Is there a break-even point between lf only tanks and ISRU?

[Vectura]

Wasn't entirely sure how to word the title. I'm not great with maths, so I'm asking. Say I have a situation where I'm using a nuclear engine, whether the stock NERV or a modded nuclear engine, which is heavy, efficient and liquid fuel only. For the sake of part count and tech tree unlocks, I want to use LFO tanks with the O drained out rather than loads of the aircraft tanks. I also don't want to install a tank contents switching mod.

In this situation, for a large long-distance orbital mothership, is there a point (likely a mass boundary) where it would be more efficient to take a tiny tank and loads of ore/ISRU converting to LF, rather than LFO tanks with the LF drained?

 

Edit: to clarify, I don't mean mining along the way, just carrying a converter and ore tanks instead of drained O regular tanks.

Edited July 30, 2018 by Vectura

[bewing]

No, 1kg of ore converts to 1kg of fuel. Ore is twice as dense as fuel, so it stores in a smaller space -- but that doesn't buy you anything. So you are never better off converting, because you have to include the added mass of the converter. (unless you mine along the way)

 

Edited July 30, 2018 by bewing

[Ultimate Steve]

36 minutes ago, bewing said:

No, 1kg of ore converts to 1kg of fuel. Ore is twice as dense as fuel, so it stores in a smaller space -- but that doesn't buy you anything. So you are never better off converting, because you have to include the added mass of the converter. (unless you mine along the way)

 

He appears to be referring to the fact that ore tanks get a better mass ratio then LFO tanks filled only with LF. Ore gets 8.5 and LFO with just LF gets only 4.6.

I tried to make a graph but it didn't work and it confuzzled me, but @Vectura I *think* I wound up with 10.9 tons, as in if your ships mass excluding fuel tanks exceeds 10.9 tons then you should add the 5 tons of ISRU equipment for optimal delta-v (don't use the small one, it's bad, and you'll need solar panels and radiators so that gets 5 tons).

 

Keep in mind that you'll need a rather large fuel tank even if you use the ISRU because the ISRU is slow and you'll want to be able to use the engine for more than a few seconds at a time.

But really, I think it would be easier just to unlock the Mk3 tanks, just grab a few biomes of science from Minmus, with just the materials bay, mystery goo, thermometer, barometer, surface sample, crew report, eva report on the surface, eva report just above, you can get 490 science per biome. If you have the seismic accelerometer and/or gravioli detector then it's more.

Edited July 30, 2018 by Ultimate Steve

[Vectura]

Thanks for the reply. Unfortunately, I have already grabbed most of the minmjus science, I am playing on a low-science multiplier save. To get the spaceplane tree unlocked I’ll need to go interplanetary again.