Craft file 1.1 wobbles / rocks about in 1.2

[mart]

Hello,

I've created a craft (Space Shuttle) all-stock in 1.1. I've have since then flown it many times (as others) in version 1.1 of KSP without any issues.

I haven't played for a few months, and now I'm dusting off my KSP and tried to load the craft file. Since it's all stock, it loads without issue of course but it doesn't fly as well anymore.

Who with CoM / CoT / SAS / Flaps and such knowledge is willing to have a look at the craft file please ?

https://kerbalx.com/mart_nl73/Space-Shuttle-Prometheus

The 2 main issues (I can PM you a video of the issues):
- During launch right when SRB occurs the shuttle starts to rock back and forth. Wobbles. It's controllable but annoying.
- In space, when I circularize orbit (I do that with Main engines for the first part), the craft becomes very very unstable and uncontrollable.

Note: The craft file has a 'hidden' RC-L01 in the nose. So there is SAS settings on the cockpit and the RC-L01.

there are also 2 flaps 'tugged' away there for CoM and landing.

Yes, some might be odd, yet this was one perfect bird to fly with in 1.1.

Regards,

Martin

Ps. It mentions on Kerbalx, it's made during 1.2 pre-release. That's not true. It was build in 1.1 and intended for 1.1 and worked in 1.1 it is however never tested in 1.2 until now.

edit:
Can confirm 'gimbal' might be a big issue with this issue
 

 

Edited March 26, 2017 by mart

[Cunjo Carl]

@mart

Took a while to play around with this and there's a couple things I've noticed, though none of them will fully solve the troubles. As a note though, this shuttle has a nice bit of energy in it. While fiddling around I was getting orbits out past Duna before dropping he main tank- I like the design!

As you mention, the main issue is the SAS trying to manage the gimbal, and this isn't the first large craft I've heard of that's become trickier to fly in 1.2. Control of yaw and roll are all heavily coupled with the vectors' gimbal, and KSP is getting in to harmonic oscillations trying to damp both of them at once. I was hoping that tossing some vernors in key places (wingtips) would give the craft enough control to not get into the wiggles to begin with, but no such luck. I have found a handful of incremental improvements to make it a bit more manageable though.

• The main tank has a flat bottom, which has become far draggier in the latest version. Putting an empty but somewhat pointy fairing on it helps reduce its drag, which seemed to help the wobble slightly.
• You can increase the flow priority of the main tank's lower sections to help keep the COM more inline with the thrust. You can do this by right-clicking the bottom section and increasing the flow-priority by 2 and the middle section by 1. It helps a bit, but not much.
• The vectors can be tilted one tick further down (more straight) without hurting launch conditions at all. It helps later on a little bit if the wiggles get more severe.

I've tried a handful of other things, but nothing in my toolbox is getting rid of the wiggles satisfactorily. I hope the little improvements above make it a bit more manageable for you. And hopefully someone will have a magic bullet for this problem. There's a huge amount of expertise in the help forums, so I'll keep my fingers crossed!

 

EDIT: Inspiration struck, and I found an ok solution. Turning off the gimbal on the lower two vector will keep it from rocking too much on the way up. It's not ideal, but it works!

Edited March 28, 2017 by Cunjo Carl
typo

[mart]

First of all, thanks for even trying and looking at it !

Yeah, gimbal is an issue as it seems. Why was that not before I wonder. Must have changed.

I have been playing with gimbal and no real succes yet, but there is so much to try. I mean, 3 engines and gimbal off or gimbal anywhere between 0 and 100 ....

And then there is the SAS we have the RC-L01 and the mk3 cockpit, do I switch some settings or disable one of them completely etc. etc.

Together with your inspiration, and you earlier tips I will try again see if I can work things out.

Again, thanks !

Mart.

[Warzouz]

That's true. When launching a riocket, I usually reduce gimbal to 30%. It's largely enough for gravity turn

[Cunjo Carl]

@mart Had one more thought, and this one finally worked! Together with some of the above suggestions, I tried an old trick... normally I consider it a bit cheaty, but it's to fix a bug and clipping isn't unusual on shuttles and spaceplanes...

If you place large reaction wheels on the bottom of the drop tank (together with disabling two of the vectors' gimbals) they'll keep you nice and stable on the way up. Rotating this into the tank above it will keep it aesthetically pleasing and aerodynamically streamlined. You'll notice the batteries placed every 4, it's important these are strutted (which is tricky) or autostrutted (which is easy) in order to keep it from producing its own oscillations. If you don't want these providing undue advantages to your maneuverability in orbit (assuming you haven't dropped the tank yet) they can be put on a toggling action group.

Not the most elegant solution, but it makes the launch a gentle and pleasant one!

I still hope someone has a more direct solution...

[mart]

Well yeah this might just be on the edge of being cheaty but if you take a look inside the nosecone you'll find 2 wings clipped so....

This is great, thanks for all your investigations and time !! As soon as I get the time to give it a go again I'll let you know how it went.

[Cunjo Carl]

5 hours ago, mart said:

Well yeah this might just be on the edge of being cheaty but if you take a look inside the nosecone you'll find 2 wings clipped so....

This is great, thanks for all your investigations and time !! As soon as I get the time to give it a go again I'll let you know how it went.

I feel like Columbo, but one more thing! If you don't mind your drop tank being a bit larger, you could actually leave the reaction wheels hanging downwards and just cover them with fairing to look nice. You have plenty of thrust and deltaV and thrust to get away with the added weight and drag. That would spare the moral crisis of clipping, which is nice. Ok, that's it! Have fun flying.

[mart]

Hi again ;-)

I have indeed experienced a very gentle lift off with quite a few reaction wheels on the back of the main tank. There is however still work to be done.

I have installed KSP 1.1 and sure enough the thing works like a beauty, no issues at all. So I screenshot all CoM/CoT arrows and stuff to make sure I have the same readings on 1.2. Which I have unfortunately.

So it really is a change in environment (atmosphere/drag) which we experience during lift off and make it rock about back and forth and swing a bit. And it's a change in SAS or gimbal or both which makes it do uncontrolled movement while in space and using the final burns for orbit.

As said, the launch can largely be solved with all your input. I thank you very much for that. However, I need to lower the gimbal during launch to 30% at least for a gentle launch while at the same time I need more gimbal once in orbit.

Gimbal alone won't cut it, I have to throttle back to at least 40 or 50% tops otherwise it spins nevertheless. So ... All in all:

- Launch is now 'acceptable' not perfect (especially if you use KSP 1.1 as intended with this craft, you can pitch / roll and correct as you wish without issues)
- Orbit correction is not yet as it should. It is however possible to make burns if you plan ahead.

I kinda wanted to fix alle issues with stock not by using any mod. I also want to 'cheat' as little as I possible can, and release this craft for 1.2. For now, I don't think it's the quality I expect for release. For me it's ok, I know now what I need to do and how to fix things. For others it's a bit of a puzzle without clear instructions.

So yeah. Still some fixing to come up with. But I'm getting there.

Regards,
Mart.

Edited March 30, 2017 by mart
typo