Fuel Tank Usage Rules? + other oddities

[Pacca]

I just had a very strange mission in KSP X3 It was basically the Minmus equivalent of Acapello 13 

 

The initial goal was really simple; just do an Apollo styled mission to drop off scientists at a science base I'd previously placed on Minmus. Things started going wrong before I even finished the ship though! When playtesting the CSM, it's engine would burn for less then a second and immediately die out! Its' body was an SM-25 Service Module, and it had various components in it, mostly fuel tanks. It had an FL-T400 attached to the upper portion of the SM-25s' interior, with reaction wheels and RCS tanks attached to it, and some extra liquid fuel + oxidizer tanks attached to it. There were also some super small oscar tanks attached to the outer nodes inside the SM-25. Here's a screenshot with some parts removed (a lower oscar tank, a radial mono propellant tank in the middle, and some radial batteries to fill in the gaps) to clearly show the important stuff

 

I was able to work around it by attaching the engine to the components inside the SM-25, offsetting it so it didn't clip, and auto-strutting it to the root part so it didn't wobble in odd looking ways. I felt really dirty doing this, but it worked...

 

Afterwards, I messed up by accidentally unstaging some stages after setting up a Minmus encounter. Since I didn't technically need them (my Minmus rockets as of late have been overkill X3) I went ahead, docked the CSM and Lander modules together, and eventually setup a stable Minmus orbit close to the surface. Somewhere along the lines, the GUI freaked out; the staging diagram on the side became empty, and it started saying that every single burn (even ones that took less the 1 m/s; keep in mind this is a Wolfhound pushing a standard Apollo setup) would take about 1 minute and 20 seconds! The problem went away when I switched to another craft and came back, but it spooked me for a while, and made maneuver nodes a fair bit more difficult to use (and reordering of staging nearly impossible; in fact, I was able to delete ALL the stages in the menu since they were all empty, and it removed the entire interface from my screen).

 

This is where the next big hiccup happened! I undocked the lander so I could let it land, but I didn't move it right away, so the docking ports were still in contact. While I was making preparations to set up the landing, the ships redocked! I went back to undock them again, and I couldn't  Neither docking port had an undock option, yet the ships were still glued together! I tried switching away and back to it, but that didn't fix the problem; they were simply glued together and I couldn't find a workaround  I ended up having to land the Lander and the CSM together!!! Shockingly, this part of the mission went relatively smoothly, despite the extra baggage 

 

After that odd landing, I did what I wanted to do and launched the Ascent stage, with the CSM in tow (thank god, this was a Minmus mission and not on a Moon with stronger gravity ) From there, I used all the remaining fuel to setup an aerobrake so I could slowly burn off my orbital speed to land safely. During this, however, I noticed that the stats page in the corner said I had fuel left, but all the engines said they were empty 

 

As it turns out, some or all of the oscar tanks (I couldn't quite tell, I was in the atmosphere at the time and not dying horribly was a bigger priority then counting up tanks) had fuel left!!! I used some of them to kill some speed by transferring fuel to the tanks that the CSMs' engine could actually use, but it was quite the hassle, since I had to clip the camera through the hull to access them. After another accidental unstaging that cost me a lot of science, I finally landed and was able to return my Pilot to Kerbin. My initial plan to leave the ascent module so the scientists could return in one mission failed, but I was able to deliver them and bring my Pilot back despite everything, so I considered it a success, even though a new return mission is needed now...

 

tl;dr: What are the rules behind how engines use fuel tanks? I thought they just used whatever was available to them either in that stage or via crossfeed, but clearly this isn't the case. Are there ways to prevent the staging GUI from breaking? And what should I do if my docking ports decide to never undock ever again?

EDIT: Here's the craft file for the mission.

Edited January 3, 2019 by Pacca

[bewing]

Wow, how do you find all this crazy stuff?

Of your list of troubles, the only one I know about is the undocking problem. It's been an issue in KSP for ages, but almost nobody can manage to make it happen twice. And unless we can do it repeatably, the devs can't fix it. They've tried. The way you fix it after it happens is by using a persistence file editing tool. If you need it in the future, just search the forum for "Can't Undock".

As far as your fuel problem goes:

1 hour ago, Pacca said:

I thought they just used whatever was available to them either in that stage or via crossfeed

^ this is correct. If it isn't working like that, then you need to post your craft file here so that other people can test it and verify the problem.

1 hour ago, Pacca said:

Are there ways to prevent the staging GUI from breaking?

The staging GUI never ever breaks. So the way you keep it from breaking is that you just use it normally and it doesn't break.

So, once again, if you can make some other funky thing happen, then you need to post your craft file so we can all verify that something is broken.

 

Edited January 3, 2019 by bewing

[Pacca]

7 minutes ago, bewing said:

Wow, how do you find all this crazy stuff?

Of your list of troubles, the only one I know about is the undocking problem. It's been an issue in KSP for ages, but almost nobody can manage to make it happen twice. And unless we can do it repeatably, the devs can't fix it. They've tried. The way you fix it after it happens is by using a persistence file editing tool. If you need it in the future, just search the forum for "Can't Undock".

As far as your fuel problem goes:

^ this is correct. If it isn't working like that, then you need to post your craft file here so that other people can test it and verify the problem.

The staging GUI never ever breaks. So the way you keep it from breaking is that you just use it normally and it doesn't break.

So, once again, if you can make some other funky thing happen, then you need to post your craft file so we can all verify that something is broken.

 

It's good to know there's a proper solution for at least one of these issues! I imagine that undocking issue could potentially ruin all sorts of stuff if there wasn't at least some workaround.

The issue appears to be consistent with the craft, so here's the .craft file. Feel free to tell me if I messed something up, this is the first time I've shared a craft publicly before. The former issue near the beginning can be replicated by simply placing the Service Modules Wolfhound onto the SM-25, and the later issue is already what the craft is suffering from; I don't know how to fix it without doing something dumb like adding fuel lines. The rest of the rocket appears to work as intended, though.

As for the staging GUI, it's fine that it's unknown; the fix was as easy as switching vehicles, so if it ever comes up again, it shouldn't be that big of an issue.

[bewing]

Well, congrats. Those are actual bugs. I've reported them to the devs.

 

[Pacca]

I tried building a new Service Module by completely throwing out the SM-25 and it's added parts and building a new one based off of a new SM-25. And it suffered from the exact same fueling issue  I decided to ditch it for a more standard setup with a rockmax tank and some radial additions, but given all the trouble the Sm-25 has given to me in the past I feel I should just stop trying to use it now...