wobbly mechjeb

[Xd the great]

yeah so when i use mechjeb to land my short spacecrafts it wobbles. mechjeb does not know how to deep throttle stuff, especially if the engines have a large gimballing range. Is there any way to solve this problem?

[Bakkerbaard]

I must admit that I haven't encountered this problem yet, so I don't know much. I usually stick on the ruggedized case, as the big MechJeb cone doesn't show in my regular parts list and it's kinda heavy. ...And that eye freaks me out. ;o)

Anyway, could it be as simple as locking the gimbals?

[Gargamel]

12 hours ago, Xd the great said:

short spacecrafts

What's a short space craft to you?

12 hours ago, Xd the great said:

deep throttle stuff,

What's deep throttle?

12 hours ago, Xd the great said:

especially if the engines have a large gimballing range. Is there any way to solve this problem?

Solved within the sentence before the question.  Reduce the gimbal range on the the engine. 

Or, fuss with the PiD settings. 

[Xd the great]

Well, without rcs, mechjeb performs better.

[Gargamel]

3 hours ago, Xd the great said:

Well, without rcs, mechjeb performs better.

Then turn off the RCS. 

[Xd the great]

will the overcontrolling of mechjeb be fixed? i have to manually thread the retrograde vector to the top of the navball

[mystifeid]

Tried build 801 yet? Smartass (up) and smart rcs (kill velocity) are turned on after landing. Drove me crazy. Went back to build 800.

[Gargamel]

22 hours ago, Xd the great said:

will the overcontrolling of mechjeb be fixed? i have to manually thread the retrograde vector to the top of the navball

1) Have you tried asking in the MJ thread?  Or submitted a bug report?

2) I have never had this happen.   Are you referring to when it zeros out it's horizontally velocity right before final descent?  If so, just wait, it's letting the RG drift to up on it's own. 

[Xd the great]

well mechjeb lands REALLY WOBBLY. may submit a video to see what is wrong.

[Gargamel]

21 hours ago, Xd the great said:

well mechjeb lands REALLY WOBBLY. may submit a video to see what is wrong.

Start with a picture of your craft, you may just have a design that doesn't play well with MJ's algorythms. 

[Xd the great]

I use the spacecrane from ground construction mod, and add some necessary aerodynamically shaped fuel tank and a battery and a 2.5m payload stower. 

[Gargamel]

Nope... nevermind this post, I was thinking of another mod.  Carry on!

 

But try a stock lander, see if it's still twitchy?

Edited June 6, 2018 by Gargamel

[Xd the great]

13 hours ago, Gargamel said:

Nope... nevermind this post, I was thinking of another mod.  Carry on!

 

But try a stock lander, see if it's still twitchy?

Yes, i use 7 wolfhound engines on 7 rocknomox 32 tanks for my mining/refueling craft. Comes with almost 5000 delta v and all the necesseties for docking. Still wobbles like crazy. Gonna upload photoes later.

[Capt. Hunt]

check that the CoM and CoT are lined up.

[Xd the great]

4 hours ago, Capt. Hunt said:

check that the CoM and CoT are lined up.

Checked, all stuff is symmetrical.

[Capt. Hunt]

12 hours ago, Xd the great said:

Checked, all stuff is symmetrical.

That's not what I asked, everything can be symmetrical and still be out of balance if the CoT isn't lined up with the CoM.  Turn on the CoT/CoM markers and check that they are aligned.

 

Also check if your RCS is balanced around the CoM.

[Someone2018]

@Xd the great Try landing without target to see if you have the same problems (on the latest build), I haven't been in the mood for KSP in the last few weeks, but I never even looked into the the guided landing (which misbehaved even more than the unguided one). Out of curiosity what do you dislike about the SmartASS and SmartRcs?

Edited June 7, 2018 by Someone2018

[Xd the great]

5 hours ago, Someone2018 said:

@Xd the great Try landing without target to see if you have the same problems (on the latest build), I haven't been in the mood for KSP in the last few weeks, but I never even looked into the the guided landing (which misbehaved even more than the unguided one). Out of curiosity what do you dislike about the SmartASS and SmartRcs?

I just dont use them.

Well, probably gonna try unguided landing tonight.

[Gargamel]

7 hours ago, Xd the great said:

Well, probably gonna try unguided landing tonight.

If you mean "land somewhere", that can often be a fix for MJ issues.  I will sometimes get close with landing at the target, and then if stuff starts going wonky, I'll take control, recover, and then use Land somewhere to finish the deal. 

7 hours ago, Xd the great said:

I just dont use them.

SmartASS is superior to SAS IMO, and is very useful, just gotta get used to it.  Only down side is that it is slower to enable versus stock SAS (a few mouse clicks vs T and a mouse click), but it does have more options. 

I used to play with SmartRCS a lot, but of late (last couple years), I haven't touched it much.   But if you are having RCS issues, try fiddling with the settings, you might find a sweet spot. 

[Xd the great]

Hmm.

4 hours ago, Gargamel said:

If you mean "land somewhere", that can often be a fix for MJ issues.  I will sometimes get close with landing at the target, and then if stuff starts going wonky, I'll take control, recover, and then use Land somewhere to finish the deal. 

SmartASS is superior to SAS IMO, and is very useful, just gotta get used to it.  Only down side is that it is slower to enable versus stock SAS (a few mouse clicks vs T and a mouse click), but it does have more options. 

I used to play with SmartRCS a lot, but of late (last couple years), I haven't touched it much.   But if you are having RCS issues, try fiddling with the settings, you might find a sweet spot. 

Will mechjeb be allowed to do a landing where the horizontal velocity is cancelled out first before the suicide burn?

Also, seems like the problem is that mechjeb does not take into the gimballed thrust.

[mystifeid]

Seven Rockomax 32 tanks @ 18t = 126t
Seven Wolfhounds @ 2.5t = 17.5t

Total Mass of Engines/Tanks = 143.5t

I suspect that whatever is sitting on top does not come close to that mass which would make it a wide bottom heavy design and these sway easily.

I was curious so I made one and cheated it into orbit around the mun and yes, it sways quite a bit during the descent. I also noticed that the CoM stayed at the same height above the CoT regardless of how much fuel was in the tanks.

So I tried replacing the 32's with a sub-assembly of two 16's and set the bottom tanks to have higher flow priority than the top ones. This way, as the fuel is burnt the CoM moves away from the CoT. I tried landing with the bottom tanks already empty and saw negligible effect on the sway.

Next I replaced the seven wolfhounds with one mainsail - at the expense of nearly halving the TWR. Around the mun with half empty tanks the wolfhounds had a TWR of 16 and the mainsail 10. Good enough.

But now the ship lands with very, very little sway at all.

It's hard to say whether this is because of the reduction in mass at the tail end or whether it's because the wolfhounds are probably not perfectly aligned but one thing is sure: 17.5t is lot of engine to hang off a mun lander.

Anyway, try reading about long rocket designs vs wide rocket designs.

Edited June 8, 2018 by mystifeid

[Darinth]

The way that mechjeb handles rotation control (a PID controller) I don't think it's possible for it to not take into account gimballed thrust. The only craft that I've had serious enough issues with mechjeb that I just wasn't able to get mechjeb to control it was a massive 'mothership' style craft. It had a central shaft and essentially had a half-dozen additional rockets hanging off the sides that could dock/undock to do things like science or fuel runs to a moon/planet. It had two of each child craft, 1 on each side to guarantee symmetry and everything was aligned down the center of mass... but still mechjeb couldn't managed to keep the craft from oscillating back and forth. Even after I played with the PID settings, it didn't work. To be fair, stock SAS only handled things a hairline better.

Mechjeb doesn't generally handle extremely agile craft or extremely sluggish craft very well. If you're on either of these extremes, you may have to adjust your PID settings or ideally adjust the craft to be more/less agile. Also, as has been said, make sure that your RCS is well balanced. As an example, a craft with a TWR in excess of 12 is almost always going to fail it's descent, because mechjeb calculates the suicide burn, knows it can stop the craft's 2000 m/s in just a few seconds, and does a fantastic job of canceling your vertical momentum. Unfortunately, physics says that when you're firing an engine with that high of a twr, being even the tiniest bit off results in the craft tilting to the side and that tends to result in a crash and burn even after canceling your vertical momentum successfully.

Check your TWR. Make sure you've got enough RCS/reaction wheels/attitude control and that it's well placed. Make sure the craft is well strutted, use auto strut. Another issue that can pop up is that if a craft 'flexes', then it can cause an issue. All SAS and mechjeb attitude control is based on your control point. If your control point is able to go out of alignment with the rest of your craft, it can cause an issue. I once built a craft with a bunch of reaction wheels attached right near the cockpit. What I didn't realize at first and laughed when I realized what I'd done, is that the reaction wheels were so powerful they were actually rotating the cockpit before the rest of the craft which was confusing the hell out of the attitude control.

Long story short, mechjeb isn't perfect, nor is stock SAS, but they're both pretty good unless dealing with extreme situations. I'll put money down that either something has gotten messed up in the install, or there's something odd with the craft itself.

Btw, depending on what you consider extremely short, you may be running into the 'too high twr' problem. An easy test/fix for this is to set the thrust limiter on the engine lower. This will reduce your TWR and if the craft lands fine that's probably your issue. The gimbal is being taken into account, it's just too powerful to be effectively balanced.

[Someone2018]

@Darinth The latest changes on mechjeb (only in git) were done to make the unguided landing work better with high TWR rockets, from memory TWR>20 was typical for my tests. Prior mechjeb did some silly things like boosting the craft into the air again or even turning the rocket a few meters above the ground to burn of a tiny bit of horizontal speed, which is usually epic fail. The whole SmartASS/Rcs thing was to do the last bit of stability control once it lands.

[Gargamel]

18 hours ago, Xd the great said:

Will mechjeb be allowed to do a landing where the horizontal velocity is cancelled out first before the suicide burn?

Also, seems like the problem is that mechjeb does not take into the gimballed thrust.

Allowed?  I've found that to be standard.  It comes in high, and then zero's it's horizontal velocity, and on low mass bodies, it has to wait some time before the velocity becomes vertical, then it starts a low speed deceleration burn.  I think suicide burn is the wrong nomenclature here, as the vast majority of the velocity has already been lost, and now it's settling down gently. 

And I've very rarely had issues with gimbals and MJ, unless it's something way overpowered in the gimbals, like a vector.  Unless I'm specifically building a ship with offset thrust, I will reduce the gimbal range on engines like the vector to like 10%.  The only time I would want high gimbal range is during a landing, but I rarely use high gimbals engine for that.   Any other time I rarely need to turn, under power, that quickly. 

18 hours ago, Darinth said:

Check your TWR.

Not only that, but check it for the body you are landing on.  Easily accessible in the DV MJ window in the VAB.  I usually just use this to make sure my mining ships have enough Dv and TWR to launch with a full load, but If I see my ship is way over powered for the body I'm trying to land on, I'll use smaller, more efficient (hopefully) engines.   With smaller more efficient engines, you get more dv for the fuel mass, and then you can reduce fuel if needed.