FAR Help. Have to fight rocket when in atmosphere.

[Doctor Mod]

My rocket always seems to want to tilt a bit off of straight east. Even with SAS on I can't seem to avoid having it want to be a bit off. Am I doing something wrong here that I'm missing? Kind of hard to do a Mun Mission when getting orbit is a bit of a fight and probably wastes fuel from constant maneuvering.

[NathanKell]

Welcome to the forums!

Yep: your TWR is way, way, way too high. You should be launching at about 1.5, second stages don't need to start much above 1, and orbital stages can be as low TWR as your patience. Also I would suggest using AV-T1 winglets, or just setting the pitch/yaw/roll response of your winglets to 0 each; gimbals will give you enough control.

Finally, you are pitching over at about 100m/s, right? Not waiting until you're high up like stock?

[Doctor Mod]

Yeah. 100 m/s

I've been able to get to orbit before with other Rockets using FAR...though they always seem to have a bit of a wobble to them...even in space. Also wow...I probably should have checked those numbers. I'll try your suggestions and get back to you.

[impyre]

For what it's worth, I don't think you're doing anything wrong. This happens to me ever since the latest changes to the stability assist system... specifically, hold prograde is the worst. You might try and see if it happens in stock for you (or the without the SAS). I regularly design my launch stages with a TWR around 2-2.5, but I don't go full throttle either.

[Doctor Mod]

I just did a launch. SAS was off. Rocket goes up...but it doesn't want to do the turn. It always seems to want to head in a south east direction. I can turn on SAS. I can hold down the keys and it just...isn't wanting to listen to me.

It slowly gets to 100 m/s and I try starting my turn. Usually I either A. Have to keep pushing buttons to keep it on track and fight it every step of the way B. get it to do the turn and then constantly have to push buttons to keep it from leaning too far and flipping out.

Having watched a video of someone doing a FAR Gravity Turn...perhaps I am trying to turn too fast. They were at 7KM up and were making very very slight adjustments slowly.

Edited March 26, 2015 by Doctor Mod

[impyre]

You need to ensure that your center of lift is aligned (vertically) with your center of mass. Try selecting the entire rocket (in the VAB) with shift-click, then rotate the whole thing 90-degrees around its vertical axis(i think that's q/e)... see if the problem changes. If so, there's an alignment problem somewhere. I've had problems with the tri-coupler causing mis-aligned center-of-lift in the past. The best solution in my opinion (and this is what I do) is to go from a single stage to a three-sym stage by using radial decouplers and struts rather than using the tricoupler. Your overall profile should be somewhat sloping, not thin and pencil-like. This design won't be as tall, and will be much more aerodynamically stable.

[Taki117]

They way I do my turn is straight up to 100m/s or 1km, then pitch east to 80 degrees, but when the velocity vector is at 85 degrees then I just set SAS to follow velocity vector and watchmy time to apoapsis.

[Doctor Mod]

Welp I figured out one issue. I was not using Gimbaled engines. That explains why I was having a .... time turning my rocket or controlling it. Torque wheels can only do so much.

[Kryxal]

Something to think about, you could use the bi-coupler with an 800 and a 400, that would bring your initial TWR down nicely and you'd have just as much fuel ... a pair of 800s would give MORE fuel and probably an even better TWR for FAR.

[Vectura]

I often have a similar seeming issue. All of my rockets, and stock ones, always rotate so that they are going south east. SAS is unable to fight it, and even 4 large fins with 100% roll control are unable to stop it turning (SAS just doesn't even try, it seems). The new SAS is just useless. It is incapable of even doing stuff like pointing at a maneuver node.

[cantab]

I just did a launch. SAS was off. Rocket goes up...but it doesn't want to do the turn. It always seems to want to head in a south east direction. I can turn on SAS. I can hold down the keys and it just...isn't wanting to listen to me.

This sounds like you have too much stability, as well as too little control authority. With the wide part of the rocket and those fins down low you'll have a rocket that self-corrects if you try and nudge it. That's good except the effect may be so strong you can't pitchover.

As for the spontaneous pitchover in a certain direction, do you have any non symmetric parts? FAR applies drag to things like OX-STATs that stock doesn't. If that's not the issue my next guess is that the tri-coupler is known for not being perfectly symmetric. Could you switch to a first stage with three cores in a line?

[WuphonsReach]

Far better to have a rocket that resists pitch-over, then one that likes to tumble. The former can be easily corrected by adding vernors / RCS thrusters to the top/bottom ends. The latter is harder to correct for.

Worst case with a rocket that won't turn is that you waste a bit of fuel due to a non-optimal ascent turn.

Either way, the CoL in the VAB has to be behind the CoM until the 1st stage is exhausted (assuming 1500-2500 m/s in the first stage). You may need to use the right-click tweak menu to empty the 1st stage tanks of fuel (starting at the top and working downward) to verify.

[cantab]

In my experience I want it not too far off neutral. Either way is fine; mild aerodynamic instability in a rocket, usually down to drag rather than lift being up front, can be kept under control with attentive piloting.

[Doctor Mod]

Far better to have a rocket that resists pitch-over, then one that likes to tumble. The former can be easily corrected by adding vernors / RCS thrusters to the top/bottom ends. The latter is harder to correct for.

Worst case with a rocket that won't turn is that you waste a bit of fuel due to a non-optimal ascent turn.

Either way, the CoL in the VAB has to be behind the CoM until the 1st stage is exhausted (assuming 1500-2500 m/s in the first stage). You may need to use the right-click tweak menu to empty the 1st stage tanks of fuel (starting at the top and working downward) to verify.

I really need to unlock RCS. Would make steering and stabilizing in space so much easier.

[Astrofox]

Get rid of the wobble problem by getting Kerbal Joint Reinforcement, and do you use procedural fairings? Also your TWR needs to be lower than 1.7, or better yet, TWR < 1.5.