It seems I have discovered the most efficient fuel tank for LV-N.

[Sharpy]

All the LF+Ox tanks have ratio of 9, but emptying them of oxidizer that drops the ratio to pitiful 4.6 The best LF tanks have full:dry mass ratio of 8.00280112

Then there's this:

While the necessary ISRU converter adds 4.5t and the one gigantor needed to run it adds another 0.3t, and you do need a small buffer tank to store the processed fuel (even if it's a tiny Oscar), the pure Large Holding Tank weight ratio is 17:2 or 8.5.

That's still not the same as dedicated mixed fuel tanks, but that's half a point better than the best dedicated LF tanks.

The break-even point with best LF tanks is at 32.9 Large Holding Tanks.

33 full ore tanks, ISRU and a gigantor give 56.1t wet mass, 4 950 units of ore, or 9900 units of LF.

Comparing, Liquid Fuselage Long is 10 000 units of LF at 57.14t wet mass.

Any ore tank above that is pure benefit.

If your LV-N powered mission is to take more than 10,000 units of fuel (more than one Liquid Fuselage Long) or you decide for other, non-optimal fuel tanks, seriously consider taking a refinery instead.

[katateochi]

there's a problem with this approach, there's a bug (what? in KSP....no!) where you can get uneven thrust (if you have multiple engines) if you run the converter at the same time as running the engines.

more on the bug here

[Sharpy]

The ISRU converter produces only 0.9 fuel/s while the engine can consume over 1.5 - of course if there's not enough fuel, it may not be distributed equally.

OTOH, ISRU+LV-N asteroid pusher, that's an ingenious idea.

[Laie]

The ISRU converter produces only 0.9 fuel/s while the engine can consume over 1.5 - of course if there's not enough fuel, it may not be distributed equally.

Nope. Look at the example pictures in the bug report: there is enough fuel to keep all engines running, yet turning on the converter leads to uneven thrust loss.

[katateochi]

The ISRU converter produces only 0.9 fuel/s while the engine can consume over 1.5 - of course if there's not enough fuel, it may not be distributed equally.

OTOH, ISRU+LV-N asteroid pusher, that's an ingenious idea.

The bug isn't about insufficient fuel being produced to keep up with the engines, even if you have a big reserve of fuel it causes some engines to run at reduced thrust while the others carry on running at full.

But yes, aside from that bug, ISRU + LV-N astro-tugs are great. It's actually made asteroid capture missions much more fun compared to how they were before, and a tug like that can keep operating indefinitely, fetching asteroids back to LKO without ever needing extra fuel brought up to it.

This is my current ISRU + LV-N astro-tug http://kerbalx.com/katateochi/AstroHopperMKII

[Red Iron Crown]

I think you may need to review your math a bit. According to my numbers:

Best LF tank is the Mk2 Liquid Fuel Fuselage, with a mass ratio of 8.017544.

Ore tank has a mass ratio of 8.5.

Breakeven point for using Ore tanks+ISRU+2 Gigantors vs Mk2 LF fuselages is at 534.5t of propellant.

This analysis ignores needing some LF tankage to store output (this could be quite small, though), and needing more than one ISRU to keep up with even one nuke at full throttle.

[Sharpy]

33 tanks of 15 tons is 495 tons. Which converts to 4950 units of ore, or 9900 units of fuel. Probably your second Gigantor pushes your math to 534 tons.

One more advantage: versatility. Need more thrust? Produce oxidizer. Need some monoprop for RCS? Here it is. Need to go light into reentry? Jetison the ore.

Damn, during my last SSTO reentry I really wished for a "jettison fuel" option.

[Red Iron Crown]

I was thrown a bit by "33 full ore tanks, ISRU and a gigantor give 56.1t wet mass". Actually calculating it out like above makes it much closer than I thought, I guess the differences are the extra gigantor and using the ever so lightly more efficient Mk2 tank.

[mikegarrison]

This obviously shows that the stock game needs the ability to select the fuel mix in fuel tanks. There is no way that it should be more efficient to bring all the raw materials and refine your own fuel enroute than to bring already-refined fuel.

(The self-fueling asteroid tug, on the other hand, is working as intended.)

I'm using the Interstellar Fuel Switch mod (necessary for Nertea's suite of mods), and it just feels right to be able to set up any tank I want as all-LF if I want it.

Alternately, stock could just add a bunch of LF-only tanks, but with the game architecture that would just load up more memory and clutter up the build menus.

[ajburges]

Egads! The large ore can does have a better empty mass percentage than the Mk2 fuselage.

Still, that little bit of efficiency (less than 6%) would be quickly gobbled by the 4 tons of ISRU module and any support modules. You need about 33 tanks to break even! At that point, you won't be able process ore fast enough to meet engine demands.

This is only useful if your mission calls for the ISRU to begin with. That's limited to SSTA, asteroid redirect, and other infrastructure independent vessels.