I have an issue with my rocket

[lalacka]

So, i made a rocket(medium sizes), putted SAS and lots of RSC(that thing with monopropellant), but at 2-3k meters its top begins to spin, after which it simply breaks off.

On the top of the rocket i placed big, big repeater from mod DMagic Orbital Science.
How can i stabilize it?

P.S.:I will upload screenshot soon.

[Snark]

Moving to Gameplay Questions.

3 hours ago, lalacka said:

but at 2-3k meters its top begins to spin, after which it simply breaks off.

I'm having trouble understanding what this means-- how does the "top" begin to spin?  Isn't the entire rocket one single rigid piece, so the whole rocket spins?

If you mean that your rocket is flipping (i.e. so that you can't keep it pointed , and it keeps wanting to rotate so the tail end goes first, or something like that), then that's a common problem in KSP and it comes down to rocket design, i.e. your rocket is aerodynamically unstable.

If that's what's happening, the fix is to redesign your rocket to make it stable.  However, fixing the instability requires understanding why it's unstable, and there are various different problems that might cause that.  So we really do need to see a screenshot in order to be able to offer more specific advice.

3 hours ago, lalacka said:

I will upload screenshot soon.

Excellent.    Will look for that, then.  Once you've posted a screenshot, there's a pretty good chance we'll be able to spot what the issue may be.

Broadly speaking:  you want the ship to be built like an arrow:  pointy and streamlined in the front, fins on the back, and CoM (center of mass) near the front of the rocket.  But we'll be able to advise more specifically after we see a screenshot.

[Johngi]

1 hour ago, Snark said:

Broadly speaking:  you want the ship to be built like an arrow:  pointy and streamlined in the front, fins on the back, and CoM (center of mass) near the front of the rocket.  But we'll be able to advise more specifically after we see a screenshot.

I agree with this one, though also remember:

To stay stable, your centre of lift must be below your centre of mass, and both of these must be directly in the centre of your rocket, no horizontal deviations.

[Snark]

1 hour ago, Johngi said:

To stay stable, your centre of lift must be below your centre of mass

This is actually not quite the case.  What needs to stay behind your center of mass isn't the center of lift, it's the center of dynamic pressure.  (You can think of this as "center of drag", sorta.)  That's a different thing from "center of lift".  (The CoL display in the VAB only includes components that explicitly have "lift" assigned to them, e.g. wings; but the center of dynamic pressure applies to all parts.  If you focus only on CoL, you're likely to get a highly misleading picture.)

Unfortunately there's no convenient VAB marker for showing where that is (for good technical reasons), so you kinda have to eyeball it.  Main thing is to make sure that your CoM is as far forward as possible, and that any fins you have behind it need to be as far behind the CoM as possible (because the effectiveness of a fin is directly proportional to its distance behind CoM).

 

[Johngi]

2 hours ago, Snark said:

This is actually not quite the case.  What needs to stay behind your center of mass isn't the center of lift, it's the center of dynamic pressure.  (You can think of this as "center of drag", sorta.)  That's a different thing from "center of lift".  (The CoL display in the VAB only includes components that explicitly have "lift" assigned to them, e.g. wings; but the center of dynamic pressure applies to all parts.  If you focus only on CoL, you're likely to get a highly misleading picture.)

Unfortunately there's no convenient VAB marker for showing where that is (for good technical reasons), so you kinda have to eyeball it.  Main thing is to make sure that your CoM is as far forward as possible, and that any fins you have behind it need to be as far behind the CoM as possible (because the effectiveness of a fin is directly proportional to its distance behind CoM).

 

That... actually makes a ton of sense lol. How on Kerbin did the devs make that work?

[Xd the great]

Other than that, add fairings, make sure that the rocket is not too long or too short, relative to the length of the rocket.

Also, engines like ks 25 is good for countering the tumbleing rocket.

And also, did you switch on RCS?

[lalacka]

I have just deleted fairing and my rocket was stable again.
Thx for tip about Center of Mass, i will test it:D