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Professor Duck

Rocket stability

Question

I feel dumb for having to ask for help on this since I haven\'t been having too many issues until now, but its getting really annoying and I don\'t know how to fix it.

Any time I tack some SRBs on the outside of my center-line fuel tank and have those lift-off first, my rockets will begin to spin (and usually pitch to the left or right). Even using SAS, ASAS, and RCS, I can\'t stabilize anything that might have a chance of going to the moon. I\'m not over-balancing anything or making anything too extreme.

Any suggestions?

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11 answers to this question

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Radial decouplers have a tendency to be a bit... flexible. Try putting struts from one booster to the next, at the bottom and at the top. See if that makes things better.

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Radial fins on the SRBS can also help, provided they\'re well aft of the decoupler.

In general decoupler torquing is a semi-intentional aspect of the game\'s design, and one approach is that if you can keep the nose on the right spot rolling actually doesn\'t matter. It\'s also guaranteed to last only 25 seconds.

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The problem is that when you light the boosters, they all hammer up against the decouplers, which bend and point the thrust outward. If that thrust gets off-center or off-axis at all, it\'ll snowball into chaos.

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It helps to have a very massive (as in mass-heavy) center of the rocket to increase the force needed to destabilize its vertical inertia into an angle.

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Here\'s my solution to the problem. It gave me a very stable, very rotation free and quite manageable rocket.

Put the decouplers as low on the SBRs as you can. The reason is quite simple: The closer the decouplers are to the engine nozzle, the lower the lever on the decouplers.

Connect the lowermost points on your SBRs with the SBRs next to it, so the 6 (or however many you use) SBRs are linked to each other with struts. This reduces the lever even more and equalizes the forces executed inwards between the SBRs.

Connect the top of the SBRs to the rocket inside with another set of struts. This is basically mostly for balancing and to lower the shear force on the couplers from the SBR trying to push 'up', less tear on the couplers.

That results usually in VERY solid booster behaviour. I had an array of 18 boosters arranged around my rocket without them going bonkers.

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It simplifies things if you put the decouplers at the top and strut the boosters together at the bottom; decoupler placement doesn\'t affect much in terms of leverage, but it means that there will be less force on the struts at the bottom (decreased mechanical advantage, don\'t feel like elaborating). It also removes the need for top struts, without decreasing stability.

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A combination of ASAS and wings mounted on the isdes of the boosters it probably the way to go. On the other hand make sure you aren\'t stacking the boosters directly on top of eachother

If all else fails, use the power of struts!

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If it\'s the solid boosters that are causing the radial attachments to bend, then I suggest struts to keep things rigid.

As said before, flexible things can cause the thrust to go off center and potentially cause spins/wrecks. Even in the best scenario it\'s wasting a bit of fuel.

If they\'re bending and you put wings on them, then it will just cause those boosters to flap more.

Wings will help AFTER you make the attachments rigid.

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Everything Mentioned above is very true.

Also, you remember that you can attack parts radially without decouplers - straight on to other parts - such connections are much more rigid and should help to reduce spin.

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Everything Mentioned above is very true.

Also, you remember that you can attack parts radially without decouplers - straight on to other parts - such connections are much more rigid and should help to reduce spin.

The trade-off being you are stuck with that mass until you eject the entire stage. It is a lot to consider when dealing with SRB\'s. Of course once you get all this nailed down, weather will get thrown into the mix. I eagerly await the videos of launches done during Hurricane Kerbinya.

Sometimes if my rocket begins to wobble or start to go its own direction during burn, I turn off the SAS and let it restabilize, then turn it back on. If it stabilizes quick enough, then the launch is saved and I actually get something more stable. Sometimes the SAS system can decide a good wobble, rotation, or lean is what\'s desired and you just need to...well...turn it off and on again.

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