How do I stop my rockets from flipping?

[Neil Kermstrong]

It's too damn annoying when you finally build a big and powerful rocket just so it becomes impossible to fly it because it won't stop flipping, and moving like a spaghetti

I have Kerbal joint reinforcement but it isn't doing anything

[pandaman]

@Neil Kermstrong  First...  To help stop it flipping.  Make sure your Center of Mass is near the top (pointy end) and your Centre of Drag is nearer the bottom (hot end). You can also add fins at the bottom to help too.  Basically copy a dart, heavy bit at the front, fins at the back.  Even without fins the dart will still fly point first, your rocket is the same, fins will give it a bit more stability, but you won't always need them, and they add drag too.

Second...  To help with wobbly rockets.  If you haven't done it already, switch on 'Advanced Tweakables' in the main menu game settings, this gives numerous extra features.  For this particular problem though, when building right click a part to open the option window, options for 'Rigid Attachment' and 'Auto Strut' will be available (iirc they may also be available in flight).  Experiment with those, I use them all the time.

A simple experiment to illustrate the need for 'heavy at the top' is to balance a sweeping brush on your finger.  With the brush end resting on your finger (heavy at the bottom) it is very hard to balance, but with the brush (heavy) end at the top it is far easier to control.  Wobbly rockets tend to be more prone to flipping too as the thrust keeps moving about relative to the CoM, like a random, uncontrollable gimballing effect.

Hope that helps.

Edited September 26, 2020 by pandaman

[Gargamel]

This Question about Gameplay has been moved to Gameplay Questions.  

 

[bewing]

Flipping is caused by either "basic" or aerodynamic instability.

What I call basic instability happens when your thrust vector does not pass through your CoM. This often happens for a noodly rocket. Eventually, your control mechanisms are unable to counteract the off-balance forces, and the rocket becomes unstable.

Aerodynamic instability is what pandaman described. Passive stability happens when the drag at the front end is low, and the drag at the back end is high. So, if possible, move the draggy bits farther toward the back. It also helps to move the heavy bits toward the front -- but doing this often contributes to noodly behavior. If draggy stuff has to be at the top, then putting some of it inside a fairing or service bay can help.

You can tell which kind of instability is your main problem by where in your ascent the rocket flips.

Noodly rockets are generally caused by building a rocket that is really tall, or that has parts that are too skinny. The skinnier the fuselage, the more likely the rocket is to bend. Creating a few (less than five) autostruts between the bottom stages of the rocket and the very top (generally the root part) can dramatically reduce bending.

A picture of your rocket could help us diagnose why your rocket wriggles.

Edited September 26, 2020 by bewing

[Neil Kermstrong]

25 minutes ago, bewing said:

Flipping is caused by either "basic" or aerodynamic instability.

What I call basic instability happens when your thrust vector does not pass through your CoM. This often happens for a noodly rocket. Eventually, your control mechanisms are unable to counteract the off-balance forces, and the rocket becomes unstable.

Aerodynamic instability is what pandaman described. Passive stability happens when the drag at the front end is low, and the drag at the back end is high. So, if possible, move the draggy bits farther toward the back. It also helps to move the heavy bits toward the front -- but doing this often contributes to noodly behavior. If draggy stuff has to be at the top, then putting some of it inside a fairing or service bay can help.

You can tell which kind of instability is your main problem by where in your ascent the rocket flips.

Noodly rockets are generally caused by building a rocket that is really tall, or that has parts that are too skinny. The skinnier the fuselage, the more likely the rocket is to bend. Creating a few (less than five) autostruts between the bottom stages of the rocket and the very top (generally the root part) can dramatically reduce bending.

A picture of your rocket could help us diagnose why your rocket wriggles.

 

1 hour ago, pandaman said:

@Neil Kermstrong  First...  To help stop it flipping.  Make sure your Center of Mass is near the top (pointy end) and your Centre of Drag is nearer the bottom (hot end). You can also add fins at the bottom to help too.  Basically copy a dart, heavy bit at the front, fins at the back.  Even without fins the dart will still fly point first, your rocket is the same, fins will give it a bit more stability, but you won't always need them, and they add drag too.

Second...  To help with wobbly rockets.  If you haven't done it already, switch on 'Advanced Tweakables' in the main menu game settings, this gives numerous extra features.  For this particular problem though, when building right click a part to open the option window, options for 'Rigid Attachment' and 'Auto Strut' will be available (iirc they may also be available in flight).  Experiment with those, I use them all the time.

A simple experiment to illustrate the need for 'heavy at the top' is to balance a sweeping brush on your finger.  With the brush end resting on your finger (heavy at the bottom) it is very hard to balance, but with the brush (heavy) end at the top it is far easier to control.  Wobbly rockets tend to be more prone to flipping too as the thrust keeps moving about relative to the CoM, like a random, uncontrollable gimballing effect.

Hope that helps.

I have placed some auto struts and turned rigid attachment on in the parts that were having issues and now it's flying like a dream! Thanks for help

got one question though, what is the best type of auto struts? noticed there are multiple

Edited September 27, 2020 by Neil Kermstrong

[Superfluous J]

30 minutes ago, Neil Kermstrong said:

got one question though, what is the best type of auto struts?

That's like asking what the best engine is. Depending on what you want, it varies.

In general though, you want the auto struts to be in places where they auto-connect to something perpendicular to what the part is actually connected to.

[Neil Kermstrong]

8 minutes ago, Superfluous J said:

That's like asking what the best engine is. Depending on what you want, it varies.

In general though, you want the auto struts to be in places where they auto-connect to something perpendicular to what the part is actually connected to.

I'm not asking about the locations, Im talking about the fact there are multiple options like heaviest part or root part

[Spricigo]

6 minutes ago, Superfluous J said:

That's like asking what the best engine is. Depending on what you want, it varies.

What I was about to say, posted "just now"

 

[Spricigo]

3 minutes ago, Neil Kermstrong said:

I'm not asking about the locations, Im talking about the fact there are multiple options like heaviest part or root part

those option are what define the "other location". To be more precise: the other part that is linked by the autostrus.

Root:usually the fist part of a craft.

Grandparent: the part two connections closer to the root part

Heaviesty: the part of the craft with higher mass.

 

It may be problematic if you have a lot of struts to a small (weak) part of ir the point of connection for several struts change (root part when docking, heaviest with fuel consumption/ore mining). But for the most part the option is to allow some degree of control to where the other side of autostruts will be.

 

[pandaman]

8 hours ago, Neil Kermstrong said:

I'm not asking about the locations, Im talking about the fact there are multiple options like heaviest part or root part

Basically it is what works for what you are trying to do.

I mostly use Root or Grandparent, but like most things, there is no 'best',  just most suitable for the current situation.

[VoidSquid]

You might also want to check out this video, @Neil Kermstrong :

 

 

[DerekL1963]

16 hours ago, Neil Kermstrong said:

I have placed some auto struts and turned rigid attachment on in the parts that were having issues and now it's flying like a dream! Thanks for help

Be careful with Rigid Attachment - it turns your rockets into glass, rigid but unbendable without breaking.  It can cause rockets to break that otherwise might not.

IMHO: As far as autostruts go, usually Grandparent (preferred) or Root (for parts physically close to the root) are the best bets.  Which part is the heaviest can and will change over the course of the flight (I.E. as fuel is consumed), and the struts will change when it does, that can lead to...  unexpected results.  Grandparent and Root only change when docking/undocking and the dynamic loads are generally very low during that process.

[jimmymcgoochie]

I always autostrut engines to the root part- it seems to help the whole rocket remain stable when physics warp is active instead of everything compressing together and in rare cases the engines end up inside other parts, which promptly explode due to exhaust heat...

When you have parts that are attached radially e.g. boosters on the sides of your core stage, autostrut those to their grandparent so they stay still relative to the core; if you put parts on top of those e.g. fuel tanks or nosecones, grandparent autostrut will lock all the boosters together which can really help with stability and stop the 'boosters pivoting around decouplers until they snap off and everything explodes' thing from happening. Be careful with autostrut to heaviest part as the mass will shift during flight- especially launches- and that can make things less stable; it's also prone to causing unfortunate side effects if you dock craft together so if you're docking stuff, turn the struts off just in case.

[Grogs]

I'm usually wary about using root or heaviest part for autostruts because those can change with fuel levels, or when you dock to another craft. For something that will just be used for takeoff like the center of the booster stage, I'll use root part. For most other things, I'll use grandfather. I mostly use them to reinforce potential weak spots like couplings and fairings.