question about rocket stability

[endl]

one of my rockets is flipping over when it starts its gravity turn, im not exactly sure whats going on. i have no lift structures (i tried using some it didnt help) my COT is in line with COM. besides those things what can cause a rocket to lose orientation like this during a gravity turn? ive gone through several set ups, and i made sure that my navigation core for mech jeb is oriented correctly.

Edited October 1, 2014 by endl

[Friend Bear]

You are probably starting the turn too sharply in the atmosphere. There are a couple things you can consider/do:

1) Get a rotating wheel stabilizer on there. SAS rocks socks.

2) If you need to turn low, get at least two wings (control surfaces) at the base for stability and control. Even with less atmosphere, there is still atmosphere.

3) Try to wait to correct above the atmosphere. If you are burning fast and far, you may extend your orbit to an undesired height, preventing this from being the best option, so use it as a last resort.

Also, I would avoid mechjeb until you are more seasoned. When you have to make complicated maneuvers in the future, reliance on mechjeb prevents a full dissection of possible problems. I don't even use it because I want to understand every aspect of everything as for fault, etc. Just a suggestion. Don't worry, so long as there is support at liftoff, every rocket is going to go straight. Its over-correction you need to babysit.

Edited September 30, 2014 by Friend Bear

[Alshain]

Friend Bear's suggestion assumes your using FAR or NEAR. You can turn 45 degrees right off the launchpad in stock without any consequences. This is because there is no air pushing against your broad side in stock, where as FAR/NEAR simulate that effect, if you turn too fast your rocket will be pretty much traveling sideways against the air which best case scenario causes you to lose control, worst case with FAR is your craft gets ripped apart.

You said your COT is in line with your COM, but how far from it? If you have a heavy payload in the front of the rocket that shifts the COM too close to the tip, the thrust is going to push the back end around. This is true in any aerodynamic case. The solution in this case is to build smarter. You can break up your payload if possible, or even (in stock) hang it on the sides from a support beam along the sides of the lifter, so the center of mass of the payload is closer to the center of mass on the lifter. This all depends on the shape and size of the payload, you want to keep your mass on either side of the lifter equal.

Screenshots would help us help you.

Edited September 30, 2014 by Alshain

[Friend Bear]

Right, sorry for that. Been using NEAR for a while so forgot.

Thank you Alshain.

As for his comments, they are dead on. Those were the problems I ran into early on...weight. If the rocket had too small a lifting body, then sometimes being wobbly was not the problem. Instead, balance was. But I also agree that at this point we should look at a screenshot.

Edited September 30, 2014 by Friend Bear

[endl]

Right, sorry for that. Been using NEAR for a while so forgot.

Thank you Alshain.

As for his comments, they are dead on. Those were the problems I ran into early on...weight. If the rocket had too small a lifting body, then sometimes being wobbly was not the problem. Instead, balance was. But I also agree that at this point we should look at a screenshot.

yea im using stock aero, far makes my vc chug. i tried bringing the lifter mass higher as well the rocket still flips.

[Friend Bear]

Another possible issue is that the rocket is not lifting fast enough. When I had narrower rockets reaching 150m/s (rough guess but just know slow is a factor) a minute in, it would topple more often than not. Put some solid fuels on there too maybe?

Edited September 30, 2014 by Friend Bear

[endl]

i have been using limit terminal v on mechjeb as i was told it was inefficient in lower atmo. i guess i could just go full bore and give a shot.

edit: nope didnt work

Edited September 30, 2014 by endl

[Friend Bear]

Not too bad an idea. Just pay attention to ISP. Plus, a high atmo ISP engine may be less efficient than a buff, low ISP engine purely because with the lower ISP engine you cover, comparably, no ground. Always have to look at weight and thrust, etc.

[endl]

Not too bad an idea. Just pay attention to ISP. Plus, a high atmo ISP engine may be less efficient than a buff, low ISP engine purely because with the lower ISP engine you cover, comparably, no ground. Always have to look at weight and thrust, etc.

welp i give up. this thing has bested me i cant really break it up because of the way things need to be interconnected.

mods: Near future series(solar,ships,construction), B9, B dynamics, 6s tubes(with extra tubes from thread), karbonite (and k+), USI survival

craft file

[Franklin]

Have you considered spamming SAS units? Their effectiveness stacks. I mean like, see what 9 giant ones do for ya.

Also it's the antithesis of efficiency, but if your rocket is top-heavy maybe throw on something equally heavy on the bottom and eject it after you're in orbit. I've used a lab module as a counterbalance before. Gonna make your rocket maneuver like a fridge, though.

[endl]

is there like a preferred ratio of sas/ton? also mechjeb doesnt use sas so will that even work. or will i have to force it to use stock sas?

[Franklin]

Could always try flying it without MechJeb.

But while MechJeb doesn't use SAS the SAS modules' spinny-turny buff is applied to whatever you attach it to regardless if you've got SAS active, so it'll play with MechJeb fine.

[-Velocity-]

Could you have a fuel flow symmetry problem? I don't know what your rocket looks like, you simply posted a picture of the payload.

[endl]

Could you have a fuel flow symmetry problem? I don't know what your rocket looks like, you simply posted a picture of the payload.

nope i used asparagus assembly from different successful lifters ive made

[SRV Ron]

welp i give up. this thing has bested me i cant really break it up because of the way things need to be interconnected.

mods: Near future series(solar,ships,construction), B9, B dynamics, 6s tubes(with extra tubes from thread), karbonite (and k+), USI survival

craft file

http://i.imgur.com/Cu5ByXX.jpg

The command pod on top is notoriously unstable at speed in the lower atmosphere due to aerodynamics. It will need extra SAS control and or fins to stabilize. Turn slowly or expect it to swap ends quickly.

The two test rockets below tests this out, the left one is stable in flight, the right one needed fins to stabilize the second stage.

[Claw]

Yes, the cupola causes problems in this configuration. You will need to put more wings (top and mostly bottom) in order to control it. The cupola's drag index is set differently than other parts, causing it to have a lot higher drag.

Cheers,

-Claw

[endl]

well you guys were wrong about the cupola. it was the USI derp pods just found out they have a drag coefficient of 1.0 and i had 12 of them

[Claw]

To be fair, we are correct about the cupola and it's drag as far as stock craft go. Personally, I'm less aware of the settings on the mods you are using since I play mostly stock. In either case, I'm glad you got it sorted out.

Cheers,

~Claw

[Sirine]

Check my sign, try it.