The buffer depot allows for 100 transport drones. When you set one up with all 100 drones, it will send a convoy of fuel trucks to fill it up, proportional to that number of drones. The problem is that the fuel capacity of the buffer depot is the same as that of the request depot, 10000. With the techs unlocked to upgrade drone capacity, it's easy to go over this limit and then fuel is destroyed and wasted. Especially if you use the startup settings to increase fuel consumption/capacity at all. With the request depot's 20 drones it's much harder to exceed that limit, and it won't happen with default settings, but with a buffer depot's 5x drones it happens much more easily.

I tried to override this limit in a mod, but make_request_recipes is called in the final-fixes stage, so there is not much that can be done (if transport drones is not a dependency that's guaranteed to load first). Perhaps the fuel in a request recipe should either be a separate parameter, or it could be a simple formula that scales with the fluid capacity settings of drones and is adjusted to an amount that will not overflow even for a buffer depot.

Hmm, looks like I was wrong about request depots. I'm not sure what I was doing earlier, but they can fit 100 drones also, or 200 for fluid recipes. So this issue actually applies equally to request depots too, not just buffer depots. So if fuel per drone is >100, then it will overflow the fuel storage amount anytime it refuel back to drone amount * fuel per drone. And I'm not sure about edge cases where the amount of fuel carried per truck causes it to round up even higher than that, so ideally there would be a safety margin above the normal request amount.

The reason that I am trying to use non-default fuel values is that I'm trying to port this to be compatible with my mod, which has limited options for liquid fuel. There is no atmospheric oxygen to combust it with, and it has to be just 1 liquid so oxygen can't be provided separately. To get around that, I'm having the drones be steam powered, but the steam is lower in energy density than vanilla steam or petrol, so it should require a bit more of it.

Does adding Transport Drones as an optional dependency work around the problem (ensuring that TD is loaded first, if installed)?