Can I transfer fuel while burning the engines?

[cephalo]

So I'm building a huge mother ship for a Jool 5 attempt, and after many hours, I have detected a major flaw in my design. I have a row of Mk3 tanks where the nuke clusters were in a certain position, and fuel lines leading from the medial and lateral tanks to the tanks where they were connected. Anyway, at some point I had to move the engines to the outer tanks, and I forgot to switch the fuel lines, so they can only use the outer tanks.

Everything is almost ready to go after much effort, and I really don't want to re-launch all the pieces. I have enough for a decent burn in the outer tanks,(650 dv when full) but possibly not enough. Can I transfer fuel while burning the engines, or will that cause problems?

[Sirine]

Yes. You can transfer fuel while burning.

[Claw]

Sirine is indeed correct. And you can do it with multiple tanks at a time.

[Sharpy]

Interestingly, while you can transfer fuel while burning just fine (done this a number of times, even in my previous asteroid tug where I'd transfer fuel through the asteroid to external engines clawed to the surface, and at such a rate that about three tanks worth of fuel were burned by the time the tanks of the external engines were filled), you can't manufacture fuel while burning. Or more precisely, you can, but the engines will only burn with power corresponding to the fuel output of the ISRU, disregarding whatever they have in attached tanks. With random exceptions too. I was wondering why the poor thrust of my last tug, some 160kN. Switched the ISRU off, and got full 4000kN immediately.

[Captain H@dock]

Can I transfer fuel while burning the engines, or will that cause problems?

The only problem you can run into (and I can't remember if this is still in 1.0.x) is when the transfer has finished and you've got more than two tanks involved.

Assuming you're got 4 tanks (A to D). Your engine is burning the content of D, and you're selecting A+B+C+D, and you select IN on D. Now, A+B+C will equally transfer their content* to D, which is probably good to keep your craft balanced. However, once D is full, the transfer will stop for all tanks but one. You'll then have only one tank (say filling-in for the content of D that is being burned (to keep D full). This will probably unbalance your craft. That's something to keep in mind.

*This is tank flow dependent, but assuming same tank model.

[tomf]

Wait, you can have transfers with more than two tanks selected now? Does it take/add equally to all the tanks that weren't the one you started the transfer on?

[Sharpy]

Yes. Rightclick the first, alt-rightclick the rest. You can either draw from all into one with "IN" or dispatch from one into all with "OUT". The drain/push isn't equal - depends on tank size (filling Oscar-B from a Kerbodyne tank takes just as long as filling another Kerbodyne tank).

Problem starts with "many to many". With six external nukes, each on its own (small) tank and three big tanks in the central tug/miner, the transfer was looking like whack-a-mole on each of the three big tanks - as one would be depleted, 80% of the fuel would land in the remaining two.

[Padishar]

Interestingly, while you can transfer fuel while burning just fine (done this a number of times, even in my previous asteroid tug where I'd transfer fuel through the asteroid to external engines clawed to the surface, and at such a rate that about three tanks worth of fuel were burned by the time the tanks of the external engines were filled), you can't manufacture fuel while burning. Or more precisely, you can, but the engines will only burn with power corresponding to the fuel output of the ISRU, disregarding whatever they have in attached tanks. With random exceptions too. I was wondering why the poor thrust of my last tug, some 160kN. Switched the ISRU off, and got full 4000kN immediately.

Yes, this is a known bug that is related to a long-standing bug in the resource scanning code in the game. Lots of very good information has been passed to the devs recently relating to this bug (and various others caused by the same underlying issue) and I have high hopes that this will be fixed in the next version.