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The Dawn spacecraft has three ion engines that are mounted such that their thrust vectors are at significant angles. You can see this in this image:

enter image description here

In the image, one of the three engines is active. Two questions:

1) Why three engines? I suspect this is for redundancy, but seems very wasteful. In addition, several other missions use a single propulsion system.

2) Why different thrust vectors? I suspect this has something to do with redundancy and making sure that the thrust vector passes through the center of mass.

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  • $\begingroup$ Does Dawn also use it's ion engines for attitude control? But the 3 thrust vectors look coplanar which would limit their utility for attitude control. $\endgroup$ – HopDavid Jan 22 '15 at 11:31
  • $\begingroup$ @HopDavid Dawn does also have 12 unbalanced configuration RCS thrusters (hydrazine, 0.9 N, 46 kg of propellant total, perturbing delta-V in SC +X, -X and +Z directions only) and now two (out of four) reaction wheels that are still operational. See e.g. Dawn Spacecraft: Do You Have Attitude Control? Related here: How will the ion thruster powered Dawn spacecraft enter orbit around Ceres? $\endgroup$ – TildalWave Jan 22 '15 at 14:00
  • $\begingroup$ From the picture, it looks as though using a different engine would just mean rotating the spacecraft. I'm just guessing here, but I think the fact that it's solar powered means that it would only use one engine at a time. $\endgroup$ – jamesqf Feb 25 '15 at 20:52
  • $\begingroup$ The ion engines must point approximately through the CG of the spacecraft, and have to be gimbaled to be able to point them exactly through the CG when operating. (The CG moves as propellant is expended.) So actually they are all pointed in the same direction, which is through the CG. This, by the way, makes them almost useless for attitude control. $\endgroup$ – Mark Adler May 16 '15 at 16:22
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Each of Dawn's three 30-centimeter-diameter (12- inch) ion thrust units is movable in two axes to allow for migration of the spacecraft's center of mass during the mission. This also allows the attitude control system to use the ion thrusters to help control spacecraft attitude.

The same article writes further to say

Two ion propulsion engines are required to provide enough thruster lifetime to complete the mission, and the third engine serves as a spare.

See more at: http://dawn.jpl.nasa.gov/technology/ion_prop.asp#sthash.IOhMDaGr.dpuf

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    $\begingroup$ But why three engines? $\endgroup$ – Erik Jan 22 '15 at 16:03
  • $\begingroup$ @Erik The article linked above answers that question; I'll update my post with a block quote $\endgroup$ – Everyone Jan 22 '15 at 16:59
  • $\begingroup$ Interesting stuff. I guess something about the ion engine degrades over time or has a low MTBF. $\endgroup$ – Erik Jan 22 '15 at 17:10
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    $\begingroup$ @MarkAdler yeah -- that quote is good and was added after my comment. $\endgroup$ – Erik Jan 22 '15 at 21:43
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    $\begingroup$ @Erik Acceleration grid erosion is unavoidable with ion thrusters. It's predictable, easily measurable through performance degradation, but sadly also certain. For a quick primer on electric propulsion, see e.g. Choueiri, Edgar Y. (2009). New dawn of electric rocket. The Ion Drive (PDF) $\endgroup$ – TildalWave Jan 27 '15 at 2:30

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