rockets flipping during gravity turn

[cosmodrome]

I'm having some serious issues with rockets flipping right when I start my gravity turn.  I didn't have this problem pre 1.0.  I just recently upgraded to 1.0.5 and the issue started.  At first I thought it was due to the fairings bug, but this last rocket has no fairings.  Mechjeb has the same problems, only earlier in the flight.  The only mods I have are KER/MJ.  I also cannot figure out how to attach an image right now   but this seems to happen with almost anything I design.  Please help because this has become extremely frustrating.  Thanks in advance.

http://imgur.com/sXfZXTX

[Ketatrypt]

more weight in the top/front never hurts. If you can't do that, try throwing on the steerable fins (AV-R8 Winglet) and, last but not least, try to keep your speed down until you get high in the atmosphere... Personally I limit my speed to less then 300m/s below 5km, 450m/s below 10km, and full throttle above 10km.. staying slow in the lower atmosphere will give better control, and it might also save a bit of fuel.

[FullMetalMachinist]

If you're just coming into the post 1.0 world, then you need to re-learn the best flight path to orbit (as we all have). You can no longer go straight up to 10km and turn right to 45 degrees. The new aerodynamic model dictates that you perform a realistic gravity turn, starting very soon after liftoff.

Common wisdom is along the lines of 5 degrees East when you're going about 100 m/s, and gradually following the prograde marker on the way up. You still be at 45 degrees East by the time you're 10km up, but you need to get there slowly by doing a long gradual turn. After that, most people give milestones something like 60 degrees by 30km, and nearly horizontal by 50km.

Also, as Ketatrypt said, keep draggy stuff like fins at the back, and heavy stuff at the front. This makes your rocket more like an arrow or dart, and it will be more aerodynamically stable. 

The rocket you have in the picture looks pretty good, you should be able to get to orbit with the right flight path. My only suggestion is to install Claw's stock bug fix modules, it fixes (among other things) the annoying "lift in front of the fairing" bug. Hope this helps! And welcome to the forums! 

Edit:

Old but informative post about flipping rockets: Link

Claw's bug fixes: Link

Post about inserting pictures: Link

Edited February 11, 2016 by FullMetalMachinist
Added links

[Snark]

You've got some significant issues with that rocket as a result of the new aero.

First and foremost, you have fuel drainage issues that are messing up your CoM. You've got two fuel tanks stacked vertically (three, counting the little conical one).  That's going to cause big stability problems, because the top tank will drain first, drastically lowering your CoM and rendering the ship unstable.  The simple fix (if you don't want to redesign/rearrange the rocket) is to disable the top tank in the VAB.  This will prevent it from draining, and the bottom tank will drain first, which will actually improve your CoM for the first part of the flight, because your CoM will go up as fuel burns.  Wait until the bottom tank is nearly empty, then enable the top tank.

(You also have that 4-ton conical tank up top.  Suggest leaving that disabled until later, too.)

Next:  Replace those fins on the bottom with something steerable, like the AV-R8 winglet.  It'll have more steering authority that way.

Orient the rocket in the VAB so that it's facing east on the launchpad instead of the default, which is north.  That way, as you make your turn, the radial boosters will be on your rocket's left and right, instead of top and bottom.  It's more aerodynamically stable that way.

Put a fin on each Kickback-- one on the left, one on the right.  Stick it way down low, as low as you can get away with.  That will really help your pitch stability during the turn.

It's hard for me to see your nuke stage up top (it's kinda obscured by all the KER stuff), but from what I can see, looks like it may be kinda draggy.  But hard to judge without a better view of it.

Anyway, those suggestions ought to do the trick.  I suspect that the major thing that's really killing you is having the top fuel tank drain first.  Fixing just that one thing may be enough, depends how draggy your payload is.

Edited February 11, 2016 by Snark

[Skylar']

use wing flaps. or don't do gravity turn. u can start slowly turning just after launch and target turn speed to get 45 degrees just at 10km i doing it all time and forgot about flipping rockets  

Edited February 11, 2016 by Skylar'

[Warzouz]

When I've flipping issues, I just increase the size of the fins (use wings). If the rocket is too steady, I reduce them. Turning them into control surfaces is also a solution as state above, but it can induce wobble and make the rocket hard to control.

Do you keep you AoA reasonable ? (less than  5°)

 

[cosmodrome]

13 hours ago, Warzouz said:

OK, I think that the gravity turn was my biggest issue.  I changed out the fins with the AV-R8 fins and added them to the Kickbacks.  Changed my assent to a more realistic flight and that seemed to work very well.  I did have some oscillations because of the payload, but that was expected.  Thanks for the help.  I'll have to try some of my other rockets to make sure I've got this down, but it looks good.

 

 

[Sovek]

Clustering engines can also help instead of using one big one I've found, while providing near the same the level of control without making the whole rocket super stable.

The following image allowed for a very stable rocket without any fins

Spoiler

Six Wildcat V's from KW offered pretty good control authority for such a heavy rocket.

[SnakyLeVrai]

Good to see you've found the butter zone  

If you notice the way rockets are made in real life, they mostly are long, thin tubes with a big ass engine at the end. The extra distance from CoM helps making thrust vectoring more effective, hence a more stable rocket.

My gravity turn starts right on take-off and, when looking at the navball, I try not to point the nose away from the green circle of prograde vector. If I did, the drag from the tip of the rocket would flip me. It means you indeed have to start your turn real early. Take some angle when you don't go too fast at first, because the speed will make drag stronger and you'll flip your rocket. It goes easier above 35km, atmosphere thins out and drag decreases a LOT.

See example below:

Spoiler

 

 

[redmonddkgamer]

I am using 1.2.2 with the Almaz/Proton craft file from Raidernick. I am using MechJeb for an ascent to 257km, with an inclination of 51.6. I didn't see a SoM problem in the editor, I think it is an ascent path problem.

[Snark]

1 hour ago, redmonddkgamer said:

I am using 1.2.2 with the Almaz/Proton craft file from Raidernick. I am using MechJeb for an ascent to 257km, with an inclination of 51.6. I didn't see a SoM problem in the editor, I think it is an ascent path problem.

Good to know. 

However, it's worth noting that the above conversation is from nearly a year ago, so I think it's pretty safe to say that the OP has long since had their problem solved-- not to mention that there have been a couple of KSP versions since then, and at least one of the problems with the OP's ship (fuel drainage issues) would be moot today anyway, due to the changed fuel-flow mechanics of KSP 1.2.

Locking the thread to prevent further confusion.  If someone else has a similar problem and wants to ask about it, feel free to open an new thread.