Rocket flipping with fairings

[Tortoise]

I have 2 rovers on the top.

I launch and go up a little bit, then it suddenly turns and starts flipping, and I don't even do my gravity turn yet. Each time I add fairings, I have a problem with my rocket for some reason. Any tips?

EDIT: I do use FAR btw.

And I just realized that im extremely nose heavy. And it launches, but as I get higher the bottom gets lighter, then suddenly I flip. And I have no solution yet :/

Edited June 2, 2014 by Tortoise

[Taki117]

The problem is that the overly large fairing is causing large amounts of Drag. What you can do to alleviate this is to build a larger rocket (to more closely match the width of the fairing) add fins on the bottom of your rocket, or lots and lots of reaction wheels.

[LordQ]

The problem isn't that it's too top heavy, the problem is it's too bottom heavy. In the VAB, check whether the Centre of Lift is behind the Centre of Mass. If not, add fins until it goes behind it. A rocket, or any plane for that matter, will be unstable if the Centre of Lift is in front of the Centre of Mass.

[Spartan-S63]

Also, make sure with such a payload that you don't vary your angle of attack too far from your prograde vector. If you deviate too far, it is liable to flip over and the heavier the payload, the more likely it is to flip in the air.

[Hodo]

The problem I am seeing isnt that you have to much weight at the bottom, or anything like that.

It is probably two fold.

1- Your load isn't even, the weight of your load at the top of the rocket is off center so it is shifting the CoM out of alignment with the CoT.

2- You have a massive cone you are trying to push through the air, this is causing some problems with drag.

Last problem that it could be but don't know because I don't know your TWR, is you TWR is TOO high for the rocket at launch. Try to reduce it and keep it around 1.2-1.3 on launch.

[cantab]

Your rocket is clearly bent in that shot. Lack of rigidity will make things harder to control. You could resolve this by switching the upper stage to use Kerbodyne tanks. Even though they're less efficient in terms of tankage they may help make your rocket more manageable.

Besides that, I suspect drag is the issue. The rocket will want to fly draggy end backwards, so try adding some fins at the bottom to compensate for the fairing up top.

[LethalDose]

As mentioned above, Fins will help by increasing the drag at the aft of your craft.

When are you starting your gravity turn? I've had trouble like you're describing in the past, too, and found it mitigated when I started my gravity turn later and added tail fins.

Finally, it looks like you have 4 x 2.5 m grey tanks in the central spar of the rocket. Why not use two orange tanks? Few joints may mitigate the wobble.

Best of luck!

[StrandedonEarth]

Yup, it needs fins. Big ones, by the looks of it. Slimming the fairing will also help, if you can. Reaction wheels may also help to fight the flip, but it needs fins to naturally fly straight, just like an arrow.

I'll assume the rocket is bending because it is flipping

[LethalDose]

I was just looking at your image and realized that your smoke trails beneath the engines are almost perfectly vertical, while the rocket is about at a 45 degree angle. If you're cranking the heading over that far that suddenly, the change in your angle of attack is going to be very high and that will cause aerodynamic instability. With big rockets in FAR, sometimes it's better to start your gravity turn early, and do it very very gradually to avoid large AoA in the atmosphere.

It may not be an "optimal" launch trajectory because you spend a slightly longer time in thicker lower atmosphere, but IMO it's worth it not to lose control of your rockets.