Spinning rockets (end over end)

[sylvie369]

Okay, I'm lost. I've been flying Kerbal rockets with fair success for quite a while, but now I cannot seem to build a rocket that doesn't start spinning wildly at some point in its flight. If thrusters are on as well as SAS, they fire in the direction that makes it spin MORE, not less. If I leave the thrusters off I still get the spinning, though later in the flight.

I suspect this is a simple problem of the relationship between CG and CP (that after I've burned enough fuel it goes backwards and causes the problem). I understand CG/CP relationships in real rockets (I've built and flown hundreds of them), just not how to maintain the necessary relationship in KSP rockets. Anyway that doesn't explain why the automatic stabilization fires the wrong way. I don't know how to solve that.

I also suspect that this has something to do with the change in physics that came with the recent update.

Regardless, I no longer have any idea how to make rockets that fly reliably. Any hints?

[Kaos]

I can often solve this problem by flying slower and to stay closer to the flight direction.

[sylvie369]

It seems to be something much more than that. I could be flying nicely in orbit, engines off, stabilization on, holding me in the direction of the flight path, and all of a sudden it starts tumbling end over end. Several times I was stably in orbit with engines off when it started happening.

Oof. And just now it started doing it in a capsule that had already safely splashed down in the water. That suggests that it has nothing to do with CG/CP relationships (it's just a capsule) or the thrusters (since there aren't any).

Hmm. And if I turn SAS off, the water starts splashing like mad. ...? The water calms if I turn SAS back on, but the capsule keeps spinning.

There's clearly something in the rocket that is forcing it to spin. It doesn't seem to be related to the rocket's environment, since I've now seen it start in the atmosphere, out in deep space, and in the ocean. In the ocean case right now the rocket (capsule) wasn't even moving when it started to spin.

Edited June 28, 2015 by sylvie369

[Kaos]

I once had the game in a mode where everything started to wobble and turn around. After restart (complete close of the program), everything worked fine again.

Another reason I can think of is that if there is a lot too much control stuff in the rocket, SAS can start to destabilize it by overcompensating minor errors. Does it work without SAS and only manual control?

[ElWanderer]

Sounds like phantom control inputs, perhaps.

With SAS off, what are the pitch, roll and yaw indicators showing? If they're not centred hit modifier-x to cancel trim (modifier is alt on Windows)

Do you have a joystick?

Alternatively, this sounds a bit like the "universe gone mad" that I've heard described after using the klaw causes a game-breaking bug (klaws are kraken-bait), but I've no experience of it myself.

[sylvie369]

I'll try that. Are you suggesting that alt-x could turn off phantom control inputs that are causing the spin?

[Kryxal]

Alt-x is the "reset trim" command, I believe. My first thought was part clipping, but that's a neat trick with just a capsule and maybe some chutes.

Is it using battery charge to make you spin (or monoprop as the case may be) or is it just happening without resource use?

[sylvie369]

Oh, I just lost another one - one that had been flying normally - to this "phantom control input" problem. I was able to slow the spinning down by rotating the rocket 180 degrees so the control forces were working against the spin. I didn't think to look at whether or not it was using battery change. Hopefully I'll remember that next time.

As the thrusters and SAS were causing the spin, the attitude indicators were showing forces. Before I manually rotated it, the pitch indicator was pinned to one side. After rotation, the yaw indicator was the one pinned.

Is this "part clipping" something I can fix? Or should I forget about that as a possible cause?

[selfish_meme]

The way to design a rocket for sustained atmospheric flight now in more realistic aero is first to make sure it is tall and slender. Second the front must be heavier than the rear. You are probably well up on these two. The third factor is it must be more aerodynamic at the top than at the end. Otherwise drag will pull the nose of the rocket around. There are two ways your nose gets more drag than the tail, by being un-aerodynamic to start with, and having no fins to create drag at the end, and second speed. If your rocket has a TWR higher than 2 and you start flying faster than the speed of sound (around 260m/s at SL I think) the drag on the nose rapidly increases, where the tail because it is occluded does not. Eventually the nose becomes more draggy than the tail and the rocket flips. Use the Kerbal Engineer mod, keep your TWR around 1.6 and stay slower than 300m/s until around 10000m.

[p1t1o]

Just on the off-chance...

Your command pods always face the right direction right? As in, the "up" of the command pod is the intended direction of flight?

Do you use multiple command pods?

I think I remember building a rocket once with an inverted command pod - this caused the flight controls to also be inverted, causing similar problems to those you describe.

Are you running stock or with mods?

[Mars Mullo]

I found that there are two simple things to that work.

1. stay in the prograde bubble all the way to about the 30 -50km altitude.

2. place stabiliser fins or even wings on the lowest parts of your main rocket (don't waste them on your SRB's) (Don't use the rudder wings as they almost seem to over - do the correction and buffet the rocket back and forth)

That's it and it worked for me.

Easiest way to keep the speed appropriate to do those things above is to have all your rockets in the lower stages with a TWR of about 1/ 1.2 > 1/1.5. Reason being is the rocket will get a hard time at the time it passes from sub to super sonic. If you slowly pass through the band it seems to keep your rocket on target instead of giving the control surfaces a quik flip into a direction outside the pro grade bubble on the UI.

[sylvie369]

Yes, my command pods are facing up. And it's clearly a matter of the thrusters/SAS pushing the rocket into the spin, not a matter of instability. I just watched a rocket flying just fine and suddenly the thrusters just knocked it over (and the SAS seems to be doing the same thing - killing the thrusters didn't help much).

I know the aerodynamics models were changed recently, and I suppose that might have something to do with it. I've been flying KSP rockets for a few years now without ever having seen this problem before. If it's a matter of those rockets being unstable, they're unstable in a way they didn't used to be. All of these rockets would have flown fine six months ago.

==================

Just now I noticed that if I send a Kerbal EVA while it's spinning, the spinning stops, until the Kerbal reenters the pod. Then it starts spinning up again like mad. I was able to cycle through that several times before finally the spinning pod flung the Kerbal off into space. Each time when he went EVA, the pod quickly stopped spinning. This was on a rocket that I'd flown out to a parking orbit about 1.1 million meters out without incident. Once out there it orbited just fine for a while, then began spinning end over end. Again, it seems to have nothing at all to do with rocket design or aerodynamics.

Edited July 3, 2015 by sylvie369

[ElWanderer]

Are these all happening to the same ship design or lots of different ones? If it happens to multiple ships with no design elements in common it's unlikely to be part clipping. All I can think of is a badly-behaved input device or mod.

[Daveroski]

Remember that the pod has reaction wheels built in. If you have too much SAS, an added SAS controller for instance, this can make smaller ships 'twitchy' they tend to respond to phantoms much more readily.

If you feel that you can reduce the SAS you have on a craft, I suggest you try it. The difference can be quite surprising.

[Kryxal]

Also note that staging off SAS modules once they're overkill is perfectly valid.

[SAI Peregrinus]

Have you used the offset tool on the command pod/part? That will cause this. I'm not sure if the pod has to be the root part for that to happen, I need to test it more.

[Bev7787]

Do you have fins added on your rocket? Also can I see your rocket. Pics plz