Rocket Flies Wonky, No Idea Why

[deltalambdamu]

I tried to build a rover for Mun. I got a good design I like. So I then build a rocket to get it there. All of my launches ended the same way. The rocket would start tilting west. Then it would start tilting north a few seconds later. ASAS, Retraction Wheels, and RCS are not enough to keep it straight, even with me pushing the buttons to point it the other direction.

I thought it might be an aerodynamics issue. In the VAB, I enable the two buttons for center of mass and center of lift. The center of gravity is slightly off center to the right of the center of mass. The center of thrust is off center to the left of the center of mass, and pointing slightly to the left. I use symmetry mode when building rockets, and this was no exception.

I thought the rover itself might've been the problem. I built a small rocket with the rover to see if the effects stayed. The rocket flew pretty straight. I removed the rover completely from the problematic rocket. SAS and RCS were enough to keep it straight, so I disabled them. When I did, the rocket did pretty much what it had been doing, but to a lesser extreme.

Just to see if it'd make a difference, I added 6 multiconnection ports (each with mass 1.6) and the only thing that changed was the center of mass. I didn't fly it, though.

I'll post the craft files tomorrow, but what are your first thoughts?

Edited September 3, 2014 by deltalambdamu

[Claw]

Pictures (or a craft file) would definitely help in troubleshooting. Pictures are usually preferred, especially if you have any mods.

However, when putting a rover on top of a rocket, it's pretty common for it to be out of balance. While the rover was built in the SPH with symmetry, it will still generally be asymmetric in the up/down direction. When you then strap it on top of a rocket, it can cause problems. The other thing is that if you put all that mass on top of a rocket, it will sometimes cause it to want to flip over.

What you really need is for the Center of Thrust (the pink marker) to be directly in line with the Center of Mass (the yellow marker).

Also, I know you said you have reaction wheels and RCS, but you may want to consider adding fins to the bottom. If it wants to flip over due to the rover's weight, fins will help and they're fairly light.

[neamerjell]

The center of [lift] is slightly off center to the right of the center of mass. The center of thrust is off center to the left of the center of mass, and pointing slightly to the left.

I believe you may have just answered your own question, in part at least. Shifted centers of mass, lift and thrust can cause all sorts of problems. I am curious how this can happen though. Are you using any procedural parts mods? I have experienced things with procedural wings and symmetry where the center of lift will go slightly left or right for apparently no good reason. I click the part, remove it and reattach it and the problem apparently solves itself. I blame a bug in the mod. Are you using any aerodynamic rules changer mods? Ferram Aerospace can cause all sorts of stuff to act differently than it does in the stock game.

Are you using 3-way symmetry? Are all your engines of the same type? Are all your fuel tanks the same mass? The last question may hold the cause of the off center mass. If you right clicked a fuel tank and dragged one of the sliders down by accident and didn't realize you had done it, that would throw off your weight distribution and possibly your engine burn times depending on your design.

(Claw beat me to this suggestion by one minute LOL!)

Finally, try adding fins to the bottom. Control surfaces are much more effective than reaction wheels while in the atmosphere.

I hope I at least gave you some starting point of where to look for the cause of your problem, but without pics or video or a craft file, I'm afraid all I can do is speculate.

Edited May 23, 2014 by neamerjell

[allmhuran]

I wonder if, perhaps, you have had the unfortunate experience of clipping a part completely inside of another part without having symmetry on, which the assembly buildings sometimes accidentally allow.

For example, say you build a simple 3 column rocket... one long core and two long liquid boosters powered by LVT30s attached radially. Then you accidentally pick up an LVT30 in 1x mode. It is possible to connect this new LVT30 so that it is entirely overlapping one of the existing booster engines. If you have done this, the part will look "extra shiny" in the VAB and when flown. It will also mean you wil have asymmetric thrust to weight. It is similarly possible to do this with fuel tanks and several other parts, with similar consequences.

[SRV Ron]

You could also have a broken strut upon launch that can cause the same issue given that you mention the instability on flight without the rover.

Broken strut caused this;

When the offending part was fixed, flight was stable.

[Sirrobert]

Usually if your rocket has a flaw, and you can't figure out why, it's a good idea to scrap it and start anew.

That will often solve the problem

[deltalambdamu]

I guess it was the game just acting really stupid. It wouldn't be the first time. But I went to make a video and noticed the game decided to delete a fuel line (actually, it just attached both ends to the same tank). I fixed it, re-attached the struts because one of them got completely deleted. I then tried to fly the rocket again to see if the issue persisted. And everything is working fine now. I have the thing in LKO getting ready for a transfer burn, though I may need to launch a refueling mission first. Here are my ships:

Rover - SPH Rover

Ship - VAB Rocket

[SRV Ron]

Fuel lines and struts can vanish under certain conditions. When it is fuel lines missing, the rocket quickly goes out of balance. With struts, things break causing instability and eventually failure.