I'm curious if I encountered a bug, or some weird design feature in the game. I met up with an E class asteroid while it was passing Kerbin on a mission to get it to Ike. This is only the second asteroid I have captured, with the first being a class B to bring into orbit around Kerbin (no problem there). Knowing this bigger mission would require much more delta V to be on hand, I designed a tug that would mine the asteroid to replenish fuel. My particular design has a small fuel tank just below the drills and ore containment, followed by the ISRU, and then more fuel tanks connected directly to the engine. There is a fuel line connecting the first fuel tank across the ISRU converter, so fuel will be drained from it as well when the engine ignites. In fact, this small tank is the first tank to be drained by default. It is also the quickest to fill with the ISRU producing fuel because it is so small. Here's the weird bug(?) I started mining the asteroid as soon as I rendezvoused. I fired my engine to change the orbit of the asteroid to intercept Duna's orbit, but after initially seeing an expected change in apoapsis and delta V, the rate of change suddenly fell to minimal. What was expected to be a 2-3 minute burn, was more like 4-6 hours. I thought something had borked with the game. I quit and reloaded a couple times, but nothing changed. My engine was producing minimal thrust despite any throttle changes, and set to 100% thrust. This was a "Mammoth" 4000 thrust beast. No way it should take 6 hours to make a 70 m/sec delta V change, even with a class E asteroid. Through a process of elimination, I found that the first fuel tank, being the smallest, had filled completely with fuel and was not draining at all despite 100% thrust. I was still actively mining the asteroid at the time. When I stopped the mining, suddenly I had full thrust available again. It seems that when that small fuel tank filled completely, the engine was only consuming the small amount of fuel actively being converted from ore... a trickle. Is this a bug others have seen?