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I have heard many times that the designers of the Voyager spacecraft neglected to account for the angular momentum of the tape recorders (on which data was stored). To compensate for it after launch, they were forced to use external thrusters every time they wanted to spin up or stop the tapes. I have been able to find a number of sources which re-tell this story (such as here and here) but none which are very credible.

How much truth is there to the story? Are any more details known? If true, how did mission control compensate for the effect?

TildalWave
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Chris Mueller
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  • One place where this story is recorded (physics textbook): https://books.google.nl/books?id=6upvonUt0O8C&pg=PA336&lpg=PA336&dq=voyager++%22angular+momentum%22+recorder&source=bl&ots=NeQTherk_x&sig=5j-O7J1XWUJGBzdEhXM-wv6u-KU&hl=en&sa=X&ved=0CDMQ6AEwA2oVChMI85L60L30xgIVY5rbCh0YnAPs#v=onepage&q=voyager%20%20%22angular%20momentum%22%20recorder&f=false – Hobbes Jul 24 '15 at 19:22
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    Reminds me of the legend that the designers of UCSD's main library didn't account for the weight of the books, causing it to slowly sink into the ground. I later found out that most colleges have a similar legend. – Keith Thompson Jul 24 '15 at 19:43
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    @KeithThompson - Closer to home, this reminds me of the completely false internet legend of the millions NASA that supposedly spent developing a space pen. – David Hammen Jul 26 '15 at 13:56
  • @DavidHammen Any idea what was the development costs to NASA before they dropped the project themselves? All the source I find only mention that NASA's development costs for the pen skyrocketed, leading to public opposition, and cutting the project, but nowhere can I find exactly how much NASA did spend (the number must be somewhere out there, but strictly speaking and extrapolating, if the number does not exist, it is as likely that they spent $100 too much as it is that they spent $5bn too much?). – V-J Jul 26 '15 at 16:43
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    A very late reply, @V-J, but that is entirely an urban myth. What NASA did do was to pay a company $4382.50 in 1965 for 34 custom-made mechanical pencils. (That would be $41200 in 2022 dollars.) People were outraged when that relatively tiny expense became public. As far as I can tell, NASA spent zero on developing a space pen. There's lots of excrement on the internet. – David Hammen Sep 25 '22 at 11:40

2 Answers2

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Edit: Yes, momentum of the tape recorder (DTR) was an issue. However, I don't think it was an issue that was overlooked in the design phase. The Voyager team knew it would become an issue for the Uranus and Neptune flybys, and had a fix in place before the Uranus flyby. I've found no indication of a "last-minute fix".

In 1973, NASA published a report "The applicability of frame imaging from a spinning spacecraft".

Another source of image motion is random vibrations of the sensor mounting platform. These are often due in part to the limit cycles in the IMC servo system and spacecraft attitude control. In addition, any moving part on the spacecraft will cause some jitter at the sensor.

..

The spin motion of the spacecraft is the only significant component of image motion, and the only component that will cause any appreciable smear.

(note: calculations in this report were made for a hypothetical spin-stabilized spacecraft at Jupiter)

So as far back as 1973, NASA was aware of moving parts on the spacecraft being a potential source of image smear. In my opinion, it's unlikely they subsequently neglected to account for the DTR.

The Voyager Neptune travel guide (PDF page 140) (document prepared by the Voyager mission planning office) has a good overview of the problem and the fix:

As Voyager cruises along in a zero-gravity environment, the start-stop motion of its tape recorder can add more jiggle to the spacecraft's natural limit cycle motion. To reduce these types of disturbances, the Voyager Flight Team has devised new software that fires the spacecraft thrusters to offset the tape recorder speed change whenever the tape recorder starts or stops.

This was more of an issue on the Neptune encounter than earlier in the flight, because of the longer exposure times required.

There's an AIAA conference paper that may contain more information on this topic in relation to the Uranus encounter, but I don't have access to that.

According to New Cosmic Horizons: Space Astronomy from the V2 to the Hubble Space Telescope and Pale Blue Dot: A Vision of the Human Future in Space (by Carl Sagan, who was closely involved with the Voyager program), this motion compensation technique was used at Uranus as well as Neptune.

For the Saturn encounter, they used image motion compensation on the Rhea flyby. In this case, they flew by Rhea so close they had to rotate the spacecraft to keep Rhea centered and prevent smearing. The Saturn press briefing doesn't talk about compensating for the motion of the DTR.

Hobbes
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  • The linked article does not say that the designers of Voyager neglected the angular momentum of the tape recorders. What it does say is that the Voyager 2 was reprogrammed to use the attitude thrusters to counter the induced jitter for the Uranus and Neptune fly-bys. This could have been the plan all along. – David Hammen Jul 25 '15 at 18:59
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This seems to be very similar to the myth of NASA spending millions of dollars to develop a space pen. One thing is certain: This did not happen (quoting from tutorvista.com):

Voyager 2, in 1986, as it flew past the planet Uranus, was set into unwanted rotation by this flywheel effect, every time its tape recorder was turned on at high speed. The ground staff at the Jet Propulsion Laboratory had to program the on-board computer to turn on counteracting thruster jets, every time the tape recorder was turned on or off.

Even worse, (from education.com),

What Caused Voyager to Point in the Wrong Direction?

While the second statement in that tutorvista.com link is true (see Hobbes' answer), the first is not. The title of the page at education.com is even worse; it's completely bogus.


At the time of the Uranus encounter, it took 2.5 hours for information sent from Voyager 2 to reach the Earth. It would have taken an additional 2.5 hours for any corrections to have been sent back to Voyager 2 -- and that's assuming the engineers could have instantaneously recognized the problem developed a plan to compensation for that undesired rotation. The Uranus encounter was of a very short duration, much like the recent fly-by of Pluto by the New Horizons spacecraft. The bulk of the encounter would have been already been over with that (minimum) five hours time between problem recognition and problem resolution.

In fact, it would have taken much longer than five hours. Propellant was the one of the most precious resource on the Voyager spacecraft. A plan that involved expending propellant would have had to have gone to the highest level for approval. The time frame needed just for this approval would have involved weeks, perhaps months. It would also have taken a good amount of time to develop the compensation algorithm. This was not a seat-of-the-pants decision.

There's another problem with the entire premise. Quoting Donald Rumsfeld, "There are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – the ones we don't know we don't know." Spacecraft attitude control systems need to deal with the known knowns, the known unknowns, and the unknown unknowns.

Had they been uncompensated, the attitude rate errors that would have resulted from starting and stopping of the tape drive would have fallen into the category of "unknown unknowns" ("process noise" in terms of Kalman filters). The attitude control system would eventually have addressed these errors, but only after the fact. The result would have been fuzzy images rather than pointing in the wrong direction. Addressing these knowable consequences of starting and stopping the tape drive as an integral part of the attitude control system moved this from the "unknown unknown" category to the "known known" category.

Perhaps this was foreseen during the design stage, perhaps not. It's hard to tell; I searched and searched. At the very minimum, someone at JPL foresaw this months in advance of the Uranian system encounter.

David Hammen
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