Rocket flipping

[Tripod27]

So, this rocket here (using it hopefully to get to Duna and back) has a bit of an issue

As soon as I begin my gravity turn, the thing just goes into a top over bottom spin and having all of my gimbals activated can't counteract it (I generally only use the middle one). Basically if I have it pointing anywhere other than straight up, it just starts flipping

I've never had this issue before, and I have no idea why it's happening (sorry for the two images btw, couldn't zoom out enough to fit it in one)

In case you're wondering, by the time I start my turn I'm at 12000-15000 meters and two of the rockets (the one closest and furthest from the camera) have already seperated

Edited June 4, 2013 by Tripod27

[rryy]

I have this problem, too, sometimes. The best solution I found for this is to start your gravity turn earlier on, around 3 km up, and gently turn over to do the gravity turn instead of turning directly to 45º at 10 km, keeping somewhat closely pointed to the surface prograde vector. Adding some winglets also seem to help.

[Supernovy]

Can we have a picture of the payload? It may be that there is to much drag at the top of the rocket.

[Sᴄɪɴᴛɪʟʟᴀᴛᴏʀ]

Its likely the centre of mass is too high, what is the weight of your upper stage? Aerospikes are quite heavy and you have some atomic engines there as well so that may be your issue.

[Vanamonde]

Have you tried some fins near the bottom?

[Tripod27]

Can we have a picture of the payload? It may be that there is to much drag at the top of the rocket.

Yeah, I'm stupid and uploaded the same image twice. Edited to show payload

BTW it does have the FAR CAS menu up in the top, but I'm not using it, it lags the hell out of my game, especially if I'm using non-stock parts

Edited June 3, 2013 by Tripod27

[Sᴄɪɴᴛɪʟʟᴀᴛᴏʀ]

The payload is overweight and is making your craft top heavy. You should try and compensate by adding some control surfaces to the base of the rocket as well as adding more struts between the payload and lifter. If that doesn't work I'd recommend a separate launch for the top of your payload which looks like a service module of some description and dock the two craft in orbit before transferring to Duna.

[Tripod27]

Ok, tried adding 3 fins to each base rocket for a total of 18 fins, and it still wasn't enough. I even turned 90 degrees so my area with no fins on it (due to staging) would mostly be effecting yaw instead of pitch, and I just couldn't keep the nose up if I turned more than a degree.

The center of mass is decently centered, as displayed both in the hangar and by the camera in game, and it's where my top ring of seperatrons is, just below the middle of the two jumbo tanks. By the time I make my turn it's moved up to about the center of the two tanks. Is that really too high?

[Sᴄɪɴᴛɪʟʟᴀᴛᴏʀ]

The thing about centre of mass is that the burning of the fuel will gradually shift it upwards and then when you attempt your gravity turn the tip of your payload becomes a pivot point and if the torque and lift of your rocket can't compensate for the craft's inclination to tip over the rocket will begin to flip. Try test launching it with the current set up but remove the service module and see if the results change.

[E. F. Kranz]

First stage is too tall. Cut it into two stages.

[Nao]

The thing about centre of mass is that the burning of the fuel will gradually shift it upwards and then when you attempt your gravity turn the tip of your payload becomes a pivot point and if the torque and lift of your rocket can't compensate for the craft's inclination to tip over the rocket will begin to flip. Try test launching it with the current set up but remove the service module and see if the results change.

Uhm, i don't think having a top heavy rocket is bad since it means also that gimbaled engines produce more torque. Its more about the top stage bending around (probably on the 3man pod to lander connection and then more on the big decoupler below). So the CoM drifts to the side.

@Op try more struts, especially try making struts between asparagus staged orange tanks and bottom payload, and then another round of struts between bottom payload and rover.

[Sᴄɪɴᴛɪʟʟᴀᴛᴏʀ]

Uhm, i don't think having a top heavy rocket is bad since it means also that gimbaled engines produce more torque. Its more about the top stage bending around (probably on the 3man pod to lander connection and then more on the big decoupler below). So the CoM drifts to the side.

@Op try more struts, especially try making struts between asparagus staged orange tanks and bottom payload, and then another round of struts between bottom payload and rover.

He disabled the gimbals on all but the center stack rocket but even with full gimbaling it can only do so much for a top heavy rocket and once it gets that inertia going as it begins to flip gimbaling won't do much to recover. If the center of mass is too high to maintain the rocket upright during the gravity turn then either weight shedding from the payload or a new stage is needed.

[Tripod27]

Guess I'll just cut it down to two payloads then, thanks guys (didn't add more struts because there really isn't room for them and I hate adding struts to asparagus stages below because as you take off pairs it wobbles unevenly)

[Nao]

He disabled the gimbals on all but the center stack rocket but even with full gimbaling it can only do so much for a top heavy rocket and once it gets that inertia going as it begins to flip gimbaling won't do much to recover. If the center of mass is too high to maintain the rocket upright during the gravity turn then either weight shedding from the payload or a new stage is needed.

I do agree that the part of the problem here might be a top heaviness of the rocket. But feel like your reasoning for this is flawed. The distance from engine to CoM doesn't influence stability in any way for theoretical fully rigid rocket. It's the bending of parts that make the CoM move off center that's causing the instabilities. And the top heavy rocket creates bigger bending forces. It would probably fly fine if just strutted right.

I just wanted to clear that out since i've seen that misconception on this forums several times.

(It's hard to judge peoples experience on forums so sorry if you know that already )

[zanzibar196]

Here is a how-to on getting the heavy lifter base you want to build right:

...and a video showing how stable it is, in real flight... (no mods!!)

[Eric S]

BTW it does have the FAR CAS menu up in the top, but I'm not using it, it lags the hell out of my game, especially if I'm using non-stock parts

Just clarifying what you're saying here, if FAR's aerodynamics are active, it will cause exactly the kinds of problems you're describing with non-aerodynamic payloads. Are you not using the menu, or is FAR not in effect when you launch?

[Sᴄɪɴᴛɪʟʟᴀᴛᴏʀ]

I do agree that the part of the problem here might be a top heaviness of the rocket. But feel like your reasoning for this is flawed. The distance from engine to CoM doesn't influence stability in any way for theoretical fully rigid rocket. It's the bending of parts that make the CoM move off center that's causing the instabilities. And the top heavy rocket creates bigger bending forces. It would probably fly fine if just strutted right.

I just wanted to clear that out since i've seen that misconception on this forums several times.

(It's hard to judge peoples experience on forums so sorry if you know that already )

No problem at all, I've been playing KSP about a year so I have a decent understanding of the mechanics but in practice you aren't going to have a fully rigid rocket unless you strut the connections between all the major parts because if you strengthen one connection it will just begin to sway at the next weakest point until you have struts everywhere. The simple fix without inflating the part count is just to shed weight to mitigate the sway effect and use separate launches. But if his computer can handle the extra parts then strutting should fix it as long as he doesn't do an overly-steep gravity turn lower in the atmosphere where drag becomes an issue.

[Nao]

@Sᴄɪɴᴛɪʟʟᴀᴛá´ÂÊ€ Ah ok agree 100%.

More OT:

One thing i've started using recently to improve rigidity is to connect *every* part in the central stack to its (stack) neighbor. Mostly with only one short strut. It doesn't provide that much rigidity but blocks any bigger bends on parts that that are prone to it like the small 2m fuel tanks, 2m decouplers or hitchhiker modules. So even under big loads the payload doesn't move too much and it returns to original position much faster.

Also about adding struts to asparagus stages, it can work if you: 1)make sure the asparagus stage itself is firmly attached to central tank so even when there is only two boosters left it doens't wobble. 4 struts are usually enough ex:

   O     < asparagus boosters
  | |    <-struts
   @     <-central stage
  | |  
   O     < asparagus boosters

Then use two struts per booster (in the same way) to connect to the payload. This way also you don't need to connect asparagus pairs to each other at all, and it will still not spin during ascent.

Cheers!

[mellojoe]

Have you verified your fuel delivery?

I just had a rocket displaying the same tendancy, and it turned out when I was doing my fuel lines, I didn't have symmetry on. Thus, only 2 of the 3 rockets were fuel transferring. I was using fuel unevenly, creating a huge imbalance. You might right click on the fuel tanks while you launch to see that they are draining correctly and in the correct order.