Fuel/Decoupler bypass in flight...

[MaverickSawyer]

Okay, gang, got a bit of a technical/gameplay head scratcher here...

 

I'm making a new crew transfer vehicle that I want to be common for both low Kerbin orbit and for missions to the Mun or Minmus, simply adding a crew compartment if I'm going to be in flight for more than 18 hours. Here, have a look:

Now, obviously, the dV takes a serious hit because of the presence of the habitat module, which is why I added the fuel tanks to the sides of it: More fuel gives more burn time and more dV. Case in point:

By adding that fuel line, I see a nearly 5-fold increase in my fuel supply. Probably far more than adequate for just about every mission I'll be using this thing on...

There's a catch, though. (This is where you guys come in.) The parachute used by the capsule is also a decoupler. Fairly straightforward and usually a good thing, as I can clear the docking port off of the parachute well in advance.

It does not, however, allow fuel to flow through it.

As the above image shows, I can allow fuel to bypass the decoupler attaching the propulsion module to the command module, and I'd been counting on the parachute doing the same. Buuuut, for whatever reason, it can't. So, I'm trying to come up with a solution to bypass that decoupler in flight after docking. I already tried using the KAS extensible pipes, and it... kinda summoned the Kraken. So that one's straight out. and, since it's one vessel,  the KAS RTS winch and DMagic's EVA Fuel Lines won't work.

I'm sure I could edit the config file to fix this, but that's... kinda cheating, in my books, and isn't exactly a challenge. I want to find an in-game solution for this.

So... anyone got any bright ideas? 

[Geonovast]

Do you have the "Fuel Transfer Obeys Crossfeed Rules" enabled in your difficulty settings?

[Aegolius13]

So the parachute is an inline part with nodes on each end, but does not allow crossfeed?  Not sure i completely understand the architecture here, but a few possible options:

1. You might be able to create a couple arms on each craft with additional parallel docking ports, similar to what people use to force docking at a certain alignment.  Those might allow fuel to flow, but I'm not not sure how the game treats those kind of connections, since it generally doesn't like loops of parts.  

As @Geonovast says, with the fuel transfer setting switched, you could manually transfer fuel into a tank on your command module to feed the engine.

2.  You could redesign the craft, i.e., by using radial parachutes.

3.  I shudder to even say this, but you could use monopropellant engines, as I believe they allow universal crossfeed.

[MaverickSawyer]

4 hours ago, Geonovast said:

Do you have the "Fuel Transfer Obeys Crossfeed Rules" enabled in your difficulty settings?

Yep. That's what I was trying to show in the third pic... that part doesn't allow for the engine to draw directly from the tanks on the hab, regardless of the settings. Yes, I can manually transfer, but that's time intensive and is difficult to keep balanced during a burn... and with a total of eight tanks to juggle, four of which are rather small; the Dumplings are cute and look fantastic in that spot, but... not a lot of storage.

4 hours ago, Aegolius13 said:

So the parachute is an inline part with nodes on each end, but does not allow crossfeed?  Not sure i completely understand the architecture here, but a few possible options:

1. You might be able to create a couple arms on each craft with additional parallel docking ports, similar to what people use to force docking at a certain alignment.  Those might allow fuel to flow, but I'm not not sure how the game treats those kind of connections, since it generally doesn't like loops of parts.  

As @Geonovast says, with the fuel transfer setting switched, you could manually transfer fuel into a tank on your command module to feed the engine.

2.  You could redesign the craft, i.e., by using radial parachutes.

3.  I shudder to even say this, but you could use monopropellant engines, as I believe they allow universal crossfeed.

It's already got a fairly high part count, due to the complicated propulsion bus (I tried to base it off of the Japanese Fuji spacecraft concept, but can't get the sizing of the fuel tanks quite right... ), not to mention the multiple RCS points due to the multi-mission nature of the design. I didn't want to have to redesign the ship between versions if I could help it.

As for the radial parachutes... Hmm. Hadn't considered that angle, tbh. Might have to look into it. And monoprop... yeah, no. Monoprop is acceptable for small stuff, but this thing's already taking enough of a performance hit as it is. No need to add to the problem! 

[The Flying Kerbal]

If I'm reading this right, the fuel lines will do what you want but you don't want to use them for aesthetic reasons?  If so then I can fully understand why, that's one classy looking little vessel you've got yourself there!

If it was me, I would seriously consider using the radial chutes, but would you be willing to place fuel lines as close to the docking port as possible, making them much less unobtrusive?  Just an off-the-cuff-thought that came into my head and I'm sure other people will come up with a much more elegant solution than that.

Again full kudos for designing such a flashy spacecraft, I really like it. 

[MaverickSawyer]

No, I can't use fuel lines because the ship is pictured in its on-orbit configuration. For launch, the habit is tucked behind the capsule so the LES tower can be installed.

[Snark]

On 3/22/2019 at 5:36 PM, MaverickSawyer said:

There's a catch, though. (This is where you guys come in.) The parachute used by the capsule is also a decoupler. Fairly straightforward and usually a good thing, as I can clear the docking port off of the parachute well in advance.

It does not, however, allow fuel to flow through it.

Yeah, the thing you're up against is the "tree" nature of spacecraft in KSP.  Except for the special "composite" parts (like the fuel duct) that connect at two points... other than that, there's always one and only one "path" between Part A and Part B.

So if you've constructed your ship in a way that puts a non-crossfeed-enabled part in that path, well... you're, ah, hosed.  (Pun intended. Sorry.)

So... things you could do:

1. Turn off the "fuel flow obeys crossfeed rules" setting.
2. Mod the part so that it does allow crossfeed.
3. Don't use that part.  Use a regular docking port, and attach some radial parachutes instead.
4. Add an additional (regular) docking port elsewhere on the pod, e.g. on the side, so that it's possible to re-dock the ships to transfer fuel, then dock them back as shown for doing the burn.

Personally, if it were my own game, I'd go with #3, myself.  For me, about 70% of the point of a docking port is that it allows fuel transfer, so I'd be less likely to use that part anyway.  But I understand that tastes differ.