Moving an asteroid - is this a bug?

[dborgo]

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?

Edited September 16, 2015 by dborgo

[p1t1o]

Sounds weird. Two things spring to mind, though they are both fairly obvious so I dont know, but just in case:

Fuel line definitely connected the right way? (Arrows pointing from small tank towards engine?)

Are you producing oxidiser at the same rate you are producing fuel? (Are both produced at once? I havn't done this type of mission since before ISRUs were invented...)

[Rath]

You need to have a bunch of fuel tanks to fill up while wait, however make sure to shut off the drills when ore tanks are full. The ISRU does not produce enough fuel fast enough to even run the LV-T 30 until the ore runs out.

This is not what you do:

___________

/ \

l l

l asteroid l

l l

l l

\________l__/

___ l Drill

l l ISRU

l___l

___

l__l Ore tank

___

/ \ Engine

Instead you need to do this:

___________

/ \

l l

l asteroid l

l l

l l

\________l__/

___ l Drill

l l ISRU

l___l

_____

l l

l l Fuel tank

l l

l____l

_____

l l

l l Fuel tank

l l

l____l

_____

l l

l l Fuel tank

l l

l____l

___

___

l__l Ore tank

___

/ \ Engine

Edited September 16, 2015 by Rath

[nekogod]

I actually believe this is a known bug, if you fire the engines whilst the ISRU is active you get something like 1% thrust. I don't believe it's because it's using the small amount of fuel being made I think it's simply ISRU on = no thrust.

[Rath]

Hmmmmm....

[KerikBalm]

Bug, don't fire enginges while ISRU is operating...

[Padishar]

Yes, this is a known bug. Hopefully this (and a number of other related bugs) will get fixed in the next update.

[Mister Dilsby]

I've had this happen with my Kranefly, which uses 6x LV-Ns. With the ISRU running, one of the LV-Ns would lose thrust and the ship would pitch/yaw away from true. If I shut down the offending LV-N, a different one would lose thrust!