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AeroGav

Make fuel drain mode switchable !

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The past few weeks I've been trying to make spaceplanes for other users on Kerbal X.    As such I've been restricting myself to designs that require no mods whatsoever to fly, but the problem I'm hitting time and time again is fuel balance.

As you may be aware , all jet engines in game drain fuel evenly , from every tank in the vessel.  If your plane is down to 50% internal fuel,  every tank onboard will have 50% of it's full capacity, regardless of size.

Eg.  your FAT-455 airplane wing will now have 300 LF out of 600 max capacity,  whilst your mark 0 fuselage will now have 25 out of 50.

Adjust your airplane so that there's as many tanks ahead of CG as there are aft of it,   then everything balances perfectly, full to empty with no handling problems.

FULL TANKS

20160501191726_1_zps7ein3a7f.jpg

EMPTY TANKS 

20160501191804_1_zpsypyrh0m2.jpg

*this plane was modified off a design Bewing sent to me, original here

Spoiler

 

20160502072932_1_zps1mwbypja.jpg


 

Now the problem is, when you fire up the NERV or any rocket engine at all , for that matter.  Rocket engines only take fuel from the stack of tanks immediately ahead of them, radially attached wings or tanks on other stacks are ignored.   Well , if that was the only problem you could run fuel ducts, though given how bugged the drag  from these are, it's not great.  But I've made some designs that kept ducts to an absolute minimum.

But the real show stopper is that the rocket engine drains the tanks sequentially.    It looks for the furthest away tank that's on the same stack as it, and empties it completely before taking so much as a drop from the others, then moves on to the next one down.   So, by emptying tanks at the front first, the plane's CG moves aft, behind the CoL and suddenly it wants to do backflips.

Other than using mods, what can you do about it?

  • use a single large tank per engine. I suppose for LF/O engines you have options.  You could mate one FT800 per Terrier engine and that'd be about the right compromise between fuel endurance and thrust for a spaceplane.  If LF only however, the biggest we have is the 400 capacity mk1 fuselage tank.  The mk2 has an 800 LF fuselage tank but it's the wrong diameter and much draggier.
  • fuel ducts.  Draggy !  Also, the tanks have to be the same size, because unlike the jet engines,  ducts don't take account of how big the tank is that they're pulling from.  It will pull (for example) 3 fuel per second from each tank whether it's a little Oscar B or a Kerbodyne.
  • locking tanks and manual transfer.    TBH,  this arrangement works pretty well if you're keeping Rick Wakeman,  Tony Banks, Geoff Downes or other legendary keyboardists chained up in your basement,  they can work all that for you while you operate the flight controls.   Unfortunately the food costs were breaking me and i had to let them go.

So devs, how about a nice little toggle on each engine so we can set fuel drain mode between sequential and balanced

I can sort of see why sequential mode exists for rockets..  gravity feed and all that, it assumes you're combining three FT400s into a single large tank, but it's not the same if the tanks are lying horizontally in an airplane.        Even in a rocket,  do the tanks not have internal baffles, multiple pick up points etc?

Seems to just be an artificial way of increasing difficulty in the VAB on early career mode.  Make us want to spend science points to unlock larger tanks even though they're exactly the same in terms of dry/wet mass ratio,  diameter, heat tolerance and fuels carried.

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In the meantime i've had another go at working around the problem in a bone stock SSTO.      Balancing the fuel tanks fore and aft so that CG doesn't shift with the balanced fuel drain of the jet engines is something i can do already - see the above post for the SPH pics of my craft fuelled and defuelled.

But what of the NERVs?

I'm going to attach them to the largest LF tanks i have access to , the FAT-455 airplane wings.   That'll be the only ducts i'm using for drag reasons.   On the front node of this engine we can also fit a single FT400.

The first thing I had to do was find out where the CoM of the wing itself lay.  This was a process of trial and error,  moving the wing back and forth until i found the location where emptying or filling the wing tank would have no effect on the vehicle's CoM.   It turns out the wing's CoM lies halfway between the rear spar and the trailing edge.   You'll notice that placing the wing in such a way is going put our CoL too far forward.  We'll need substantial lifting surfaces to the rear to correct for this if we're building a canard. 

20160502195735_1_zpstohi38sh.jpg

The NERVs will also pull from the 400 LF fuselage tanks mounted to their front node when the wings are empty.  Being a regular shape, the CoM of this part is more obvious.   I arranged things so that the CoM of these tanks and the main wings and the vehicle itself are all in a line.   The NCS adapters at the front of the stack are locked,  they can be part of our minmus injection fuel once orbit is made.    1000 LF per NERV should get us to orbit.

I built some "angle iron" tail booms out of Big S strakes to give both a rear lift surface and a yaw stabilizer, but they weren't near enough to correct for the wing placement,   so a horizontal wing plank was added to the top.   Another strake to seal up the unsightly gap between front fuselage, sponsons and the canard, and it's actually looking a lot nicer than i thought it was going to , for something based on a random combo of mk1 fuselages and airliner wings..

20160502203703_1_zpsnkbwg8k9.jpg 

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