Interplanetary ship design problems.

[Vanamonde]

I spent about 12 hours building this ship in the VAB, breaking it into launch segments, and assembling it in orbit.

But unless somebody can come up with some insightful advice for me, it's looking like a total write off. The problem is that the game can't figure out what is supposed to be attached to what through the docking rings. I built it as a core,

docked to a drive section,

and the surrounding section is just 6 orange tanks with a docking ring at each end. But is it just impossible to get the game to consider those tanks linked fully at both front and back? At first, it claimed the engines had no fuel, because it couldn't seem to find a path from engines through docking rings to tanks. I fixed that by telling it to undock the front docking ring on each orange tank, which seemd to force the software to consider the lower docking ring the prime one, meaning the engines were able to trace a path to fuel through the orange tanks. And as I understand it, the next time the game loads that ship, it should consider docking rings that are touching to be liked, right? So I exited the game and started it up again. But the ship wobbles under thrust now because, as you can see in this pic,

the front docks of the tanks are swinging freely, even though they indicate themselves as docked and show no gaps when not under stress, as you can see here.

It also picks up a roll around the long axis, for some reason, and that tiny little strut with the antennas on it seems to give the ASAS fits, and it struggles to hold a heading even though that assembly only masses about 0.561 mass units on a ship that totals around 250 mass units.

So is there a way to save this, or is this whole design concept unworkable?

[That guy who builds stuff]

Are you opposed to using mods?

http://kerbalspaceprogram.com/quantum-strut/

[Wait- Was That Important?]

They're not actually docked- when using multiple docking ports, you have to dock, undock, align yourself, and dock again multiple times to get them all docked, the key is precision.

You can click on each one and it'll say "docked", that's when you know you've docked them all simultaneously.

[Ringer]

Docking several ports at a time is tough, but since you have 6 there, they should be more or less forced into place. Also, what Wait said is true; right-clicking one will give you the "Undock" and "Disable Crossfeed" options if they're actually connected.

If everything is in fact correctly docked and you still can't get the crossfeed working, it's probably a part hierarchy problem. If I'm not mistaken, the docking port on your core doesn't just have to be a child part of the tank (you attached the port to the tank), it also has to be BELOW it, since this is how fuel normally flows; from the top tank downward. Maybe your six smaller fuel tanks on the core think they shouldn't automatically pump fuel through the docking ports, because the ports are on the top. You have the core sideways in the editor, so I can't tell.

I also highly recommend the Quantum Strut; it's one of only 3 mods I use. The requirement to place new ones in EVA is cool and somewhat more realistic, too; you have to work for your space-built awesomeness! A similar mechanic for attaching fuel lines in space would be the best.

Since it took you so long to make, my heart goes out to you if you can't get this ship up and running. I have built something similar that had natural fuel flow from the core out to all docked drive engines, so it's definitely possible. However, that was my second attempt, the first being foiled after several hours when I made my docking approach and realized I forgot to place any docking ports on the core

[Vanamonde]

I have a irrational dislike of mods, so that's out.

I understand that only one pair of docking rings on each assembly is the official, full-strength one, but shouldn't the seconday one still be strong enough to transmit fuel flow and keep the upper end of the tank from waggling around?

the key is precision.

The core and drive section have bi-couplers for that very reason. They can't NOT be precisely aligned, and even if the tanks were when I first placed them, exiting and returning to the flight should cause them to be re-generated as a precise alignment, right? And it's possible multiple re-dockings might fix it, but there's a good reason I can't. I tried it once already, and one of the frakking tanks considered itself ENTIRELY undocked and began to float away. One of trickier feats of piloting I've ever achieved was siddling the re-assembled ship back up to that stupid tank and getting it to nestle into the gap that was the precise length of the tank itself. So thanks for that suggestion, but I really don't want to risk undocking the segments again.

Also, what Wait said is true; right-clicking one will give you the "Undock" and "Disable Crossfeed" options if they're actually connected.

They do give me the menu options, even though they clearly are not properly linked.

It is most likely a hierarchy issue, which is annoying because I used a similar setup on my previous IP ship, and it worked fine. And actually, I don't see why this doesn't. Fuel should flow from the upper orange core to the lower orange core because they're directly connected, then through the fuel lines to the upper whites, then vertically through the docking links to the side oranges and side whites, and then straight into the engines. The middle white tank on the core doesn't directly feed to anything, but I was planning to right-click move its fuel to the central orange tanks as needed. So despite appearances, the fuel flow on this ship is actually quite linear.

But I actually believe this design just won't work, and it was a forlorn hope that I was overlooking something simple. Thank you anyway, gentlemen and/or ladies.

[Merinsan]

I've had a problem with docking like this also. I got everything docked, all points said they were docked, but fuel was not being drawn through all tanks. Previously I've seen someone say that even if all are docked, in reality only 1 is. I don't know if that is correct, but it would explain things.

[mpink]

LOL

This is far more fun and entertaining than the Challenges threads at the moment.

Solution

On the drive section right click on the dual docking clamps and disable the cross feed for both the clamps.

This will result in a failure to retrieve fuel from the center tank on the drive section but you can transfer this manually by right clicking on the tank and then alt+ right clicking on another tank and transferring the fuel manually.

In the future place your fuel ducts on your drive section.

[RoboRay]

Disliking mods is one thing... but disliking mods that are made for an unfinished game to deal with incomplete features or problems the devs simply haven't gotten around to addressing seems to be cutting off your nose to spite your face. A lot of key components in KSP were once only doable with mods. Then, the mods are "retired" when Squad finally puts that feature into the game. Many of them were even on Squad's "to do" list.

But suit yourself.

[Vanamonde]

cutting off your nose to spite your face

Well, I did say it was an irrational dislike of mods. But since it came up: 1) I don't want to become dependent on something that may be broken when the next update comes out. 2) Look around the forum and you can see that players are frequently posting things like, "HELP! Why does Mod X cause my Kerbals to spontaneously combust!" Who needs the headaches? and 3) I like the challenge of trying to get things done with stock parts, and when a feature is added to the game that allows me to do something I couldn't previously do, it's like opening a present on Christmas day. You wouldn't want to open your presents early, would you?

disable the cross feed for both the clamps.

I'm afraid I don't see what you're suggesting. In hopes of preventing loops that would confuse the fuel flow, I made the lowermost centerline tank a dead end that doesn't feed into the others, planning to manually pump its contents into the tanks above it when they ran low. So blocking it shouldn't change anything because it's not part of the fuel flow anyway. But yes, in the follow-up design I'm planning, the fuel will all flow in a linear fashion from front to back, and from centerline to sides at the rear. Basically, I'm going to rebuild this ship backwards, with the core integral to the drive and a separate piece at the nose.

[Eric S]

From what I've seen, getting fuel to flow well through the bi/tri/quad couplers is generally a pain. That and part count are why I use the Common Berthing Modules addon instead of multi-port docking.

The only thing I can think of trying to solve the fuel flow issue without a mod would be to modify the config file for the multi-couplers you use to remove/alter the "NoCrossFeedNodeKey = bottom" line.

[RoboRay]

Well, I did say it was an irrational dislike of mods.

LOL, you got me there.

My rule is generally "If I can do it with stock parts, I do it with the stock parts". I also only use mod parts that are balanced with the stock parts, or add features that are clearly missing and will be coming eventually. When the stock part or method appears, I generally have no more use for the mod.

Example? I used to use a variety of rover mods like the DEMVs and CARTS stuff. Now, I use only the stock rover wheels. I don't feel like I've lost anything by not waiting for stock powered wheels. In fact, it was the ability to drive around on the surface that made me realize months ago that the planets and moons aren't plain and boring... they actually have some impressive terrain features. I just thought there wasn't much to see because I hadn't been getting more than a few hundred meters from my landing site. Using the mod parts until the stock parts were available gave me a way to explore and reasons to go places.

[Eric S]

My rule is generally "If I can do it with stock parts, I do it with the stock parts".

Me, not so much. I see nothing sacrosanct about stock parts.

I also only use mod parts that are balanced with the stock parts, or add features that are clearly missing and will be coming eventually. When the stock part or method appears, I generally have no more use for the mod.

This I can agree with more. If I'm using a part specifically because it's got better stats (as opposed to more appropriate stats), then something is wrong. The last time I looked at both KW Rocketry and the Nova Punch Pack, I found parts that I was happy to use that filled niches that the stock parts didn't, and I found parts that were categorically better than certain stock parts, which I stayed away from.

One example is the Common Berthing Mods. Rather than deal with the unrealistic solution of multi-port docking and the way the multicouplers affect fuel flow, I'd rather use something like a stronger docking port, as long as there are tradeoffs, and in Fusty's CBM module, there are tradeoffs (admittedly, the higher price isn't really a tradeoff until we get campaign mode). So I'm happy to use those parts when I'm doing orbital docking to build larger craft.

Another one would be some custom fuel tanks I made for my munar lander. I wanted to do an apollo style mission, complete with two stage lander, and any way I did it just wound up with either one of the stages having enough fuel that I didn't need two stages, or a stack of 1.25m parts under a 2.5m lander can, which just didn't look right. So I made some thin, low capacity 2.5m fuel tanks specifically for that craft. They were perfectly balanced compared to stock parts as far as volume vs capacity and volume vs dry mass.

I'm more lenient about functionality that may or may not be coming. I use the Kethane mod and ISA mapsat stuff to give me more of a purpose for being where I'm going. I'll be switching to the stock resource extraction and processing stuff as soon as it comes out, even though it will be less convenient. On the other hand, I've heard nothing about the devs doing anything like ISA mapsat. Like you, I used to use the BobCat CART stuff, but other than the individual seat, I haven't used any of that since stock rover wheels came out. Likewise, I've heard nothing but speculation and rumor about any kind of robotics, but I'm perfectly willing to use Damned Robotics or the Robot arm pack where appropriate.

I'm still doing a monthly stock Mun landing and return (well, not completely stock, I do use KER, but it's information only) just to make sure I haven't picked up any bad habits or addon dependencies.

As for the statement about an irrational dislike of mods, having my own list of irrational desires, goals, standards, etc., I'm fine with other people having their own :-)

[CalculusWarrior]

I used to use a lot of mods myself: MechJeb, KW Rocketry, etc but recently I've gone back to using only the stock parts. I feel that it sort of takes away from the game, as some of the parts packs are so massive that I found myself not actually using the stock engines and fuel tanks. I felt that if I wanted to do anything, then I could do it with the stock parts, as that's what they were designed to do. I even found myself struggling to make even a Munar transfer without the aid of MechJeb and the powerful rockets. However, mods such as ISA MapSat, Kerbal Engineer, and the Kethane pack add a new dimension to the game, while not feeling so 'cheaty' as adding more powerful engines. Kerbal Engineer is my absolute favourite, as I've learned a lot about delta-v and TWR which has expanded my abilities greatly.

Basically, I understand your hesitation to install mods, but be aware that there are mods out there that expand on one's experience in the game, rather than detract from it.

[Ringer]

The core and drive section have bi-couplers for that very reason. They can't NOT be precisely aligned, and even if the tanks were when I first placed them, exiting and returning to the flight should cause them to be re-generated as a precise alignment, right?

Ah-hah! I think that's the problem! They CAN be mis-aligned with a bi-coupler, which I didn't figure out on my head, but realized very soon when I first dual docks. I made you a quick drawing:

I was able to fix my double-dock on the second try; while re-docking it, I simply rammed the ports together harder and they got mashed together enough that the clamps took hold.*

Now any number of decouplers greater than two DOES force them all to line up; as long as one port lines up with its counterpart, so must the others.

*That's how she likes it

[Tristavius]

Actually, the three port can also be off... it's very possible for only one to connect if you're just slightly off, and the others to be at a slightly off-angle as per your above sketch. If you have 2 of 3 connected then the third is defiantly aligned though.

[Vanamonde]

The bicouplers are fixed to their ship segments in a rigid way, so if they were not lined up with each other, the ship segments would not be, either. This would be easily visible, as the two columns of radial parts would deviate from each other. But they do not, so I'm sure alignment is not the problem. Thank you for the suggestions, though.

However, some experimentation has revealed that fuel flow is very weird. This configuration works just fine, with the upper tank draining first, and then the lower one.

But in this configuration, the engine drains the lower tank and refuses to believe it has access to the upper tank, which remains untouched.

Here, the engine drains the central tank and doesn't believe the side tanks exist.

But in this configuration, everything works fine and the engines drain the tank from outermost to innermost.

But fuel flow is flummoxed by bicouplers again in this configuration, and the engine is convinced it has no fuel at all.

All tanks feed to the engine in this arrangement, which is what my successful interplanetary ship has been using.

But that's hard to scale up, so I think I'm going to use an elaboration of this arrangment on my next ship, for more fuel storage and longer range. This one also gives the engine access to all those fuel tanks. But that's going to jack the part count up, badly.

So the moral of the story is, multi-couplers do not transmit fuel, and can't be between the engines and the fuel supply. Also, docking rings transmit fuel vertically, but not horizontally. Or something.

Edited May 1, 2013 by Vanamonde

[Eric S]

So the moral of the story is, multi-couplers do not transmit fuel, and can't be between the engines and the fuel supply. Also, docking rings transmit fuel vertically, but not horizontally. Or something.

Yes, this is the exact fuel flow problem I mentioned a few messages back as the reason I use the Common Berthing Modules addon instead of multiport docking in my large craft.

For liquid fuel and oxidizer (other fuel types have a more relaxed fuel flow), the only things that move fuel between separate stacks are fuel lines or multi-couplers, and the direction of flow is restricted with multi-couplers to the point that if you have two multi-couplers facing each other, even without docking ports between them, you can't get fuel to flow through both multi-couplers in the same direction. Note: for purposes of this description, docking ports do not separate a stack into two separate stack, but multi-couplers do, sort of.

If you examine the config file of various multi-couplers, you'll find a section that looks like this:

// keeps fuel from flowing FROM the bottom nodes (prevents stack imbalances and such)

NoCrossFeedNodeKey = bottom

I suspect that removing this line might fix this problem, at the risk of causing other problems. From what I've seen, having multiple routes for fuel between engines and tanks can lead to non-symmetrical fuel usage.

[Vanamonde]

Yes, this is the exact fuel flow problem I mentioned a few messages back

Sorry I wasn't clear. The pics I posted were to test and then to confirm that you were correct about the multiconnectors being a big part of the problem.