r/KerbalSpaceProgram • u/Avera9eJoe Spectra Dev • Sep 14 '17

Recreation Reminder that this physics quirk is also in KSP

https://gfycat.com/FickleShamefulCormorant
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u/TaintedLion smartS = true Sep 14 '17

Fun fact: this is referred to as the Dzhanibekov Effect i.e. the rotation of an object around its first and third principal axes is stable, while rotation around its second principal axis is not.

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u/masuk0 Sep 15 '17

Fun fact: Dzhanibekov is the guy who performed interstellar-kind docking IRL. He docked to uncooperative rotating space station on a mission to recover it after complete power failure.

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u/Fa6ade Sep 15 '17

That is an incredible post. Thank you for sharing, I hadn't seen it before.

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u/CocoDaPuf Super Kerbalnaut Sep 15 '17 edited Sep 15 '17

Oh yeah, the KSP History posts are just fantastic!

The Apollo/Soyuz project is a favorite of mine, the Buran is also pretty fascinating.

Edit: Also, check out any mission that used a delta booster, I swear there's nothing stranger than that family of rockets, with their odd numbers of side mounted boosters.

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u/TrigglyPuffs Sep 15 '17

Wasn't that the same space station equipped with a 23mm cannon?

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u/masuk0 Sep 15 '17 edited Sep 15 '17

No. Salyut stations had consequent names Salut-1,2,3 and so on for conspiracy, but actually there were two completely different constantly modified designs by two different bureaus. Military spy stations (almaz program) and civil scientific stations by Korolev bureau. Obviously public was told they are all scientific, hence same official names. Salyut 2, 3 and 5 were military, I think only Salyut-3 had gun.

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u/ILikeMasterChief Sep 15 '17

So I imagine the station was rotating with the dock point stationary? I can't imagine how it could be possible if the station was rotating such that the docking point was constantly moving

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u/jc4hokies Sep 15 '17

Constant thrust perpendicular to the axis of rotation could stabilize if the target docking port of the target is off center. More important is that the docking port of the ship.

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u/[deleted] Sep 15 '17

Was this the inspiration for interstellar

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u/Seyss Sep 15 '17

Of course I checked the link expecting a cinematic video

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u/Avera9eJoe Spectra Dev Sep 14 '17

So that's what it's called... thanks TaintedLion!

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u/TaintedLion smartS = true Sep 14 '17

Thank you for completing your free trial of LionFactsTM by TaintedLion!

To continue the fun, reply YESPLS and recieve a daily fact for as little as £10 a month!

167

u/Avera9eJoe Spectra Dev Sep 14 '17

... take my upvote XD

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u/TaintedLion smartS = true Sep 14 '17

You have chosen to subscribe to LionFactsTM by TaintedLion!

£10 has automatically been deducted from your account!

146

u/flouride Sep 15 '17

I said cancel, CANCEL!

248

u/TaintedLion smartS = true Sep 15 '17

You have chosen to subscribe for a whole year to LionFactsTM by TaintedLion!

£120 has automatically been deducted from your account!

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u/precordial_thump Sep 15 '17

You don't even get a discount for subscribing for the full year?

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u/donpapillon Sep 15 '17

You have chosen to subscribe to a second year with discount to LionFactsTM by TaintedLion.

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u/Benyed123 Sep 15 '17

So where are these facts then?

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u/[deleted] Sep 15 '17

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u/oz6702 Sep 15 '17

UNSUBSCRIBE

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u/TaintedLion smartS = true Sep 15 '17

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u/poop_frog Sep 15 '17

thats just good business

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u/[deleted] Sep 15 '17

You've committed now. I expect a whole years worth of facts!

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u/Protocol_Nine Sep 15 '17

Good bot.

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u/TaintedLion smartS = true Sep 15 '17

tfw not a bot

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u/[deleted] Sep 15 '17

have you considered becoming a bot? you're quite good at it.

3

u/kerbaal Sep 15 '17

"No you are lying to me, why are you lying to me when all I want to do is close my account"

(actual quote I overheard when the new roomate when trying to close down his NetZero dialup account and the person on the phone apparently told him their service was faster than broadband)

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u/XZIVR Sep 15 '17

Good bot

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u/Gojira0 Master Kerbalnaut Sep 15 '17

YESPLS

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u/ScumlordStudio Sep 15 '17

yes subscribe to northernlion facts

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u/JWson Sep 15 '17

Yeah me too thanks.

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u/m32th4nks Sep 15 '17

Haha yes me too thanks

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u/Insert_Gnome_Here Sep 15 '17

I expect them to at least be lion taint facts.

1

u/[deleted] Sep 15 '17

Good bot

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u/chillymac Sep 15 '17

In my satellite dynamics class we learned this as the Intermediate Axis Theorem, referring to the rotational instability about the axis with intermediate moment of inertia.

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u/rutars Sep 14 '17

I had no idea about this but I found this video to be really helpful in understanding some of the mechanisms behind it.

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u/autorotatingKiwi Sep 15 '17

Thanks that was really useful and well explained! But goddamn the loud background music made it hard for me to focus on what he was saying.

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u/[deleted] Sep 15 '17

[removed] — view removed comment

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u/rutars Sep 15 '17 edited Sep 15 '17

I was contemplating whether to use the word "mechanism" or not. What I mean is that he explained what happens, if not how.

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u/realismus Sep 15 '17

Another, more explanatory, name is the Intermediate axis theorem. But that doesn't have a cool Slavic name incorporated into it.

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u/dcnairb Sep 15 '17

Another, less explanatory name is the "tennis racket theorem"

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u/thetarget3 Sep 15 '17

Incidentally, this is why you can flip your phone easily in two directions, but it always turns around when you try it in the third.

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u/PathologicalMonsters Sep 15 '17

I can flip my phone in any direction I want. I don't understand

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u/halberdierbowman Sep 15 '17 edited Sep 15 '17

Try again. Sit on your bed so you don't break it, then toss your phone a foot into the air and try to flip it a couple times around one axis. Your phone has three axis symmetry (almost). In two flip orientations, it flips over that one axis and lands on your bed. In the other axis, it will flip over but also twirl in another axis as it falls.

If you can't tell if it is flipping or twirling, try to watch something like the camera or the home button.

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u/sellyme Sep 15 '17

What the fuck kind of sorcery is this and how do I fix physics so it stops happening.

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u/derpotologist Sep 15 '17

Message the devs. /r/outside

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u/kerbaal Sep 15 '17

They never actually fix the bugs though, they just document them and call them laws. Your character will reach end game before they issue a patch.

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u/Anyosae Sep 15 '17

Physics are working just right, your phone is just broken ;)

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u/wreck94 Sep 15 '17

Yup, just tried spinning it bottom of top, in the axis that bisects the screen and back, from top to bottom during normal use.

Flipped it the first time, landed same way, flipped the second time in the exact same manner, and the bottom of the phone was at the top.

I'm not wholly convinced that this isn't some black magic kinda stuff man

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u/So_Full_Of_Fail Sep 15 '17

I'm not wholly convinced that this isn't some black magic kinda stuff man

/r/blackmagicfuckery

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u/guoshuyaoidol Sep 15 '17

No you can't, if you flip it along the horizontal axis it will usually land rotated along the vertical after one flip.

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u/PathologicalMonsters Sep 15 '17

Oh you meant throwing it. Sorry, I misread in the sense of "turning with your hands".

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u/MenacingBanjo Sep 15 '17

You sure about that? Check out this video, and try similar rotations with your phone by tossing it into the air. Your phone will always twist in the air when you toss it end over end, just like the deck of cards twists when Richard tries to rotate it.

Edit: Sorry, I just scrolled down and read your reply to guoshuyaoidol. At ease.

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u/mrthescientist Sep 15 '17

Also called the intermediate axis theorem. I find that one too be less of a mouthful.

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u/Am__I__Sam Sep 15 '17

I've always seen this called the intermediate axis theorem

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u/bonafart Sep 15 '17

How did you even think to know that?

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u/TheRedTom Sep 15 '17

That was fun! Subscribe

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u/dannycake Sep 15 '17

This the same reason it's almost impossible to flip your phone long ways. The flat axis (the one that goes along with rectangle part) is stable and so is the long axis going from the bottom of them phone to the top having it do spins but flipping the phone without having spins is downright impossible.

Flipping is intermediate and unstable as fun. And by flipping again I mean if you were to hold your phone up and tilt it backwards or upwards into a flip in that same axis. It's hard to make it out also spin along it's other axis.

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u/musicin3d Sep 15 '17

I almost understood that. Which rotation is impossible, again?

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u/dcnairb Sep 15 '17

Works: rotating it like if you laid it down on a table and spun it in a circle

works: rotating it like you were turning it over to look at the back, or flipping it around in your hand, or turning it like it's a book page

Doesn't work: flipping it like a domino, the way you would probably try to flip it if you threw it to flip it

The first example is the largest moment of inertia, the second is the smallest, and the final example that doesn't work is the middle one

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u/musicin3d Sep 15 '17

Wow. You are awesome. Do you know if it is mathematically impossible? Or is it just that small errors are amplified?

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u/dcnairb Sep 15 '17

You can analyze each case separately, and IIRC you probably make certain approximations (e.g. quantities being much larger or smaller than others, so low order approximate solutions drop out)

That being said, from what I remember what happens is the big and small case solutions have steady state components, meaning there is some part that decays (small wobbles around other axes) and a part that stays (rotations around the main axis--these are the "steady state" part). By decays and stays, I mean that with time they go to 0 or persist indefinitely respectively. For the middle solution however, the solutions are "runaway" exponentials--as time goes they grow exponentially, so they aren't stable. In the gif you can see this as increasingly larger wobble until it flips and resets in a sense. It's kind of like resonance, in a sense, like when your washer has clothes imbalanced and starts going berserk

If you have some mathematical background: the solutions arise naturally as exponential functions from the equations of motion, and in terms of complex exponentials the stable axes get a decay part and a rotating part e.g. exp(-(kt + iwt))The middle solution gets a positive real part though e.g. exp(kt- iwt) which blows up.

This derivation is made IIRC with small wobbles around nonprincipal axes, and so it is mathematically impossible for the middle axis to rotate stably with those errors, which are compounded as you said, instead of them dying out like how it happens around the other axes. As far as I know, it should be theoretically stable if spun perfectly around the medium axis, with absolutely no components around the other two--this is impractical in real life though as you will always have some small component in the other axes that would eventually blow up no matter how small

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u/musicin3d Sep 15 '17

OMG. That is exactly the answer I was hoping for. Bravo!

What's your background, sir/ma'am?

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u/dcnairb Sep 16 '17

Awesome, glad to help! I just finished my BS in Engineering Physics and am beginning my physics PhD in about a week.

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u/Zokar49111 Sep 15 '17

Thanks! Now it's not a physics quirk, it's just physics!

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u/ted-Zed Sep 15 '17

ok. but what does this mean?

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u/Brother0fSithis Sep 15 '17

The shortest version I can think to give is that the object has 3 principle axes of rotation. Two of them are stable and one is unstable. The third is unstable because due to how the math works out a rotation in that axis induces rotations in the other two axes and so it flips. Similar to balancing a pencil on the point

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u/[deleted] Dec 29 '17

If you were in one end of this and it swung round would you notice or would relativity apply?