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UnusualAttitude

Spaceplane fuel flow issue.

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Hello All,

New player here. I'm from the flight simulation crowd. I bought KSP a couple of months ago and I've been having a huge amount of fun designing and flying spaceplanes, so thanks for the inspiration and advice of all the SSTO designers on this forum.

In my career campaign, I got to Mun and Minmus using small landers that fit neatly into the Mk2 cargo bay, but I was quickly restricted by its limited size so I designed the ExHaul class twin-boom spaceplane to lift unshielded payloads. Here she is delivering Station Alpha to Kerbin's North Polar Icecap.

Wpual0V.png

Designed and flown using NEAR, she's a bit of a pig to handle but she will lift 15 tonnes to LKO, and at least 6 tonnes to Munar orbit.

However, she has a small fuel flow problem, as demonstrated by this static burn...

F8O0UPv.png

Firing just the RAPIERs. Forward tanks are locked off as reserves, but with all other tanks open, the forward bicoupler tank drains first on the port boom whereas the aft bicoupler drains first on the starboard boom, creating a fuel imbalance. This can be checked by locking one tank or the other, but it's an annoyance to have to worry about this during a long burn into orbit:rolleyes:

Is there something I'm missing about the way fuel should flow ? Any ideas or fixes ? Will provide more pics or download if required, thanks a lot in advance.

Using; NEAR, Kerbal Engineer Redux, KAS, Fine Print.

Cheers, UA.

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Use some fuel lines to connect all the fuel tanks in a closed circle. That should make them all drain evenly. Just remember to always have your fuel supplies distributed evenly across the empty centre of mass. You really don't want it to have shifted when you re-wnter the armosphere.

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A useful resource: http://forum.kerbalspaceprogram.com/threads/64362-Fuel-Flow-Rules-%280-24-2%29

The TLDR version is that fuel always drains from the farthest tank first, and you can use fuel lines to manipulate which tank is farthest. In this situation, though, you're getting screwed by a symmetry bug. The bicouplers seem particular prone to attracting them.

You can do assorted design tricks to get around this, or you can just take the easy way out: TAC Fuel Balancer (good) or Goodspeed Fuel Pump (better, but a tiny bit more complex).

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Since you've already modded your game with NEAR I'll assume you're not afraid to mod it slightly further.

TAC Fuel Balancer will not permanently solve your fuel balance but it can keep it balanced during flight.

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Thanks for these suggestions. I was hoping to avoid the use of too many fuel lines, as they really do spoil the looks of a spaceplane (not that she's a particularly pretty spaceplane in the first place!). I will look into TAC, as the ability to dump fuel is appealing for landing after an aborted launch, although not very environmentally friendly...

By the way, I'm considering going from NEAR to FAR. Do you think that this sort of unshielded payload solution would work, or would she start falling to bits as soon as I go through Mach 1 ?

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Thanks for these suggestions. I was hoping to avoid the use of too many fuel lines, as they really do spoil the looks of a spaceplane (not that she's a particularly pretty spaceplane in the first place!). I will look into TAC, as the ability to dump fuel is appealing for landing after an aborted launch, although not very environmentally friendly...

By the way, I'm considering going from NEAR to FAR. Do you think that this sort of unshielded payload solution would work, or would she start falling to bits as soon as I go through Mach 1 ?

Should be fine, although you might lose some of the radial bits off the payload if you go too fast too low.

In most respects, FAR is actually easier than NEAR. NEAR is simpler, but simple doesn't necessarily mean easy. The mach effects of FAR make it a lot easier to slow spaceplanes down during reentry.

The only legitimately tricky bit of FAR was aerodynamic failures, and that is much less of a threat now with the new wing strength tweakable.

As mentioned, I'd take Goodspeed over TAC-FB. They're both good, though.

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OK, I'll give FAR a try then. Thanks again WanderFound, your content is a particular source of inspiration for me and many other spaceplane builders, I'm sure.

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As for payloads that large, you will definatly want to switch to an inclosed cargobay like the ones in B9 expecially if you are going to use FAR.

I run FAR, and have had quite a bit of experience designing SSTOs in FAR. I have put over 120tons into orbit inside the cargobay of one of my largest SSTOs.

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