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Crossfeed Issues


Scarecrow71
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I'm trying to build a craft that has multiple stages...but I'm not using decouplers between them.  As part of the Jebediah Space Program challenge, you are only allowed to use parts from Jeb's Junkyard manufacturer, which means no decouplers.  So I'm building a multi-stage rocket to get into orbit, and the thought is to have the upper stage overheat and blow apart the lower stage when the lower stage runs out of fuel.  Unfortunately, the upper stage's tanks are emptying into the lower stage, and so I can't ever activate the upper stage.

Is there a way to turn off crossfeed on parts that don't allow for this to happen?  I do NOT want my upper stage tanks draining into the lower stage, but there are no parts from Jeb's Junkyard that have the explicit option to turn off crossfeed.  Is this possible, or do I need to find a different way to accomplish this?

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Normally engines themselves do not allow cross feed.  So if your upper tanks are separated from the lower tanks by an engine, cross feed shouldn’t occur.  If it is, you might have a game setting set weird, not sure what that could be though.

If you have offset parts, that could be contributing to the problem, perhaps the game thinks the tanks are connected?

Otherwise, the part action window (PAW) of the tanks should allow you to ‘shut off’ the tank- click on the little green arrows to turn them into red X’s.  That function might even be able to be tied to an action group.

My last idea might be the one that is easiest to implement.  Set the tank burn priority in the PAWs, which can be done either in flight or in the VAB.  You want the Priority number of the lower tanks to be higher than the upper tanks, so that they (lower tanks) burn their fuel first.

By the way, the fuel burn priority can also help you with rocket stability on tall rockets.  A rocket becomes more stable if it’s COM moves upwards, and less stable if it’s COM moves down.  So for tall rockets, I sometimes find it helpful to burn from the lower tanks first, which moves the COM upwards.

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My upper stage is an FL-TX900 and a Cheetah; the lower stage is an FL-A215 Adapter, 3 FL-TX900fuel tanks, and a Bobcat.  And the upper tank is draining into the lower.

What else is really odd is that, if I use MechJeb for ascent guidance, MJ fires the upper engine as part of the first stage.  This is all in sandbox, and I didn't alter any settings when creating this particular sandbox game. 

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Not sure what MechJeb is doing to you, I’ve never used it.  Try changing the fuel flow priority numbers, and make sure the upper engine isn’t activating before you want it to.  For me, proper staging would solve that problem, but is sounds like MechJeb is doing something weird?

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2 hours ago, Scarecrow71 said:

My upper stage is an FL-TX900 and a Cheetah; the lower stage is an FL-A215 Adapter, 3 FL-TX900fuel tanks, and a Bobcat.  And the upper tank is draining into the lower.

What else is really odd is that, if I use MechJeb for ascent guidance, MJ fires the upper engine as part of the first stage.  This is all in sandbox, and I didn't alter any settings when creating this particular sandbox game. 

As its no separator mechjeb assumes its on the same stage. You get the same issue parallel staging vacuum engines you want to activate them  independent on separating boosters. 

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46 minutes ago, magnemoe said:

As its no separator mechjeb assumes its on the same stage. You get the same issue parallel staging vacuum engines you want to activate them  independent on separating boosters. 

Even when I stage manually, the upper tank drains into the lower one.  I am going to try doing this in my install that has no mods and see what happens.

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So I tried this again this morning, but this time in an unmodded game.  Same results:  the upper tank starts draining into the lower tanks, even though I've got an engine between the upper and lower stages.  I staged manually when I saw this was happening, and as soon as the lower tanks exploded (one of them), the bobcat engine stopped firing as it was out of fuel.

@18Watt, is there any chance you can test this on your end?  Sandbox, normal difficulty, no changes to any settings.  If you can build the craft I have above and just see what happens on your end, just to rule out that it's something in one of my games?

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1 hour ago, Scarecrow71 said:

@18Watt, is there any chance you can test this on your end?

Well, this is interesting, and not the behavior I expected at all.  I'll post some screenshots after I do a little more testing, but it does not seem to be specific to the parts you are using.  

Give me an hour or so to get screenshots posted.  I think I do have a solution for you though.

Edit:  Most of my screenshots will be with the engines you are using, although I think I can duplicate what you are seeing with any engine or fuel tank combination.  It's not what I expected, and not sure if it is intended behavior or not.

 

Edited by 18Watt
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5 minutes ago, 18Watt said:

Well, this is interesting, and not the behavior I expected at all.  I'll post some screenshots after I do a little more testing, but it does not seem to be specific to the parts you are using.  

Give me an hour or so to get screenshots posted.  I think I do have a solution for you though.

Edit:  Most of my screenshots will be with the engines you are using, although I think I can duplicate what you are seeing with any engine or fuel tank combination.  It's not what I expected, and not sure if it is intended behavior or not.

 

Yeah, I tested this with multiple engines (Cheetah, Terrier, Swivel, Reliant) and got the same behavior every single time.  Modded or unmodded.  Sandbox, Science, Career.

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Okay, here's what I'm seeing:

  • Engines appear to allow fuel to flow through them in either direction.  I only looked at 5 different engines, but all the ones I looked at allow fuel to flow through them in either direction.
  • That came as a surprise to me, because we normally use decouplers between stages, which DO prevent crossfeed.  Or docking ports, which CAN prevent crossfeed.  
  • I am not seeing a selection to prevent engines from cross feeding.  
  • I can show you how to fix your ships though.

Test 1, and Fuel Delivery Overlay:

Spoiler

Test rig in VAB.  I enabled Fuel Delivery Overlay (which is kinda buggy sometimes..).  As you can see, fuel appears to be able to flow through the Cheetah in either direction.

ELwEqe6.png

Second photo is with Bobcat (lower engine) running, and Cheetah (upper engine) inactive.  Fuel is ONLY flowing from the lower tank.

zu8IOlA.png

Next photo is with the Bobcat shut off, and the Cheetah (upper) running.  Fuel is still only draining from the lower tank.  This is because the fuel flow priority number on the Lower Tank is higher (10), while the fuel flow priority number on the upper tank is low (-10).

zyqAJi8.png

Test 2, with Fuel Flow Priority numbers equal (zero on both):

Spoiler

In this test, I set the tank flow priority number on both tanks to zero.  The ship should (and does) drain fuel evenly from both tanks, regardless of which engine is running.

First screenshot, the Bobcat is running, and fuel is draining from both tanks.

wk5zSpZ.png

Next screenshot, the Bobcat is inactive, the Cheetah is running.  Fuel is still flowing equally from both tanks.

O9D5zlr.png

Third test, Upper Tank has higher flow priority: I think this is what you have been seeing...

Spoiler

This next test is probably what you have been seeing.  I've set the upper tank to have a higher fuel priority (1).  The lower tank has a flow priority of zero (0).

Sure enough, with the Bobcat running, fuel is flowing from the upper tank only.

o79VmUl.png

With the Bobcat idle, and the Cheetah running, fuel is still flowing from the upper tank only.

cF2o7LJ.png

What you need to do is look at the Flow Priority settings on your tanks.  For whatever reason, when you built your ships, KSP defaulted to setting your upper tank to a higher priority number than the lower tank.  (Did you build your rocket from the bottom up?). KSP normally does a good job at guessing which tanks should have higher priority, but it's just guessing and sometimes gets it wrong.

Fuel Priority can be set in the VAB/SPH, and can also be adjusted during flight if needed.

What you want is for the tank(s) in your Upper Stage to have a LOWER Flow Priority number than any of the tanks in your Lower Stage.  

If the tanks in your Lower Stage all have a Flow Priority of zero (0), then the Flow Priority number of your Upper Stage tank(s) needs to be less than zero.  A value of -1 would work in that case.

Try playing with Flow Priority to see if you get your ship functioning properly, and let me know..

 

Edited by 18Watt
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Some more examples of Fuel Priority:

Test 1, Priorities all equal:

Spoiler

I have 4 tanks.  The Flow Priority on all of them is the same (-10).  They should all burn equally.

rls0g1V.png

And they do.  The engine burns from each tank equally.

57MPEI9.png

Test 2, Tanks Have Different Flow Priorities:

Spoiler

For this test, I have 4 tanks.  The uppermost tank has a Flow Priority of -10.  The next one is -9, then -8, and finally the bottom tank is -7.  The engine will draw fuel from the highest priority tank first.  When that tank is empty, it will start feeding from the next highest number.

OhQoYzC.png

Engine running, and is getting fuel from the tank with the highest Flow Priority number, in this case -7.

p1giHNY.png

When that tank runs out, the engine starts getting fuel from the next highest number, -8.

BOziQUA.png

Then the next highest number, -9.

ZkqKJxM.png

Finally the other 3 tanks are drained, and the engine feeds from the tank with the lowest Flow Priority number (-10) last.

QsQlnvt.png

Flow Priority is a handy tool to put in your bag of tricks.  It's really useful when trying to manage the movement of the COM of a vessel.

  • For spaceplanes, the movement of the COM as you burn fuel is critical.  If the COM moves too far as you burn fuel, the aircraft can become difficult to control, or even uncontrollable.  On a plane with multiple fuel tanks, it's handy to specify the order from which fuel will be consumed.
  • For rockets, if the COM moves aft (downwards), the rocket can become aerodynamically unstable.  (Think of trying to shoot an arrow backwards..).  I like to have the COM on my rockets move upwards if possible, at least while they are in the atmosphere.  So I set the Flow Priorities so that fuel will be burned from lower tanks first.  That moves the COM up during climb as fuel is burned, or at least prevents/minimizes the COM from moving down, and causing stability issues.
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I built my ship from the top down, so I assume that KSP dealt with flow priority correctly.  However, I'll have to test this when I get some time, most likely later today (Mrs. Scarecrow71 made me take the day off today so we can do errands).  I will certainly let you know what happens when I rebuild and retest!

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@18Watt

Well, I've got some bad news on this front.  I rebuilt the rocket, and I checked the flow priority values in the VAB.  As you can see in the following image, the upper tank has a flow priority of -10, while all of the lower tanks have a flow priority of 0.

Z8VEKM0.png

So I took this thing for a test drive, knowing that the flow priority is set correctly.  Check the following video; fuel flows from the bottom tanks correctly...until they run out, at which point fuel starts flowing from the upper tank into the bobcat.  You can see this happen at about the 3:50 mark.

I apologize for the graininess of the video; I'm still relatively new at taking video and uploading it to YouTube.  Anyhow, you can see that the fuel empties out of the upper tank without staging near the end of the video.  Is this a bug that has never been reported?

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3 hours ago, Scarecrow71 said:

fuel flows from the bottom tanks correctly...until they run out, at which point fuel starts flowing from the upper tank into the bobcat.

Yes, that is how it works:  Fuel will draw from the highest priority tanks.  When they are empty, it will draw from the next available tank.  Unfortunately, since engines do not prevent crossfeed, the tank above your engine becomes available.

The only ways I can think of (at the moment) to prevent that are:

  • Turn off the tank (turn green arrows to red X's), and only turn it on when you're ready to use that fuel.
  • Deactivate the lower engine when it's fuel has run out, so that it doesn't drain the upper tank.
  • Place a part between the engine and lower tank which can stop crossfeed.
  • The only part in Jeb's Junkyard that has that ability is the inflatable airlock.

Normally you could get a lot of good tips by looking at how other players approached the challenge,  I think that's a fairly new challenge though, and I'm not sure how many members have responded.  I haven't tackled that one (yet).

I do remember staging without decouplers a long time ago, and was scratching my head wondering why I didn't run into this exact problem when I tried it (years ago).  I figured it out-  I was using solid rockets (SRBs) as the lower stage, so of course the lower SRBs would not use the fuel from the upper tanks, because it's not the right fuel type.

 

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2 hours ago, 18Watt said:

I do remember staging without decouplers a long time ago, and was scratching my head wondering why I didn't run into this exact problem when I tried it (years ago).  I figured it out-  I was using solid rockets (SRBs) as the lower stage, so of course the lower SRBs would not use the fuel from the upper tanks, because it's not the right fuel type.

Food for thought: do SRBs allow liquid fuel to crossfeed through them...?

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3 hours ago, 18Watt said:

Unfortunately, since engines do not prevent crossfeed, the tank above your engine becomes available.

To me, thats a bug.  Especially considering the engines are in different stages and nonfuel lines are in play.

3 hours ago, 18Watt said:

The only part in Jeb's Junkyard that has that ability is the inflatable airlock.

I am unaware on how to use that part effectively.  I can only get one side of it to ever connect.

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10 minutes ago, Scarecrow71 said:

To me, thats a bug.

Yeah, me too.  I did not expect it to work that way.  However, at this point I think it's probably the intended behavior.  I would probably have it function differently if I was making the game though.

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1 hour ago, 18Watt said:

Yeah, me too.  I did not expect it to work that way.  However, at this point I think it's probably the intended behavior.  I would probably have it function differently if I was making the game though.

Parts have config files, yeah?  Could we turn off crossfeed there?

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@18Watt

So I ran a test this morning on the Bobcat engine.  I altered the CFG file for the Bobcat engine, changing the following entry:

fuelCrossFeed = True

I made this entry to be:

fuelCrossFeed = False

I then launched KSP and built a rocket using the Bobcat engine for both the upper and lower stages.  I did this because I only wanted to alter 1 engine as a test.  And the results were...I wouldn't say entirely expected, but based on what's been happening here, they were what I was thinking would happen.  Disabling crossfeed on the engines pretty much makes them paperweights.  That is, without crossfeed, no fuel even gets into the engines, which means they aren't burning or thrusting or doing anything.  This means that we have to have crossfeed turned ON for the engines.  This, therefore, makes this behavior expected.

It's a bummer that fuel flows from the upper tanks into the lower tanks, but it is what it is.  I will have to come up with a way to prevent this from happening on future flights.  But, at the very least, I'd like to thank you for your help in this!

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55 minutes ago, Scarecrow71 said:

Disabling crossfeed on the engines pretty much makes them paperweights.

I suspected that, thanks for confirming my suspicion!

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On 1/4/2022 at 1:50 AM, Fraktal said:

Food for thought: do SRBs allow liquid fuel to crossfeed through them...?

Yes, i have used this to keep the core stage full until booster separation. Remember to enable crossfeed on the booster decouplers

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11 hours ago, Blaarkies said:

Yes, i have used this to keep the core stage full until booster separation. Remember to enable crossfeed on the booster decouplers

Oh, right. Now that you mentioned it, I remembered using that design choice too. Was wondering because to this very day, I still don't understand why Squad made the choice of not allowing crossfeed through KV pods even though every single other command pod in the game allows it.

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Yeah the crossfeed rules are like english grammar rules

How does do everyone get around crossfeed limitations (and the difficulty setting enabled) when you have a tank at the front, and lots of hab modules and stuff in the middle, and then more tanks/engines at the back. I have been trying to run a fuel pipe neatly along the sides, but it needs a bend at 1 point because of tank size differences. The question is basically: what do you use as an intermediate connector part between fuel pipes that span curves?

I used the Ant engine for that, it seems to work pretty well. Cheap, small and light. I don't know if it handles other fuel types though? :/

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2 hours ago, Blaarkies said:

I have been trying to run a fuel pipe neatly along the sides, but it needs a bend at 1 point because of tank size differences. The question is basically: what do you use as an intermediate connector part between fuel pipes that span curves?

I think the little cubic octagonal strut also allows fuel to flow through, and it's a physicsless part.  At least it used to work- before engine plates were introduced I used to mount multiple engines on cubic octagonal struts to get say a cluster of 3-6 Spark engines under a fuel tank.  And the engines worked fine.  Ymmv.

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