Wobbling and Tumbling

[SiliconPyro]

I've got some serious wobble issues. It seems like every rocket I make, add son as any radially mounted large tank gets near empty (typically a stack of at least three), I stay losing control. The ship will start wobbling, then tumbling end over end. Sometimes it's like a flat spin and totally uncontrollable. With my latest design, I have to cut engines when the fuel tanks are half full, stage, then accelerate again.

I wish I could upload a craft file, but not until tomorrow. I'm just venting here so I can sleep. LOL.

[OtherDalfite]

Try adding struts going from the decoupler (first stage) to a fuel tank in your second stage. The wobble is caused by the decoupler having a low force to keep the upper stages in place. As for the fuel tank and engine problem, again use struts to hold each one together radially and on a higher stage. Hope this helps!

[lucidLemon]

Do you have fins at the top of your rocket? They always screwed me over when I was first experimenting with fins.

[Gunhed]

Can we have a picture of the rocket?

[opticalshadow]

sounds like a few things could be it.

struts, do you have them? and if so are they correct? just connecting two peices isnt enough, you need to connect them in a fasion that prevents moving. crisscrossing the tops and bottoms with each other stops twiting, also for your bigger peises try to run a strut from the top of one peice to the nose of your ship, and the same with the bottom.

your weight ratio may not be symetrical all around. if it isnt it will turn topple and wobble. if you have an engine on one side, and that side isnt the bottom, you need an engine mirror on the other side, same with anything, the weight has to be even or it wil effect flight. if you have parts connected by decouplers, especially if the parts weigh alot diffrent and are of different size, make sure you have enough struts eraseing any leeway.

where are your control units (if any) if you have a long rocket, and you only put fins on lets say the top, it will wobble alot because the only point of control is coming from there.

[OxfordTheCat]

Try adding struts going from the decoupler (first stage) to a fuel tank in your second stage. The wobble is caused by the decoupler having a low force to keep the upper stages in place.

Wouldn't this prevent decoupling, or cause an extremely unpredictable decoupling as struts break (or not break) ?

I've never tried this because I always assumed it would have catastrophic results.

[maltesh]

Struts, like fuel lines, break whenever the attached subassembly decouples.

[SiliconPyro]

I have to stop asking questions as I'm falling asleep. No good can come of it.

I theorized that a lack of proper struts was to blame and reinforced without getting a screenshot. Unfortunately, the shots I did take were long distance shots of the vertical climb. I'll do a launch and see if the problem is still there. Thanks for the replies.

[SiliconPyro]

Okay, the problem is gone. I solved it by adding crossmembers between the large radial tanks as you can see in the below image. This doesn't solve my Delta-V problem, haha, but at least I'm making it to orbit with some sort of stability. I think I'm going to add some SRBs and another set of tanks.

[Nibb31]

I think that you have too much tankage. Try removing fuel tanks instead of adding more. I find that the optimal mass of fuel for those 2m engines is 2 big tanks. Adding more fuel tanks only decreases your thrust to weight ratio and is counter productive. In other words, you will burn longer, but you won't increase your delta-V because of the mass penalty. If you really need that much fuel, then add more engines and stages to get rid of the mass, but don't use more than 2 tanks on each engine.

Also, use a fuel pipe to feed those 4 radial tanks into the central core. That way, you can fire all your 4 engines for liftoff without using the fuel from the central tanks. Your radial tanks will empty sooner, but you will be going faster and higher by the time you shed them, and your central core stage will still have its tanks full.

[Analogy]

I'm having a similar problem with this attached craft. Works fine up till near the end of the third stage, then it starts tumbling all over the place. Everything's braced pretty well. Putting winglets on causes things to collide with each other on stage separations.

[SiliconPyro]

Thanks for the advice, Nibb31. I added tanks and SRBs and it did a little for me, but I forgot I used to run fuel lines like that. I haven't played a lot for a while, just getting back into it. Do you think I should put engines under the top two tanks, then a decoupler, then two more tanks with engines? I'm gonna give that a shot. It's stable now at least, so it's time for optimization. In all honesty, I'm awful at rocket design. I just throw stuff together until it works. I need to do more math during the design stage.

[cardgame]

Ever since I started putting two struts between all of my outer stages, I've had zero roll problems.

Place one strut on the top and one on the bottom – connect all the outer stages together. Works for SRB as well as liquid stacks.

[Analogy]

Ever since I started putting two struts between all of my outer stages, I've had zero roll problems.

Place one strut on the top and one on the bottom – connect all the outer stages together. Works for SRB as well as liquid stacks.

I'm already doing that in my design, can you look at it?

[SiliconPyro]

My rocket has evolved a bit and if I could keep it under control, I'd be making orbit with plenty of fuel, but I've still got problems. Here's my craft file. Have fun.

[EndlessWaves]

I did a test run and it exploded in orbit when mechjeb instantly throttled the engines up to full power to circularise because you have too much thrust at that point.

EDIT:

I had a further play with it. You didn't say what it was supposed to do exactly but if you're trying to get the top section into kerbin orbit (the poodle engine and everything above) then the lower half isn't needed and working on the efficiency of the upper half is sufficient, I've attached a revised version that can make it to orbit.

[ATTACH]34301[/ATTACH]

I mainly just rerouted the fuel lines slightly but I also made a couple of other tweaks like moving the sepratrons further round so there's less chance of their damaging the full fuel tanks and removing the nose cones from the lander (which are currently decorative only and should only be used when you're confident you can handle the extra weight).

Edited October 5, 2012 by EndlessWaves

[SiliconPyro]

The intent of the craft is a round trip to the Mun. A visit to the stranded Kerbals near the arch to be specific. I actually got my craft there, but there is definitely not enough fuel to get home, let alone make Mun orbit.

[EndlessWaves]

That ship definitely has enough fuel to land on the Mun and get back to kerbin even if you have to land somewhere difficult.

I've just taken it on a trial run and it did it, although your landing legs didn't make it. If you want them on the outside tanks then I'd reverse the direction of the fuel lines, you'll need more fuel for the trip to the mun than the trip back - you've got more engines pushing more weight on that leg.

[Luke321]

try to use mechjeb and roll all the time:

http://www.youtube.com/watch?v=VNNWLGYe894&feature=youtu.be

[Bluejayek]

I think that you have too much tankage. Try removing fuel tanks instead of adding more. I find that the optimal mass of fuel for those 2m engines is 2 big tanks. Adding more fuel tanks only decreases your thrust to weight ratio and is counter productive. In other words, you will burn longer, but you won't increase your delta-V because of the mass penalty. If you really need that much fuel, then add more engines and stages to get rid of the mass, but don't use more than 2 tanks on each engine.

Also, use a fuel pipe to feed those 4 radial tanks into the central core. That way, you can fire all your 4 engines for liftoff without using the fuel from the central tanks. Your radial tanks will empty sooner, but you will be going faster and higher by the time you shed them, and your central core stage will still have its tanks full.

It depends on what you are trying to do. Sometimes you really do need all that fuel. This craft for example has 8 tanks per engine, and needs all of that fuel. It does make extreme use of SRB's though.

Edited October 6, 2012 by Bluejayek
Spoilered

[Xellith]

It depends on what you are trying to do. Sometimes you really do need all that fuel. This craft for example has 8 tanks per engine, and needs all of that fuel. It does make extreme use of SRB's though.

Forgive me for sounding like an asshat, but I really cant think of a way to phrase this without sounding arrogant or something - but isnt having more than 1 SAS module pointless? I see 6(im asuming there are at elast 8) on that rocket design I think.

[SiliconPyro]

Forgive me for sounding like an asshat, but I really cant think of a way to phrase this without sounding arrogant or something - but isnt having more than 1 SAS module pointless? I see 6(im asuming there are at elast 8) on that rocket design I think.

SAS modules use torque to maintain course. So more can be better if placed in positions where they have leverage. Only the ASAS uses thrust vectoring to do it's job, which is why it is less useful without thrust vectored engines.