Fins are counter productive.

[KrazyKrl]

I think the problem you're having is the fins, is your rocket occludes some of your fins from the airstream, reducing their effectiveness. Keep your rocket pointed very close to prograde; or you'll start to stall like an airplane.

As for the Fins-west, nofins-east issue, I believe that is something with phantom forces; and the eastward rotation of the planet. Try to offset your fins into your rocket a bit; to reduce their stability.

Also, you are experiencing something that is used with real rockets. A true gravity turn. Real rockets would start their eastward orientation shortly after clearing the tower.

Also, if you're using a gimbal-equipped core engine; try setting your control fins to "roll only".

Too little stability = your rocket's body lift will cause stability issues

Too much stability = no control

Too little control = inability to change AoA much, but is not much of a problem; unless you get too far from prograde... then you can "stall" your rocket, just like an airplane.

Too much control = Just be careful, or you'll rip your rocket apart.

Edited May 10, 2015 by KrazyKrl

[Temeter]

I see what you mean, but it does not explain the Fins->West/No-Fins->East issue.

Under FAR 0.15, it does not happen.

FAR calculates lift quite a bit different. As for the west/east things, I did explain it to you. Minimal forces get extrapolated because of the rockets instability. Who know what it is? Maybe a side effect of the nosecone reverting the tilt before achieving a 'stable' speed, maybe control surfaces lowering the COL so far that the effect gets denied and reverted. Maybe the planets rotation.

Of course. Building an optimal rocket was not the point.

Talking about why a terrible rocket doesn't work well is kinda pointless, don't you think?

[gogozerg]

Talking about why a terrible rocket doesn't work well is kinda pointless, don't you think?

Then you're in the wrong thread.

I'd like to understand the behaviour of the aero model, even if the problem shown here is not relevant when flying a well engineered and balanced rocket.

[BlackHat]

Of course. Building an optimal rocket was not the point.

Talking about why a terrible rocket doesn't work well is kinda pointless, don't you think?

His "poor" design identified a real problem, What is probably a bug that needs to be fixed.

And improved design will not fix the problem, only hide it.

[Tarheel1999]

Less likely a bug and more likely due to how planets and terrains are created in KSP. Factor in rotation and gravity and you have your answer.

http://forum.kerbalspaceprogram.com/threads/120331-Fins-are-counter-productive?p=1922669&viewfull=1#post1922669

[djnattyd]

I've just noticed this happening using a rocket built using the new tier 0 fins.

[Engineer of Stuff]

Um, what did I just read? Is someone hatin' on fins?

[gogozerg]

Um, what did I just read? Is someone hatin' on fins?

Then read again.

[herbal space program]

It's an engineering issue. You need a better T/W ratio and actual control authority.

I had exactly the same problem with a rocket that had >1.3 TWR and took off like a cannon shot. Even if I locked the SAS prograde right at the outset and then didn't touch anything, as the rocket hit around 100m/s it started to flip and there was no way to stop it. The same rocket sans fins flew just fine. It's a problem with either the symmetry system or the aero model. In the real world, there is no reason a rocket with fins at the bottom going 100m/s through thick atmosphere would want to flip more than one without fins. Try it throwing a dart or shooting an arrow some time!

[Rickenbacker]

Less likely a bug and more likely due to how planets and terrains are created in KSP. Factor in rotation and gravity and you have your answer.

http://forum.kerbalspaceprogram.com/threads/120331-Fins-are-counter-productive?p=1922669&viewfull=1#post1922669

That's what I'm thinking too: the fins make the rocket go straight up, but the planet rotates away underneath it. And since the navball in ground mode is relative to... well, the ground, it shows a tilt to the west.

[NecroBones]

I made a small test, and there may be something to this, though I didn't see it happen as egregiously. I built a small rocket, using the Mk1 pod with a small parachute, heat shield, decoupler, and a pair of FL-T800 tanks, with a Swivel underneath. I launched with and without fins, with SAS turned off, and not touching the controls.

Without the fins, it flies straight up, until around 6km of altitude, where it will become unstable and tumble out of control.

With the fins, it leans to the west a little, but never loses control. By the time it exits the lower parts of the atmosphere, the veering toward the west stops, and it continues to fly straight (at the westward heading).

I'm wondering if this is due to the planet's rotation. The lateral air movement would conceivably hit the fins harder, pushing the lower end eastward, causing the rocket to steer to the west.

[herbal space program]

I made a small test, and there may be something to this, though I didn't see it happen as egregiously. I built a small rocket, using the Mk1 pod with a small parachute, heat shield, decoupler, and a pair of FL-T800 tanks, with a Swivel underneath. I launched with and without fins, with SAS turned off, and not touching the controls.

Without the fins, it flies straight up, until around 6km of altitude, where it will become unstable and tumble out of control.

With the fins, it leans to the west a little, but never loses control. By the time it exits the lower parts of the atmosphere, the veering toward the west stops, and it continues to fly straight (at the westward heading).

What I'm talking about is not at all subtle. However I was using the AV-11 winglet and not those basic fins, and my rocket was also quite a bit taller. I have to run now but I'll post a screenie later....

[gogozerg]

By the time it exits the lower parts of the atmosphere, the veering toward the west stops, and it continues to fly straight (at the westward heading).

When speed increases, fins start to do their fin job.

[GoSlash27]

What I'm talking about is not at all subtle. However I was using the AV-11 winglet and not those basic fins, and my rocket was also quite a bit taller. I have to run now but I'll post a screenie later....

I don't know if this sheds any light on it, but I've noticed that active control surfaces are way overpowered in 1.02 and lag noticeably in displacement.

This can set up weird oscillations and twitchy handling, especially if you have SAS engaged.

I have been running absolutely minimal control surfaces on my spaceplanes and don't normally use fins on my rockets.

HTHs,

-Slashy

[NecroBones]

When speed increases, fins start to do their fin job.

Well yes, but they should also be helpful at moderate speeds as well. The rocket didn't veer west at all without the fins (until it lost control altogether, so clearly the fins are keeping it stable). The westward turn with fins was something I was able to reproduce.

[Shadowlord]

Did anyone in this thread try using six or eight fins instead of four?

My initial 1.0.2 finless rockets were flipping on their own, even with SAS on, and adding just four fins was ineffective, but when I went up to six or eight, that was enough to hold them stable as long as I wasn't trying to tilt them in the atmosphere (much, anyways). Of course now I have a rocket which is cool with no fins, but it's an asparagus - so its center of mass doesn't migrate the way normal rockets' CoM's do.

[bakanando]

That's the next thing I wanted to do, I played minecraft all day so haven't tested it yet.

[magnemoe]

Did anyone in this thread try using six or eight fins instead of four?

My initial 1.0.2 finless rockets were flipping on their own, even with SAS on, and adding just four fins was ineffective, but when I went up to six or eight, that was enough to hold them stable as long as I wasn't trying to tilt them in the atmosphere (much, anyways). Of course now I have a rocket which is cool with no fins, but it's an asparagus - so its center of mass doesn't migrate the way normal rockets' CoM's do.

have used two sets of fins after each other many times, i found that rockets with large fairings or no fairings needs more fins as the front drag is large, if you have an oversize fairing you can also not use an stubby second stage in the atmosphere, in this setting you would want to get higher than 40 km before staging, same if you want to drop fairing and do the last 1 km/s with the payload who might have LV-N

[Rakshasa]

The AV-11 winglets worked perfectly for me in a triple configuration, a non-gimbaled rocket with two booster that tended to flip over at high speeds.

Not every rocket should have fins, and not every rockets should not have fins. Basically the error lies between the user and keyboard.

[oglommi]

I had problems with some rockets flipping over. Closing the upper tanks and opening them higher in the atmosphere made the rockets more top heavy and solved the problem.

[Warzouz]

Not every rocket should have fins, and not every rockets should not have fins. Basically the error lies between the user and keyboard.

Basically the error lies between the CHAIR and keyboard.

[NecroBones]

Basically the error lies between the CHAIR and keyboard.

Ah yes, ye old "PEBKAC": problem exists between keyboard and chair

Anyway, as far as it appears, fins do seem to be adding stability and appropriately keeping rockets from flipping out of control. What the OP was really pointing out though, is that with the fins (and no SAS or control input), there's a reproducible turn to the west. Without the fins, as long as it stays stable, the trajectory remains vertical. My quick little test experienced this as well, just less pronounced than what the OP was seeing.

[SAI Peregrinus]

Just did some testing. Rocket was (top to bottom)

Mk-16 parachute, Mk1 command pod, TR-18A stack decoupler, FL-T800 tank, FL-T800 tank, LV-T30 engine. 0-8 AV-T1 winglets.

The path of the rocket SHOULD go west, due to the Coriolis effect, and the fins will tend to orient it to go straight through the airstream. That would cause a westward tilt in the lower atmosphere.

Rockets were all launched at full throttle, SAS off, no control input.

Stock, T1 launchpad:

0 fins: Wild tumble at mach 1.

3 fins: Tilts west.

4 fins, +: Tilts west.

4 fins, X: Tilts west.

6 fins: Tilts west.

8 fins: Tilts west.

Tier 2 launchpad:

0 fins: Tumbles wildly at mach 1.

4 fins: Tilts west.

Tier 3 launchpad:

0 fins: Tumbles wildly at mach 1.

4 fins: Tilts west.

nuFAR, Tier 3 launchpad:

0 fins: Tumbles wildly at ~100m/s.

4 fins: Tilts west.

All rockets thus performed as expected given a wind velocity of 0 relative to Kerbin's surface.

[herbal space program]

So now when I threw together an Mk-1 capsule, three T800 tanks, and a swivel engine, the AV-11 winglets worked fine. This seems to be some sort of problem that is unique to really tall rockets without gimbaled engines. I will investigate further a bit later to see if I can isolate what causes the problem....

[NecroBones]

All rockets thus performed as expected given a wind velocity of 0 relative to Kerbin's surface.

Thanks for the additional data. I suspected it was a Coriolis effect.