Fuel Management in SSTO Spaceplanes (Crossfeed Disabling?)

[wisnoskij]

Hello All. I am having huge problems with managing the fuel of spaceplanes with both LF only engines and LF+Ox engines. I have TAC Fuel Balancer installed, and one solution seems to be to set my rocket tanks to IN for both Ox and LF, but is there a stock solution to jet engines taking LF from your rocket tanks?

Edited June 7, 2015 by wisnoskij

[G'th]

If you're having problems with your center of mass moving, it'll be less complicated to use an empty RCS tank paired with a full one to help balance the plane out throughout the flight. Unless you have very, VERY heavy RCS usage during your flight, the loss in propellant in those tanks won't have as big of an affect on your COM compared to the draining of your fuel tanks.

Another good tip for SSTO's is to try and figure out how much delta V you want/need to get into orbit and then to carry out your mission. This number should represent all your dV requirements from the jet/rocket engine switch to landing. Depending on where you want to go, anywhere from 2k-3.5k dV will give you a good margin and some usefulness. Once you have this number figured out, all you then have to do is put in enough LF to bring your plane up to Jet engine cutoff. It requires a lot of playtesting and a more standardized ascent profile (IE, almost the same every time), but it will simplify fuel management if you set it up so that the LF drains without affecting your COM. (Which can be done by having the LF be right on the COM)

[Col_Jessep]

I use RCS Build Aid: http://forum.kerbalspaceprogram.com/threads/35996-1-0-2-RCS-Build-Aid-v0-7-1

The mod shows you the center of mass and the dry center of mass. Keep them close together. Try to design your entire spaceplane around one large center tank and run fuel lines from it to the engines if necessary. To avoid ugly, long fuel lines it is sometimes useful to pipe your fuel into the wings and from the wings to the engines.

[wisnoskij]

My problem is not fuel use unbalancing the craft.

All of my tanks are connectioned (I have three stacks of the second level plane parts), jets on the outside, reliant's on the inside. The jets take their fuel from the rocket tanks, then when I start my rocket engines, for some reason it cannot even access most of the fuel and promptly runs out of fuel to burn (that is can access). Is there a way to use fuel lines on already connected tanks to redirect which tank a particular engine will prefer to take fuel from?

[Red Iron Crown]

Fuel lines can direct which tank(s) a non-RAPIER rocket engine draws from, but both jet engines and the RAPIER do not follow the fuel crossfeed rules. They will draw liquid fuel (and oxidizer, in the case of the RAPIER in rocket mode) from all active tanks within a stage evenly. Stage in this context being defined as the number of decouplers between the tank and the root part of the vessel. Basically, there are only two strategies to manage which tanks these engines draw from:

1. Use decouplers between the tanks you want to empty first and the root part, even if you do not plan to decouple them.

2. Selectively disabling fuel tanks in the right click menu for them, then enabling them when you want them to drain.

[wisnoskij]

Ahh, interesting. I thought it was acting strange.