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I'm starting to see a pattern. Both this answer about Cassini liquid propellant engine and this answer about ion thrusters discuss a particular necessity for gimbaling engines to keep thrust aligned with the spacecraft center of mass (on average).

While thrusters could be used (in at least some cases) to correct for mis-aligned main engine thrust, it's a waste of propellant compared to simply pointing the engine's thrust in a direction that doesn't produce a torque on the spacecraft to begin with.

Is it pretty much a given that all (or at least practically all) deep-space spacecraft have gimbaled engines? Or have some used vectored thrust, or found other solutions?

edit: to clarify, I'm asking about actual existing deep-space spacecraft, not hypotheticals or future plans or ideas. If it's still on the ground but almost finished and funded for launch, that's close enough to being an existing spacecraft for this question.

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The Mars Reconnaissance Orbiter and the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft have six non-gimbaled main engines, relying on smaller attitude control thrusters to counter any torques.

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  • $\begingroup$ It seems five is the minimum to accomodate one failure, but six does so much more effectively. Thanks for putting this to rest. $\endgroup$ – uhoh Nov 30 '16 at 6:49
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For deep space craft, another alternative to a gimbaled main engine is to use differential thrust with multiple engines; four small thrusters in square or diamond layout gives straightforward two-axis steering by firing them in different duty cycles (or throttle levels). If no large-delta-v maneuvers are needed (e.g. the spacecraft is performing flyby rather than orbital insertion) this can be done with the attitude control thrusters and there might be no "main engines" at all.

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  • $\begingroup$ I didn't ask if future craft would have to have gimbaled engines, I've asked if it's a given that deep space craft have gimbaled engines. I will add the words "actual, existing" somehow, but I didn't mean hypothetical or future craft, I mean ones out there in space now. $\endgroup$ – uhoh Nov 30 '16 at 0:39
  • $\begingroup$ Having 4 thrusters is quite common for satellites. $\endgroup$ – Antzi Nov 30 '16 at 1:00
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    $\begingroup$ @uhoh deep space spacecrafts are satellites too, and they also use such kind of arrangements. $\endgroup$ – Antzi Nov 30 '16 at 2:25
  • $\begingroup$ @Antzi are you sure? I thought the use of "satellite" was a shortening of "artificial satellite" (to distinguish from moons which are indeed satellites) and we do not usually talk about satellites of the sun or satellites of the galactic center. I don't think deep space spacecraft (while in deep space) are (correctly) considered satellites. Anyway, if you know of a deep-space spacecraft with such kind of arangement please post it specifically as an answer, including a link to the description of the multiple, non-gimbaled engines. $\endgroup$ – uhoh Nov 30 '16 at 2:39
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    $\begingroup$ For exemple, Rosetta seems to be such a space craft. Altho I couldn't find any evidence that it had a gimbal system, I also couldn't find any evidence that it does not. $\endgroup$ – Antzi Nov 30 '16 at 2:42
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One example of a deep-space spacecraft that uses multiple fixed thrusters pointed in different directions instead of a gimbaled engine is New Horizons.

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  • $\begingroup$ The link does not describe vectored thrust, or indeed any mechanism that aligns engine thrust with center-of-mass. $\endgroup$ – Nathan Tuggy Dec 30 '16 at 0:07
  • $\begingroup$ in aerospace engineering, "vectored thrust" refers to a jet engine that can direct its thrust by changing the shape of its nozzle. IIRC, all of New Horizons' thrusters have a fixed attitude (no moving parts). I've never heard them described as 'vectored thrusters'. $\endgroup$ – Hobbes Dec 30 '16 at 8:58
  • $\begingroup$ I've edited my answer to avoid use of the term "vectored thrust", which I didn't know applied only to engines that change nozzle shape. I believe it is now accurate. $\endgroup$ – Katie Bechtold Jan 19 '17 at 22:02

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