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I heard the following quote: "A rocket to the moon spends only 7% of the time on the correct course, the rest of the time it is off course and needs to be corrected and put back on course."

This is a nice anecdote in relation to how important it is to keep correcting your own course in life, have a destination in mind but keep checking if you are on course and correct.

Examples of similar quotes:

The Apollo moon rockets were off course 97% of the time. Yet they still reached their chosen destinations – and returned to earth – with pin-point precision and timing.

Source: Don't Go Into Small Business Until You Read This Book by John Counsel

Did you know that an Apollo rocket is actually on course only two or three percent of the time? At least 97% of the time it takes to get from the earth to the moon, it’s off course. Put another way, for every half hour the ship is in flight, it is on course for less than sixty seconds.

Source: Better Networker

They are nice examples but are they really true. Where would these percentages come from?

Oddthinking
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carolineggordon
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    Actually (if I listened to those silly sentences) to me that would mean that you don't really need to have a destination in mind at all most of the time, because at the end someone will guide you to the Moon anyways. :P – nico May 02 '12 at 13:26
  • @nico: That depends on whether you identify yourself narrowly with the CSM or more widely, perhaps with an Apollo mission as a whole. Anyway, the metaphor leaks quite a bit ;) – Piskvor left the building May 02 '12 at 14:42
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    Describing a system in the normal fluxuation of it's feedback loop as *"off course"* just proves that they guy who wrote the book was a business major because he couldn't hack it in a slipstick course. – dmckee --- ex-moderator kitten May 03 '12 at 02:31
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    How is "the right trajectory" defined? Given a narrow enough definition, one could probably claim that the Apollo rocket was off course until the final adjustment was made when entering lunar orbit. With what I expect would be a more useful definition of "moving closer to the target," it was probably on course close to 100% of the time. – Flimzy May 03 '12 at 04:31
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    I guess the author defined "the right trajectory" as having its nose pointed at the target rather than being on the calculated trajectory designed to intersect the target at the desired future location in space along the target's trajectory in orbit :) – jwenting May 03 '12 at 09:19
  • I think a similar claim is made in one of the pragmatic programmer books. – Andrew Grimm May 05 '12 at 23:13
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    @jwenting If the criteria for being "on course" is "nose pointed at the target" then the criteria is rather silly. There's zero atmospheric drag in cislunar space so there's no need at all to point your craft's nose at its destination. There is a need to keep the antenna pointed at Earth though, and to keep the heat shield warmed with heat from the sun. And none of that takes into account the need to "lead the target" and have your spacecraft travel to where the moon is going to be rather than where it currently is. – GordonM Jul 20 '15 at 10:05
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    I think the main message you can get from these quotations is "business analysts have no idea of rocket science". – Philipp Jul 21 '15 at 09:24
  • @GordonM: Another criterion for being "on course" could be "on a course which will actually touch the moon". The only time the LEM would be on a course to actually touch the moon would have been after it separated from the command module and was making its descent. Prior to that time, it would have (like the command module) been on a course that would miss the moon, since of course the objective of the command module wasn't to collide with the moon. – supercat Aug 01 '15 at 17:34
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    That quote is one of the silliest justifications I have ever heard for not planning something properly and a complete disrespect for everyone that took part on the moon missions. This quote is the reason behind the fact that most business analysts are consultants instead of managers. Or rocket engineers. – T. Sar May 16 '17 at 14:36
  • It's too bad that the questioner last visited this site over two and half years ago. The accepted answer should not have been accepted as such. It is incorrect. – David Hammen Nov 26 '18 at 14:58

2 Answers2

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Because of the incredible cost of getting propellant out of the Earth's gravity, I am pretty certain the moon rockets were almost always perfectly on course, with occasional corrections.

According to NASA, the launch phase gets the Apollo rockets to Earth's orbit, then there is a burn to join what they call the Trans Lunar Coast Phase, during which no propellant is expected to be fired. The next burn is at the Moon end to join a Lunar orbit.

The calculations on that page are all about making sure the phases line up with the minimum burn required to transition from one to the next, which is why launch windows are so critical.

One of the indicative diagrams from that page.

enter image description here

A comparison of lunar landing strategies can be found here. Again, it is heavily focused on savings in propellant, or as they state it, delta V (change of velocity):

... the TLI maneuver for the WSB is greater than for the standard ballistic transfer, there can be a lunar capture DV savings of about 25% when capturing into a Lunar orbit. The Hiten mission 17 (originally called Muses-A) performed such a capture in October, 1991

Rory Alsop
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  • Rory, I find this surprising, so I just want to make sure I understand it. You are saying the Apollo rockets did *not* undergo frequent corrections, but instead there were only three, relatively short, periods the rockets were used - to launch, to head toward the moon, and then to stop when they got there. That's remarkable. – Oddthinking May 03 '12 at 01:04
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    @Oddthinking, I'm wondering if the correct interpretation of the statistic isn't that the rockets were only 'pointing at' the moon for 7% of the trip... – Benjol May 03 '12 at 05:30
  • @Benjol: Not a bad thought - that may have been how the mistake was made. I assume that the vehicles point towards where the moon is *going* to be, and that's probably not within the circumference of the moon most of the time. Needs some evidence though. (Also, that means the re-tellers of this (alleged) fact are drawing invalid conclusions.) – Oddthinking May 03 '12 at 05:41
  • @Odd - that's what all the documentation I can find says, yes. Vast screeds of calculations to plan the course exactly so they didn't have to make corrections except at these key points. Some minor ones, sure - but it sounds like the maths/physics is good enough that it took all the gravitational forces into account. And in space, you don't ever need to point at where you ar4e going - your rocket nozzle just needs to point opposite to the direction you need force applied. – Rory Alsop May 03 '12 at 07:53
  • Hmm, I wonder what the author actually meant by "moon rocket" - if the launch vehicle (Saturn V) was meant, then the quotation might have made *some* sense - *that* was indeed making path corrections during ascent to Earth orbit (if the author would call the second stage Iterative Guidance Mode "path correction", that is), but the origin of the 7% figure is still unclear. – Piskvor left the building May 03 '12 at 11:45
  • I was pointed to this [site](http://www.ibiblio.org/apollo/) which is simulation software for rocket launches. Not sure it helps me as I don't really want to spend time investigating it. – carolineggordon May 03 '12 at 11:50
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    I think we can count this myth as busted based on this information. There are in fact not frequent corrections just a couple of points at which the burn happens, presumably during this burn there may be many mini corrections going on. But the general idea that you are making constant corrections is wrong. – carolineggordon May 05 '12 at 08:51
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    @Oddthinking, why do you think it's remarkable? It seems intuitive to me. Gravity is an extremely weak force, and space is a near-perfect vacuum; with no significant gravity perturbing your course and no friction to speak of, there would be no reason to continue to fire the rockets. In fact, continuing to fire the rockets would be a _bad_ idea, as you'd have to apply approximately the same amount of force in the opposite direction to land. This would cost a _lot_ of fuel, and rockets are already 90+% propellant! – Brian S Apr 30 '14 at 14:16
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    @Brian: Just to be clear: I am not at all suggesting the answer is wrong, just that it was a pleasant surprise to me. While I didn't expect that gravity or friction would drive the craft significantly off-course, I am shocked at the precision involved. It strikes me as hard to get your trajectory right to within a degree, and any error of a degree early in the flight would be easily corrected early and catastrophic later, encouraging the use of frequent small corrections rather than infrequent large ones. I reiterate: My intuition was clearly wrong. I love it when that happens. – Oddthinking Apr 30 '14 at 14:42
  • This answer misses the midcourse corrections. From a perfectionist point of view rockets are off course (deviating from expectations by a non-zero amount) 100% of the time. From a practical point of view, even though rockets do indeed deviate from expectations 100% of the time, it would be incredibly stupid (and wasteful of propellant) to constantly fire thrusters so as to keep the vehicle exactly on the planned trajectory. The midcourse corrections are good enough, and perfection is of course the enemy of good enough. – David Hammen Nov 26 '18 at 14:52
  • @DavidHammen - I agree, however in this case perfect could also be used when the craft arrives in the burn window correctly, and exits on track. The point is that burns are at specific points, not continuous. – Rory Alsop Nov 27 '18 at 15:32
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This image shows the Apollo 13 timeline, derived from the official log:

Apollo 13 Timeline.

As you can see, they only course-corrected four times over the course of the 143-hour mission.

The Apollo 11 mission had five scheduled course corrections, three of which were considered unnecessary by mission control when the window arrived. Of the two course corrections that actually occurred, one was only a three-second burn and the other doesn't specify how long it lasted.

So no, I wouldn't say they were only on course 3% of the time.

Oddthinking
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Brendan
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    +1, but given Apollo 13 did not go according to plan nor complete its mission, can we meaningfully include it? – Oddthinking Apr 30 '14 at 01:44
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    @Oddthinking - Well, they still went to the moon, right (and back, thankfully)? The question really only mentioned _getting_ to the moon, after all. If anything, I'd expect a craft with serious issues to need _more_ course corrections, to be off-course more often (whether from just being off in the first place, or having some mid-transfer trouble). So this makes a nice counterpoint to a "no problem" run. – Clockwork-Muse Apr 30 '14 at 13:46
  • @clockwork depends on the kind of failure. Anything that doesn't make the ship lose mass into space (or to lose mass in a different way than desired) can't change the ship's trajectory either. You can miss a maneuver, accidentally fire less or more or in the wrong direction, but an oxygen tank rupture that doesn't leak air into space doesn't cause a need for a course correction – John Dvorak Jul 25 '15 at 15:49
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    @JanDvorak: A discovery that the craft cannot be relied upon to use its rockets in the intended fashion could cause a need for a course correction to ensure that the craft's actual abilities will suffice to get it where it needs to go. – supercat Aug 01 '15 at 03:35
  • I would expect that in some sense the original claim could be accurate if "correct course" is interpreted to mean "a course which would result in its reaching the surface of the moon", since most of the time the LEM would have been attached to the command module which was, of course, never on a course to actually reach the moon's surface *since that wasn't its job*. – supercat Aug 01 '15 at 16:32
  • Much better than the accepted answer, but still a bit off. The reason for midcourse corrections is that a spacecraft is always off-course. Always. How much off-course is a different question, and whether that deviation is worthy of correction is yet another question. Perfect is the enemy of good enough. Apollo 11 missed the intended landing site by four miles, which was good enough. – David Hammen Nov 26 '18 at 16:18