• 0
putnamto

quick question about tipping rockets.

Question

what is it that determines wich direction the ship will tip?

like say i have a ship that i know has a really tall heavy payload, but the payload is symetrical, yet 100% of the time when its going to tip in flight it always tips to the north, why is this? what determines the direction the rocket will tip if its going to tip?

Share this post


Link to post
Share on other sites

7 answers to this question

Recommended Posts

  • 1
10 hours ago, putnamto said:

what is it that determines wich direction the ship will tip?

Tiny asymmetries, including any itty-bitty fluctuations as the physics engine processes things like control-input oscillations, etc.

It's the nature of instability.  Try balancing a pencil on its point-- it'll fall over every time. But which direction will it fall over?  Answer:  in whatever direction the original "error" was, and since that original error may well have been literally microscopic, then from your perspective it might as well be random.

Instability works because if you err from the "perfect", then the size of that error feeds directly into the force that increases the error.  It's a positive feedback loop that amplifies even the most microscopic initial perturbations into a macroscopic rocket-flip.

Of course, if the rocket is inherently asymmetric-- e.g. you've got an antenna mounted to one side, which adds a bit of drag on that side-- then it'll flip in that direction every time.  But if the rocket is inherently unstable (e.g. has its CoM too far down, or whatever), then it's going to flip, even if it's "perfectly symmetric"-- making it "symmetric" just means that the flipping will be random instead of biased towards one particular part.

In short:  The direction that the rocket flips isn't super meaningful, and in a lot of cases you might as well think of it as random-- it's not really of any more significance than "which direction does a leaf flutter when you drop it".  The main thing is to design the rocket so that it's stable, rather than unstable, so that it doesn't want to flip in the first place.  ;)

Share this post


Link to post
Share on other sites
  • 0
Posted (edited)

In the real world, it would probably either be coriolis forces or whichever direction the wind is blowing. In KSP, it's probably more like "whichever direction gets calculated first".

 

Edited by bewing

Share this post


Link to post
Share on other sites
  • 0

There could be a number of explanations for what happens to your rocket:

  • Do you have side boosters? Do they have fuel lines connected to something else like another booster, or the center core? Make sure the symetry has not bugged out. Sometimes, one of the fuel lines is disconnected. The booster does not empty as fast as the other one and the CoM gets offset, tipping the rocket over because of misalignment between CoM and thrust vector.
  • Make sure the rest of the rocket is well aligned with the thrust vector. You can use KER or RCS Build Aid to see if your engines produces any torque, when in the VAB or SPH.
  • Check for lifting surfaces producing torque,
  • Make sure you have enough force to counter residual torques, with thrust vectoring or RCS on your rocket to help it to stabilize itself.

Crashed enough rockets because of those to know how to handle it :D

It has no reson to tip to the north in particular... just rotate it 180° before launching, it will tip to the south...

Share this post


Link to post
Share on other sites
  • 0
6 hours ago, Snark said:

Tiny asymmetries, including any itty-bitty fluctuations as the physics engine processes things like control-input oscillations, etc.

It's the nature of instability.  Try balancing a pencil on its point-- it'll fall over every time. But which direction will it fall over?  Answer:  in whatever direction the original "error" was, and since that original error may well have been literally microscopic, then from your perspective it might as well be random.

Instability works because if you err from the "perfect", then the size of that error feeds directly into the force that increases the error.  It's a positive feedback loop that amplifies even the most microscopic initial perturbations into a macroscopic rocket-flip.

Of course, if the rocket is inherently asymmetric-- e.g. you've got an antenna mounted to one side, which adds a bit of drag on that side-- then it'll flip in that direction every time.  But if the rocket is inherently unstable (e.g. has its CoM too far down, or whatever), then it's going to flip, even if it's "perfectly symmetric"-- making it "symmetric" just means that the flipping will be random instead of biased towards one particular part.

In short:  The direction that the rocket flips isn't super meaningful, and in a lot of cases you might as well think of it as random-- it's not really of any more significance than "which direction does a leaf flutter when you drop it".  The main thing is to design the rocket so that it's stable, rather than unstable, so that it doesn't want to flip in the first place.  ;)

well thought out response, thank you.

it may not be random, but i was just meaning that its confusing how a perfectly symetrical craft will always tip one way, and not even in the way i expect it to.

for instance ive got a rocket(metephorical) thats just a jumbo, a mainsail, and four boosters, we will call the boosters N, S, E, W, NS decouple first, so i would assume if the rocket would tip after this point the rocket wold tip towards the sides with less thrust, or no boosters, yet more often than not they tip towards the side that still has a booster.

am i just wrapping my head around this wrong?

 

Share this post


Link to post
Share on other sites
  • 0

I still say your mental error here is giving too much physical credibility to the game's aero modeling.

 

Share this post


Link to post
Share on other sites
  • 0
Posted (edited)
13 hours ago, putnamto said:

for instance ive got a rocket(metephorical) thats just a jumbo, a mainsail, and four boosters, we will call the boosters N, S, E, W, NS decouple first, so i would assume if the rocket would tip after this point the rocket wold tip towards the sides with less thrust, or no boosters, yet more often than not they tip towards the side that still has a booster.

am i just wrapping my head around this wrong?

 

Yes. The rocket being symmetrical means the centre of thrust is going to be firing through the centre of mass at all times - so long as it decoupled symmetrically. Toggle the indicators on in the VAB and take off the boosters manually, the two CoT and CoM balls and arrows don't move. Move the boosters and engines around, see what happens in different configurations. When the thrust doesnt point through thr CoM any kore, that induces a spin (small differences can be overcome with reaction wheels, engine gimbals, or RCS)

In fact, your assumption is the opposite of what can happen - with the boosters on just the E and W side, those are the only sides that are subject to drag peculiarities or thrust misalignment, causing a turn.

As for the direction the rocket does tip, that's likely due to the initial launch angle. Anything other than bang on 0° and the CoM will be to one side of the CoT (imagine looking up at the rocket from a point directly below it on the ground) and gravity will pull the rocket over. Now, KSP tends not to load in things instantaneously and the physics engine may mean your rocket wobbles slightly before lift off - this mean a launch angle of not 0° which means it'll eventually start turning.

Edited by BudgetHedgehog

Share this post


Link to post
Share on other sites
  • 0
15 hours ago, putnamto said:

it may not be random, but i was just meaning that its confusing how a perfectly symetrical craft will always tip one way, and not even in the way i expect it to.

The answer is that the craft probably  isn't 100% perfectly symmetrical.

Apart from the usual floating point stuff, I have a faint memory that there once were differences in the rigidity of attachments, depending on whether they were the "original" piece or the symmetry counterpart. This would allow to outside boosters flex more or less, thus incucing a slight tilt.

I just tried it with a pair of the long boosters attached to an orange tank by means of one decoupler plus one strut (strut near the bottom). Roll out (no clamps), wait a few seconds, ignite. The vessel will reliably tilt away from the part I placed, and towards the symmetry counterpart.

Just did a few attempts, plus I have mods installed. So YMMV.

Share this post


Link to post
Share on other sites
This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Answer this question...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.