Mechjeb suicided my rocket!

[Loren Pechtel]

I'm on Minmus, trying to launch to meet up with my tanker in orbit.

I launch, kaboom.

I reload and watch the launch from the close up view.  Mechjeb uses the reaction wheel (it didn't light the engine or RCS) to flip my rocket over and then lights the engine when it's pointed about 120 degrees from the vertical.  Of course it goes boom.

It's also doing some pretty horrible orbital matches when I do a launch to rendezvous.  My fueler is in a 89 degree orbit, there should not be a major plane change burn needed!

Even with manually waiting until the target orbit passed overhead but then with a manual launch burn of about 80 m/s I used over 500 m/s to do the rendezvous.  (And, major curses, I ran out of RCS fuel.  I have to send up a resupply rocket to recover my Kerbel and the boatload of science up there.)

[Frybert]

Did you have your RCS turned on?

[Loren Pechtel]

17 minutes ago, Frybert said:

Did you have your RCS turned on?

No, I was letting Mechjeb fly the ascent.

[Frybert]

MJ won't turn your RCS on for you, you have to do that manually.

[nobodyhasthis2]

1 hour ago, Loren Pechtel said:

I'm on Minmus, trying to launch to meet up with my tanker in orbit.

I launch, kaboom.

I reload and watch the launch from the close up view.  Mechjeb uses the reaction wheel (it didn't light the engine or RCS) to flip my rocket over and then lights the engine when it's pointed about 120 degrees from the vertical.  Of course it goes boom.

It's also doing some pretty horrible orbital matches when I do a launch to rendezvous.  My fueler is in a 89 degree orbit, there should not be a major plane change burn needed!

Even with manually waiting until the target orbit passed overhead but then with a manual launch burn of about 80 m/s I used over 500 m/s to do the rendezvous.  (And, major curses, I ran out of RCS fuel.  I have to send up a resupply rocket to recover my Kerbel and the boatload of science up there.)

@Frybert is absolutely correct on RCS. It will restrict to reaction wheels alone otherwise. 

Also need to point out that it likes to land a maneuver node and hold nice and steady. It does through a feedback loop that requires good control authority. If your RCS is not balanced or there is also a problem. Weak or modded saturated reaction wheels will also lead to delays in finding the node quickly. Plus if there is low RCS fuel anyway for manual maneuvers that is not going to help either. This can cause trouble on launch because it would normally just miss the burn time. However in a launch it has already committed to finding the node. In atmosphere the control surfaces can make up for the other two systems but not here obviously.  

The automatic pilot is running a selection of modular scripted functions. So if it does not get into the right plane. It will correct by running the plane change script for it after the launch. This is where you might want to take it off autopilot and assess the situation briefly. Another rendezvous scripts or tiny manual tweak might help. It can also generate the launch guidance node to help as well so you can see where wants to go. It is not a fan of direct approach to rendezvous without orbing a few times. 

It will fly a very generic curve to orbit (which can also be tweaked) that does not take into account the handling characteristics of an individual rocket. By default it also goes for 0.1 m/s accuracy and that a little too low in some situations. Minmus is very forgiving and that is something you could take advantage of in ways Mechjeb can't. 

That is not say it does not work. It will do the job of several mods and is very useful. It just needs to be watched very carefully and needs a bit of extra fuel now and again. 

 

Edited February 28, 2016 by nobodyhasthis2

[Loren Pechtel]

4 minutes ago, nobodyhasthis2 said:

@Frybert is absolutely correct on RCS. It will restrict to reaction wheels alone otherwise. 

Also need to point out that it likes to land a maneuver node and hold nice and steady. It does through a feedback loop that requires good control authority. If your RCS is not balanced or there is also a problem. Weak or modded saturated reaction wheels will also lead to delays in finding the node quickly. Plus if there is low RCS fuel anyway for manual maneuvers that is not going to help either. This can cause trouble on launch because it would normally just miss the burn time. However in a launch it has already committed to finding the node. In atmosphere the control surfaces can make up for the other two systems but not here obviously.  

The automatic pilot is running a selection of modular scripted functions. So if it does not get into the right plane. It will correct by running the plane change script for it after the launch. This is where you might want to take it off autopilot and assess the situation briefly. Another rendezvous scripts or tiny manual tweak might help. It can also generate the launch guidance node to help as well so you can see where wants to go. It is not a fan of direct approach to rendezvous without orbing a few times. 

It will fly a very generic curve to orbit (which can also be tweaked) that does not take into account the handling characteristics of an individual rocket. By default it also goes for 0.1 m/s accuracy and that a little too low in some situations. Minmus is very forgiving and that is something you could take advantage of in ways Mechjeb can't. 

That is not say it does not work. It will do the job of several mods and is very useful. It just needs to be watched very carefully and needs a bit of extra fuel now and again. 

 

Nobody has explained why Mechjeb would choose to flip over my rocket while it was sitting on the ground, though.  I was a few seconds from ignition when it tried to turn a cartwheel.  No visible exhaust so it could only have been the reaction wheel.  It was like it was trying to head through Minmus to get where it wanted to go.

[nobodyhasthis2]

18 minutes ago, Loren Pechtel said:

Nobody has explained why Mechjeb would choose to flip over my rocket while it was sitting on the ground, though.  I was a few seconds from ignition when it tried to turn a cartwheel.  No visible exhaust so it could only have been the reaction wheel.  It was like it was trying to head through Minmus to get where it wanted to go.

It still only has access to what the reaction wheel gives it. if you can tip it over just a fast manually that is ok and if you can't. Well it is a case of hello Mr Kraken. 

 It can also generate the launch guidance node to help. That is what it's future intention is going to be. Remember it is generic solution. I have never hap Mechjeb flip a rocket on Minmus but it does not pay attention to hills either.  

Also see http://forum.kerbalspaceprogram.com/index.php?/topic/130937-mechjeb-ascent-guidance-failure/#comment-2432055 just double check the version and seek help help from dev.

Edited February 28, 2016 by nobodyhasthis2

[Frybert]

Really the only way we're going to know for sure whats happening is if you can get some video for us. I can 95% guarantee you that if you can show a video of whats happening someone can tell you whats going wrong in a heartbeat.

[Loren Pechtel]

5 minutes ago, Frybert said:

Really the only way we're going to know for sure whats happening is if you can get some video for us. I can 95% guarantee you that if you can show a video of whats happening someone can tell you whats going wrong in a heartbeat.

I'll see if I still have a save around.

The rocket works (except the materials bay--my scientist can't do anything with it)--this was the second trip up to the tanker with multiple hops per landing.

[Yemo]

7 hours ago, Loren Pechtel said:

I'll see if I still have a save around.

The rocket works (except the materials bay--my scientist can't do anything with it)--this was the second trip up to the tanker with multiple hops per landing.

If you have SETIrebalance installed, that prevents your scientist from collecting goo and science jr (except if you also have UnmannedBeforeManned installed, in that case this rebalancing feature is deactivated).

@Loren Pechtel

edit: Oh, and on topic, yes mech jeb ascendence has great difficulties at the moment. It burns until apo is above the threshold and then totally stops thrusting even if that results in a loss of control, so it most likely flips without ridiculous (stock) RCS. If that flip lowers your apo below the threshold even by 10 meters, mech jeb burns again regardless where the rocket is pointing, essentially failing the ascent...

Try an acceleration limit of 20 instead of 40 and adjust the ascent path to 55. And activate corrections or something. Not pretty but less horrible.

Edited February 29, 2016 by Yemo

[Loren Pechtel]

17 minutes ago, Yemo said:

If you have SETIrebalance installed, that prevents your scientist from collecting goo and science jr (except if you also have UnmannedBeforeManned installed, in that case this rebalancing feature is deactivated).

@Loren Pechtel

edit: Oh, and on topic, yes mech jeb ascendence has great difficulties at the moment. It burns until apo is above the threshold and then totally stops thrusting even if that results in a loss of control, so it most likely flips without ridiculous (stock) RCS. If that flip lowers your apo below the threshold even by 10 meters, mech jeb burns again regardless where the rocket is pointing, essentially failing the ascent...

Try an acceleration limit of 20 instead of 40 and adjust the ascent path to 55. And activate corrections or something. Not pretty but less horrible.

No, I don't have that mod installed.  He could collect from the goo, just not from the science jr.  I couldn't interact with the science jr in any fashion.

Despite the failings I brought home enough science from that mission to complete the tree (even after a docking calamity that left my lander damaged) and so I decided to start again with the difficulty bumped up.

This wasn't a case of burning until apo was above the threshold--my rocket was tipping over on the ground before the rockets were even lit.  It flipped it and lit the booster heading into the ground.

I downloaded the MechJeb source and had a look to see if I could see why it's very badly behaved when I try to land while not in orbit.  It's going to take a more careful perusal to understand but I can see it's blindly doing steps without seeing if that's appropriate.  I really wish I could say "land here" while sitting on the ground and have it understand I want a parabolic hop.  Scansats will show you the biomes--but only on the big map and you can't land with that.  Hand flying there's no reasonable way to target a desired biome other than with the cheat menu.

[Yemo]

@Loren Pechtel: Different case, same principle I guess. It seems to follow a strict rule (like if apo 10m below threshould, start burn) regardless of the actual effect of that action.

However mech jeb is a really big project and the circumstances changed a lot recently (atmo, heating) and 1.1 will nearly certainly break stuff again. So it is very understandable that putting effort to optimize it now has small rewards considering it does not last very long until it needs to be touched again.

Under those circumstances mech jeb dev is really great!

And after 1.1 hits, offering support by feedback, suggestions and contributions (in code or otherwise) will make it great again.

[Loren Pechtel]

4 minutes ago, Yemo said:

@Loren Pechtel: Different case, same principle I guess. It seems to follow a strict rule (like if apo 10m below threshould, start burn) regardless of the actual effect of that action.

However mech jeb is a really big project and the circumstances changed a lot recently (atmo, heating) and 1.1 will nearly certainly break stuff again. So it is very understandable that putting effort to optimize it now has small rewards considering it does not last very long until it needs to be touched again.

Under those circumstances mech jeb dev is really great!

And after 1.1 hits, offering support by feedback, suggestions and contributions (in code or otherwise) will make it great again.

Fixing it's propensity for flying into the planet and it's poor choice of orbits when targeting something in a polar orbit shouldn't be broken by whatever is in the 1.1 update.  If it goes bonkers again on me I'm going to see if I can figure out why.

[Zhetaan]

Did you, perhaps, set your MechJeb module to 'Control from Here'?  Did you dock with your fueler using a radial port and set it to 'Control from Here' for docking?  Is your Kerbal in a non-command pod, or is your command pod in a weird orientation?  If your point of control is in a weird orientation (which can happen if you forget to transfer control away from a radial docking port, for example), and MechJeb doesn't know the difference, it may be trying to orient the ship according to that control point.  In the case of a radially attached port or the MechJeb pod itself, it means that the rocket tries to orient sideways.  The engine's thrust is perpendicular to that, and the end result is a cartwheel.  It's worse if your point of control is on the bottom and MechJeb thinks you're upside down.

Edited February 29, 2016 by Zhetaan

[Loren Pechtel]

8 minutes ago, Zhetaan said:

Did you, perhaps, set your MechJeb module to 'Control from Here'?  Did you dock with your fueler using a radial port and set it to 'Control from Here' for docking?  Is your Kerbal in a non-command pod, or is your command pod in a weird orientation?  If your point of control is in a weird orientation (which can happen if you forget to transfer control away from a radial docking port, for example), MechJeb doesn't know the difference and will try to orient the ship according to that control point.  In the case of a radially attached port or the MechJeb pod itself, it means that the rocket tries to orient sideways.  The engine's thrust is perpendicular to that, and the end result is a cartwheel.  It's worse if your point of control is on the bottom and MechJeb thinks you're upside down.

Pretty hard to select a docking target while you're still on the ground.  it's hard enough in space!  (And why can't it default if you have only one!)

I have yet to build a rocket with a radial docking port.  The lander was a can, a fuel tank, a RCS tank and jets, an engine, a probe core, a docking port, one of every science instrument, legs and a ladder.  Next time it will also have lights.

[wasml]

What Zhetaan said. If the probe core was on upside down I could see this happening - easy way to check is look at the nav ball before launch. If the probe core is upside down the orange side of the nav ball will be visible - if not the nav ball will be on the blue side.

[Loren Pechtel]

5 minutes ago, wasml said:

What Zhetaan said. If the probe core was on upside down I could see this happening - easy way to check is look at the nav ball before launch. If the probe core is upside down the orange side of the nav ball will be visible - if not the nav ball will be on the blue side.

Except this was a lander that had already been hopping around Minmus.  I have no idea of how to rearrange a rocket in flight and the only Kerbel on the scene was a scientist anyway.  Also, I was able to fly it up manually.  It flew as expected and MechJeb had no problem guiding it to the tanker.  (At least until it ran out of monopropellant.  Why couldn't the Kerbel use a tow rope to haul it in?  They were that close.)

Edited February 29, 2016 by Loren Pechtel

[wasml]

If MechJeb has been hopping the lander around then this probably isn't the problem. Checking the nav ball is just a diagnostic to try and locate the problem - if it shows the lander pointing the wrong way you've found the problem and there's not much you can do about it (other than flying manual). If the orientation is correct then I'm not sure what is wrong.

[Zhetaan]

The only other thing I can think of is that you somehow accidentally put the navigation into target mode, and MechJeb is trying to thrust according to your velocity relative to your tanker.  If that isn't it, then I'd have to call it a Kraken attack; MechJeb shouldn't be doing this on its own.  After all, the whole point of using an autopilot is that it knows which way is up as well or better than you do.

[Loren Pechtel]

1 minute ago, Zhetaan said:

The only other thing I can think of is that you somehow accidentally put the navigation into target mode, and MechJeb is trying to thrust according to your velocity relative to your tanker.  If that isn't it, then I'd have to call it a Kraken attack; MechJeb shouldn't be doing this on its own.  After all, the whole point of using an autopilot is that it knows which way is up as well or better than you do.

It was targeted on the tanker because I was launching to it--but I had done that before and it worked, albeit not very well.  It messes up enough that I've about decided to simply launch into an orbit under it and use the rendezvous to actually catch it.

[artwhaley]

Firstly... I know it's frustrating to have your rocket crash...  but I want to start by reminding folks again that mechjeb is a volunteer project that has taken thousands of hours to put together..  for nothing but fun.  So... please remember to keep your tone appreciative of what it DOES do... and not accusatory.  You have no RIGHT to mechjeb.  You paid squad some money for the right to the stock game.   Everything else you get is someone being REALLY kind and sharing their hard work with you... so when you come to the forums asking for help, it's always better received to approach things from the "Hey, I know mechjeb works for 10,000 of you, under 100,000 situations, but... I've found a situation where it's not working for me, and I'd appreciate some help.  Oh, btw... as it asks for in the mechjeb thread's first post - here's my .log file and .craft file so I can get support."    I just bring it up because I know from my limited experience... that it can feel like a thankless job, because nobody thinks to start a forum post the 1000 launches that mechjeb got perfect for you and saved you hours of time and tedium....  they just post when it doesn't do what they think it should.  I'm sure it wasn't your intention to be ungrateful, and I admit I'm oversensitive to it, as we just see a lot of it as mod makers... 

 

I think the 'control from here' folks are most likely right.  That's the only time I've ever seen mechjeb flip over on launch.  Does your navball show sky?  If it shows planet, that's your problem.  Mechjeb is assuming you're already upside down and trying to right you before it burns  The 'control from here' part's axis is the ONLY way Mechjeb can figure out what part of your rocket you intend to have 'up' right now.  Also... do make sure you don't somehow have another of the mechjeb autopilot windows activated but closed.. if it's trying to fly a rendezvous or plan a landing at the same time as you're trying to ascend... weird things could happen...]the code doesn't let you do TOO much of that mistake, but it's worth checking.

 

We'd REALLY need to see screenshots, video, and logs to help much more, but I've never heard of the flipover behavior from anyone else, so I think we'll find the answer in the log, craft or screenshot.  

 

Also, there have been a LOT of upgrades to the launch to plane and launch to rendezvous code lately, so make sure you're on the right build of mechjeb, by checking that forum thread....  and... are you doing a launch with circ, then reverting so mechjeb can learn your ship's time to orbit?  

 

 

[sarbian]

On 29/2/2016 at 3:02 AM, Loren Pechtel said:

I downloaded the MechJeb source and had a look to see if I could see why it's very badly behaved when I try to land while not in orbit.  It's going to take a more careful perusal to understand but I can see it's blindly doing steps without seeing if that's appropriate.

I am eagerly awaiting for your fix submission.

[cicatrix]

On 2/29/2016 at 6:54 AM, Loren Pechtel said:

Except this was a lander that had already been hopping around Minmus.  I have no idea of how to rearrange a rocket in flight and the only Kerbel on the scene was a scientist anyway.  Also, I was able to fly it up manually.  It flew as expected and MechJeb had no problem guiding it to the tanker.  (At least until it ran out of monopropellant.  Why couldn't the Kerbel use a tow rope to haul it in?  They were that close.)

I think the problem was in the orientation of your control part (probe core). Did you detach anything from your lander? Was there a tug vehicle in orbit?

Your control part (probe core) should be oriented 'upwards' (facing in the opposite direction of your thrust vector). If you have it upside down MJ would try to orient your PROBE CORE onto the 'correct' vector and if your engines happen to point in the wrong direction after this - well, that's just too bad.

[Loren Pechtel]

5 hours ago, cicatrix said:

I think the problem was in the orientation of your control part (probe core). Did you detach anything from your lander? Was there a tug vehicle in orbit?

Your control part (probe core) should be oriented 'upwards' (facing in the opposite direction of your thrust vector). If you have it upside down MJ would try to orient your PROBE CORE onto the 'correct' vector and if your engines happen to point in the wrong direction after this - well, that's just too bad.

This was a hopper, IIRC on it's third launch from Minmus after the last refueling.  The rocket flew properly on the previous flights.

[sarbian]

Well, you can repeat it all you want but this is the obvious problem that leads to your issue. Without screenshot, video or save we can't do more.