Why the hell is this rocket drifting?

[ROXunreal]

My Mungoer I craft is suffering from a mysterious ailment for which a diagnosis has been completely elusive, despite my best efforts.

The rocket ALWAYS drifts to the same side, despite the following:

- the rocket is symmetrical

- all booster thrust limiters are at 100%

- all booster fuel is 100%

- turning off SAS does not help

- liquid engine is turned off (so no gimballing)

- I rotated the rocket in the VAC, it always drifts to ITS same side regardless, so Kerbin's rotation/gravity are not at play here

- the only asymmetrical part is the upper stage with the antenna, solar panels and thermometers, however I removed those and left just the pod to test it, rocket drifted regardless.

- I use stock physics.

I put on winglets and they help to counteract the drift, but once I jettison my BACC boosters, the drift overpowers the winglets and I veer off course terminally just before the S1 SRB's run out of fuel.

The only thing left to try as far as I know is aerodynamic cones, but as I understand their positive effect is almost unmodelled in KSP, and I am sitting right next to my launch pad mass limit of 140t anyway (adding two more sepratrons makes the rocket exceed the mass limit). Also that the rocket always drifts to one side has to mean something.

This is the rocket and what it looks like when I try to take off:

I'm utterly perplexed.

Edited January 9, 2015 by ROXunreal

[Streetwind]

Does the bottom left corner show control input? If SAS is on it should show control input to counteract the drift but with SAS off it should not show any. If you see anything else than that you might have accidentally activated a trim setting, or you have some sort of bug. Try zeroing out trim by pressing alt+X.

Does the drift stop when you're on the final pair of trashcan boosters (after jettisoning the giant S1's)? Sometimes strutted parts, if not strutted in just the right way, can twist minimally against the flexible connection and cause off-center thrust. And when you're on the trashcans, you lost all the strutted parts providing thrust, so that effect should stop. If it doesn't, check if it stops when you're on liquid fuel alone - ideally in orbit (try HyperEdit, it's great for testing). If your craft tilts sideways even then, you definitely have an imbalance in your spacecraft. Perhaps you accidentally clipped a part into it so that it has become invisible. Do you remember if during construction, you accidentally "lost" a part while trying to attach it? It's probably sitting inside the rocket now. Use the center of mass and center of thrust overlays in the VAB to check if both of them are in line.

As a side note: are your trashcan boosters even producing thrust? The decouplers below them are on the wrong way around, and are potentially obstructing the nozzle, causing KSP to think your engines should produce zero thrust. Turn them upside down so that they fall away with the BACCs.

EDIT: I advise testing things in orbit because when launching, gravity will automatically continue to make you tilt over if you're not going perfectly straight up (see: gravity turn). That way you might not notice if your drift stops because you're already tilted way to the side at a very low altitude and slow speed, causing you to simply keep tilting by gravity.

Edited January 9, 2015 by Streetwind

[Starhawk]

Looks like you've got a decoupler still attached to the bottom of your RT-10 booster in the pics. That doesn't look like the source of the problem, but it's definitely behaviour you don't want.

Contrary to what Streetwind says (sorry) the decoupler looks like it's on the right way. Maybe it's clipped into the Rockomax tank?

I would recommend going in to the VAB and stripping the ship to it's smallest stage (that has propulsion) and test that. If you get no 'pull' effect, then go back in and strip it down to the next smallest stage and launch and test that. Lather, rinse, and repeat. You will at least know which exact portion of the craft has the problem.

Invisible parts clipped inside can definitely cause problems like this.

Good luck!

Edited January 9, 2015 by Starhawk

[DeMatt]

In regards to the decouplers that Streetwind and Starhawk mention... it looks like you've accidentally clipped them into the RT-10s. So the bottom of the RT-10 is connected to the bottom node of the TR-18A, when you meant to attach it via the top node.

As to the "drift" problem... you have very little turn authority as-is. The winglets, especially the ones on the RT-10s, are positioned way too far forwards to provide rotational force - move them as far down the boosters as you can. The fact that you have only the Mk1 Command Pod for torque suggests to me that you're going to have problems turning once you're out of the atmosphere. Consider adding Reaction Wheels.

A few more comments... I also wonder at why you have the RT-10 + BACC combos, instead of a second pair of S1 SRB-KD25k's. What're the Separatrons (the ones on the lander) for, lighting? The Poodle is probably overkill for such a lander - an LV-909 may have only a quarter the thrust, but it's a quarter the mass. You probably don't need eight LT-1 landing legs, try six or even four. BACCs in general tend to be a poor choice versus the other two solid boosters; it might actually be worth seeing how well the rocket flies without that outermost quartet of BACCs.

Final thought: you can post up your .CRAFT file on a filehosting service like Dropbox; if you aren't using mods, then other players can download it and try flying it themselves.

[MarvinKitFox]

Actual cause:

You have a symmetry error, some small component has been "duplicated" by symmetry, right onto itself.

This is NOT fixable.

Junk the top part of your ship, re-build it, and the problem is likely to vanish.

Other trivia: your small SRB's are dragging a decoupler, this will cancel its function as a booster.

The decoupler is just upside-down.

[Starhawk]

In regards to the decouplers that Streetwind and Starhawk mention... it looks like you've accidentally clipped them into the RT-10s. So the bottom of the RT-10 is connected to the bottom node of the TR-18A, when you meant to attach it via the top node.

That's it exactly it, DeMatt. You hit the nail on the head. I just went into the VAB and checked, and it matches the pictures.

ROXunreal, when you attach the stack decouplers, they can attach in two positions. Only one of these two will allow correct functioning of the components. The decoupler needs to be placed in the LOWER of the two possible positions. The next booster will then attach (and decouple with the decoupler) properly.

Back to the unbalanced flight... Parts invisible because they are duplicated onto themselves or clipped completely inside other parts still seems like the most likely cause.

[ROXunreal]

Thanks for the help, I guess it was indeed a clipped part. The problem persisted after I attached the decoupler to the small SRBs properly.The BACC were not at fault as the problem persisted (and got very much worse) after dropping the BACC stage. My guess is that something in the large SRB was at fault, as the liquid fuel stage was stable when tested separately. I rebuilt the rocket from the liquid fuel stage downwards and the problem vanished.

Does the drift stop when you're on the final pair of trashcan boosters (after jettisoning the giant S1's)? Sometimes strutted parts, if not strutted in just the right way, can twist minimally against the flexible connection and cause off-center thrust.

I never got to the stage where I would activate the RT-10 without the rocket already being grossly off course, so I wouldn't know.

As to the "drift" problem... you have very little turn authority as-is. The winglets, especially the ones on the RT-10s, are positioned way too far forwards to provide rotational force - move them as far down the boosters as you can. The fact that you have only the Mk1 Command Pod for torque suggests to me that you're going to have problems turning once you're out of the atmosphere. Consider adding Reaction Wheels.

I needed a pair of winglets on the RT-10 as I needed some control after I jettison the BACC stage, there was no where else to put them on the upper stage. I did later lower the attachment point of the RT-10 on the liquid rocket as a response to this. No room before launch pad mass limit for adding reaction wheels, or anything really.

A few more comments... I also wonder at why you have the RT-10 + BACC combos, instead of a second pair of S1 SRB-KD25k's.

Mass limit, I thought all those boosters would provide more thrust than just four big SRBs (couldn't add more S1's due to mass limit).

What're the Separatrons (the ones on the lander) for, lighting?

I like to have them as an initial boost to the east right after surface takeoff from the Mun. I could lower the number of landing legs to six and rearrange the Sepratrons to match the layout, I wouldn't go with only four given that my Mun landings are often rather imperfect.

The Poodle is probably overkill for such a lander - an LV-909 may have only a quarter the thrust, but it's a quarter the mass.

I don't know if it would be sufficient, maybe with this fuel tank it would be.... I used it with the T-200 tank on my first Mun landing ever some weeks ago and it was very insufficient to get home from the Mun. Taller small diameter fuel tanks are out of the question due to stability upon landing.

I'll toy with the 909 and see how it goes. Turns out this setup is no more efficient in getting to orbit than other things I tried before, but I'm only testing at this point anyway. I need to get the main liquid fuel rocket to orbit with at least half the overall fuel remaining for the Mun transfer and orbit. So far at best I can manage getting to the Mun with the liquid rocket but having to use the lander itself to enter a stable orbit, leaving no fuel for returning home.

Edited January 9, 2015 by ROXunreal

[Starhawk]

Thanks for the help, I guess it was indeed a clipped part.

No worries, I had that same thing happen to me, and it had me baffled for a while. Glad you got it solved!

Mass limit, I thought all those boosters would provide more thrust ...

A thought or two about this. If mass limit comes into play, consider this. Despite their high thrust, SRB's are very heavy for what you get out of them. I've tended to find that multi-staging SRB's tends to lead to diminishing returns very quickly.

You might try playing around with more liquid fueled stages.

As far as the LV-909 goes...

I don't know if it would be sufficient, maybe with this fuel tank it would be.... I used it with the T-200 tank on my first Mun landing ever some weeks ago and it was very insufficient to get home from the Mun. Taller small diameter fuel tanks are out of the question due to stability upon landing.

You are absolutely right about not wanting tall, skinny landers. Short and wide is the way to go.

That said, the LV-909 is way shorter than the poodle, allowing you to place your landing gear higher, resulting in your centre of mass being lower. With a low enough centre of mass, you should even be able to get away with only four legs.

As for sufficient, it only matters if the engine can get the mass of your lander off the munar surface. From the looks of it, it should easily be sufficient (even without the separatrons ). Given the same amount of fuel, and the fact that it is so much lighter, the LV-909 will be much more efficient and will thus get you much farther than the poodle.

Good luck!

[LethalDose]

It sounds like the issue's been resolved, but I would also recommend some reaction wheels just to make the whole thing generally more controllable.

SRBs don't have gimbal capabilities, your aerodynamic surfaces are pretty close to the CoM on the longitudinal axis, I don't see RCS, and the only part with a reaction wheel I see is the pod, which is really weak compared to the size of the vessel. I suspect even if you get the drift-in-the-same-direction issue resolved, it's still gonna be tricky to control.

[ROXunreal]

As long as it's balanced it will fly straight enough until I drop the SRBs, acceptable drift can be corrected with winglets. After that the liquid fuel stage is easily controlled with engine vectored thrust in the atmosphere, whereas in space it can turn with SAS, not really fast but then again, I don't need fast rotation abilities in orbit. Once I dropped the large tanks, the rest rotates pretty fast just with the pod torque.

The LV-909 advice was completely sound, mission complete

It was also ridiculously easy to land on the Mun with this engine as it's not as sensitive to small thrust inputs with this weight above it, much easier to control rate of descent, and more than enough to not only get home but to actually land on Kerbin as well and retrieve the parts (even though I had a rough landing, my own fault). I still like my Sepratrons though, even if just for fun

This was the winning design in the end. I did as was suggested above (before reading it) - added a small liquid stage between the big Skipper stage and the lander.

Thanks again all.

[Starhawk]

Congratulations!

I still remember the elation of my first successful Mun landing.

Happy flying!

[ROXunreal]

Far from my first as I did a number in Science mode, but still my first in career. I do remember that elation though. Actually Minmus was the first non-Kerbin body I ever landed on, for obvious reasons.

[Starhawk]

When I started, I didn't know the obvious reasons, and went to the Mun first. Went to Minmus later and thought, 'Well, wasn't that easy!'. That was before I even discovered maneuver nodes.

[technion]

You realise if you start playing with FAR and need a better controlled ascent.. just how hard it is steering with SRB's as your main thrust. They are fine in addition to a rocket, but replacing one pair of those with tanks and T-45 engines would be a significantly better option. Not to mention, you probably have far more thrust than you need already, and substituting that for a rocket with extra dv would make this a much better flying rocket.