Confused by side boosters, course deviation and wobbling

[DerekJ]

Hi everyone

I'm relatively new to the game and currently struggling with building any kind of simple rocket containing side boosters. I'm not trying to do anything extravagant now, just trying to get a good launch into equatorial orbit. I seem to have two problems that I can't completely solve.

Problem 1: After turning east after liftoff and turning on SAS-prograde the rocket deviates from its heading and heads off-course quickly resulting in a predicted orbit not 0 degree inclination. This issue doesn't occur if I remove the boosters completely, showing that the side boosters are the problem. I'm 99% sure I don't have any asymmetry in the design.

Problem 2: Sometimes the boosters wobble after launch causing all sorts of problems, sometimes catastrophic. 

Here's what I've already tried:

- Playing around with exact position of radial decouplers on the main stage, and the position of the boosters on the decouplers. I can sometimes mitigate the issue of wobbling but it's counter-intuitive since in the picture below I have everything seemingly "centred" so it should be ok??? Yet there's still wobbling.

- Moving booster orientation from N/S to E/W (that shouldn't make any difference right??)

- Removing fins from main stage (doesn't help)

- Changing solid boosters to liquid ones (same issue with course deviation)

- Adding struts to connect the booster pairs together with 2x symmetry (no matter how many struts I add there's still a significant problem, should I connect struts to the main stage or just between the boosters)

- Adding launch stability enhancers (doesn't help at all)

Any advice appreciated. Really getting frustrating since this issue never seems to come up in videos of other people using them.

Here's a picture:

[Vanamonde]

Neither the boosters nor that type of fin can pivot to help with steering. I suspect your problem is a matter of having to apply a lot of corrections and counter-corrections, causing flexion. Use the pivoting fins, if you have them yet. Yes, strut the upper part of the booster to the central stack. Just one each should be enough to eliminate most of the trouble. And otherwise, just try to steer gently. 

Welcome to the forum.

[HvP]

In addition to what @Vanamonde said, which is true, your fins aren't steerable - your solid rocket boosters are the only thing firing on your first stage and THOSE aren't steerable either. They don't have a gimble to redirect your rocket's course. You don't have much control until you drop them and switch to the stage with what I'm guessing is an LV-45 engine which DOES have a gimble to pivot and direct the rocket's course.

So, like Vanamonde said, replace the fins with steerable fins, and/or maybe move the LV-45 "Swivel" engine down into the first stage so that it fires with the side boosters to give you more steering control. Just be sure to right click on the boosters and turn down your thrust so that you aren't accelerating too fast with all three going. This might not be necessary if the steerable fins give you enough control to steer the ship though.

Edited May 17, 2020 by HvP

[overkill13]

Strut the nosecones rather than the boosters.  I know it looks terrible, but the boosters are already fairly well secured by the decouplers.  The game simplifies physics in ways that makes things weird, like strutting parts that already have a direct connection having little effect.  I've built many a rocket with virtually the same design and I always strut the nose cones.  Actually I autostrut them, but the difference should be minimal.

That pod is not horizontally symmetrical in the orientation you have it sitting in.  There's a ladder and window on top that doesn't exist on the bottom.  The game does some calculations for the aerodynamics on the surfaces facing into the airflow so that could be causing a pull to the north or south.  I'm seeing something hanging off the "bottom" side of the pod too, near the parachute, that would be the left side in the photo.  That could be pulling.  Rotate your nose cone so the window faces N or S and you should only get pull in the vertical direction during flight.

Keep in mind that I have NEVER seen a flight come off the pad and turn east with precisely 0 inclination without my input and most of the time, even with active piloting, it doesn't go below around 0.04 degrees on my KER readout.

Your rocket is simple and pointy, there will be more gravity losses than aerodynamic ones in this design.  Fire both boosters and the main engine at launch and get away from Kerbin as fast as possible, while also enjoying the gimbal on that Swivel engine.  You shouldn't need the fins at all in this case.  Use fuel flow priority to drain the fuel at the bottom tanks too which will help keep the rocket pointed prograde.  I used to concern myself with airspeed on launch, but I have seen far less delta-v consumed with a max speed launch than slow and steady with concerns about breaking the sound barrier, in these designs.

Completely irrelevant to the topic, but do you like that shroud on the heat shield?  I hate them, you should be able to move the decoupler to a slightly higher attachment point and eliminate the checkerboard shroud.  I also wonder about their aerodynamic effects since it sticks out from the side of the rocket.

Edited May 17, 2020 by overkill13

[DerekJ]

Thanks everyone.

I put struts on the boosters and more importantly added steerable fins. I had a very successful launch to orbit on my first attempt. Amazing that one detail changed so much. 

Later I'll play around with the orientation of the pod.

DJ

[Klapaucius]

3 hours ago, DerekJ said:

Thanks everyone.

I put struts on the boosters and more importantly added steerable fins. I had a very successful launch to orbit on my first attempt. Amazing that one detail changed so much. 

Later I'll play around with the orientation of the pod.

DJ

For rockets, I tend to set everything to "rigid attachment" (note: rigid is not something to use on aircraft because they become too brittle on landing) and I always autostrut. As you get to bigger rockets, you wind up with the damp noodle syndrome otherwise.  If you don't already have this set, you need to go into your settings page and activate "advanced tweakables" to have these options.

[HvP]

That's good news @DerekJ!

Welcome to KSP and happy flying!