How to build stageable ion ship?

[Foxster]

I've been messing with a ship recently that has some ion engines in the mix. I would have liked to make the ion bit bigger, with more stages. However, ion has the screwy fuel feed mechanics that seem to make fuel ducts not work and tanks get emptied from all over the craft in seemingly random ways.

How can you make multi-stage ion craft?

[Taki117]

The easiest way is to simply disable the tanks in an upper stage by clicking the green arrow next to the amount

Alternatively you could go into the resource file for Xenon and change the fuel flow type.

[jarmund]

TAC Fuel Balancer will help you out here. It allows for auto-transfering in and out of tanks as you please.

[Foxster]

Thanks, guys.

Don't want to install another mod just for this so I guess its a matter of disabling the flow with the green arrows.

[jarmund]

Just for the record, the mod is very useful in general. I for one use its aajto-balance feature a lot for planes and fuel barges

[Red Iron Crown]

Xenon's fuel flow works like this:

Set stage # for each tank by counting decouplers between root part and the tank.

Drain fuel evenly from all tanks with the same stage #, starting with the highest.

So arranging tanks around a central core is problematic, because they all have one decoupler between them and the root part and thus get counted as the same stage #. Better to attach two tanks with radial decouplers and then make stacks under those tanks.

[Mister Dilsby]

My (failed) Eve ascent ship has multiple staged ions that worked, sort of. Here is the .craft file: http://kerbalx.com/kuzzter/Evoglide 2B.craft" target="_blank">download Evoglide 2B</a>'>http://download Evoglide 2B

If you want to fly the silly thing, EVA the pilot and he will be able to board the command seat from the lander can's ladder. Then throttle up, spacebar to eject the lander can and tower: space again to light the aerospikes and release from the portable pad, climb at any angle up to 45 degrees and eject spent rockets with spacebar, spacebar yet again and you are now running on ions in Stage 4. I have the outer two pairs of ion engines attached to droppable pylons. Intent was to run on 9 engines, draining only from the tanks on the outer pylons, then run on 7 engines, draining only the second outer set, then eject those and run on 5 engines for rest of flight.

This is where ion staging gets strange. It seems to treat all the detachable tanks as one group regardless of what stage they are in. In other words it will drain from the Stage 4 and Stage 3 tanks since those are all on the other side of separators, but not from the core set of tanks permanently attached in Stage 2. My workaround in this case was to simply disable all the tanks in Stage 3, as shown:

When the Stage 4 tanks are nearly empty, I start feed from one of the Stage 3 tanks in each pylon. That way when Stage 4 is empty and I drop the tanks, the remaining 7 engines still get fuel. Then I enable all the rest of the tanks in Stage 3. From that point everything works as you'd expect--only the Stage 3 tanks drain, and when you drop them Stage 2 will start feeding.

Hope this helps!

ETA: RIC's explanation makes perfect sense. All of my pylons each have one decoupler between themselves and root, therefore they all feed together. Adding a second decoupler to the outermost pylons should make the Evoglide 2B stage as intended.

Edited April 1, 2015 by Mister Dilsby

[Red Iron Crown]

^^ It's because there is only one decoupler between each tank and the root part. See my post above.

[Foxster]

Confirmed.

This craft uses the bottom set of tanks first...

Not ideal but now I know how it works I can do things with it. Thanks.

[Mister Dilsby]

^^ It's because there is only one decoupler between each tank and the root part. See my post above.

^^^I did, I did, you ninja'd me you ninja you!

[Foxster]

Umm, seems its not as simple as that after all. Things get screwy if you have more than one stage of ion engines.

This works OK...

The fuel goes from the bottom, then the middle then the top tanks.

But this doesn't work...

It has the decouplers in stages from the bottom up just like the previous one but after the first two stages (5 & 6) it starts using Xenon from all the remaining tanks, as you can see from the delta-V stats screen where stage 4 uses all the remaining fuel.

I'm a bit stumped now.

I can use ions for a top 25kdV stage but I was hoping to have multiple ion stages for, say, 50kdV.

[Dazpoet]

Just "close" the tanks you don't want it to drain from manually? There should be a small green thing next to the resource (Xenon in this case) in the right-click menu. Click that so it becomes red and it shouldn't drain from there until you click it back to green again.

[Mister Dilsby]

Did you get to the 7-stage config by copying and pasting a subassembly from the 4-stage config? If so I wonder whether the stage precedence somehow got copied along with the parts...one way to check would be by hand-building a trial stage 5 onto the 4-stage and seeing if that works.

[-Velocity-]

In stock, the xenon tanks need to be separated by decouplers. I created a "refuelable", massive xenon-propelled ship that had paired, xenon fuel assemblages attached to the ship via docking ports, and with each fuel assemblage pair separated by a decoupler. (A "fuel assemblage" was a subassembly I created that had like 100 xenon tanks tightly packed together with Clampatron Sr. docking ports at each end. Effectively, it is just a very large xenon tank.) When a fuel assemblage goes empty, xenon fuel begins to get drawn from the next set of fuel assemblages down the line, and the empty fuel assemblage is discarded. (It is possible to refuel the fuel assemblage, but it would be HIGHLY tedious.) The ship is "refuelable" in that once it's returned to Kerbin orbit, I can launch a rocket containing new fuel assemblages and dock them onto the ship with a "construction drone".

So basically, xenon tanks and xenon tank groups just need to be separated by decouplers to get them to feed your engines in a staged manner. If I remember correctly, crossfeed enable/disable had no effect on xenon fuel, which is why I had to use decouplers.

Just be careful, and save a lot, because if you press the spacebar accidentally, you can split your xenon ship in half... the decouplers are only there to force the xenon fuel to flow correctly, in a staged manner, and are NEVER intended to actually be activated, because doing so splits the ship apart. Obviously, it would be best if Squad made xenon fuel follow the same rules as regular fuels, but it is what it is

Oh, and all those people saying to "just disable the tanks"- umm, that doesn't work so well when your ship weighs like 300 metric tons and has like 750 xenon tanks or whatever ridiculous number my ship ended up having. I don't remember.

OH and I never actually launched this ship into orbit because some new patch came out, but I proved the concept was viable on the ground.

Edited April 1, 2015 by |Velocity|

[Foxster]

Did you get to the 7-stage config by copying and pasting a subassembly from the 4-stage config? If so I wonder whether the stage precedence somehow got copied along with the parts...one way to check would be by hand-building a trial stage 5 onto the 4-stage and seeing if that works.

Yea, that occurred to me and I did just what you said and built the bottom stage manually again and it still did the same.

- - - Updated - - -

In stock, the xenon tanks need to be separated by decouplers. I created a "refuelable", massive xenon-propelled ship that had paired, xenon fuel assemblages attached to the ship via docking ports, and with each fuel assemblage pair separated by a decoupler. (A "fuel assemblage" was a subassembly I created that had like 100 xenon tanks tightly packed together with Clampatron Sr. docking ports at each end. Effectively, it is just a very large xenon tank.) When a fuel assemblage goes empty, xenon fuel begins to get drawn from the next set of fuel assemblages down the line, and the empty fuel assemblage is discarded. (It is possible to refuel the fuel assemblage, but it would be HIGHLY tedious.) The ship is "refuelable" in that once it's returned to Kerbin orbit, I can launch a rocket containing new fuel assemblages and dock them onto the ship.

So basically, xenon tanks and xenon tank groups just need to be separated by decouplers to get them to feed your engines in a staged manner. If I remember correctly, crossfeed enable/disable had no effect on xenon fuel, which is why I had to use decouplers.

Just be careful, and save a lot, because if you press the spacebar accidentally, you can split your xenon ship in half... the decouplers are only there to force the xenon fuel to flow correctly, in a staged manner, and are NEVER intended to actually be activated, because doing so splits the ship apart. Obviously, it would be best if Squad made xenon fuel follow the same rules as regular fuels, but it is what it is

Oh, and all those people saying to "just disable the tanks"- umm, that doesn't work so well when your ship weighs like 300 metric tons and has like 750 xenon tanks

I kinda have that already with the first craft above. Each of the bottom stages of tanks are separated by decouplers and, as I said, that works OK. The difficulty comes when trying to stage multiple sets of engines too.

[-Velocity-]

Yea, that occurred to me and I did just what you said and built the bottom stage manually again and it still did the same.

- - - Updated - - -

I kinda have that already with the first craft above. Each of the bottom stages of tanks are separated by decouplers and, as I said, that works OK. The difficulty comes when trying to stage multiple sets of engines too.

Odd. I built an "interstellar probe" that I launched away from Kerbol at something like 100 km/s. I had staged ion engines on that one. They worked just fine.

Edit- maybe just try a single set of tanks per ion engine, not staged tanks for each engine set. Less efficient, yes, but by a whole lot? Not really.

Also, did you actually test this configuration in game? Those delta-V stats by Mechjeb could easily be incorrect, especially for something more "arcane" like staged ion engines, which is a relatively new feature. I've seen the delta-V stats bug out before or otherwise have incorrect information. Alternatively, those stats could in FACT be correct, because you simply have the craft staged incorrectly- like the decouplers are out of order or something.

Edited April 1, 2015 by |Velocity|

[Foxster]

I'm getting tired and a bit punchy on this. Think I'll start again tomorrow.

Thanks for the help.

[Mister Dilsby]

I'm getting tired and a bit punchy on this. Think I'll start again tomorrow.

I can definitely feel that! When you get back to it here's another thought: could it matter what decouplers and tanks the algorithm 'finds' as it traces a path back to root? That is, would it work if you laid it all out in one line with no branches like so:

[ROOT][T1]=[T2]=[T3]=...[Tn]

(where [T] are tanks and = represents one decoupler)

rather than having multiple branches like this, and trying to differentiate by adding decouplers:

[T1]=[ROOT]==[T2]

...Hypothesis being that the multibranch method reaches a limit in number of stages but the linear method does not.

Edited April 2, 2015 by Mister Dilsby

[Red Iron Crown]

If you have ion thrusters in different stages it confuses the dV calculators like MJ and KER. They assume that an engine will burn all fuel available to it before staging the engines away, and with ions that means draining the entire vessel including stages above it. It will calculate drop tanks without thrusters just fine, but once you get to a stage with jettisonable thrusters in it the simulation behaves strangely.

The workaround is to disable the thrusters in all but the last stage. This will yield correct dV numbers but the TWR for those lower stages will be incorrect.

[Aethon]

Lol. Here are the last few stages of my Sundiver. It requires some babysitting, turning on and off tanks, fuel transfer etc., if you want it to perform efficiently, but it works. Below the tree stage is an LVN. I'm not sure whether KER is calculating the dv correctly, but I think it is. Dv actually goes up at times during the mission.

[dontbesilly]

Spiral Staging (Asparagus) drains fuel as planned, from the outside to the inside, with xenon tanks - no fidgeting with fuel on/off switch.

KER shows it "not", MJ does show it.

Tested on the landing pad.

 

Engine block removed to give a better view (which obviously does not show any staging stats).

 

[FleshJeb]

I need to experiment to see if batteries are stagable...