Fuel System Challenge

[Hotel26]

Good day, all.

I am facing a challenge with the fuel feed system and am hoping to receive some expert advice!

This link is to an unmodded .craft file for KSP 1.3.1.  The craft is the B payload for Operation Beach Head loaded atop a standard Aquila launcher/transit vehicle.

 

Problem Statement:

The lowest 3 engines ignite during the lift-off stage and are intended to burn out together.  To make this work in KSP versions 1.0.5 (which it did), fuel lines had to be run from the lower tanks to the next-level higher *engines*, rather than the tanks above.  (This was due to one of the peculiarities/flaws of the original KSP fuel feed system.)

Since 1.2.1, 1 tank on the lower deck runs out before the other two.  I investigated this in 1.3.1 with the fuel feed overlay system and verified that the fuel lines are all correctly attached and everything seems fine.  In addition, I can see no way with the new fuel priority system to prioritize this any differently.

Any advice, insight, expertise will be greatly appreciated and acknowledged!

[And yes, I did promise that Aquila is the ugliest ship in the Kerbal universe.  If anyone has created an uglier Gorgon, I'm sure we'd all enjoy seeing it!!]

Edited February 4, 2018 by Hotel26

[Hotel26]

OK.  Maybe I didn't repeat your changes faithfully.  When I did what I thought you said, I saw engines A & B drawing fuel from both tanks A & B.  But I may have had the priorities the wrong way around.  I'll retry it.

I want A & B to run on fuel from A only.  Then stage AA away and run B & C on tank B only.  Then stage BB away and run C on tank C remaining.  Asparagus staging.

Did you actually achieve that?

So, after running the test bed, my understanding is that I should set the pri of the lowest three tanks (A) to 20, say.  The next three tanks up (B) to 10.  And the top three tanks (C) to 0.  And remove the fuel lines.

Yes?

Edited February 6, 2018 by Hotel26

[Hotel26]

And no go after taking the superfluous fuel lines out.

Any chance of you posting your modified craft file?

[Rodhern]

On 2/4/2018 at 10:23 AM, Hotel26 said:

... I'm sure I didn't understand the fuel priority system very well!

Anyone that do understand the fuel flow priority system well probably deserves some kind of medal. Do we even know someone like that? (I guess with a forum like this, there is bound to be a few though).

Edited February 6, 2018 by Rodhern
spelling

[Flavio hc16]

i would adveice you to download the fuel pump mod, that show the fuel priority in a way more intelligent way

[Spricigo]

19 hours ago, Rodhern said:

Anyone that do understand the fuel flow priority system well probably deserves some kind of medal. Do we even know someone like that? (I guess with a forum like this, there is bound to be a few though).

• Fuel can flow both ways between normally attached parts (parent/child), one way along fuel line linked parts and don't flow between parts linked only by strut connectors
• Docking ports or decoupler with crossfeed off blocks the flow of fuel.
• Each engine will drain fuel from the fuel tank with higher priority except if the path(partA, attached to partB, attached to partC, ....) to it is blocked
• If the engine have access to multiple tanks with same priority, equal amounts from each tank will be drain.

 

• moving a stack one stage up/down change flow priority by =/-10

*

What i suspect the problem with the original craft may be:

• Staging messing with flow priority
• Expecting it to work as if it had fuel lines (booster stop when its tanks  run out of fuel), when using decoupler with crossfeed (boosters use fuel from central stack)

 

 

[swjr-swis]

On 2/5/2018 at 11:38 AM, Hotel26 said:

Nevertheless, for all that, I poured as much ingenuity and love as I could into Aquila and I will mourn it.  I'm not done on the fuel problem.  (...)

Then I will make a launchpad test bed.  It will have 3x small engines running from 3 tanks, A, B, and C, and with A feeding B and B feeding C.  This is basic asparagus staging and it should work.  At start, all engines should run off fuel in A and consume it all before touching a drop out of B and C.  A's engine will quit, fuel starvation, as expected.

If I can get past the fuel problem, I will rebuild this ancient bird...  it has what Gizmo will never have: personality.

After reading through this thread and seeing the above, I felt I couldn't let this be the end of the long illustrious career of the Aquila lifter. Especially because I agree with your statement here: even though fuel flow did change in significant ways since 1.0.5, the good ole traditional asparagus staging should still work exactly as one expects it to.

So, with that in mind, I did a rebuild from scratch of the lifter in 1.3.1, using your shared file as example. Then I copied the rover payload from yours as a subassembly and placed it on the lifter (with a few minor tweaks - antennae added and wheels/fairings slightly redone to prevent the wheels from blowing when the fairings are staged). My resulting craft file is on the dropbox link below.

https://www.dropbox.com/s/ja08gr8cy28aheq/Beach Head B 131.craft?dl=0

I couldn't get your original craft to work quite right simply by editing it - I suspect a combination of broken symmetry and some save error sneaking in during your previous edits. I decided it would be faster to rebuild it from scratch instead of trying to debug the issue.

It's not identical... I did do a few things differently, without -I think- affecting its overall concept and personality. The in-game description text should list all differences. But if I understand you correctly, I think it now works the way you wanted it to, with the 'traditional' asparagus type of fuel flow, automatic engine cut-off, and staging. You'll have to let me know if it proves a worthy continuation of the dynasty.

Long live Aquila VIII.

[Hotel26]

13 minutes ago, swjr-swis said:

Long live Aquila VIII

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Your generosity, sir, is amazing!!

It flies!  Aquila flies again!  Yahoo!!!!

Edited February 8, 2018 by Hotel26

[swjr-swis]

A few separate points that didn't seem to get answered:

 

On 2/4/2018 at 10:23 AM, Hotel26 said:

what are "quad-couplers"?

The part you used to fit the clusters of LV-N engines below the X200-16 tanks. The game assigns a lot more drag to them than one would expect - if your ship/PC can take the part count, it's almost always better for drag to avoid them altogether, and place engine clusters through the use of radially attached nose cones. In your craft though, you also picked the 1.25m-to-4 adapter (TVR-2160C), which means there is a node mismatch between the tank and the quad-coupler - adding even more drag. In the rebuild I posted, I replaced the quad-coupler by the TVR-400L, so at least the nodes are matched.

 

On 2/4/2018 at 10:43 AM, Hotel26 said:

the thermal; systems, for example, are for long burns of the nukes for interplanetary transit insertions...

I tested with the radiators disabled: I did a continuous full throttle burn of the LV-N stage and burned through the entire fuel without a single heat warning anywhere. Heat does not seem to be an issue anymore, so you may be able to do without them (or replace them with the smallest ones). The only thing I could think of that I did not test is how it fares when burning much closer to the sun.

 

On 2/4/2018 at 11:35 AM, Hotel26 said:

OK.  Think about the scenario where the payload platform has 6 peripheral tanks and I wish to place the load ABABAB to keep the mass distribution symmetrical.  It's going to end up as 6 individual placements.  That's OK.  Only discrepancy would be with rotations not being exactly the same, for example.

The way symmetry is forced by the stock game, it won't let you override an already existing symmetry in a stack other than placing things from a certain point individually. In the case of this craft, I think it works best building the entire lifter with a 2x3-way symmetry, and then place the payload individually - ensuring balance/symmetry manually. Which I think is very doable.

 

On 2/4/2018 at 4:00 PM, Hotel26 said:

I have to put a lot more stuff on it.  Mainly though nukes and I won't get enough solar sails on it even 2 orange tanks tall, so I will use my motorized solar sails (16 x Gigantor) that stow for take-off/aero-braking but deploy for ion flight.

If you still plan on adding ion drives as well as nukes (which I don't think would be very efficient, mind you, but don't let that hold you back), I would suggest using fuel cell arrays instead of a large number of solar panels. You are lobbing a huge unused capacity of LFO (the nukes do not need the Ox!)  tankage into space, and fuel cells could use a fraction of that to cover all your EC needs, including powering the ion drives. Note that even though the Gigantors can theoretically provide more EC per part than the fuel cell arrays, typically you won't be able to get all solar panels to generate optimally, and the farther from the sun you get, the better the fuel cells will do in comparison.

 

On 2/5/2018 at 11:19 AM, Hotel26 said:

That get damaged when the fairings blow off.  So the fairings are going to have to go.  Drag, drag, drag.

Consider configuring the fairings to 'clam shell' mode, with a bit more than the default force, lower the segments to 2, and rotate the fairing such that the parting line is diagonal across the lifter lattice - the two shell halves should then part to the 'sides' of the corner and never touch any other part of the craft.

 

On 2/5/2018 at 11:38 AM, Hotel26 said:

Secondly, it's not easy in KSP to run a fuel line in other than a straight line.  You cannot connect it from a tank to one above bypassing an engine in between. 

Actually, you can. I can think of two methods:

1. More parts and less 'cheaty' for those who have strict no-clipping policies: cubic octagonal struts let fuel flow through - so simply attach one of those to the side of the decoupler below the engine, then run one fuel line from the lower tank to the strut, and one fuel line from the strut to the top tank. You may need to gizmo translate the strut a bit to make them connect well, then translate back after they are connected.
2. Less parts, less drag: with the translate gizmo, first shift the engine a good bit aside. Place a single fuel line from the top side edge of the lower tank to the lower side edge of the higher tank. Now with the translate gizmo again, hit space to reset the engine back in it's default place. The fuel line may visually clip a bit, but for the game's purposes it is connected properly between the tanks. You can even hide the fuel duct entirely if you so wish with the translate gizmo, even moving it to where it literally passes through the center of the decoupler and engine - it will still work. Just remember it's there, and if you ever reconnect the stack, you may need to redo the fuel ducts.

[Hotel26]

On 2018/02/08 at 9:40 AM, swjr-swis said:

A few separate points that didn't seem to get answered:

A master class!!  Gracias.

I'll put some time into study of your work.  I'll carefully dissect it and make a part map.  Then rebuild it several times to become familiar with the techniques.  In addition, I'll use the rebuild to gizmo-late the outer tank ring out a bit to provide more clearance for payload above.

Note that 2x Aquila (2 are required for all the payload in Operation Beach Head) can be docked tail-to-tail to make a space station.  Going interplanetary, I gang 2 , 3 or 4 Aquila together with the for'ard Aquila carrying the payload.  Under these circumstances, the nukes may have a whole orange tank full of LF to burn, although rarely in one go.  I think this is where the radiators are required.

I really liked where you put the large nosecones and the 2x3-way symmetry insight.  I'll post a couple of screen shots of your work for others to see.  You very much stayed within the spirit and personality of the original which indicates a great deal of artistry.

I want to express a good deal of gratitude also to @foxster for staying with this and to note that his comments about "broken symmetry" seemed to have been on-track.  I was interpreting them just as independent remarks, I think.

It's a real thrill to have this machine back in fighting form.  It will be Lucky Aquila VII, because I had just incremented to 7 specifically for finding a cure to the staging problem.

 

Edited February 17, 2018 by Hotel26

[swjr-swis]

5 hours ago, Hotel26 said:

I'll put some time into study of your work.  I'll carefully dissect it and make a part map.  Then rebuild it several times to become familiar with the techniques.

There really shouldn't be much to study about it - it follows your build, so it's mainly your own techniques. The main exception is that I used 2x 3-way symmetry all throughout the lifter, which seemed like the natural way to achieve your stated behaviour. With using symmetry comes the structural side benefit of auto-struts linking in symmetry as well, which allows minimizing the use of external struts. Almost everything else is a copy from your craft.

I left all fuel flow priorities exactly as default, making the fuel ducts the only parts responsible for the desired asparagus flow. It works exactly as one would expect and how it always used to before they made the fuel flow changes - the ducts bypass the crossfeed separations allowing multiple sets of engines to feed off the tanks that will stage off first, so the craft always has full tanks left after every staging event. And since they do this with one-way flow only, the engines cut off automatically when the stage tanks are empty, which serves as a cue to stage.

The comms set is a fudge and a bit of an afterthought. I wanted to add comms and relay capability (motherships generally need both) but without significant changes to the rest of the craft to accommodate it, and the result isn't exactly pretty. 4x RA-2 + 2x 88-88 pointing outwards from inside a 2.5m service bay, placed between the bottom center docking port and the orange tank, would have been a better solution.

[Hotel26]

6 hours ago, swjr-swis said:

There really shouldn't be much to study about it

It was very easy!  I did a rebuild and I made the change I've been wanting to make (but hadn't trusted myself to a rebuild): which is that I pulled open the top payload deck to allow spacing for 2.5m cargo that contain radial protusions such as engines and legs.  I took some screenshots and post them but the family relationship between Gizmo and Aquila becomes very apparent.

With 2x Aquila launches, I can put all of Operation Beach Head into LKO.  On 7 payload platform docks, there will be sufficient room to consolidate the whole payload.  Then gang the second, now-empty Aquila with the first to start forming up the interplanetary "road train".  Then a tanker comes up to refuel it.  I've used up to 4 Aquila for the longer missions.

Screen shots this evening...

[Hotel26]

This is the end of the chapter, I think.  Some screenshots, left-to-right, top-to-bottom:

• Aquila 7 showing the spacing now in the payload deck
• 2x Aquila ganged as a "road train" for interplanetary travel
• 2x Aquila docked as a fuel station upon arrival at the destination
• Aquila Gizmo in a 6-way configuration showing its family resemblance

 

Edited February 9, 2018 by Hotel26