Asymetric fuel usage

[Rodyle]

Hello there everyone!

I've been working on a SSTO people carrier capable of going to the moon, and I think I've worked out most issues in it. However, one issue keeps biting me in the ass. After the outermost fuel tanks have been emptied (including the hidden one in the back), the fuel drainage of the tanks becomes asymmetric, making it quite dangerous for every kerbonaut inside it, since it starts pulling to the right at 25 KM while travelling at 1.7km/s. I have basically tried everything, including taking the entire craft apart looking for errant fuel lines. I hoped someone here may be able to find the fault in it.

Thanks in advance!

Link to craft file: http://pastebin.com/zXd2Yn03

Edited February 17, 2014 by Rodyle

[capi3101]

Okay...so from your craft file, it looks like you've got an interplanetary spaceplane SSTO design (gotta be specific - you CAN make single-stage rockets, and those ARE SSTOs).

Starts pulling to the right at 25,000 meters going 1700 kps....what's your IntakeAir status at that level? Is it less than 0.21?

I see that you're using three Turbojet engines - so I automatically suspect asymmetric thrust prior to flameout.

16 Ram Air Intakes - five per engine. Yeah - suspect you're getting close to the operational ceiling of the jets.

Here's what you do: when the pull begins, throttle back to 2/3 and see if you can get it back on course. If you can, then that's definitely the issue you've got. If not, then something else is going on. Every time it starts to pull to the right, throttle back another tick. Eventually, you'll get to the point where you're no longer accelerating; that's when you light your rockets. Continue throttling back until you're no longer accelerating with the rockets lit. Then kill your jets, close your intakes, throttle up full and make the final push for orbit.

Try that; see if it works. If so, great. If not, we'll discuss your craft a bit.

Edited February 17, 2014 by capi3101

[Rodyle]

Okay...so from your craft file, it looks like you've got an interplanetary spaceplane SSTO design (gotta be specific - you CAN make single-stage rockets, and those ARE SSTOs).

Starts pulling to the right at 25,000 meters going 1700 kps....what's your IntakeAir status at that level? Is it less than 0.21?

I see that you're using three Turbojet engines - so I automatically suspect asymmetric thrust prior to flameout.

16 Ram Air Intakes - five per engine. Yeah - suspect you're getting close to the operational ceiling of the jets.

Here's what you do: when the pull begins, throttle back to 2/3 and see if you can get it back on course. If you can, then that's definitely the issue you've got. If not, then something else is going on. Every time it starts to pull to the right, throttle back another tick. Eventually, you'll get to the point where you're no longer accelerating; that's when you light your rockets. Continue throttling back until you're no longer accelerating with the rockets lit. Then kill your jets, close your intakes, throttle up full and make the final push for orbit.

Three Jet engines and two Rapiers, actually. So yeah, I'm working over the operational limit of the engines. I shut down engines whenever a flame-out is imminent, or at least throttle down until it's no longer an issue. Sure, I sometimes get some pull to either side because of this, but this is not the issue. The issue is actually what I explained. After the outer tanks are emptied, the engines start to drain the fuel assymetrically, which slightly offsets the centre of mass, which produces as pull to the right which becomes stronger the longer I fly the plane, since the CoM offset becomes more pronounced.

EDIT: Album to show the issue:

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As you can see: everything's fine until the outer tanks are emptied.

Edited February 17, 2014 by Rodyle

[Claw]

I think it's because you're trying to split that hidden MK1 fuel tank. When you have an odd number of engines firing, and the fuel rules, your odd engine is causing your plane to draw fuel from the right inboard and hidden MK1 tanks and leaving your left inboard tank untouched (or less touched as the case may be). You can shut down this center engine or lock out the hidden MK1 tank as a mid-term solution.

You might need to rework your fuel lines a little so they aren't feeding backward and forward so much. As a minimum, try to change how your hidden MK1 is tied into your fuel system. It's usually less confusing to combine multiple tanks to a single point and redistribute, than to try to take single tanks and split or cross connect multiple times.

This might help too, although the fuel rules can be a bit complex.

[Claw]

In fact, I just removed the split from your hidden MK1 and connected it with a single fuel line to the bulkhead in front of it (inside the clipping). That fixed the asymmetric fuel feeding, although I didn't look to see what that does to your CoM during flight.

EDIT: Nice bird btw.

Edited February 17, 2014 by Claw

[Rodyle]

In fact, I just removed the split from your hidden MK1 and connected it with a single fuel line to the bulkhead in front of it (inside the clipping). That fixed the asymmetric fuel feeding, although I didn't look to see what that does to your CoM during flight.

EDIT: Nice bird btw.

Trying it out right now. It seems to work out so far. You have my eternal thanks for the link to the fuel rules (I'm the kind of guy who actually learned the pathing algorithms in Transport Tycoon by heart) and a proper mention in the description of the Roc.

[Claw]

Trying it out right now. It seems to work out so far. You have my eternal thanks for the link to the fuel rules (I'm the kind of guy who actually learned the pathing algorithms in Transport Tycoon by heart) and a proper mention in the description of the Roc.

Transport Tycoon = 2 thumbs up. You should check out Open-TTD.

Good luck with your craft!

[Rodyle]

Transport Tycoon = 2 thumbs up. You should check out Open-TTD.

Good luck with your craft!

Yeah. I basically am married to Open-TTD. Me and a few friends still play it on our yearly two-week LAN party.

Anyhoo: question answer.

[m4rt14n]

Yeah. I basically am married to Open-TTD. Me and a few friends still play it on our yearly two-week LAN party.

Anyhoo: question answer.

Open TTD is awesome. Anyways sorry for hijackimg thread. Lol.