Stability

[T3E]

Hello Guys,

I worked the whole night and the day on a rocket to bring 3man capsule + Hitch Hike container + General Equipment to the Mun.

All in all its about 18t.Rocket mass is about 230t.

The thing is, I cant even reach the point for my gravity turn since the rocket is quite early heading in some direction instead of straight up in the sky.

Every important part is made symmetric (just some antennas not).

I removed the asperagus system even tho i double checked the fuel ducts (so fuel tanks are used in the right order).

cant figure it out.

Could it be in general about my heavy payload? The wrong COM/COL?

Its all centered tho.

Can provide with pictures if its necessary, but it might be a more general problem.

Edited May 25, 2013 by T3E

[Panichio]

So even without the asparagus system it is straying off-course? Hmm, do you still use some fuel ducts, though?

The other problem I can think of right now is maybe the CoM is too high, and that is causing the top-heavy rocket to be unstable. As always, pictures would help a lot.

[T3E]

Yes there are no fuel ducts left on this stage.

I heard a little bit about the relation of CoM to CoL (and the "distance" between them) but I dont know exactly.

The screens provided show the main rocket, with just some solid boosters to get the TWR high enough (and to show that its certainly not my noobishness about the asparagus system )

Note that I also zoomed "in" the rocket and checked if they are really centered - they are both.

The whole rocket + the turn I cannot explain nor want:

The turn gets worse and worse, its just the beginning in the picture.

Btw, the center of mass was with the additional 6 tanks for the asparagus around the decoupler at the top of the orange tank.

Edit2: Here the picture with the center of mass with all tanks used in the first place inclusive all engines.

Edited May 25, 2013 by T3E

[Pursuedtank]

You should be able to drop all the SRB's and boosters at the same time because you turn them all on at the same time, that is unless there are decouplers that I can't see.

It looks to be a very long craft that has trouble righting itself due to the thrust and center of mass and all that. It is a very large craft that you should be able to reduce down to make it more stable in while flying, you may also want to consider building to the sides rather than up.

Edited May 25, 2013 by Pursuedtank

[T3E]

Another thing is that you might be able to drop all the SRB's at the same time because you turn them all on at the same time

So the Center of mass should be lower?

The decouplers are from the asparagus system i just didnt sorted them new for the test (since i didnt even reach the point of dropping them...).

[Panichio]

Again, it might be because the rocket looks top-heavy, and the first stage might be far below the CoM, causing it to be unstable when there is some wobble in the rocket.

EDIT: Alternatively, the orientation of your winglets might be a little off, although I can't tell from the picture.

[T3E]

Removed the winglets and tried to lower the CoM by orientating the side tanks as low as possible. CoM now in the upper part of the Jumbo tank, still the same problem.

But I guess you got a point with the "top-heavy" thing, it seems that with every wobble the rocket turns in the same direction a little more.

Looks like I have to re-design the whole thing, crap!

Thanks for the input and pointing my mistakes out

[Panichio]

Removed the winglets and tried to lower the CoM by orientating the side tanks as low as possible. CoM now in the upper part of the Jumbo tank, still the same problem.

But I guess you got a point with the "top-heavy" thing, it seems that with every wobble the rocket turns in the same direction a little more.

Looks like I have to re-design the whole thing, crap!

Thanks for the input and pointing my mistakes out

Glad to help! Give us an update if you successfully redesign it. It'll help out others with the same problem.

[chemieglennie]

I agree with the top heavy thing. I've changed the general design of my ships so the interplanetary/transfer stage is built to the side of the lander rather than underneath making it all more compact.

Often consists of two of the big grey tanks with nuclear engines.

Also use loads of struts down to the bulky stage.

[T3E]

Okay (quite early) Update

I reloaded my Jupiter I which was actually designed for a smaller payload (rover only) and according to that had much smaller upper stages. (with CoM at the upper part of Jumbo Tank as well).

So I simply put my new payload on the old rocket and the whole "off course" problem disappeard.

On the other hand it MIGHT be also my sloppy working on the stacked engines on the bottom of the Jumbo Tank (even tho i used symmetric-mode).

At the very End, Jupiter II was a failure and the responsible Engineer-Kerbal was demoted to "Test-Pilot"

Picture of the old-new Rocket (I hope it can deliver it also to the Mun, test pending)!

I agree with the top heavy thing. I've changed the general design of my ships so the interplanetary/transfer stage is built to the side of the lander rather than underneath making it all more compact.

Often consists of two of the big grey tanks with nuclear engines.

Also use loads of struts down to the bulky stage.

Yeah I somehow tend to attach only at the first stage tanks at the sides, might be the design thing due to real life creations

About the Struts, i have problems to attach them if the tanks are all at the same size, ak

Cheers

Edith says that the size of the upper stages looks quite the same, so maybe it was only my sloppy work at the stacked engines, lol.

Edited May 25, 2013 by T3E

[MR4Y]

You only make top-heavy rockets on FAR.

[Vanamonde]

1. There is a lot of wiggle in the engine-to-decoupler joins in the upper part of your rocket. That's going to allow the mass to bob around and force your guidance to fight a lot of transient deviations. 2. As other have noted, the length of the rocket allows the mass to swing around far from the parts that are attempting to exert control. 3. Fins in the middle of the ship have very little leverage to help steer the ship. 4. And SRBs are static and cannot contribute to steering, so you have a heavy, long rocket, and just about he only thing trying to keep it under control is the single Mainsail. But the good news is, those are all fixable problems.

[maackey]

Vanamonde beat me to it

To expand on his explanations:

1.) Put horizontal struts between your SRB's to prevent wiggle like |-| or |X|. This will severely help your stability problems.

2.) Long rockets aren't necessarily bad, you just have to have more gradual turns (in real life and in FAR especially).

3.) Fins should be placed as far from the center of mass as possible, to maximize the stabilizing force they can give.