When I dock, everything explodes. What can I do?

[FinalFan]

So I've been going through career stock and I happened to notice that I was in the middle of an Eve transfer window.  This inspired me to get really ambitious and try to do an Eve land and return mission.  I designed the whole monstrosity myself—it's probably way oversized because I tend to do that, but according to the math it should be able to put my kerbals back into Eve orbit where they will rendezvous with another group. 

Obviously a setup rated for Eve is capable of SSTO, which I did, but I didn't really plan very well so I needed to send a refuel mission.  I just so happened to have a couple of very large tankers lying around which seemed perfect.  I sent them to the Eve ship and very, very carefully set up a docking maneuver that was close to the normal-antinormal ideal.  I came in, decelerated to 0.1 m/s, and ... the Eve ship spontaneously exploded.  What? 

[edit:  TinyPic is dead; long live Flickr.  Original link for insanity posterity:  http://i64.tinypic.com/ibi0i0.jpg ]

Even Jebediah was shocked.  I wasn't really sure what would provoke such a violent reaction.  I mean, sure, the part count was getting a little high, but the docking maneuver was almost as gentle as humanly possible, and I'd already spent many minutes in physics range.  Maybe the reaction wheels are the problem?  I turned SAS off and docked on pure RCS ... the final connection was noticeably more wobbly, and it took longer after the connection was made for the explosions to begin, but ultimately the result was no different. 

This seems very stupid.  Is there anything I'm doing wrong other than "don't build such big stuff"?  Is there anything left worth trying—for example, could it make a difference to completely shut off the reaction wheels instead of just shutting off SAS?  If I decide to cheat, is there an easy way to tell the game to fill up the fuel tanks? 

Additional information:  the explosion is not catastrophic in that the vessel as a whole survives.  The vessel begins to tear itself apart but settles down after certain sections are torn free. The sections in question were intended to be the stuff I detach when preparing to depart Eve.  Mining equipment, some science stuff, AND the majority of my reaction wheels.  But turning the authority down to 0/100 and setting them all to Pilot Only doesn't seem to make a difference.  Is it possible that it would help to switch to the Eve ship as the main vessel (i.e. point-of-view ship) when the docking is about to succeed?  The refueling ship is the one with RCS but I think it's not possible to avoid the explosions with the way things are now. 

Edited September 23, 2019 by FinalFan
TinyPic is dead; long live Flickr.

[bewing]

Generally, this is an autostrut problem. So the first question is always "How many autostruts are you using in your designs (for BOTH docked ships)?" Then, "Did you turn off all your autostruts before you docked?" If so, "How many landing legs/wheels are included in your design (because they automatically use autostruts and you can't turn them off)?". Then, "What is the most massive part on your combined ships, and where is it?"

 

[FinalFan]

Thanks for the input.  I have not intentionally been using autostruts; is it off by default?  (I have a fair number of struts on the Eve ship but are these distinct from autostruts?)  There are 16 landing legs, and they do seem to be on the parts that want to die.  As for the most massive part ... there are a lot of big ones on the Eve ship, but since the fuel tanks are mostly empty the heaviest single part is probably a full S3-7200 fuel tank on the refueling ship (I didn't have the S3-14400 tech when it launched). 

Edited September 21, 2018 by FinalFan

[bewing]

18 minutes ago, FinalFan said:

Thanks for the input.  I have not intentionally been using autostruts; is it off by default?  (I have a fair number of struts on the Eve ship but are these distinct from autostruts?)  There are 16 landing legs, and they do seem to be on the parts that want to die.  As for the most massive part ... there are a lot of big ones on the Eve ship, but since the fuel tanks are mostly empty the heaviest single part is probably a full S3-7200 fuel tank on the refueling ship (I didn't have the S3-14400 tech when it launched). 

Yes, autostruts are off by default. You have to change a setting before they activate.

OK, do you have any fuel tank of a similar capacity to the s3-7200 on the Eve ship? Before you dock, if you transferred all your fuel on the Eve ship into a single tank, would that one tank now be your most massive part?

 

Edited September 21, 2018 by bewing

[FinalFan]

2 minutes ago, bewing said:

Yes, autostruts are off by default. You have to change a setting before they activate.

OK, do you have any fuel tank of a similar capacity to the s3-7200 on the Eve ship? Before you dock, if you transferred all your fuel on the Eve ship into a single tank, would that one tank now be your most massive part?

Yes, I have enough fuel that a S3-14400 in the Eve ship's center stack will outweigh a S3-7200.  I take it you believe that may help. 

[FinalFan]

11 minutes ago, bewing said:

Yes, autostruts are off by default. You have to change a setting before they activate.

OK, do you have any fuel tank of a similar capacity to the s3-7200 on the Eve ship? Before you dock, if you transferred all your fuel on the Eve ship into a single tank, would that one tank now be your most massive part?

I filled one of the large tanks in my Eve ship's center stack, and that seems to have done the trick!  Thank you very, very much.  Fortunately, the refueler was completely devoid of struts. 

[bewing]

Yeah, the way landing leg/wheel autostruts work is that they attach your landing leg directly to the most massive part on your current vessel. When you dock, suddenly the combined vessel may have a new "most massive" part. At that point, all the autostruts on the one vessel must be detached from the old most massive part, and forcibly attached to the new most massive part. There are large forces involved in doing this, and it can be an extremely violent process -- as I think you have just seen with your ship. Preselecting a known most massive part, and forcing it to NOT change during the docking can help.

 

 

Edited September 21, 2018 by bewing

[FinalFan]

1 minute ago, bewing said:

Yeah, the way landing leg/wheel autostruts work is that they attach your landing leg directly to the most massive part on your current vessel. When you dock, suddenly the vessel may have a new "most massive" part. At that point, all the autostruts on the one vessel must be detached from the old most massive part, and forcibly attached to the new most massive part. There are large forces involved in doing this, and it can be an extremely violent process -- as I think you have just seen with your ship. Preselecting a known most massive part, and forcing it to NOT change during the docking can help.

Thanks for telling me the theory behind what was going on.  I love to learn that kind of stuff, and it might even come in handy someday. 

[Gordon Fecyk]

Does turning Advanced Tweakables off also turn autostruts off completely? Even on landing legs and wheels?

[bewing]

16 minutes ago, Gordon Fecyk said:

Does turning Advanced Tweakables off also turn autostruts off completely? Even on landing legs and wheels?

No. The ones on landing legs and wheels are currently permanent. That's what caused FinalFan's problem -- he has Adv Tweakables off, but the landing legs were still autostrutting. I think turning off Adv Tweakables just makes it so you can't change the current autostrut setting for each part.

 

[Gargamel]

15 hours ago, FinalFan said:

Obviously a setup rated for Eve is capable of SSTO

As a side note to the topic, this is not obvious.  All my even landers have been asparagus staged ascent vehicles.   Used a good chunk of the fuel (without staging) for the transfer and orbit, which reduced it's landing weight, and then an ISRU rover refueled the launch vehicle. 

[FinalFan]

1 hour ago, Gargamel said:

As a side note to the topic, this is not obvious.  All my even landers have been asparagus staged ascent vehicles.   Used a good chunk of the fuel (without staging) for the transfer and orbit, which reduced it's landing weight, and then an ISRU rover refueled the launch vehicle. 

Quick double check, I meant to say Kerbin SSTO.  It's most definitely not Eve-SSTO capable (4 planned ataging events after takeoff).  

However, in retrospect I also agree it is not unimaginable that an Eve ascent vehicle would need help to make Kerbin orbit without shedding any of its planned Eve stages.