Backflipping Rocket

[Gooru]

At first i have to say i really really hate the new aerodynamics. It nearly killed the game for my i played three times more than any other game in the last 15 years.

The main problems is, i simply don´t understand why certain things happen to my crafts.

 

One thing for example are the backflipping rockets.

You build your payload, put it on top of a rocket and launch.

Of course you´ll forget the stabilizers and it will flip when you start your gravity turn - no problem.

Fly a little bit slower to minimize dynamic preassure and add some controlsurfaces at the back.

Things get better, the rockets get stable and your controlsurfaces hardly manage to keep it from flipping.

 

But then - Baaaaaammm - all over sudden all the aeroforces change diretion and try to flip your rocket the over way around and before you can do anything it makes a backflip.

What is this?

i don´t know why it is happening and i don´t know what this behavior has to do with real aerodynamics.

I googled to find out if this effect exists in reality but found nothing.

What mysterious kerbal-backflip-force is simulated by the engine when this happens and what can i do to counter it?

[Venusgate]

I imagine you have multiple tanks in your atmo stage.

If this is the case, the fuel drain behavior for engines is always "drain the tanks furthest to nearest."

What this means for you is that the Center of Mass keeps dropping as you go up, and when your CoM if below the visual center of your rocket, it wants to THROW that heavy end toward space. Like trying to throw a dart backwards.

What you can do:
MAOR fins.
MAOR SAS
Shorter rockets - the atmo drag isnt going to totally ruin you. experiment with short fat ugly rockets until you get a feel for what you can get away with.
DON'T throttle down until your Ap clears the atmo - especially if you have vectoring engines (like the LT-45) - unless you have a starting TWR of 5+, or are launching a flying saucer, the amount of drag you're fighting will be less than the amount of gravity you need to get away from. The longer your engines are burning away form the gravity, the more it's draining from your dV.
 

[Gooru]

53 minutes ago, Venusgate said:

I imagine you have multiple tanks in your atmo stage.

If this is the case, the fuel drain behavior for engines is always "drain the tanks furthest to nearest."

What this means for you is that the Center of Mass keeps dropping as you go up, and when your CoM if below the visual center of your rocket, it wants to THROW that heavy end toward space. Like trying to throw a dart backwards.
 

No. I just have one single big Tank down at the engines. The CoM moves to the top when fuel is burned so it should get more "darty" while burning.

My problem isn´t that there is atmo drag. The problem is that the drag suddenly and randomly changes direction so one can´t react fast enougth to counter it.

First it pulls (or pushes) the nose down and then suddenly from one millisecond to the other it pulls it up so the rockets gets out of control and flips backwards.

What causes this sudden change of that force?

Edited December 5, 2015 by Gooru

[Gooru]

1 hour ago, Venusgate said:

DON'T throttle down until your Ap clears the atmo - especially if you have vectoring engines (like the LT-45) - unless you have a starting TWR of 5+, or are launching a flying saucer, the amount of drag you're fighting will be less than the amount of gravity you need to get away from. The longer your engines are burning away form the gravity, the more it's draining from your dV.
 

I have no problem to fight the gravity and enougth dV.

But if i throttle too fast the dynamic pressure will make my rocket flip if there is too much AoA.

I i throttle down the rocket is much more stable.

Edited December 5, 2015 by Gooru

[Harry Rhodan]

A picture of your rocket might help.

[Gooru]

Sorry, it seems i can´t just insert the image. but here is a link.

http://imgur.com/WjzehEG

Edited December 5, 2015 by Gooru

[Harry Rhodan]

I see that you use a low TWR approach so I guess your rocket is trying to flip as soon as you hit Mach 1? At that point there's a sudden spike in aerodynamic pressure that might topple you over. And with most of your fuel in the first stage your CoM will be pretty low even if it is slowly rising again, while the lighter payload is extremely long and will give the stronger aerodynamic forces more leverage against you. The three big reaction wheels might even intensify the problem: they are extremely light and just make the payload even longer.

There's also an ongoing problem with an offset center of lift on fairings so be sure to use the inofficial bug fixing mods: http://forum.kerbalspaceprogram.com/index.php?/topic/87737-ksp-v105-stock-bug-fix-modules-release-v105a2-14-nov-15/

[Gooru]

Wow, thanks.

That patch worked like a charm. The Rocket is stable like hell now, without even taking care of preassuer ans AoA.

Thanks to claw for fixing what the devs messed up.