1.2 Fuel Lines?

[RX2000]

So I have a couple of questions about fuel lines in 1.2.

Its my understanding that with the new fuel flow system, you dont really need fuel lines at all anymore? Or whats the purpose of them now that apparently, you can just set up asparagus staging without actually running lines? Or what am I missing?

Discuss.

[moogoob]

I suppose they're still useful for drawing fuel through parts without crossfeed - like decouplers.

(unless decouplers DO have crossfeed, then I'll eat my words with a side of rice pilaf and a nice Cabernet Sauvignon.)

[Jovus]

Also - I might be wrong about this - but I think that if you just set up fuel priorities, your outside engines won't cut off when the outer tanks are out of fuel, whereas if you use fuel lines they will.

[Gaarst]

6 minutes ago, moogoob said:

I suppose they're still useful for drawing fuel through parts without crossfeed - like decouplers.

(unless decouplers DO have crossfeed, then I'll eat my words with a side of rice pilaf and a nice Cabernet Sauvignon.)

Bon appétit !

[Leafbaron]

16 minutes ago, Gaarst said:

Bon appétit !

I think you  mean to say Bone apple teeth* haha

[RX2000]

32 minutes ago, Jovus said:

Also - I might be wrong about this - but I think that if you just set up fuel priorities, your outside engines won't cut off when the outer tanks are out of fuel, whereas if you use fuel lines they will.

Mmm I'll have to check this tonight when I get home.

I must admit, I actually have just still been using fuel lines like I always have. I havent really done much testing on my own to see what happens in 1.2 now if I get rid of them.....

[Sharpy]

They allow unidirectional flow.

Try making an asparagus-staged rocket with the new fuel crossfeed. It WILL work, except the engines in the stages that should be dropped will continue to burn - drawing fuel in opposite direction. You need to figure when to stage them by observing fuel level in their tanks. Arduous, tricky, unreliable.

Or disable crossfeed in decouplers and use the pipes.

Edited October 26, 2016 by Sharpy

[RX2000]

If you tell the flow to go from the outside tanks to the inside then shouldnt the engines not have anything to burn when they run out? Or why would the fuel go back out to those engines?

[Jovus]

Just now, RX2000 said:

If you tell the flow to go from the outside tanks to the inside then shouldnt the engines not have anything to burn when they run out? Or why would the fuel go back out to those engines?

Because you're not specifying flow direction with the new rules; you're just specifying flow priority.

[Aegolius13]

40 minutes ago, Jovus said:

Also - I might be wrong about this - but I think that if you just set up fuel priorities, your outside engines won't cut off when the outer tanks are out of fuel, whereas if you use fuel lines they will.

That's correct.  I've now had to watch my asparagus staged boosters for fuel levels, because you can't just dump them when they turn off.

I also had my second stage eat all of my third stage's fuel because I wasn't watching my fuel levels. Meaning if you have an engine that doesn't burn and does not need to be crossfed fuel, it's a good idea to disable crossfeed to/from that part.  

[RX2000]

Mmmm ok so fuel lines ARE still necessary then for some things.

[coredumpster]

1 hour ago, Jovus said:

Also - I might be wrong about this - but I think that if you just set up fuel priorities, your outside engines won't cut off when the outer tanks are out of fuel, whereas if you use fuel lines they will.

This, as I found out the hard way lugging my LFO boosters almost all the way to orbit wondering why they hadn't yet cut-off before the main.   ( Didn't have KER for 1.2, so was just kerbaling my dV for each stage. )

If you want one-way flow, you need to use the fuel lines.  If you want two-way flow, and plan to watch your LFO booster tanks carefully to know when to stage them, use decoupler cross-feed.

[rcp27]

Presumably if you turn cross feed off and use fuel lines you can do asparagus the old fashioned way?

I prefer to eat my words with fava beans and a nice chianti

[Leafbaron]

no need to turn off crossfeed. I made an asparagus rocket last night with piping and when the tanks go dry the engines just down with no adjustment to fuel flow priority or toggling crossfeed. I believe fuel pipes overwrite any existing fuel flow components. 

[coredumpster]

7 minutes ago, Leafbaron said:

no need to turn off crossfeed. I made an asparagus rocket last night with piping and when the tanks go dry the engines just down with no adjustment to fuel flow priority or toggling crossfeed. I believe fuel pipes overwrite any existing fuel flow components. 

Decoupler crossfeed is off by default.  You have to explicitly turn it on if you want to play around with it and fuel flow priorities.

[Leafbaron]

2 minutes ago, coredumpster said:

Decoupler crossfeed is off by default.  You have to explicitly turn it on if you want to play around with it and fuel flow priorities.

haha well then nevermind!

[Xyphos]

place a docking port jr on your tank, disable crossfeed, add a cubic strut to the docking port, add a fuel line going from the other tank into the cubic strut, bingo! old asparagus staging.

Edited October 21, 2016 by Xyphos

[RX2000]

19 minutes ago, Xyphos said:

place a docking port jr on your tank, disable crossfeed, add a cubic strut to the docking port, add a fuel line going from the other tank into the cubic strut, bingo! old asparagus staging.

Wait so if I just use fuel lines now how is it different then it was before?

[fourfa]

Currently, the dev version of KER doesn't understand the fuel flow rules.  I'm using fuel lines to get an accurate delta-V count, then removing them before launch.  I don't use MJ's dV display, but I believe it's also based on the same logic.

[Leafbaron]

why wouldn't KER and MJ undstand the new fuel flow system. If you have 2 tanks and you drain tank 1 first and then tank 2 or vise versa the Mass_start and Mass_end are still the same. In the basic dV equations thats all that matters

dV= ln(Mass_start/Mass_end)*isp*9.81m/s. I use the MJ dev version in 1.2 and it gives me accurate readouts. 

[Draco T stand-up guy]

5 hours ago, Aegolius13 said:

That's correct.  I've now had to watch my asparagus staged boosters for fuel levels, because you can't just dump them when they turn off.

Haven't had that problem but then I've been continuing to use fuel lines.

[magnemoe]

Fuel flow also depend on staging unlike in 1.13
My classic mun lander, core and two set of drop tanks. 
At normal staging it behaves normally.


Now if I want to know total ISP then used as an ssto, this is nice if you can refuel in orbit and do follow up landings. 

Here the core empties out first as changing the staging changes the fuel flow priority. 
 

[Aegolius13]

5 hours ago, Draco T stand-up guy said:

Haven't had that problem but then I've been continuing to use fuel lines.

I think that's normal on both counts.  With the fuel lines, the fuel can't flow the opposite way, so when the outer tanks are empty those engines are dead.  But with the crossfeed, fuel can go either way.  It will burn the high priority tanks first but send fuel the opposite direction once they're empty.  So each option has its advantages and disadvantages.  

[Daveroski]

If you have a long multi-tank bottom stage as one often does early in the game, setting the fuel of each tank to drain from bottom to top makes for a much more stable ship. It keep your COM high up on your rocket. (Easier to balance a sweeping brush on it's lighter pole end than it's heavier head end.)

If you have a craft which needs to be balanced throughout it's flight then prioritising the tanks can be very useful. One can work out the order in which to drain tanks to allow the COM to remain in a specific area.

In some cases which I have also found to be useful, one can build a craft so that when it is time for refuelling, it's COM is in the sweet-spot for RCS docking.

It is particularly useful in both these areas when balancing a craft with heavy NIRV engines.

[RoboRay]

On 10/21/2016 at 11:54 AM, Sharpy said:

They allow unidirectional flow.

Try making an asparagus-staged rocket without the new fuel crossfeed. It WILL work, except the engines in the stages that should be dropped will continue to burn - drawing fuel in opposite direction. You need to figure when to stage them by observing fuel level in their tanks. Arduous, tricky, unreliable.

Or disable crossfeed in decouplers and use the pipes.

The Smart Parts mod has a fuel-level sensor that you can put on a tank to trigger staging or an action group when that tank is empty.

If you need to minimize part count, one sensor part may be able to replace several fuel lines.

Edited October 25, 2016 by RoboRay