Very Very Annoying Bug (Or something)

[Stellarium]

Does anyone know what is happening? When my SSTO takes flight from the runway, after a few seconds it violently veers off to the left or right-hand side, which side is seemingly at random. I double-checked everything I could, (Center of mass/flight/thrust, symmetry of the spaceplane etc) and I can't seem to find an answer to why this is happening. Any help would be appreciated greatly, as this is such an annoying thing that keeps happening to all my larger planes.

If I need to send a video or something more then just tell me.
The only mods I have installed are KAX, Kerbal Atomics, and Stockalike Station Parts Expansion by Nertea_01.
My game version is: 1.12.5.

[NexusHelium]

I'm assuming you have a stabilizer, right? Those can mean the big big difference of whether your plane wants to stay where you want it to be.

[Vanamonde]

Moved to the KSP1 subs and good luck with your issue.  

[Stellarium]

7 hours ago, NexusHelium said:

I'm assuming you have a stabilizer, right? Those can mean the big big difference of whether your plane wants to stay where you want it to be.

I have 2 of the shuttle tail fins, some small canards, and about 10 elevons on the entire thing.  Although, I do have most of the elevons nearing the center of the plane, because I found it tedious filling the entire back of the wing with them. But perhaps thats the flaw with my design! (The thing has huge double-delta wings).
Also, I forgot to take stabilizers into account when the plane just violently veered to one side or the other, I thought it could have been a bug because of how sudden it was once it flew.
I'll keep this thread/topic thing up to date with my progress. 

[king of nowhere]

it it happens after you've taken flight, it is most likely a problem with aerodinamics.  you need to post pictures of your plane(s) to get more specific help

[Stellarium]

9 hours ago, king of nowhere said:

it it happens after you've taken flight, it is most likely a problem with aerodinamics.  you need to post pictures of your plane(s) to get more specific help

I would just upload a YouTube vid, but I am currently experiencing, ah... "technical difficulties" moving to my new Google account, so I'll just send a Google Drive link to the actual video file: https://drive.google.com/file/d/1OYa0kQsDKMmiaFnjQggq3al4AWWOP9yn/view?usp=sharing

Oh, and sorry I forgot 2 last mods, MRS (Modular Rocket Systems) and Kerbal Engineer Redux.

[Stellarium]

If you're wondering about the weight of the plane, (due to the massive fuel tanks) I distributed the fuel so that it had enough to lift off and also get to orbit lol

[miklkit]

Where are the COM and COL?

[DeadJohn]

@Stellarium I wonder if your name is related to the astronomy software. Anyway, that video you shared has me thinking a few things that might help diagnose the problem.

Maybe the plane is stalling. Build more speed on the runway before taking off. A video of your takeoff process would help so we can watch your airspeed and flight profile.

Check the settings on all your control surfaces. I usually assign only one axis per control but KSP is pretty forgiving even if I don't do that. Rudders can be set to only yaw. Elevons would technically control pitch and roll, but consider having most of the horizontal surfaces set to pitch, while the controls near the wingtips handle roll (they get more leverage as they are further from the centerline). Canards should be mostly pitch, but if you need extra roll they can be pitch and roll at the same time.

Try larger canards. You'll get more lift and the plane might be more responsive. Start with something excessive like the large Mk3 shuttle tail as canards just to see what happens.

Is the plane or its cargo flexing? Add autostruts in key places or use a joint reinforcement mod.

Is anything near the nose exceptionally draggy? Maybe the plane is flipping due to drag.

Empty the cargo bay and try taking off. Does it fly better? If yes then you've got a mass  issue that might be solved by more wing or more engines, or maybe it's just a balance issue that might be solved by shifting the cargo or wing placement,

Give the CorrectCoL mod a try. It shows what's supposed to be a more accurate assessment of the center of lift and mass, it estimates whether your aircraft will be stable at different speeds and altitudes, and can help know whether you have enough wing. Open its window, turn on CoM and CoL marker balls, and share a screenshot.

 

 

[Stellarium]

13 hours ago, DeadJohn said:

@Stellarium I wonder if your name is related to the astronomy software. Anyway, that video you shared has me thinking a few things that might help diagnose the problem.

Maybe the plane is stalling. Build more speed on the runway before taking off. A video of your takeoff process would help so we can watch your airspeed and flight profile.

Check the settings on all your control surfaces. I usually assign only one axis per control but KSP is pretty forgiving even if I don't do that. Rudders can be set to only yaw. Elevons would technically control pitch and roll, but consider having most of the horizontal surfaces set to pitch, while the controls near the wingtips handle roll (they get more leverage as they are further from the centerline). Canards should be mostly pitch, but if you need extra roll they can be pitch and roll at the same time.

Try larger canards. You'll get more lift and the plane might be more responsive. Start with something excessive like the large Mk3 shuttle tail as canards just to see what happens.

Is the plane or its cargo flexing? Add autostruts in key places or use a joint reinforcement mod.

Is anything near the nose exceptionally draggy? Maybe the plane is flipping due to drag.

Empty the cargo bay and try taking off. Does it fly better? If yes then you've got a mass  issue that might be solved by more wing or more engines, or maybe it's just a balance issue that might be solved by shifting the cargo or wing placement,

Give the CorrectCoL mod a try. It shows what's supposed to be a more accurate assessment of the center of lift and mass, it estimates whether your aircraft will be stable at different speeds and altitudes, and can help know whether you have enough wing. Open its window, turn on CoM and CoL marker balls, and share a screenshot.

 

 

Thanks for all the help, and here's the second drive link for the takeoff: https://drive.google.com/file/d/1w57_Ue7IJDFvJcwp6vYry_0V9UjyCM1d/view?usp=sharing

And here's for the COL and COM markers: https://drive.google.com/file/d/1pqQyaWKZb3uWu7HvrvUKxbN5IY2G_7_i/view?usp=sharing

[DeadJohn]

5 hours ago, Stellarium said:

And here's for the COL and COM

Those markers show the CoL way in front of the CoM. That causes unstable flight because the heavy end wants to flip to the front. Lift should be behind the Mass.

On the other hand, KSP sometimes shows markers incorrectly. Toggling them a few times, making a trivial edit to the plane, and/or exiting the SPH can sometimes fix it. The CorrectCoL mod can also help. I suspect the markers could be wrong based on how your overall plane looks but I emphasize that's merely a guess.

I see you have a lot of rotated Mk2 parts. I've never done that on such a large scale and maybe it causes weird aerodynamic effects during flight or distorts the CoL marker.

If you conclude that the Lift is really that far in front of the Mass, then make a backup save of your plane and start shifting parts around. Move lifting parts backwards and heavy parts forwards. Consider how the mass will change as you burn fuel and unload cargo; you don't want a stable plane to become unstable before you land.

[miklkit]

The .mov video would not load for me so I have no idea how it takes off.  I have never seen the CoL so far forward on any of my designs.  I have never built anything like that so large either though.  I have built some small ones with the rotated parts before and they both fly normally.  This pic shows that they do create a small amount of lift when rotated.  Methinks that at takeoff speeds the lift would be small tho.

[miklkit]

I DLed the video and was able to watch it that way.  The CoL seems to be reported wrong as the plane is going plenty fast to take off and does not flip.  It may be possible that that many rotated MK2 parts are pushing it sideways as they create a goodly amount of lift.

[Stellarium]

Finally, the problem has been completely resolved!

The MK2 parts were disrupting the CoL, and so the COL was also miscalculated for whatever reason, but thanks to everyone's help I was able to get the plane nice and stable!

It took a lot of iterations but it's very rewarding to have the satisfaction of flying it into orbit! 

[DeadJohn]

On 2/7/2024 at 5:56 PM, Stellarium said:

The MK2 parts were disrupting the CoL, and so the COL was also miscalculated for whatever reason, but thanks to everyone's help I was able to get the plane nice and stable!

Congrats on getting your plane working.

I now think your problem can be summarized as "too much vertical stabilizer (rudder) at the nose". The Mk2 parts and CoL marker may have been working correctly. The rotated Mk2 parts near the nose were acting exactly like vertical stabilizers and overwhelming the real stabilizers at the tail. As soon as the plane yawed a little bit it wanted to flip.