Timeline for What percent of time is a rocket to the moon on the right trajectory?
Current License: CC BY-SA 3.0
14 events
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| Nov 27, 2018 at 15:32 | comment | added | Rory Alsop | @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. | |
| Nov 26, 2018 at 14:52 | comment | added | David Hammen | 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. | |
| Apr 30, 2014 at 14:42 | comment | added | Oddthinking♦ | @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. | |
| Apr 30, 2014 at 14:16 | comment | added | Brian S | @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! | |
| May 5, 2012 at 8:52 | vote | accept | carolineggordon | ||
| Nov 27, 2018 at 15:21 | |||||
| May 5, 2012 at 8:51 | comment | added | carolineggordon | 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. | |
| May 3, 2012 at 11:50 | comment | added | carolineggordon | I was pointed to this site which is simulation software for rocket launches. Not sure it helps me as I don't really want to spend time investigating it. | |
| May 3, 2012 at 11:45 | comment | added | Piskvor left the building | 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. | |
| May 3, 2012 at 7:53 | comment | added | Rory Alsop | @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. | |
| May 3, 2012 at 5:41 | comment | added | Oddthinking♦ | @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.) | |
| May 3, 2012 at 5:30 | comment | added | Benjol | @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... | |
| May 3, 2012 at 1:04 | comment | added | Oddthinking♦ | 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. | |
| May 2, 2012 at 15:01 | history | edited | Rory Alsop | CC BY-SA 3.0 |
added 604 characters in body
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| May 2, 2012 at 14:46 | history | answered | Rory Alsop | CC BY-SA 3.0 |