rocket instability in space.

[endl]

so i have a small vessel that im testing out after inserting it into orbit i want to see if it can make it to my space station as a ferry between two orbiting stations. problem is that when i start my rendezvous maneuvers i get massive spinning and the node flys off typical of over acceleration. is there like a TWR to avoid when building a vessel for orbital maneuvers. i know placing the engine too close to COM causes torque i tried moving it back but now it spins on a different axis. so i guess i need some more info on how powerful to make the engines. im using some radial engines from the D dynamics pack atm.

[Aethon]

If you could post a good, daylight screenshot of your craft, it would help us, help you.

[endl]

i dont think youll get much out of it but here you go

[merlinux]

Is your center of thrust aligned with your COM? hard to tell from this pic.

And about TWR I generally aim for something around 1.

Edited October 3, 2014 by merlinux

[KerikBalm]

Can we get a pic from the rear of the craft?

Your vessel doesn't have much in the way of reaction wheel torque. Probe cores have very low torque.

I suspect your engines are slightly above your center of mass, which will make a torque that the probe's tiny reaction wheels have no chance of countering

[Epthelyn]

It doesn't look as if anything is off center, or at least from this angle it doesn't; as KerikBalm suggested an alternative angle would be useful.

Are any parts clipping? That's the only reason I can think of right now that would explain why an apparently stable rocket would spin out; the CoT can be anywhere* as long as it's thrusting through the CoM, in line with the CoM etc.

*There are reasons why that's not entirely true, but in your case it doesn't really matter

[Red Iron Crown]

Can we get a pic from the rear of the craft?

Your vessel doesn't have much in the way of reaction wheel torque. Probe cores have very low torque.

I suspect your engines are slightly above your center of mass, which will make a torque that the probe's tiny reaction wheels have no chance of countering

That's my suspicion, too. The SP+ parts are asymmetric with a CoM that is off center toward the bottom IIRC.

Edited October 3, 2014 by Red Iron Crown
Grammarz.

[Alshain]

Yes, what Red Iron Crown said is the issue, I had the same issue before, the engines are directly in line with the seams of the fuselage, but those seams aren't centered, the bottom section has less mass than the top due to asymmetry. You can counteract it by rotating the engines, I used a pair of 24-77 aimed just right to counter act the rotation it caused. RCS Build aid can help (it does engines too).

[endl]

[Red Iron Crown]

It doesn't look like off center thrust from that angle, must be something else going on. I'm not familiar with those engines, are they gimbaled? If you lock the gimbals, does the behavior still happen?

[Stratzenblitz75]

If you aren't against mods, RCS Build aid is a must have. While your center of thrust doesn't look too offset, it can still be ever so slightly offset. Even a slightly offset center of thrust can cause your vessel to spin. This is where RCS Build Aid comes in. It tells you how much torque your engines (and of course, your RCS thrusters) are inducing. Also, the distance between the CoT from the CoM does not effect torque; What is important is whether the direction of thrust is offset from the CoM. Think of the direction of thrust as a line that passes through the CoT. If this line does not intercept the CoM, then you will have rotation.

[r_rolo1]

Hum, do you have massless parts in that design ( can't really tell from the pics ) ? The VAB CoM display does not take in account if a part is massless or not and is calculated like if the massless parts had the mass that is stated in the parts menu. That, depending on the difference , can make the real CoM to be quite far away of the displayed one.

Edited October 3, 2014 by r_rolo1

[endl]

Hum, do you have massless parts in that design ( can't really tell from the pics ) ? The VAB CoM display does not take in account if a part is massless or not and is calculated like if the massless parts had the mass that is stated in the parts menu. That, depending on the difference , can make the real CoM to be quite far away of the displayed one.

its as you see it space plane + with engines from b dynamics, disabling the gimbals has helped the torque rotation but now mechjeb can not control burn time, it hits the apex then instead of cutting off it goes full burn with the burn time going up until the orbital plane starts going off alignment