SAS Failure

[phantom000]

I launch my rocket and when i reach about 100 M/S i pitched to one side so that the rocket is bearing around 90 degrees. That's when the SAS looses control. Not all the time, about every 3rd or 4th launch. It usually doesn't go all at once, it wobbles a little, then a little more, a little more, a little more then suddenly it's pointing in the wrong direction. That is when i just quit and revert back to launch or whatever.

So any ideas on what the problem is? I added a picture because i don't know if this is a game bug or if it's just a badly designed rocket.

 

Edited July 9, 2016 by phantom000

[Aegolius13]

Looks like you have three radially mounted TwinBoars.  That is a crap-ton of thrust given your payload does not look too large.  Are you going full throttle?  Usually around a 1.5 thrust-to-weight ratio is good, so you might want to either throttle back or, better still, go to a lighter-duty radial booster arrangement. 

Also, you generally don't want to pitch over to 90 degrees that early.  Start with maybe 10-20 degrees, and then let gravity turn your rocket the rest of the way (throttle adjustments help to fine-tune - go slower and you'll tip faster).  SAS Hold Prograde also helps keep everything in line. 

You may also want to manually pump fuel forward to keep your center of pass towards the top of the rocket.  This promotes stability.  

Also, not 100% sure it's still the case, but those Rockomax nose cones are known to be pretty draggy.  You might want to try the 2.5m to 1.25m fuel tank or adapter, and a 1.25m nose cone.  The less drag at the front, the less flippy your ship will be.  

Finally, I doubt this is causing your problem, but you might be able to cut the reaction wheels from the boosters. Vectored thrust is generally fine to steer on launch.

 

[Snark]

(Just to be clear, when you say "the bearing is 90 degrees", you mean "I go east", not "I turn 90 degrees off prograde so I'm going horizontally", correct?)

The issue here is likely that you're not aerodynamically stable.  Can you show a screenshot of your rocket with the CoM displayed?

A few potential concerns:

• You've got 3-way symmetry.  That has the potential to be unstable.  Have you considered going to 4-way or 2-way?  One idea:  Change the 16-ton tank on your central stack to a 32-ton.  Go down to just two radial boosters instead of 3, positioned so they're on the north/south sides of the rocket as you go east.  Add fuel ducts so that the center stack drains the boosters.  Stage the center engine so that it fires at liftoff, along with the boosters.  I suspect you'll get considerably better dV that way, in addition to a more streamlined shape and better stability.
• You've got fuel tanks stacked on top of your radial boosters.  Those fuel tanks are going to drain first, moving your CoM downwards, which is a Bad Thing-- it makes you less stable.  Do you really need those extra tanks?
• The amount of stability that the fins provide goes up with their distance behind the CoM.  Suggest moving your radial boosters way, way downwards on the rocket.
• That Mainsail looks overpowered for the center stack-- consider replacing with a Skipper.  You'll save a few tons of dead weight, you'll move your CoM upwards, and you'll get better fuel efficiency.

 

[Vanamonde]

A couple of things:

I never build anything in 3x symmetry. It's squirrely. 

And you might have better control results with canards for fins rather than the stablizer-with-rudder combo part. 

[bewing]

In 1.1.3, I have actually had better luck with 3x symmetry than 2x. My 2x stuff tends to yaw in the "thin" direction for no reason.

But all your engines have gimballing -- so you shouldn't need aerodynamic control surfaces at all. If you like having fins, maybe you could try the small delta wings (no control surfaces), instead of the delta deluxe. It may be the case that the winglets are not helping the engines to keep you going straight.

[Snark]

14 hours ago, bewing said:

In 1.1.3, I have actually had better luck with 3x symmetry than 2x. My 2x stuff tends to yaw in the "thin" direction for no reason.

Well, it's not "no reason", it's because 2x symmetry doesn't give particular stability on the "thin" axis.  But that's easily dealt with:

• Make sure that the "fat" axis is aligned north-south, i.e. your two boosters are on the north and south sides of the rocket as you launch.
• SAS hold prograde on the gravity turn helps to maintain yaw stability.  Doesn't need a lot, because you're pointing perfectly due east the whole time.
• A modest pair of fins on the back of the rocket are generally enough to provide yaw stability in such a case.

 

[SpacedCowboy]

Yes, I agree with all the above advice with the exception of the Rockomax nose cones, they're really not that bad.

I would lose those ASAS wheels on top of the radial boosters, (keeping the center one).

Also, if there's nothing under that fairing up top, I'd lose that also and replace it with a nose cone (a lot less weight)