Spaceplane Fuel Feed question

[Anton P. Nym]

I'm having the damnedest time getting the fuel feed to work correctly on this bird, and I'm frankly baffled so I'll throw this out to the group to see if there's a solution.

The above is intended to be a crew shuttle to and from LKO. Jet fuel tanks are numbered and rocket fuel tanks are lettered. Fuel lines go 1-2, 4-2, 5-2, 3-B, 3-C, B-A, and C-D.

Before takeoff I shut off flow from tank 1 (as a reserve and counterweight against CoM shift); the intended flow is that tanks 4&5 get drained first, then tank 2, and then a bit of tank 3 before orbit insertion. The aerospikes should drain tanks B&C first, then A&D. After re-entry (on dry rocket tanks, preferably) it should run on tank 3 until nearly dry, then I can open up tank 1 if necessary to bring the bird home.

What happens is that tanks 4&5 end up feeding the centre turbojet alright, but the port and starboard turbojets feed off of tanks B and C respectively unless I shut off fuel feed to B and C. If I do shut off B & C fuel flow runs as intended during jet flight... but that leaves me with only half my rocket fuel available during orbital insertion unless I risk asymmetric fuel loading in-flight and switch the tank flow on during the burn. (Not to mention that the aerospikes first drain from tank 3... but I can solve that problem by switching 3 off during rocket-powered ascent.)

Is there a way to fix this and get the fuel to flow as planned?

-- Steve

edited to add: The solution turns out to be putting in-line separators on both ends of tanks B and C, then re-running the fuel lines 3-B and 3-C so they directly feed the turbojets. Adds a bit of weight, but not enough to hamper the flight.

Edited August 20, 2013 by Anton P. Nym
solution found and recorded

[Ekku Zakku]

Have you tried running fuel lines from 3 to each of the side turbojets? Regardless, it shouldn't matter as all the liquid fuel your jets can access are in 1 through 5, B, and C, and all the liquid fuel + oxidizer your aerospikes need access to are in A through D, and everything available flows to where it needs to go. Unless 4 and 5 are drop tanks and you need to drop as early as possible, I don't see the problem.

Also, great design! It looks beautiful, very well made =3

Edited August 20, 2013 by Ekku Zakku

[Johnno]

The fuel logic often gets confused if you go from out-in-out or in-out-in. Solution would be to remove fuel lines 3-B and 3-C, separate the jets from the rocketfuel tanks and then give them fuel lines from 3. You could either stick a decoupler/separator between the tank and jet, or mount them onto tank 3 with cubic octostruts or radial connectors.

Another option (but more work and potentially changes your CoM too much) would be to swap places on 4&B + 5&C, and making sure that they're not stacked (connected to eachother). Then your fuel lines from 3 should work fine and you could just connect B-A and C-D.

Then again, do you really need 4&5? You could swap them for cooling thingies or intake tubes, or just structural fuselage pieces. If you want to keep the RAM instakes where they are now. 2&3 should be more than enough to feed 3 turbojets, my 3 person SSTO has 4 engines on 2 of those tanks.

[Anton P. Nym]

Have you tried running fuel lines from 3 to each of the side turbojets?

That was my first reaction; despite hooking the fuel lines directly to the jets, they still preferentially drew from B & C.

Unless 4 and 5 are drop tanks and you need to drop as early as possible, I don't see the problem.

The problem is that unless 4 & 5 are empty plus 3 still contains fuel, the bird's CoM during rocket ascent is ahead of its CoL and it becomes tail-happy. Nearly lost crew to that...

Also, great design! It looks beautiful, very well made =3

Thank you!

The fuel logic often gets confused if you go from out-in-out or in-out-in. Solution would be to remove fuel lines 3-B and 3-C, separate the jets from the rocketfuel tanks and then give them fuel lines from 3. You could either stick a decoupler/separator between the tank and jet, or mount them onto tank 3 with cubic octostruts or radial connectors.

I may consider octostruts onto tank 3, moving the RCS tanks to the rear of tanks B & C to make space, or perhaps putting separators onto B&C and then mounting the jets. I hadn't thought of either solution on my own.

Another option (but more work and potentially changes your CoM too much) would be to swap places on 4&B + 5&C, and making sure that they're not stacked (connected to eachother). Then your fuel lines from 3 should work fine and you could just connect B-A and C-D.

That one I did think of myself, but as you point out CoM fears made me write that off. I may end up trying that if the above doesn't work.

Then again, do you really need 4&5? You could swap them for cooling thingies or intake tubes, or just structural fuselage pieces. If you want to keep the RAM instakes where they are now. 2&3 should be more than enough to feed 3 turbojets, my 3 person SSTO has 4 engines on 2 of those tanks.

I may be an iffy pilot; the couple of times I've put this bird up into orbit and then juggled with the fuel flows manually, 2, 4, and 5 all ended up dry and 3 was lightly tapped (20 or 30L consumed IIRC.) My first attempt didn't have 4&5, so tank 3 ran dry and the resulting CoM shift made it uncontrollable when I lit up the aerospikes. The quandary was that I could just burn 1 on the ascent, but doing so shifted CoM too far aft during ascent and risked instability before transition to rocket-powered flight.

Anyway, thanks for the food for thought; it's given me something to try out.

-- Steve

[Ekku Zakku]

Ah, then yes, put decouplers or something in between B and C and their respective turbojets, then the fuel lines running to those turbojets from 3 would work fine.