Riddle me that... Summersault

[Tokay Gris]

Had a similar problem with a simpler rocket. Looking at it again, found that a pipe was disconnected, and fuel flew asymmetrically, quickly disbalancing the rocket.

Possible. But with this, the rocket would spin out of control completely, would it not?

But it doesn't. It does the summersault, then balances itself and goes straight up.

Edited September 25, 2013 by Tokay Gris

[Tokay Gris]

Make sure those mainsails sit below everything else. They will push your center of thrust over your center of mass if they sit too high.

The rocket has an inherent instability in this way, granted. But what I do not understand is that it rightens itself after the looping and goes straight up.

With usage of fuel, the COM should move downward and thereby enhance the instability.

Which it doesn't.

I do have an instability problem further up the stages, which I tried to counter with the RCS (which works, sort of).

But here, it did the woopty-do... then went straight up.

Might be Mechjeb. Like I said, I tested the launch with the RCS on and it went IN direction of the loop, not against it, as I would expect.

[Tokay Gris]

I have to redesign the Mainsail-stage anyway. As was expected, it overheats.

The new part will be further down. Maybe this changes the stability.

Since this vessel is bound for Eve (And I still have trouble solving the parachute-problem) I need full power on these engines.

But I am still wondering.... Sometimes it works just fine. Sometimes not.

[mellojoe]

Do you have a control pod turned sideways? Like a MechJeb unit on the side of a pod? Or a sideways command pod?

It could be trying to "see" the horizon sidways and right itself. Make sure you "control from here" on one of the command modules that is facing up from the start.

[Tex_NL]

Sorry Temstar but I have to agree with Tokay. It s neither the aerospike nor the nose cone. I haven't used either in a long, long while and I still have the same problems.

Same goes for SAS units. They too can not be the problem as the problem occurs both with and without them.

@ Tokay

Next time you run into this problem could you please try to launch the exact same vessel manually (with the MechJeb unit attached but not using it.) and post your findings. I'll do the same as soon as I run into it again.

Just enable SAS (T) and let it go straight up. If it goes off course with MechJeb in control and is stable without we can be sure it is MechJeb.

[csanders]

You know how when you hit the launchpad, you have to wait a few seconds before the rocket "drops". When it drops, the physics is kicking in.

Are you zooming the camera around before it drops? After seeing the video how they handle floating point precision issues, I think doing that may cause issues (something about how they handle frame's of reference for position and velocity). I can make my rockets explode if I zoom out fast in map view when the rocket drops. Maybe you're doing that, or something similar, and it causes a crucial part to break. Try not touching anything until the rocket drops.

(I'm kinda grasping at straws here, I admit)

[MircoMars]

Check if your navball points straight up with 90° on the right side. if not, select command-capsule or top docking port and "control from here".

sometimes (but not always) the navball thinks mechjeb is the command part and points in stupid directions.

[boomerdog2000]

Yeah if you use mechjebs SAS and the roll number you put in isn't the roll according to the ship it will try and roll as soon as it takes off, which can be problematic to say the least during certain launches.

I've found that with command pods placed normally in the VAB (no rotating) and active as the vessel "control from here" 90 degrees is generally the mechjeb roll setting you want to use.

[Tokay Gris]

Next time you run into this problem could you please try to launch the exact same vessel manually (with the MechJeb unit attached but not using it.) and post your findings. I'll do the same as soon as I run into it again.

Just enable SAS (T) and let it go straight up. If it goes off course with MechJeb in control and is stable without we can be sure it is MechJeb.

Will do... but since the problem appears and disavppears seemingly at random, a lot of tests will be needed.

[Tokay Gris]

Check if your navball points straight up with 90° on the right side.

Did that, of course. Since I ran into that trouble quite a few times (especially with rovers).

Yeah if you use mechjebs SAS and the roll number you put in isn't the roll according to the ship it will try and roll as soon as it takes off, which can be problematic to say the least during certain launches.

Not using any "roll" with mechjeb.

[Tokay Gris]

To conclude at this point:

I think I can rule out the obvious problems (parts breaking off, wrong command point, wrong fuel lines, roll in mechjeb, and so on).

That leaves the option with the zooming (haven't tried that yet, but usually I don't zoom around during launch, at least not untill the vessel has "settled".

And Mechjeb as the source of this looping.

Will continue testing....

[Tex_NL]

For the last week I shut off MechJeb as much as possible before I left a craft and for a long time I did not run into this problem. But today I suddenly did.

I am now absolutely, guaranteed, 100%, without a doubt sure this is MechJeb related!

It has nothing to do with design flaws, structural failure, asymmetric thrust or anything like that. It is MechJeb!

I launched the same refueler with MechJeb as I have done many times before. This design never brought me any problems so I know it is no design flaw. And it went seriously off course. Reverting to launch and launching without MechJeb it went up like if it was on rails. Just to be sure I reverted again and again it launched OK, at a few km altitude I turned MechJeb on and immediately it went off course again.

[mellojoe]

You might have a setting in MechJeb that is staying persistent. Something like ascent guidance turned on? Or have it trying to turn to Normal or Radial direction? Even if a MechJeb window isn't open, it can still manipulate the ship. For instance, if you leave "limit to terminal velocity" check, but dont open the Utilities tab, MechJeb will still limit your throttle. Try deleting MechJeb and reinstalling it. That should clear out any misc settings.

maybe?

[ackroe]

try putting just one SAS on it see if that works

[boomerdog2000]

What exactly are you telling MechJeb to do? Are you just using it's ascent autopilot, or its SASS?

[Tex_NL]

Would the three of you please go back to page one and START READING EVERYTHING!!!. Both Tokay and I have been talking about the ascent guidance from the start.

It has NOTHING to do with SAS. It is a bug in MechJeb! Indeed something in MechJeb stays active and it is throwing the ascent guidance off course.

[Tex_NL]

I already knew this problem resides in MechJeb's Smart A.S.S. but now I believe what part is to blame: Force Roll.

Every single time my rocket goes off course and recovers on it's own it ends up with the 'north line' on the navball pointing exactly straight up. This can't be a coincidence, if it was random the roll angle would end up random as well.