Fuel Ducts creating massive amount of drag?

[brolgarh]

I started a new career mode with .23 and the new rocket I created seems to have a strange bug with fuel ducts.

When I have fuel ducts my rocket begins to lean to one side around ~8km above Kerbin SAS tried to prevent it with yaw regulation but it cannot do anything against it because the force seems too strong. When I try to steer manually the rocket flips completely.

When I remove the four fuel ducts the rocket launches perfectly into space and can be controlled like you would expect.

Is this a bug? Because I cannot imagine a small fuel duct creates such a massive drag.

Here are both my ships - one with fuel ducts and one without them: http://ge.tt/api/1/files/1OVpleA1/0/blob?download

Can anyone re-create this behavior?

(In like 1km height throttle down the liquid engines so that they will run out with the solids (not optimal at the moment I know)).

Thank you

Edited December 21, 2013 by brolgarh

[Motokid600]

Hi, sorry I don't have .23 installed yet, but by fuel "ducts" do you mean lines? If so then it sounds like you have uneven fuel drainage with said lines. How are they arranged?

[brolgarh]

The fuel lines are arranged like this: S4->S3->S2->S1->S0 (center) (each one tank, so not symmetrical like in the third picture you posted).

I expect a very small drag from those lines of course, but my rocket has a drag I cannot compensate at all.

Would be nice if anyone could re-create that with my files.

[Motokid600]

Are you dropping each stage as it depletes?

S4->S3->S2->S1->S0 (center)

This is your problem. As your rockets ascending its draining fuel from only the S4 tank. So by time it runs out and S3 starts draining the rocket is WAY off balance due to the empty tank on one side vs the full tank on the other.

Edited December 21, 2013 by Motokid600

[Murph]

Are you dropping each stage as it depletes?

This is your problem. As your rockets ascending its draining fuel from only the S4 tank. So by time it runs out and S3 starts draining the rocket is WAY off balance due to the empty tank on one side vs the full tank on the other.

Yes.

Asparagus staging will only ever work cleanly if it's symmetrical from pad until the last liquid booster is dropped. From the sound of it, you're likely not only getting a problem with the CoM moving due to asymmetry, but also asymmetric thrust as soon as S4 is empty and its engine stops.

Edit: The simple fix is to do S2->S1->S0, with S2 and S1 each being 2 symmetrical liquid boosters. That turns it into normal asparagus staging. Of course, you might then have a lack of total thrust at the point that the S2 boosters are empty.

Edited December 21, 2013 by Murph

[thereaverofdarkness]

Sometimes removing the fuel ducts and putting new ones on fixes the problem. My old spaceplane suddenly began depleting unevenly after I put the RAPIER engines on it. I took the fuel lines off and put new fuel lines right back in the same places and it drained evenly just like it used to.

[brolgarh]

Ok this was the case it seems. The rocket tilted to one side even before dropping the S4 tank, so I didn't see the problem as with the tank itself.

But it then has to be the weight of the used fuel in that tank - I did not expect this to be such a huge factor.

Now with symmetrical booster drain it works fine (well except that the rocket does not reach orbit with my payload, but that is another story )

Thank you for your help.

[numerobis]

FYI: Fuel lines aren't "physically significant"; they have no mass, and therefore no drag.

[blizzy78]

well except that the rocket does not reach orbit with my payload, but that is another story

"It looks like you're trying to build a rocket."

[Dave Kerbin]

Three things to consider:

Fuel ducts have no mass or drag (they are one of a few special small parts that don't have any effect on the simulation to help with performance)

If you drain from a fuel tank on one side of your ship you must drain equally from a tank on the other side, otherwise your rocket will weigh more on one side then the other and tip over. Your SAS system can only compensate for very small imbalances. Fuel makes up most of the mass of your ship so it is never small.

Sometimes when placing fuel ducts (especially using symmetry) one of them might be blocked. Check to see that all your fuel lines actually go where you wanted them to, there might be a blocked line that prevents one of the tanks from draining.