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I recently stumbled upon the question Why did New Horizons have to be spin-balanced to grams-level precision? (With quarters!) and the accepted answer written by an authoritative source (the guy who created & performed the test).

I am puzzled by this statement:

However most spacecraft (New Horizons and Ladee included) spin about their minimum axis of inertia

I take "minimum axis of inertia" to mean the principal axis with the lowest moment of inertia. However, looking at the configuration of the New Horizons spacecraft, I intuitively (if moments of inertia can be intuitive) expect the spin axis (Y) to be the maximum axis of inertia:

New Horizons

  1. The New Horizons Spacecraft, Fountain et al. https://www.boulder.swri.edu/pkb/ssr/ssr-fountain.pdf

This is confirmed by Guerra et al.[2] which states:

The nominal spin rate of New Horizons is ω0 = 5 rpm, and the location of the centre of mass is the origin of the reference frame [...]. In this reference frame the principal moments of inertia are A = IXX = 161.38 kg/m2, B = IYY = 402.12 kg/m2, C = IZZ = 316 kg/m2 [19].

Where reference 19 is a private communication with "G. H. Fountain" (author of previously sourced "The New Horizons Spacecraft").

Is the comment made in the accepted answer wrong? Am I misunderstanding what minimum axis of inertia means?

  1. Estimating the thermally induced acceleration of the New Horizons spacecraft, Guerra et al. (2017) (archived)
BrendanLuke15
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    Minimum/maximum perhaps related to the https://en.wikipedia.org/wiki/Poinsot%27s_ellipsoid representation ? And you sure don't want the tennis racket theorem coming in to play so don't rotate around z... – Jon Custer Nov 02 '21 at 19:27
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    @BrendanLuke15 Two other possible explanations, both wild guesses really: i) the diagram could be incorrect (though it looks plausible to me) or ii) there are different mission phases with the rotation about different axes. You could add a link this question from jon harrison's answer to see what he has to contribute. – Puffin Nov 02 '21 at 21:20
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    @Puffin generally a good idea, but it seems they are a "one and done" kind of user as all of their reputation is from that one single answer. – BrendanLuke15 Nov 02 '21 at 21:55
  • This is not my comfort area, but I note that in Harrison's sentence you quoted, there is the mentioning of "most spacecrafts" and more importantly the "LADEE included". For LADEE, the spin-axis is quite obvious. If you can find a document giving the 3 inertial moments of LADEE along the principal axes, then hopefully your problem is solved. – Ng Ph Nov 02 '21 at 23:06
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    I'm a co-author of Guerra et al. and IIRC, we had to contact G. H. Fountain because we couldn't find New Horizons' moments of inertia anywhere. I believe the most probable explanation is that he was thinking about the semi-axis of the ellipsoid, as referred by @Jon Custer, and misspoke – Paulo Gil Nov 04 '21 at 09:27
  • @Paulo Gil, independently of the correctness of Harrison's statement for New Horizons, what should we "take home" conercerning his statement "Most spacecrafts spin about their minimum axis of inertia". True or false? – Ng Ph Nov 05 '21 at 21:14
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    @Ng Ph, I don't have a statistic but would say false. It's not a good idea because vibrations in antennas and other flexible parts make the S/C to loose kinetic energy of rotation and the axis of minimum moment of inertia becomes unstable. This is well known since Explorer-1. Rigid antennas help, but I would say it's always a liability. So, unless you can't avoid it it's better to use the axis of the largest moment of inertia. – Paulo Gil Nov 06 '21 at 23:00
  • @Paulo Gil, I don't have statistics neither. But perhaps Cassini is the exception that confirms the rule? On may be I am wrong in interpreting that Cassini spins along the z-axis(?) Hope somebody can clear this too ... – Ng Ph Nov 07 '21 at 18:53
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    @NgPh Cassini was 3-axis stabilized (i.e., spinning stability does not apply) – BrendanLuke15 Nov 07 '21 at 18:59
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    @BrendanLuke15, ah right! My bad. – Ng Ph Nov 07 '21 at 19:19
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    @NgPh, you mean New Horizons? I don't think so. We modeled the shape of the S/C and it would be very unlikely that it would be an exception. Also, for deep space I don't know any good reason to use the minor axis as rotation axis. I open the possibility for some kind of odd need, but it generally is a bad idea. In Explorer series they solve the problem by making the antennas rigid, but it was to solve the previous design flaw. – Paulo Gil Nov 08 '21 at 21:14
  • @Paulo Gil, I meant Cassini (because I interpreted wrongly that it is spin-stabilized. It has a cylinder shape similar to LADEE). – Ng Ph Nov 09 '21 at 09:40

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