More Spin Woes

[rockbloodystar]

I've read the other two spin threads, and none seem to be addressing the kind of spin i have got on a craft.

The craft is stable while buring through its stages. As soon as the engines idle back to coast to Ap it develops a flat spin that accelerates.

Any clues?

[andrewas]

Pictures? Or at leat a description of the craft?

[Warscribe]

Did you leave the SAS on? Do you have RCS enabled? Do you have enough monopropellant fuel?

[rockbloodystar]

The "lifter" (which has launched other payloads before)

is 7 mainsail engines under two fuel tanks each. Attached to that is two stages of short solid boosters.

The payload is a big fule tank, and a maneuver pod to attach it to the space station i'm building.

I'm using mechjeb, and am using more and more SAS units to add to the available torque.

[rockbloodystar]

How can I upload the file?

[rockbloodystar]

I have lots of RCS, it eats too much fuel for that to be a viable solution.

[rockbloodystar]

http://i1284.photobucket.com/albums/a572/rockbloodystar/screenshot0_zps96feaaf2.png

[rockbloodystar]

[Spatzimaus]

There are a few possible reasons for this:

1> You have parts clipping. Depending on which part, this can cause your craft to torque itself as the two clipping pieces try (unsuccessfully) to resolve their collision. This can't be fixed in-flight, really; you'll just need to go back into the VAB and fix the design to not clip.

2> There's a bug where if you go into accelerated time mode while the craft is trying to adjust its position (either from SAS or because RCS jets were firing), the game might not clear that velocity correctly after leaving the time acceleration. If your craft has flaps, you turn off SAS, and they don't return to their default position, then you're bugged. If your vessel rotates while in time acceleration, you're bugged. Exit to the Space Center and re-select your vehicle, and it'll clear this.

3> If you weren't quite out of the atmosphere and your craft isn't perfectly symmetrical, a spin is very easy to initiate. Especially common if your struts aren't symmetrical. Will only happen in-atmosphere, though.

3a> This also happens if you use a flat aerodynamic surface as a separator. For instance, I've often used the rectangular wing segments to link non-ejecting fuel tanks and engines to my centeral core. These surfaces sometimes are treated as having distinct "up" and "down" sides, so using them vertically can cause a spin. Again, this can only happen in-atmosphere, so if this is happening to you while in orbit, it won't be this.

But the biggest cause, in my experience, is just that the game has a problem with really huge craft. Maybe it's just rounding off certain forces to a given number of significant figures, and that the remainder is enough to, over time, generate a torque that overcomes whatever SAS you have. Regardless, I've had this happen on my ultra-huge boosters (like my 4000-ton heavy lift vehicle); by the time I'm ready to stage, the thing's starting to spin pretty severely.

[NannerManCan]

Oh and also you only need one asas for the whole ship. Since asas has no torque of its own

[Noticeably FAT]

I'd start removing stuff before I'd add more. You design looks very similar to one I have, but I'm lifting a little more with a little less. If you can wait another hour I'll clean it up and post a link. Until then I'd:

Get rid of the little SRBs. Replace each of those clusters with one, maybe two of the bigger SRBs.

Drop your control fins down to the bottom (I'm not sure why they work better there, but they do).

Add some more fins to your central tank, that way you'll still have some control as you drop your asparagus stages. By that time you ought to be about out of the atmosphere, but they should help.

Get rid of the 8 way symmetry on your RCS, go with 4 way.

Get rid of all those little RCS tanks, put one big one on your center orange tank, and then whatever you think you'll need on the payload.

You don't need nearly that many quantum struts, they're pretty strong.

ETA: I just noticed that you are using MechJeb, so I don't need to get rid of it. I don't think I'm using any other mods on this craft, so here it is

Edited April 25, 2013 by Noticeably FAT

[Nexustrimean]

Do you have enough electrical power? if your batteries died during ascent, you would loose all control.

[bonyetty]

Attach some Winglets no need to waste Mono. Also disabling engine gimble on boosters just before separation can also help I have found. Just remember to turn the gimble back on.

[NeoMorph]

Yeah... ASAS is the computer while the SAS are the gyros.

[rockbloodystar]

I'd start removing stuff before I'd add more. You design looks very similar to one I have, but I'm lifting a little more with a little less. If you can wait another hour I'll clean it up and post a link. Until then I'd:

Get rid of the little SRBs. Replace each of those clusters with one, maybe two of the bigger SRBs.

Drop your control fins down to the bottom (I'm not sure why they work better there, but they do).

Add some more fins to your central tank, that way you'll still have some control as you drop your asparagus stages. By that time you ought to be about out of the atmosphere, but they should help.

Get rid of the 8 way symmetry on your RCS, go with 4 way.

Get rid of all those little RCS tanks, put one big one on your center orange tank, and then whatever you think you'll need on the payload.

You don't need nearly that many quantum struts, they're pretty strong.

ETA: I just noticed that you are using MechJeb, so I don't need to get rid of it. I don't think I'm using any other mods on this craft, so here it is

Thanks for that detail, i'm currently trying the modifications. Unfortunately i can't load your file at the moment as i have mechjeb2 installed, not mechjeb 1. i'll do that shortly.

hCheers.

[Noticeably FAT]

D'oh. I keep forgetting that there are two versions of MJ now. Also, bonyetty brings up a good point on gimbaling. Too much (especially with mainsails) can cause a lot of problems. That ship I posted is set up to turn gimbaling off on all but the center stack using action group 0.

[rockbloodystar]

SOLVED (ish)

i changed from having seven mainsailes each beneath two tanks to 13 mainsails each under one tank.

The shorter wider design seems to be much morestable, i've now added that fuel tanks and three atomic engines to the multipart ship.

Many thanks all.