Asparagus staging with drop tanks

[Laughing Man]

Okay so I have a rocket with a jumbo tank in the middle with a mainsail and six radial tanks with an LV-T30 each. The six radials are asparagus staged and work perfectly fine, I tested them and the fuel flows correctly. Now here is where things go wrong; I added six SRBs on the outside of the radials and on top of those I added fuel tanks, the idea being to feed fuel from these into the radials before the SRBs putter out. I tried asapargus-ing the fuel from the SRB-mounted tanks, but when I do this it mucks up the fuel flow completely with tanks from the inner radials draining when they shouldn't.

Apologies for the crude drawing, but you can see where I'm going with this. Basically, I want the tanks to empty by number order. I've checked and rechecked the fuel lines, they are all running the correct way and I added them one at a time without symmetry. I don't really know why this isn't working.

[Tortoise]

I believe it's because all of the tanks are the same size and they are feeding off eachother equally.

Tank 1,2, 3, and 4 are all connected. So they are all gonna equally use up each of the tank's fuel and run out at the same itme.

[Laughing Man]

Would it work if I used Toroidals or Oscar-Bs then?

[Red Iron Crown]

Tank size shouldn't matter. Are you sure the inner tanks are draining? I ask because if you just look at the fuel bar in the staging list it might make you think that the inner stages are draining, but if you right click on the tanks you'll see they're still full. This happens because the fuel bar shows percentage fuel remaining in all tanks that flow to that engine.

[Superfluous J]

I had a similar problem with my Astro-Glider (don't ask. Well, do ask, but I can't tell you for a few hours. It's uploading right now) lifter. For me it didn't cause any issues but the tanks drained oddly.

I think it's because you're connecting the fuel lines to the top tanks of the central stack instead of the bottom ones. Try connecting tank 3 to the bottommost tank of stack 4. If that doesn't work, try reconnecting stacks 4, 5, and 6 by their bottom tanks instead of top ones.

And if that doesn't work I got nothing.

[psyper]

If you still cant get it working and you dont mind installing mods it might be worth giving goodspeeds Automatic Fuel Pump a try. You can set your outer tanks to value 1 and have them set to auto-pump and they will empty out first prior to all the others (fuel always flows from higher numbers to lower ones)

http://kerbalspaceprogram.com/goodspeed-automatic-pump/

http://forum.kerbalspaceprogram.com/threads/67163-0-23-5-Goodspeed-Automatic-Fuel-Pump-v2-14-1

TAC Fuel Balancer is also a popular one that people use.

http://forum.kerbalspaceprogram.com/threads/25823-0-23-0-23-5-TAC-Fuel-Balancer-v2-3-22Dec

[John FX]

Don`t give up yet. There are many fuel flow rules...

Have a look at that link and see if you have done one of the `special cases`

[Nao]

Wait... that bug is still in the game?

The problem here is that fuel flows through radial decouplers that hold the boosters (just like through fuel line). To fix this put the fuel lines on the boosters (from booster to another booster) or alternatively attach the outer circle to inner by the drop tanks. Both solutions eliminate the fuel loop created with decoupler working as a fuel line.

Alternatively you could include a part that has no fuel crossfeed (like a decoupler) between booster and fuel tanks on top of them, that shuts off the radial decoupler ability to transfer fuel.

[mellojoe]

I think it's because you're connecting the fuel lines to the top tanks of the central stack instead of the bottom ones. Try connecting tank 3 to the bottommost tank of stack 4. If that doesn't work, try reconnecting stacks 4, 5, and 6 by their bottom tanks instead of top ones.

I think this is the solution. It looks like you are draining fuel from the top of the stack, whereas the engine is connected to a tank at the bottom. Therefore, it is pulling fuel out of that bottom one first, before pulling out of the top one. The top one is what all the radial asparagus tanks are dumping into, so they won't drain until you start needing fuel out of that top tank.

Change your fuel lines to the bottom of the stacks. Even on the radial ones. Sine you have two tanks on top of each other, you want the top one to drain into the bottom one, and your fuel pipes are sort of mucking up the routing. Put your yellow fuel lines on the bottom of each stack.

[Pecan]

Or, to ensure one path for all the fuel; 1 -> 2 -> 3 -> 4 top:4 bottom -> 5 top:5 bottom -> 6 top:6 bottom -> centre.

[Claw]

Hmm, I think I ran into this before and I simply put all the fuel lines at the bottom. That way the fuel search tree looks to the fuel lines, then up the parent tree. (Without getting fancy and stretching bottom to top or top to bottom...)

I can't recall if the fuel will feed down through the SRB.

Kasuha is the fuel guru.I bet he would know.

[nobbers12345]

You can asparagus SRB's?

Hmm, didn't know,

[Superfluous J]

You can asparagus SRB's?

Hmm, didn't know,

You can't asparagus with SRBs, at least not in the way I think you mean. You can have a rocket with asparagus staging that also uses SRBs. This is what the OP did. You cannot pump SRB fuel around with yellow fuel lines (or anything else).

[Laughing Man]

I think it's because you're connecting the fuel lines to the top tanks of the central stack instead of the bottom ones. Try connecting tank 3 to the bottommost tank of stack 4. If that doesn't work, try reconnecting stacks 4, 5, and 6 by their bottom tanks instead of top ones.

I already ditched this lifter and designed a new one, but I wanted to get to the bottom of this so I made a test vehicle. Just as you said, connecting via bottomost seemed to work. Pics to demonstrate:-

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I connected tank 3 to the bottommost of tank 4 and also connected 4, 5 and 6 by their bottoms and by god somehow it worked! The outer tanks get drained quick as hell, then 4, 5 and 6 like they should however the top tanks of 5 and 6 start to drain when they shouldn't albeit at a very slow rate, speeding up when it's their "turn" in the asparagus. I still have the top tank of 6 connected to the main tank instead of the bottomost tank so I might try connecting it via bottommost as well and see if that fully eliminates the problem.

EDIT: Connected the bottom of the last asparagus to the central tank. The tops of 5 and 6 are still draining at a slow rate when it isn't their turn. I had a look at this topic and tried installing the fuel lines from outwards-in to see if that would solve the problem, but no dice. I just did another test and they actually drain from the very start only very very slowly until it's their turn in the asparagus. The bottom tanks drain perfectly however. Very odd.

Edited April 16, 2014 by Laughing Man

[SRV Ron]

You can't asparagus with SRBs, at least not in the way I think you mean. You can have a rocket with asparagus staging that also uses SRBs. This is what the OP did. You cannot pump SRB fuel around with yellow fuel lines (or anything else).

Actually, he has fuel cans on top of the SRBs that are drained, then dropped with the staged SRBs. The problem is with the way the fuel cans are being drained. They are hooked up OK but the sim is having issues with where they connect to the next tank.

[Laughing Man]

I'll make a video later if anyone is still interested, thanks for all the helpful replies!

[GoatRider]

Wouldn't it make sense to connect the drop tanks straight in to the nearest tank, since that's the order it's being used?

[Nao]

I'll make a video later if anyone is still interested, thanks for all the helpful replies!

You didn't read mine thou. Your new rocket has the same problem as the last one. Just move the fuel lines from drop tanks to boosters and it will work like a charm.

[EdFred]

I think you need to connect from the bottom of the "outermost" tank, to the top of the next stack you want to drop. It appears that some of your engines pull from from multiple tanks, rather than the outermost tank.

[Laughing Man]

You didn't read mine thou. Your new rocket has the same problem as the last one. Just move the fuel lines from drop tanks to boosters and it will work like a charm.

I did read it. The boosters are solid-fuel and the tanks on top are liquid fuel, the idea is to transfer fuel from those booster-mounted tanks to the inner set of tanks if you get me. Also, I'm pretty sure decouplers don't auto-crossfeed if the wiki is anything to go by.

I think you need to connect from the bottom of the "outermost" tank, to the top of the next stack you want to drop. It appears that some of your engines pull from from multiple tanks, rather than the outermost tank.

If I understand you correctly, you want me to change from this:-

To this:-

I did this and it made no difference whatsoever. The asparagus staging still works as intended, but the top tanks of the last two asparagus stages still drain when they aren't supposed to, at a very slow rate. Your answer got me thinking however and I've sort of managed to come up with a solution. I kept the inner tanks asparagus-staged and simply connected the SRB drop tanks to the bottoms of each inner booster and voila it worked. The droptanks all drain at once while the inner boosters stay full until the asparagus staging kicks in. Here's some screens for clarity-

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This still leaves me without the ability to double asparagus however, should I ever need it. Anyhoo...thanks for the replies!

One more question - how do I change image size? I don't want to fill the screen with those shots...

EDIT: Nvm, the post preview made it look like the images were filling the screen. :/

Edited April 17, 2014 by Laughing Man

[Nao]

I did read it. The boosters are solid-fuel and the tanks on top are liquid fuel, the idea is to transfer fuel from those booster-mounted tanks to the inner set of tanks if you get me. Also, I'm pretty sure decouplers don't auto-crossfeed if the wiki is anything to go by.

I did this and it made no difference whatsoever. The asparagus staging still works as intended, but the top tanks of the last two asparagus stages still drain when they aren't supposed to, at a very slow rate. Your answer got me thinking however and I've sort of managed to come up with a solution. I kept the inner tanks asparagus-staged and simply connected the SRB drop tanks to the bottoms of each inner booster and voila it worked. The droptanks all drain at once while the inner boosters stay full until the asparagus staging kicks in. Here's some screens for clarity-

This still leaves me without the ability to double asparagus however, should I ever need it. Anyhoo...thanks for the replies!

One more question - how do I change image size? I don't want to fill the screen with those shots...

EDIT: Nvm, the post preview made it look like the images were filling the screen. :/

Here is a post from a year ago i made on the problem: link - there is a picture describing the fuel flow. (unfortunately i can't find even older thread where we discussed it in more detail).

What i meant is to connect the SOLID boosters to each other with no fuel line connected directly to liquid drop tanks.

The problem with your rocket is that there is a fuel loop because fuel from the small drop tanks can flow naturally by the fuel line AND downwards through the solid booster. The booster part itself is fuel crossfeed capable so it's normal, the thing that enables fuel flow is the STACK connection between liquid drop tank and solid booster. (This is designed for situations like when you have some parts between engine and fuel tank in a stack, the engine can still take fuel form the tank).

Then the fuel can flow through other fuel crossfeed capable parts like radial decouplers and similar, creating loops which wreck havoc on fuel flow patterns (which is more of a bug but it's not easy to fix).

Now since the fuel loop starts AT the solid booster (by fuel flowing through the decoupler), if you connect fuel lines from the part that has that decoupler (solid booster) the fuel line gets full priority and the radial decoupler is never used to transfer fuel, eliminating the loop. The fuel flow will go like this:

- liquid droptank -> (stack connection) -> solid booster -> (fuel line) -> solid booster -> (fuel line) -> solid booster -> (fuel line) -> inner liquid booster.

Alternatively i proposed moving the radial decoupler up to the liquid drop tank which would have the same effect of disabling fuel flow through it. Or disabling the whole fuel flow through crossfeed parts by removing the stack connection between liquid droptank and solid booster (place a decoupler between them or just attach the droptank radially).

I'm glad that you were able to solve your problem thou Cheers!

[Superfluous J]

Now since the fuel loop starts AT the solid booster (by fuel flowing through the decoupler)

Under what other circumstances does fuel flow through a decoupler? I've never set up SRBs with fuel on top of them, but I attach liquid fuel tanks via radial decouplers all the time and if they allowed fuel flow that would cause some weird problems in most every rocket I've ever launched, I think.

[Nao]

Under what other circumstances does fuel flow through a decoupler? I've never set up SRBs with fuel on top of them, but I attach liquid fuel tanks via radial decouplers all the time and if they allowed fuel flow that would cause some weird problems in most every rocket I've ever launched, I think.

Hmm never given this much thought... But I think it's because when the engine asks for fuel, only the parts connected by stacking and fuel lines can answer. When it finds a fuel tank, then that tank starts asking around for fuel (in a stack and fuel lines) with the previous part in a chain (here engine) excluded. Then the next found fuel tanks asks again and the cycle repeats until there is no more fuel tanks available. It's a fail safe process.

The problem occurs when we introduce parts that aren't fuel tanks or fuel lines, but can create a stacking connection with the previous parts. Now at this point the system starts searching for any fuel tank, not only through fuel line and next in a stack part but also radially. This can start only on a part that doesn't have (relevant) fuel on it.

And there is also another problem of creating a loop. That is the radial connections and fuel lines must create a loop, and for standard designs it rarely happens since fuel lines and radial decouplers are usually connected to the same parts, creating only a "double flow" system and not an actual loop.

TL DR: for a bug to occur, there needs to be a part without fuel but connected in a stack with a fuel tank, AND radial decouplers need to be connected to different parts than fuel lines creating an actual loops and not just double flow systems.

[Laughing Man]

Here is a post from a year ago i made on the problem: link - there is a picture describing the fuel flow. (unfortunately i can't find even older thread where we discussed it in more detail).

What i meant is to connect the SOLID boosters to each other with no fuel line connected directly to liquid drop tanks.

The problem with your rocket is that there is a fuel loop because fuel from the small drop tanks can flow naturally by the fuel line AND downwards through the solid booster. The booster part itself is fuel crossfeed capable so it's normal, the thing that enables fuel flow is the STACK connection between liquid drop tank and solid booster. (This is designed for situations like when you have some parts between engine and fuel tank in a stack, the engine can still take fuel form the tank).

Then the fuel can flow through other fuel crossfeed capable parts like radial decouplers and similar, creating loops which wreck havoc on fuel flow patterns (which is more of a bug but it's not easy to fix).

Now since the fuel loop starts AT the solid booster (by fuel flowing through the decoupler), if you connect fuel lines from the part that has that decoupler (solid booster) the fuel line gets full priority and the radial decoupler is never used to transfer fuel, eliminating the loop. The fuel flow will go like this:

- liquid droptank -> (stack connection) -> solid booster -> (fuel line) -> solid booster -> (fuel line) -> solid booster -> (fuel line) -> inner liquid booster.

Ok, I think I understand you correctly here. I moved the outer fuel lines from the liquid fuel tanks to the SRBs. To my utter bewilderment and disbelief the outer tanks drained exactly as they should, one after another just as the asparagus staging was designed for it to do. Unfortunately the old problem cropped up again and the top 2 tanks of the last 2 asparagus stages started draining slowly. Again I want to itterate that the "double aspargus" works as intended at this point, but this problem still persists.

Alternatively i proposed moving the radial decoupler up to the liquid drop tank which would have the same effect of disabling fuel flow through it. Or disabling the whole fuel flow through crossfeed parts by removing the stack connection between liquid droptank and solid booster (place a decoupler between them or just attach the droptank radially).

I'm glad that you were able to solve your problem thou Cheers!

This is the second thing I did. The outer liquid fuel tanks are connected to the radial decouplers with the SRBs mounted underneath and the fuel lines connected from SRB to SRB. I'm still kind of dumbfounded, but it completely solved the problem. The outer tanks drain one after another and the inner asparagus staging also works perfectly, with all tanks staying full until it is their turn to be drained. I launched the rocket twice just to double check and right-clicked the tanks over and over just to be sure. Here's some more pics for clarity:-

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I never actually expected liquid fuel to move through solid rockets let alone have your solution solve my problem completely! What weird things you discover in this game. Thankyou sooo much! All of this kind of begs the question though - why are decouplers crossfeed capable?!

[Nao]

I never actually expected liquid fuel to move through solid rockets let alone have your solution solve my problem completely! What weird things you discover in this game. Thankyou sooo much! All of this kind of begs the question though - why are decouplers crossfeed capable?!

The answer to that and many other questions is 42.

Also actually it's not the "decouplers" but every other radially attachable part (except fuel tanks) that can do it. I did make a suggestion in bug report thread like 1,5 year ago, to disable fuel crossfeed on most parts (that still allows them to transfer fuel if there are fuel lines attached to them) but i guess it went on unheard.

I thought what I'd do was, I'd pretend I was one of that deaf-mutes. That way i wouldn't have to submit any bug reports to really old problems, that will not get fixed either way. (i'll be sad if you don't get the reference)