problem with the external fuel pumps

[xRaffle]

i was loosely following Rhadamant let's play as a tutorial to the game, i have about 50 hours of playtime by now, and i managed to land on the mun, but as im about to leave the mun i realized that my rocket with three engines that have asparagus staging have emptied my middle fuel tank that was supposed to be my last rocket on my way home, i really wanna know where did i messed up during assembly if anyone here could help me i would be glad, please be free to ask me for images or anything you need

 

[Snark]

1 hour ago, xRaffle said:

my rocket with three engines that have asparagus staging have emptied my middle fuel tank that was supposed to be my last rocket on my way home, i really wanna know where did i messed up during assembly

So, basically what you're trying to do is have the fuel draining inward (so that the radial tanks drain first, and you can stage them away), but instead you have it draining outward (with the central stack draining first), yes?

If that's what you're experiencing, most likely what you've done is gotten your fuel ducts backwards.

Fuel ducts transfer fuel in only one direction.  (In the direction of the little arrows printed on the side.)  When you're placing the fuel duct in the VAB, "end 1" is the one you place first, and "end 2" is the one you place second.

The fuel will flow from end 1 to end 2.

So, if you want your fuel to drain inwards for asparagus staging-- i.e. you want the radial tanks to drain first, leaving the center tank full-- then what you should do when placing the fuel ducts is to first place the duct end on the radial tank, then place the second end on the center tank.  It matters which end you place first.

(There's another way to do asparagus staging, which involves enabling fuel crossfeed on your radial decouplers instead of using fuel ducts, which I can go into if you like.  That sort of fuel crossfeed is in two directions, so the fuel management is handled slightly differently.  Let us know if you'd like to go into details for that type.)

 

[xRaffle]

i think that's not the issue, i took a screenshot showing the direction in game, i changed to look gray but that's is just a cosmetic  variant right?

https://imgur.com/a/4C6eI7f

 

[jimmymcgoochie]

It’s impossible to tell with the graphics settings you’re using, the arrows are too blurred to make out which direction they’re pointing.

The best way to fix this is to open up the resource panel (the fuel icon in the top right), then select liquid fuel and oxidiser on the list to show all parts containing those resources, then click the In/Out buttons to pump the fuel into the core tank. Then dump the boosters- barring a really bad design, a Terrier with a FL-T800 is more than enough to return from the Mun’s surface to Kerbin for re-entry and there’s no point dragging those boosters along when they’re stealing fuel from the core.

I’d send a Kerbal out on EVA to take the data out of that science jr and store it in the command pod though, because that’s going to explode on re-entry even with the heat shield on the bottom. Materials bays have low heat tolerance and are much lighter than command pods, so trying to re-enter with one attached to your pod is a bad idea- not only is it very difficult to get a materials bay to survive the heat of re-entry, it also destabilises the craft which will want to go heavy (pod) end first, potentially cooking your parachutes. They’re expensive (in career mode anyway), but you’re better off using an EVA Kerbal or an experiment storage unit to remove their science samples before discarding them.

[Mythos]

11 hours ago, Snark said:

fuel ducts

I loved messing around with those. Now I realize that I haven't done asparagus for quite a while and just went into the game to test some.

And I have to tell you, that this lovely little ducts are a relic of the past. You can just enable crossfeed on your decouplers and the fuel flow priority will be arranged and updated according to your staging (you don't even have to have advanced tweakables on and mess with the priorities yourself).

Edit: Well in that case all engines stay on because fuel feed goes both ways, and you have to watch the fuel levels to know when to stage, but it's the same principle.

Edited June 30, 2021 by Mythos

[jimmymcgoochie]

30 minutes ago, Mythos said:

I loved messing around with those. Now I realize that I haven't done asparagus for quite a while and just went into the game to test some.

And I have to tell you, that this lovely little ducts are a relic of the past. You can just enable crossfeed on your decouplers and the fuel flow priority will be arranged and updated according to your staging (you don't even have to have advanced tweakables on and mess with the priorities yourself).

Edit: Well in that case all engines stay on because fuel feed goes both ways, and you have to watch the fuel levels to know when to stage, but it's the same principle.

I tend to use decoupler crossfeed for things like SRBs with fuel tanks on top as there’s nothing using the fuel on the SRB side to confuse the system; for all-LF boosters it’s still easier to use fuel ducts as the delta-V readouts and fuel levels on the stage list are accurate and the ducts prevent fuel going the “wrong” way.

[Snark]

12 hours ago, xRaffle said:

i think that's not the issue, i took a screenshot showing the direction in game, i changed to look gray but that's is just a cosmetic  variant right?

Hmm.  I've taken the liberty of cleaning up the image a bit for visibility, and yeah, it looks to me as though the arrows are pointing in the correct direction (from the radial booster, towards the central stack).

So, yeah, that looks to me as though that ought to do exactly what you expect it to.  Not sure what's going on, here. 

A few questions, as a sanity check:

1. What are the fuel contents of each of those tanks, as it's sitting there in this picture?  e.g. for radial tanks and the central tank, is each one full? empty?  partial?  (and if so, roughly how full?)
2. You're lifting on all three engines, yes?
3. What's the status of the fuel crossfeed settings on your radial decouplers?  Enabled, or disabled?  (With the fuel ducts present, you want the crossfeed disabled.)
4. What's the drainage pattern, exactly, as you ascend from the surface?  For example, is it draining only the central tank while the radial ones stay untouched?  Or is it draining all three tanks, simultaneously?  (And if the latter, is it draining them at the same rate, or is one tank draining faster than the others?)
5. Do all three engines stay burning the whole time it's ascending, or do any of them conk out from lack of fuel at some point while others stay burning?  (e.g. when one tank runs dry but others still have fuel)

(FWIW, I tend to agree with @jimmymcgoochie that your current engines-and-tanks setup looks like overkill for coming home from the Mun-- you should be able to get home just fine from the surface with only that central tank and one Terrier, assuming that the tank has a reasonably full fuel load.  So you shouldn't actually need asparagus in this situation.  However, if you're having design issues with getting asparagus to work, then it's probably worth sorting that out, so it'll work for you in the future when you need it more.)

[king of nowhere]

fuel tanks have a set priority. Open them with right click to see the details, you'll see for each one a priority value. the tank with the highest number will be drained first.

if you don't manually tamper with them, the game will assign priorities authomatically. and it most often does a good job of it. but maybe in your case it did screw up

[herbal space program]

@xRaffle, Do you by chance have crossfeed enabled on your decouplers? That combined with incorrect flow priority settings might result in what you're dealing with.

6 hours ago, Mythos said:

And I have to tell you, that this lovely little ducts are a relic of the past. You can just enable crossfeed on your decouplers and the fuel flow priority will be arranged and updated according to your staging (you don't even have to have advanced tweakables on and mess with the priorities yourself).

Edit: Well in that case all engines stay on because fuel feed goes both ways, and you have to watch the fuel levels to know when to stage, but it's the same principle.

I don't really consider them a relic myself,  because disabling crossfeed and using those is considerably simpler (at least to me) than managing flow priority across a complicated vessel is. Also, as you pointed out, having to keep an eye on the fuel level of the side tanks while managing your ascent profile can be kind of a pain.

Edited June 30, 2021 by herbal space program

[xRaffle]

i made some tests with just the lander part of the rocket and king of nowhere was right, the problem was in the priority value of the tanks, thanks for the help everyone, i learned a lot with all the answers :))

On 6/30/2021 at 5:04 PM, king of nowhere said:

fuel tanks have a set priority. Open them with right click to see the details, you'll see for each one a priority value. the tank with the highest number will be drained first.

if you don't manually tamper with them, the game will assign priorities authomatically. and it most often does a good job of it. but maybe in your case it did screw up