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I've just read in Eshleman et al 1977 Radio Science Investigations with Voyager that the voyagers have:

...a novel attitude-control thruster configuration that minimizes accelerations along the Earth-spacecraft line-of-sight;

I am guessing that the goal would be to minimize any tiny amount of delta-v that would affect doppler measurements of velocity, since there is so much science available in these precision measurements.

Does this just mean that the "novel attitude-control thruster configuration" is designed to balance the impulse from the thrusters used so that only torque is applied about the spacecraft center of mass? If so, how was this novel minimization accomplished?

If it doesn't, then what does it mean?

uhoh
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  • I think the answer to your question is on page 258 of this paper, but I don't understand it well enough to write an answer: http://users.cecs.anu.edu.au/~Jonghyuk.Kim/teaching/KF%20Voyager.pdf – Organic Marble Jan 21 '19 at 14:44
  • @OrganicMarble that's chock-full of technical goodies, I'll enjoy digging to that for sure, thank you! – uhoh Jan 21 '19 at 15:46
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    I've done a fairly extensive search, but can't find any information on how the thrusters are installed. Despite the paper referenced by @OrganicMarble I've found several texts that say the thrusters are operated in pairs, but offer no detail beyond that. That paper does not go into detail on thruster configuration either. – Hobbes Feb 07 '19 at 12:09
  • @Hobbes thanks for looking into this further. There are comments on How are Voyager's 16 thrusters oriented? but no answers there either. Quite a puzzle! I wonder if it's time to start looking for 3D models and calculating moments ourselves. Just need to 1) collect all these diagrams, 2) get a coordinate system and 3) a reference that states the coordinates of the center of mass in that system. – uhoh Feb 07 '19 at 13:15
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    The available diagrams show where the thruster groups are, but don't have enough resolution to show in which direction the nozzles are pointed. – Hobbes Feb 07 '19 at 13:33
  • @Hobbes organic marble's link's Figure 4 has labels "pitch, roll, yaw" and RussellBorogove's diagram in this answer has some nice color-coded groupings taken from somewhere, there must be more out there as well. There's also the Attitude Control Modes diagram in http://spaceflight101.com/newhorizons/spacecraft-overview/ – uhoh Feb 07 '19 at 13:53
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    The Voyager Backgrounder https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810001583.pdf and all other references I've found say that there are 12 jets in two systems of 6, only one system is used at a time. Now if you only have 6 jets, they obviously can't fire in pairs (other than in the trivial sense that one fires to start a motion and the opposing one fires to stop it) because you only have 1 jet / direction / axis. – Organic Marble Feb 08 '19 at 03:38
  • @OrganicMarble I was about to say that you can always turn the spacecraft in one direction, but then remembered momentum unloading would need bidirectional thrusters. – uhoh Feb 08 '19 at 03:43
  • Yeah, it would be kind of hard to stop your rotation if your jets only pointed in one direction.... – Organic Marble Feb 08 '19 at 03:44
  • @OrganicMarble oh that was a silly thing to say! never mind :-) – uhoh Feb 08 '19 at 03:45
  • That's New Horizons, no? – Organic Marble Feb 14 '19 at 03:15
  • @OrganicMarble I've made a GIF of opposing thrusters so I'll remember next time. This one is New Horiiiiiiiizohons... https://i.stack.imgur.com/D7iHh.gif from https://youtu.be/j3Jm5POCAj8?t=106 (loud music alert) – uhoh Feb 14 '19 at 03:17
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    Ah. Nice job! I always forget to look at my memory joggers :( – Organic Marble Feb 14 '19 at 03:52
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    @OrganicMarble - For attitude control, you only have to worry about the 3 rotational degrees of freedom, so 6 thrusters is enough. They are operated in +- pairs, according to the diagram in your first comment. The paper mentioned in the question also has this to say about the thrusters: "the new thruster configuration improves spacecraft navigation while aiding celestial mechanics investigations." This is in section 7, where it seems that the "novel" here just means "new" - as in, it's a different design than what they had before. – IronEagle Jan 23 '21 at 01:23
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    @IronEagle all of my posts here have supported my argument that 6 thrusters were enough. I am not sure what point you are trying to make, if you are supporting my arguments, thanks! – Organic Marble Jan 23 '21 at 01:35
  • @IronEagle bounty expires in 22 hours and there's a 24 hour grace period after that. I think there is enough information in comments and links here to write up an answer; it would be great if the bounty didn't go to waste :-) – uhoh Jan 28 '21 at 06:58
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    @uhoh - thanks for the push, I think I may have actually figured it out. – IronEagle Jan 29 '21 at 00:43
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    @uhoh this popping back up reminded that years ago while looking into this I actually found a 3-view mechanical drawing that shows the locations of all 12 thrusters. Sadly I no longer remember exactly where I found the drawing :( But here is a crop showing the 12 thrusters and a crop showing the drawing nameplate. https://i.imgur.com/53QJwDJ.jpg https://i.imgur.com/xtsVWFz.png – Organic Marble Jan 29 '21 at 02:04
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    @uhoh I was especially fascinated that all 4 pitch thrusters are basically co-located. – Organic Marble Jan 29 '21 at 02:06
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    @uhoh I suspect the "novel configuration" may have to do with the fact that the pitch thrusters are tilted 45 degrees. Note the yaw and roll thrusters - which are going to provide the unwanted translational movements perpendicular to the antenna's line of sight - are not similarly tilted. Only the pitch thrusters - which would cause translation along the line of sight - have that tilt. But I'm gonna have to think about how that would help. – Organic Marble Jan 29 '21 at 02:11
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    @OrganicMarble see discussion below current answer, I'm thinking that it would not help, and my original quote is wrong. – uhoh Jan 29 '21 at 02:14
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    @uhoh I suspect that quote is in error as well. – Organic Marble Jan 29 '21 at 02:18
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    @OrganicMarble - I would tend to agree that the 45 degree would be the novel thing... normal control would have them in line with the antenna. But I also don't understand why the 45 degrees? – IronEagle Jan 29 '21 at 02:19
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    The only thing I can think is that it does cut down the amount of acceleration along the line-of-sight versus the same thruster being aligned along the line of sight. So, for some values of "minimizes" ... – Organic Marble Jan 29 '21 at 02:21
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    In fact, I bet that is exactly what they are talking about. If I can crank out some numbers I may write an answer. – Organic Marble Jan 29 '21 at 02:22
  • @OrganicMarble Go for it! – IronEagle Jan 29 '21 at 03:01

2 Answers2

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This answer is somewhat speculative and relies upon a generous interpretation of what "minimizes accelerations" means.

I obtained a three-view drawing of the Voyager from a link in this answer What does it mean when the Voyagers "switch thrusters"?

enter image description here

I've cropped and annotated a portion of the drawing dealing with the pitch thrusters.

enter image description here

Somewhat unusually the pitch thrusters are not mounted perpendicular to the axis they are designed to produce rotation about. Instead they are tilted approximately 45 degrees. Voyager's yaw and roll thrusters do not exhibit this tilt.

Since all the documents I've found state that in Voyager's attitude control system, a single jet fires to induce a rotation, an unwanted translation necessarily also occurs. By tilting the pitch jets 45 degrees, the amount of translational acceleration along the antenna line of sight (LOS) is reduced by ~30%.

The red line I added to the drawing represents a hypothetical thrust vector from a pitch jet. The green line is its component in the direction of the antenna LOS. The purple line is the same thrust vector directed along the LOS and serves to show the ~30% increase in thrust along the LOS compared to that produced by the tilted jet.

Is 30% a significant minimization? A skeptic might point out that the tilt also allows the jets labeled "-P1" and "-P2" to minimize their impingement on the back of the antenna. But a good design feature can serve more than one function.

Organic Marble
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"Novel" here means in respect to other planetary missions, and also the thruster fuel: the previous missions to the outer planets (Pioneer) were spin-stabilized, and previous missions to the inner planets used cold-gas thrusters. A quote from later in the same paper in your question:

The Voyager spacecraft incorporate several improvements to equipment and design that will enhance the radio science investigations with respect to the previous planetary missions ... the new thruster configuration improves spacecraft navigation while aiding celestial mechanics investigations." (emphasis mine)

Not much else is mentioned in that paper in regards to the thrusters, but I believe it is just this distinction between Voyager and the previous planetary missions. The MR-103 hydrazine thruster was also relatively new, with the first flight of the thruster only in 1974, and this paper was published in 1977.

In other words, it's "novel thruster" configuration, not "novel thruster configuration".

Missions to inner planets before 1977 by the USA, with stabilization methods:

  • Mariner: 2 sets of 6 cold gas thrusters, 3 gyros (link) (Mars, Venus)
  • Pioneer: spin-stabilization (link) (Jupiter, Saturn)
  • Viking: cold-gas thrusters (link) (Mars)
uhoh
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IronEagle
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    hmm... the novel whatever "minimizes accelerations along the Earth-spacecraft line-of-sight" and that seems to suggest to me at least that the novelty provides a way to control attitude with a minimum amount of thrust along the line-of-sight direction. That would be important to allow for attitude control during a long transmission for ranging purposes (they can be ten hours or longer for some spacecraft) without causing a Doppler-shift glitch. I really thing there is something more to this, like the pitch and yaw thrusters have minimal thrust along the boresight direction of the antenna. – uhoh Jan 29 '21 at 00:54
  • @uhoh - there may be, but there doesn't seem to be much in the way of documentation - I would mention that a spin-stabilized craft has none, so perhaps that's what they are comparing to. Perhaps one of the paper authors is still alive? – IronEagle Jan 29 '21 at 01:05
  • Actually, the paper in the first link under the question Campbell, Synnott and Bierman 1983 Voyager Orbit Determination at Jupiter says "A design flaw of the spacecraft is that the exhaust plumes from the positive and negative pitch attitude thrusters, which have velocity components along the radiometric measurement direction." Search for "thruster" in the paper for several discussions. – uhoh Jan 29 '21 at 01:11
  • If in your previous three examples the thrusters were not perpendicular to the antenna boresight or "radiometric measurement" direction, then Voyager's configuration would be unique! – uhoh Jan 29 '21 at 01:11
  • https://space.skyrocket.de/img_sat/viking_orbiter__1.jpg from https://space.skyrocket.de/doc_sdat/viking-1.htm – uhoh Jan 29 '21 at 01:16
  • 4 hours (plus 24 hour grace period) left on the bounty timer, but no pressure :-) – uhoh Jan 29 '21 at 01:23
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    @uhoh - interesting, continuing your quote: "...This fact was known before launch, but it was believed that the effect would be negligible. The conclusion of a postlaunch study was that, in fact, the plume impingement effect is significant." So the thrusters are not perpendicular to the radiometric direction (which would have been minimal), but were apparently balanced in the design, which missed this effect. – IronEagle Jan 29 '21 at 02:06
  • did you see OM's comments just now under the question? Yikes they look like about 45 degrees and do not appear at all balanced! So they certainly will impart some acceleration along the radiometric direction. I wonder if the quote in the question is just plain wrong? – uhoh Jan 29 '21 at 02:13