Asparagus Staging Question

[StevieC]

I've noticed that when i use the stock Kerbal X rocket, asparagus staging seems to work perfectly fine with no trouble, but when I build my own asparagus-staged rocket, it rolls uncontrollably until the last of the radially attached boosters are jettisoned, and only then can I correct the persistent rolling. Has anyone besides myself noticed this problem, and has anyone found a way to correct it?

[KerbonautInTraining]

Is it rolling in one direction as if you had Q or E held? Do the booster engines have a gimbal? Pics of the ship would help a lot.

This belongs in "game play questions" btw.

Edited February 22, 2016 by KerbonautInTraining

[Johnny Wishbone]

It has been my experience that the Vector engine seems to be very susceptible to inducing unintended roll, even with the gimbal severely reduced (down to as much as 5%). I've stopped using them because of it.

It could be a part issue like that, or it could be how you have your fuel flowing from one another in the "asparagus" stages. The changes to the fuel flow rules in the last couple releases seem to have broken or handicapped a lot of the older design styles.

 

Edit to add: pictures of your design usually help diagnose issues.

Edited February 22, 2016 by Johnny Wishbone

[Friend Bear]

On 2/21/2016 at 9:02 AM, StevieC said:

I've noticed that when i use the stock Kerbal X rocket, asparagus staging seems to work perfectly fine with no trouble, but when I build my own asparagus-staged rocket, it rolls uncontrollably until the last of the radially attached boosters are jettisoned, and only then can I correct the persistent rolling. Has anyone besides myself noticed this problem, and has anyone found a way to correct it?

This happened to me a while back, but only because I was using an odd number of boosters per asparagus stage; three or five. I feel like it is simply because of programming and preventing perfect symmetry. 

 

Try to make sure your asparagus is setup with two, four, six, and so on boosters depleting and dropping per stage. While I am fairly certain it is simply the boosters needing to be an even number, making the stages even keep the process easy to keep track of.

[StevieC]

oh I have six boosters, in fact the rocket I was building happened to be a simplified version of the Kerbal-X (using NecroBones' FuelTanksPlus mod to replace the three X200-16 tanks of the central booster with a single X200-96, and replacing each asparagus booster's FL-T400 tanks with a single FL-T1200 tank, and adding the FuelTanksPlus nose-cone fuel-tank to each asparagus booster).

Edited February 22, 2016 by StevieC

[FullMetalMachinist]

57 minutes ago, StevieC said:

oh I have six boosters, in fact the rocket I was building happened to be a simplified version of the Kerbal-X (using NecroBones' FuelTanksPlus mod to replace the three X200-16 tanks of the central booster with a single X200-96, and replacing each asparagus booster's FL-T400 tanks with a single FL-T1200 tank, and adding the FuelTanksPlus nose-cone fuel-tank to each asparagus booster).

It's funny that you're using a mod by NecroBones, as he happens to have a good video on this very problem on YouTube. 

 

 

[Jhawk1099]

Perhaps some fins would help?

[StevieC]

it already has the same number of fins, in the same arrangement, as the stock Kerbal X. The stock Kerbal X itself does not do this, but even if i just take the lower-stage of the Kerbal X and make a subassembly out of it, and slap that onto a different upper stage, that rolls too.

[ForScience6686]

Just a guess here, but I've noticed this only on vehicles with asparagus boosters, as fuel and occasionally struts are put on independently.  Would that cause issues with aero stability?  For example in angle snap mode, it only applies to the first attachment point and attracting the other end usually puts it off center or at an angle.  That is my guess at this problem.  Fortunately it doesn't hinder launches much as I'm generally on follow prograde through the boosters, but it can make it hard to line up your launch trajectory.

[Temstar]

It's caused by the boosters flexing. The uncontrolled roll problem has been known since the earliest days of asparagus staging in KSP. Putting fins or gimbaled engines will not fix this, you need to make the boosters more rigid to fix the roll:

Bottom reinforcement:

 

Top reinforcement:

Without reinforcement struts like this under acceleration long boosters will warp themselves into a subtle helical shape, which is the cause for the roll. Roll control authority on the boosters without reinforcement actually makes the roll worse because it simply causes worse warping of the booster instead of transmitting the correcting roll force to the centre core.

[StevieC]

hmm, then why is it absent from the stock Kerbal X only to rear its ugly head the moment i change even the tiniest thing on said Kerbal X?

[Temstar]

Don't know. Could be a lot of other lucky factors that work together to disguise the problem but I bet if you strut it well you will solve the problem at its root without resorting to voodoo rocket building. 

[FullMetalMachinist]

Just be cautious with the MOAR STRUTS answer, as struts have pretty horrible drag, and can really suck some serious dV out of your rocket if you're no careful. 

[Garibaldi2257]

I only ever use 1 strut per medium sized booster (orange tank, parachutes and engine) and it stops flexing for me.  its likely you'd need more struts for larger boosters.