Fuel Ducts and drag

[AeroGav]

Astronauts, lace your boots before flight !

Seriously, since cargo bays occlude radially mounted engines and air intakes so regularly, i thought i'd try get this working in our favour for once.  Offset those wings inside the cargo bay and run a few fuel ducts between them.    Blast off the pad with not much fuel and a stupidly OP engine, see how high we get before falling back.  Do the same with the ducts removed.   After all, both ends of the duct are inside the cargo bay so there should be no drag, right?

With Ducts - max altitude 44,058.

Without ducts - max altiude 67,486

It's an issue because ok, nobody really needs this many ducts.  But I'm trying to build the most efficient stock SSTO I can, and  there's a few factors i'd like to combine to make something great -

*big S delta wings, since their mass:lift rating ratio is the same as all the other wing parts.  And they hold LF too, as fuel tanks they have the same fuel mass:dry mass ratio as any other tank

*generous wing area, for low angle of attack at 25km + altitude, therefore low drag in the closed cycle part of the ascent 

*... due to the low drag and wing supported flight, low twr, high ISP NERV engines can be used, leveraging the LF capacity of the wings.  Little or no oxidizer to be used.

*generous wing area leading to low landing speeds offworld - when refuelled this SSTO can go to Duna and Laythe safely

What messes all this up is the fact that NERV (all rocket engines for that matter) will not pull any fuel from wing tanks without ducts.  In fact even if you just use dry wings and a stack of fuel containers above each engine, you're going to have horrible CG issues because the rockets empty the tanks front to back.  It looks like you're just meant to attach one large tank to each engine and discard both when empty.

Also, if your wings aren't doing any thing for you fuel wise,  they become dead weight, which is i guess why people fly to orbit with tiny stub wings that can barely take off given the entire length of the runway.   Like I said, I prefer larger wings that enable better STOL performance in my SSTOs..

Spoiler

What's the answer   , if you 're trying to be 100% stock?

Transferring fuel manually is out of the question, the average user has their hands full with the flight controls, trying to maintain a good ascent profile (myself included). 

Fuel tanks can't be bound to action groups so you can't lock stuff off to eg. force the jet engines to use the wing tanks during air breathing phase.  

You could build small like this (two of the biggest wings, one engine, so we need only two ducts).  But that wing has lower heat tolerance, which i can work around, but isn't going to be easy to fly for most people.   Also it's not going to be a big enough craft to put the sort of loads most people want up there.

Spoiler

 

 

 

 

 

 So,  all I have left are mods that'd disqualify me from any K prize or that might put off new users downloading the craft from KerbalX.

eg. tweakscale, so i only need one pair of wings and 2 fuel ducts

eg. auto fuel balancer like GPOSpeedPump

 

A WIP SSTO.  I've noticed that if I set prograde assist and drop my AoA to zero, it has no effect on the drag from fuel ducts - unlike drag from fuselage parts.  The fuel ducts of this design cost about 6% more total drag here.

 

Edited April 24, 2016 by AeroGav

[bewing]

If I was designing the plane just for myself, I think I'd probably do the manual fuel transfer. AFAIK, as long as the engine is on (and therefore using fuel), you only have to start the fuel xfer once and it continues indefinitely. So it's not something you really have to pay attention to during important phases of the flight. It's just a one-time thing when you have a moment, to start the fuel xfer. (Well, one-time for each wing tank.)

[AeroGav]

18 minutes ago, bewing said:

If I was designing the plane just for myself, I think I'd probably do the manual fuel transfer. AFAIK, as long as the engine is on (and therefore using fuel), you only have to start the fuel xfer once and it continues indefinitely. So it's not something you really have to pay attention to during important phases of the flight. It's just a one-time thing when you have a moment, to start the fuel xfer. (Well, one-time for each wing tank.)

Hmm is that deffo how it still works?

I've done manual alt-click fuel transfers before and if you click the dialog boxes off the screen the transfer stops.  Like if you select another tank

[Kerbart]

1 hour ago, AeroGav said:

After all, both ends of the duct are inside the cargo bay so there should be no drag, right?

I don't think offset works that way. From what I understand offset placement is merely optical, not physical. That would mean that for the drag model, those fuel lines are still on the outside. Something to experiment with as well, perhaps?

[mrclucks]

55 minutes ago, AeroGav said:

Hmm is that deffo how it still works?

I've done manual alt-click fuel transfers before and if you click the dialog boxes off the screen the transfer stops.  Like if you select another tank

I've noticed this in 1.0.5, however with dialog boxes being tab-able in 1.1 can't you have them both tabed, start the fuel transfer, and if you click off of them they will both/all still be on screen so they should continue.  

Can anyone playing 1.1 confirm this works or not?

[AeroGav]

46 minutes ago, Kerbart said:

I don't think offset works that way. From what I understand offset placement is merely optical, not physical. That would mean that for the drag model, those fuel lines are still on the outside. Something to experiment with as well, perhaps?

I think it works that way for most things except cargo bays, which do check if an item should incur re-entry heating or not, and also whether you're trying to game the system with engines and intakes within bays.  But I suppose that says nothing about whether drag should be incurred, which is dependent on the parent part of an item , and also it's possible that different , simpler rules might apply for small parts like this that were formerly physicsless.

[bewing]

Things that are attached to the inside of a cargo bay are not supposed to have any drag. This was an issue during beta, but it got fixed and heavily tested. (Bug 8666.)

However, a fuel duct or strut that is attached to both the inside of a cargobay and to a wing (that must be attached outside) would seem to be a special case. I don't know what would happen in that case -- it may work, it may not, or a kraken may come and eat your ship.

As far as pinning a fuel transfer context menu goes -- there is the little problem that you can only pin a context menu for one item of a set that got attached with symmetry. One wing of a pair. To be able to pin two context menus, you might have to attach the wings separately (with the subsequent alignment difficulties). Or the fuel transfer may not stop when the window closes -- I haven't tested that yet.

[Jetski]

Offset tool can be used to fix alignment issues easily if you want to place them singly

[cubinator]

5 hours ago, mrclucks said:

I've noticed this in 1.0.5, however with dialog boxes being tab-able in 1.1 can't you have them both tabed, start the fuel transfer, and if you click off of them they will both/all still be on screen so they should continue.  

Can anyone playing 1.1 confirm this works or not?

Yes, in 1.1 you can right click on a part, then click on a little button in the top right of the dialog box and it will not go away when you right click another part. This can be done for any part that has a right-click menu. Today I used it to watch my barometer and thermometer as I ascended into space. You can move the boxes around, too, so they are in convenient places.

[Choctofliatrio2.0]

6 hours ago, AeroGav said:

Hmm is that deffo how it still works?

I've done manual alt-click fuel transfers before and if you click the dialog boxes off the screen the transfer stops.  Like if you select another tank

It works for me, I think. Course, it may be because 90% of the time dialog boxes I open are unclosable, no matter what I do.

[Foxster]

I've been unsure since 1.0.5 (I think it was then) whether the drag of ducts and struts is a bug. They certainly didn't used to be anything like as draggy as they are now. 

I checked a couple of parts a bit ago and the fuel ducts had more drag than the fuel tanks they were attached to. 

Edited April 25, 2016 by Foxster

[bewing]

Yeah, there's a bug report about that. It's also true that the tiny drogue chutes have similar, massive drag when undeployed. Makes them much less valuable.

[mrclucks]

10 hours ago, cubinator said:

Yes, in 1.1 you can right click on a part, then click on a little button in the top right of the dialog box and it will not go away when you right click another part. This can be done for any part that has a right-click menu. Today I used it to watch my barometer and thermometer as I ascended into space. You can move the boxes around, too, so they are in convenient places.

I know this, it's one of the many reasons why I think 1.1 was such a good update(reverted back to 1.0.5 for reasons) but what I was asking was if you could start a fuel transfer and then click away from the box and the transfer continue

[suicidejunkie]

22 hours ago, AeroGav said:

Offset those wings inside the cargo bay and run a few fuel ducts between them.    Blast off the pad with not much fuel and a stupidly OP engine, see how high we get before falling back.  Do the same with the ducts removed.   After all, both ends of the duct are inside the cargo bay so there should be no drag, right?

With Ducts - max altitude 44,058.

Without ducts - max altiude 67,486

This may be a silly question, but...

Did you account for the *mass* of the fuel pipes?

 

[cubinator]

3 hours ago, mrclucks said:

I know this, it's one of the many reasons why I think 1.1 was such a good update(reverted back to 1.0.5 for reasons) but what I was asking was if you could start a fuel transfer and then click away from the box and the transfer continue

Yep, fuel transfer works with that.