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Rockets Gently Rolling During Launch


The Flying Kerbal
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I'm trying to figure out why my rockets always seem to go into a very slow, gentle spin while flying through the atmosphere during launch.

Now I am the first to admit that when it comes to rocket design I'm not a prodigy of Wernher Von Braun, but at the same time, with the number of rockets I've stitched together these last few months I would expect to get the odd design right - even if only by sheer chance!

This roll is not so serious as to pose a major threat to the launch, but it's just annoying constantly needing me to correct it with the Q & E keys.  I've tried rockets with control surfaces, gimbals, RCS during launch... you name it but still this rolling persists.

I suppose it's possible this is something in the game - a bug maybe(?) - but as I don't see messages from other people complaining about it I don't believe it is.  I had thought it was related to the amount of acceleration the vessel was experiencing during the flight to space, but I've tried every possible level of throttle, from full heel-to-the-steel and the vessel resembling a blast furnace with all the heating effects, to a level were it was barely increasing velocity, but all to no avail.

As I've said, it isn't really spoiling things, but it is irritating, especially if I'm trying to launch into an inclined orbit to match that of a ship I'm planning to rendezvous with (for example), something I'm pretty rotten at at the best of times.

If any of you have experienced this, know what causes it and how to counter it, I'd really appreciate if you share your thoughts with me.

Thanks all. 

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37 minutes ago, The Flying Kerbal said:

I'm trying to figure out why my rockets always seem to go into a very slow, gentle spin while flying through the atmosphere during launch.

Now I am the first to admit that when it comes to rocket design I'm not a prodigy of Wernher Von Braun, but at the same time, with the number of rockets I've stitched together these last few months I would expect to get the odd design right - even if only by sheer chance!

This roll is not so serious as to pose a major threat to the launch, but it's just annoying constantly needing me to correct it with the Q & E keys.  I've tried rockets with control surfaces, gimbals, RCS during launch... you name it but still this rolling persists.

I suppose it's possible this is something in the game - a bug maybe(?) - but as I don't see messages from other people complaining about it I don't believe it is.  I had thought it was related to the amount of acceleration the vessel was experiencing during the flight to space, but I've tried every possible level of throttle, from full heel-to-the-steel and the vessel resembling a blast furnace with all the heating effects, to a level were it was barely increasing velocity, but all to no avail.

As I've said, it isn't really spoiling things, but it is irritating, especially if I'm trying to launch into an inclined orbit to match that of a ship I'm planning to rendezvous with (for example), something I'm pretty rotten at at the best of times.

If any of you have experienced this, know what causes it and how to counter it, I'd really appreciate if you share your thoughts with me.

Thanks all. 

I have had this same problem in the past.  Would you by chance be using a design that has 3 boosters and 3 tail stabilizer fins?  If so, I found that adding a 4th of each of these, to make the craft more symmetrical, tends to stabilize the annoying roll you mentioned and made the launches much smoother, and a little more efficient since your not having to make adjustments the whole time.  This is just from my experience, I am also no prodigy.

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I have this problem as well, but only on certain rockets. It generally occurs when I have multiple SRB's. I place my first set of boosters, strut it, add my sepatrons (if I think I need them), then Alt-left click the entire thing and place the next set with the copy of the first. I'm adding this second set by hand, so it's never quite lined up vertically with the other set(s). This, I believe, is my culprit. I could make sure it's lined up better, but I usually don't bother. It lifts just fine, but if they're off more than usual, it'll spin on the ascent. Still doesn't really effect the launch, so I just deal with it (too lazy). Don't know if this is your problem, but I'm betting it's something similar. The relation of the engines to one another or some such.

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First make sure you haven't forgotten about a game controller that is still connected. You wouldn't be the first to have a joystick or yoke somewhere in the corner that is still attached.

18 minutes ago, The Flying Kerbal said:

... I would expect to get the odd design right - even if only by sheer chance!

Hehe. That is such an underrated technique.

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45 minutes ago, Ncog Nito said:

I have had this same problem in the past.  Would you by chance be using a design that has 3 boosters and 3 tail stabilizer fins?  If so, I found that adding a 4th of each of these, to make the craft more symmetrical, tends to stabilize the annoying roll you mentioned and made the launches much smoother, and a little more efficient since your not having to make adjustments the whole time.  This is just from my experience, I am also no prodigy.

I've noticed the same tendency. It seems to me that an odd number of fins can create asymmetrical lift when the control surfaces are engaged. If you want to steer using the fins then I would use 4 instead of 3. Of course, if your fins or other surfaces are angled then it will naturally spin.

Having too much control authority with SAS on can cause the SAS to oscillate due to over-correction -- lowering the engine gimbal degree, or reducing the power of reaction wheels can minimize it.

There are the obvious culprits like off center mass, off center drag, or off center direction of thrust.

Having a too low thrust-to-weight ratio may cause the nose of your rocket to drop too soon. Then if you or SAS oversteers to compensate you end up see-sawing. Even a slight yaw angle can turn this into a precession movement.

Are you using the MechJeb mod in "ascent guidance" mode? For some reason that mod has a bug where it will always roll the craft on the way up for the first couple kilometers.

Edited by HvP
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1 hour ago, Rodhern said:

Hehe. That is such an underrated technique.

I wonder why...

to the op: for any issue craft related post a image. There is several mistakes a inexperienced designer/pilot can make that experienced ones may spot in a glance.

Quote

 I had thought it was related to the amount of acceleration the vessel was experiencing during the flight to space, but I've tried every possible level of throttle, from full heel-to-the-steel and the vessel resembling a blast furnace with all the heating effects, to a level were it was barely increasing velocity, but all to no avail.

 

In my experience the cause of the problem almost never is the amount of thrust. Common  problems are unbalanced drag , shifting mass, to much control authority, bad trajectory, lack of structural rigidity and thrust alignment. In some cases one can still manage to fly the vessel just fine at lower thrust, but the problem its those factors, not thrust.

Anyway, we need a image to know for sure what your case is.

Edited by Spricigo
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SAS seems to not control for perfectly 0 stability in the roll dimension. I don't know why not. But if your craft wants to roll just a little bit, SAS will allow it.

Edited by bewing
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I had a problem with side mounted boosters. If they are not strutted and rigidly connected, their thrust can be be ever so slightly off center. Try placing two struts on the very bottom of the booster tanks and see if that helps.

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