alone to the last graf. ;) I'll let my answer stand as a partial. $\endgroup$ Commented Nov 29, 2016 at 14:10

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