Timeline for Why doesn't the Falcon-9 first stage use three legs to land?
Current License: CC BY-SA 4.0
24 events
| when toggle format | what | by | license | comment | |
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| S Sep 21, 2022 at 17:07 | history | suggested | abbassix | CC BY-SA 4.0 |
doing some clean-up with typos, etc.
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| Sep 21, 2022 at 13:34 | review | Suggested edits | |||
| S Sep 21, 2022 at 17:07 | |||||
| S Sep 13, 2022 at 20:27 | history | suggested | Glorfindel | CC BY-SA 4.0 |
broken link fixed
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| Sep 13, 2022 at 19:13 | review | Suggested edits | |||
| S Sep 13, 2022 at 20:27 | |||||
| Aug 20, 2019 at 18:06 | comment | added | Mark Foskey | In the same vein as Darel Hoffman's comment, a three-legged stool is far more common as a metaphor for stability than an actual object. Stools generally have four legs. | |
| Aug 20, 2019 at 7:44 | comment | added | Schwern | @DarrelHoffman Note that for an office chair stability is more important than weight, five legs rather than four. For a rocket weight is paramount, four legs rather than five. | |
| Aug 20, 2019 at 2:40 | comment | added | AJMansfield | To extend this, consider a simple description of each leg as a simple cantilever, then the beam deflection $δ = F L^3 / (3 E I)$. (F=force, L=length, E=elastic modulus, I=beam moment, equal to the width times the cube of the thickness.) If you use three legs with the same cross-section 1.414 times as long each bearing 4/3 as much load, they will deflect 1.885 times as far. To ensure they don't deflect further, you'll need to make the legs 1.235 times thicker. Which, combined with the change in length and number, results 1.3 times as much leg mass on the craft. | |
| Aug 19, 2019 at 20:50 | comment | added | Darrel Hoffman | @T.Nel Are you sitting in an office chair right now? Look down. If it's like most office chairs, you'll notice it has 5 legs. Chair manufacturers realized this ages ago. So long as you're on a reasonably flat surface (as most offices and landing pads are), more legs gives more stability, not less. I've seen some 4-legged office chairs, and they are definitely more prone to tipping if you lean in the wrong direction. | |
| Aug 19, 2019 at 18:19 | comment | added | BruceWayne | @s3raph86 - relevant XKCD | |
| Aug 19, 2019 at 15:42 | vote | accept | Star Man | ||
| Aug 19, 2019 at 15:42 | comment | added | Star Man | This is a great answer, especially when you calculated how much longer the legs have to be to maintain the same stability. Thank you. | |
| Aug 19, 2019 at 14:48 | comment | added | Jean-Bernard Pellerin | This answer is great. Thank you for the explanation, diagrams, and equations. I just had to comment, upvoting isn't enough praise. | |
| S Aug 19, 2019 at 14:26 | history | suggested | Fabian Röling | CC BY-SA 4.0 |
replaced link to stolen content with link to original
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| Aug 19, 2019 at 13:25 | comment | added | Fabian Röling | WTF? CNN just blatantly stole SpaceX's entire video and uploaded it onto their own channel! I edited your answer now to link to the original. And I reported the copy… for spam. Because there is no correct report reason for this, for whatever reason. | |
| Aug 19, 2019 at 13:21 | review | Suggested edits | |||
| S Aug 19, 2019 at 14:26 | |||||
| Aug 19, 2019 at 13:06 | comment | added | T.Nel | Great answer extending the question to why not go with 3 legs but longer. I was wondering why not 5 legs halfway through your answer. | |
| Aug 19, 2019 at 10:56 | comment | added | s3raph86 | Great answer. Mine would’ve just been “I tried it in Kerbal Space Program and it was a damn pain” | |
| Aug 19, 2019 at 1:59 | comment | added | Schwern | @uhoh Done. Four is the magic number. | |
| Aug 19, 2019 at 1:58 | history | edited | Schwern | CC BY-SA 4.0 |
Solve for more legs.
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| Aug 19, 2019 at 1:44 | history | edited | Schwern | CC BY-SA 4.0 |
added 902 characters in body
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| Aug 19, 2019 at 1:36 | comment | added | Schwern | @uhoh Good point. I'll work out the math for how much longer three legs have to be to give equivalent stability as four. | |
| Aug 19, 2019 at 1:33 | history | edited | Schwern | CC BY-SA 4.0 |
added 902 characters in body
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| Aug 19, 2019 at 1:08 | comment | added | uhoh | This is a good point especially if you consider it has to land on a very small boat. In order for three legs to have the same resistance to tipping, they would have to extend farther away from the rocket and so increase the chance of missing the edge of the boat, not to mention they'd have to be longer and/or stronger+heavier depending on the implementation details. | |
| Aug 19, 2019 at 0:56 | history | answered | Schwern | CC BY-SA 4.0 |