Riddle me that... Summersault

[Tokay Gris]

I have no idea what causes this.

Sometimes it happens, sometimes the same rocket goes straight up.

I would understand it if the rocket completely lost control, but it doesn't.

It does wild turns in the beginning, then steadies.

But only sometimes. Other times the same vessel (nothing broken off, I checked) goes straight up, without problems.

Looks like this:

And after that... goes up like on rails....

[Tidus Klein]

huuumm I have had the same problem before only way less dramatic then this (like maybe five or so degrees)

Try putting some of the launch stabilizes on your rocket because ( what i think it is) is that your rocket never sits perfectly still with out them and the sas over compensates, just a thought.

[Kasuha]

I guess we need to know more about the rocket. It's hard to guess from just the smoke trail.

[Tokay Gris]

It happens with different rockets. Sometimes more, sometimes less.

But this one looks like this in the VAB:

It is supposed to be an Eve-Lander. The problem arose during testing on Kerbin.

[Tokay Gris]

The "launch summersault" is in the first stage. Only the four Mainsails firing at that time.

[Tokay Gris]

I also noticed that the RCS system (installed to counter turning in later stages) fires IN direction of this turn. Not against. It is working FOR that summersault.

It doesn't fire to counter it.

[Tokay Gris]

I do not understand this... Just did another test. Exact same vessel.

Goes up straight as an arrow.

Didn't even need the RCS.

[Tidus Klein]

your going to hate me but i would just start from scratch and rebuild it.

or just launch with out SAS on? you dont have to have it you know.

[Tokay Gris]

I am not hating you....

I have similar problems with other vessels.

Ten launches without trouble and suddenly, the rockets do summersaults.

[Kasuha]

My experience is that sometimes it is better to start with SAS off and only turn it on after the rocket gained some speed. Have you tried to start without SAS? Or maybe all your successful starts were when you forgot to turn SAS on?

Also maybe the problem are the four reaction wheels at top of the four tanks. Perhaps they are too strong for the rocket? You have vectoring engines at the bottom after all, don't you?

The game's stabilization method (SAS) is rather crude and inaccurate and sometimes makes the rocket even less stable.

[Tex_NL]

I am pretty sure this is a MechJeb related problem. For some reason MechJeb gets disorientated during the first stages of launch as it fights between the ascent vector and the last setting used on another vessel.

To prevent this make 100% sure you remove all settings on previous ships before launching a new one.

Edited September 24, 2013 by Tex_NL

[Amoun]

That is weird....

It's the same code, should be identical.

I would rely on the fact that it could simply be a bug due to parts maybe?

I would think that maybe its the 4 SAS units you have, the 2 and 2 are pointing in different direction, so that could be throwing off the orientation for the SAS?

Maybe reaction wheels over those?

That's just my guess...because its the same vehicle, same code, its not like we have weather issue like IRL or something...so I would guess it's the conflict.

[Tex_NL]

SAS can not be the problem. Since 0.21 all ships have only one center of SAS; the command node. Some parts are still called (A)SAS but they are nothing more than reaction wheels. They add torque but do not control SAS.

I know for sure SAS can't be the problem as I too run into this bug occasionally and it also happens on launches without any additional SAS/reaction wheels.

[Tokay Gris]

I am pretty sure this is a MechJeb related problem.

Possible. But why does it sometimes work and sometimes it doesn't?

I launched a bunch of orbital tankers to 200km orbit. One after the other. No change at all. Just "Ascent autopilot" and go.

Some did work, some did crazy spirals directly after launch.

This vessel did start out without the four ASAS modules. Only after the rollercoasterride did I add them.

Didn't change anything.

Will try by hand next time. Since this vessel has the same trouble I have with all my Eve-projects: The 'chutes rip it apart.

[Tex_NL]

As I already said earlier it is my experience that MechJeb is somehow fighting itself. As if it has difficulties choosing if it should follow the ascent guidance or the latest settings in Smart A.S.S.

If there is not previous setting (either you just started playing or you have removed the settings) then all goes well. If there is MechJeb struggles.

[Temstar]

I can see you have a lot of aerspike engines, these are the culprits. You see for a rocket to be aerodynamically stable it must have more drag on its rear than at the front. That's why rockets have nose cones at the front and fins at the back. It's also the same reason why arrows have feathers (called "fletchings") at the back. When the arrow flies through the air if there's any deviation between the direction it's flying in vs the direction the arrow head is pointed at the fletchings will exert a force to pull the arrow back into alignment.

Now in KSP most parts have a default drag value of 0.2, the actual drag they case on the rocket in flight is then their mass times 0.2. However when you look at aerospike engines you'll notice that they have a drag of 0.1. By putting lots of aerospike on the tail end of your rocket you create a vehicle that's more aerodynamic on it's rear end than at the front. So when this rocket is launched it's as if you're trying to throw an arrow with the fletching end first towards something. This configuration is known as "dynamically unstable" because once you have any angle of attack where the rocket is not exactly pointed at the direction it's going (say, when you start gravity turn) the aerodynamic forces actually force the rocket to go further off course rather than try to put it back to its heading.

This is another engineering challenge for Eve return vehicle since you have to use some aerospike engines due to their great Isp in the atmosphere and thus you get dynamically unstable rockets. To correct this I recommend a lot of aerodynamic control surfaces so that the greater altitude control overcomes the instability.

Once you're in the upper atmosphere where there's less air aerospike will cease to be a problem since the destabilising force becomes negligible.

[Tokay Gris]

I can see you have a lot of aerspike engines, these are the culprits. You see for a rocket to be aerodynamically stable it must have more drag on its rear than at the front.

Sounds reasonable. But isn't it.

I have the EXACT same trouble with my fuel vessels. And they don't have any aerospikes.

They are as common as you can get. Big boosters on the bottom, smaller going upwards.

And still the same: Sometimes they work like an arrow and sometimes they do sumersaults.

Might just be MechJeb. But I kinda rule out the aerospikes. I KNOW they don't have gimbal.

But the problem start when they aren't even firing.

With this vessel, it just starts with the four Mainsails. Nothing else. And THEN it does the woooooheeee!

After the first two burn out, four aerospikes are started. (Reason is the TWR)

But the problem is before they are even started.

So I think I can rule that out.

[Temstar]

No no, aerospike drag value thing affects your rocket regardless of weather or not they are running. Drag for parts are calculated the same no matter what the part is actually doing, you can't cheat drag by just turning off an engine.

Easy way to test would be to replace aerospike with say replace them with turbojet engines and watch what happen to the rocket during that period of flight before the aerospikes ignite. Turbojet is roughly the same mass but with 0.2 drag, so if you see the rocket then to go much straighter than you know the fault is with aerospike engines.

[Tokay Gris]

I have the SAME trouble with rockets without aerospikes.

So that can't be it.

[Temstar]

I have the SAME trouble with rockets without aerospikes.

So that can't be it.

It can be caused by other things, for example the medium and large nose cones both have 0.3 drag, so sticking those on the front of your rocket actually destabilises it. Cupola is the worst at 0.4 drag and 4.5 tons, however cupola has some pretty beefy reaction wheels so it goes a long way to counter its own instability.

[Tokay Gris]

It can be caused by other things, for example the medium and large nose cones both have 0.3 drag, so sticking those on the front of your rocket actually destabilises it. Cupola is the worst at 0.4 drag and 4.5 tons, however cupola has some pretty beefy reaction wheels so it goes a long way to counter its own instability.

Same here. None of these are on the rocket.

Sometimes it works, sometimes it does the "Saturday Night Fever".

[AlexL]

I've had a few, issues, that seem a bit like what you're describing, though mine were a bit more explosive...

When you launch, try hitting control from here on the pod. If MechJeb is at an angle then it seemed to get it's angle and the angle of the rocket confused. Also, double check all you're fuel lines. Incorrect plumbing has lead to the time-travel assisted rescue of many a kerbal.

[Mr.Rocket]

Defiantly use SAS the whole time, and I would suggest putting both steerable and non steerable wing on it, and lots of them

[Kulebron]

It happens with different rockets. Sometimes more, sometimes less.

But this one looks like this in the VAB:

It is supposed to be an Eve-Lander. The problem arose during testing on Kerbin.

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.

[ArmchairGravy]

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.