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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?

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    $\begingroup$ 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. $\endgroup$
    – Hobbes
    Feb 7, 2019 at 12:09
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    $\begingroup$ The available diagrams show where the thruster groups are, but don't have enough resolution to show in which direction the nozzles are pointed. $\endgroup$
    – Hobbes
    Feb 7, 2019 at 13:33
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    $\begingroup$ The Voyager Backgrounder 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. $\endgroup$
    – Organic Marble
    Feb 8, 2019 at 3:38
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    $\begingroup$ @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! $\endgroup$
    – Organic Marble
    Jan 23, 2021 at 1:35
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    $\begingroup$ In fact, I bet that is exactly what they are talking about. If I can crank out some numbers I may write an answer. $\endgroup$
    – Organic Marble
    Jan 29, 2021 at 2:22

2 Answers 2

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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.

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    $\begingroup$ Yes this is likely to be the best answer I think. $\endgroup$
    – uhoh
    Jan 29, 2021 at 4:53
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    $\begingroup$ @uhoh thanks! Of possible interest, I uploaded the entire blueprint to archive.org/details/blueprint-voyager $\endgroup$
    – Organic Marble
    Jan 29, 2021 at 15:43
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    $\begingroup$ Now I'll have to go out and buy a bigger monitor. $\endgroup$
    – uhoh
    Jan 29, 2021 at 21:39
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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)
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    $\begingroup$ 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. $\endgroup$
    – uhoh
    Jan 29, 2021 at 0:54
  • $\begingroup$ @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? $\endgroup$
    – IronEagle
    Jan 29, 2021 at 1:05
  • $\begingroup$ 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. $\endgroup$
    – uhoh
    Jan 29, 2021 at 1:11
  • $\begingroup$ 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! $\endgroup$
    – uhoh
    Jan 29, 2021 at 1:11
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    $\begingroup$ @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. $\endgroup$
    – IronEagle
    Jan 29, 2021 at 2:06

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