Help me understand this overheating issue

[PaleoGamer]

I'm playing around with a couple of designs and have found an odd overheating issue. Look at these two rockets.

SNAFU a

SNAFU b

These two are identical except for the the lower stage. The "a" variant has a Rocomax Jumbo-64 on the top and and a Rocomax X200-32 on the bottom. With the "b" variation the two tanks are reversed.

If I fly the "a" variant I can run the Mainsail engine at full throttle until the tank is empty with no problems. The overheat gauge appears but never passes the halfway mark.

If I fly the "b" variant at full throttle the engine fully overheats and explodes before I can reach 2000 meters.

Why does the overheating of the engine depend on which tank it happens to be attached to?

Edited May 31, 2013 by PaleoGamer

[Tokay Gris]

I have no idea what the reason is, but this is well known. I am not sure, but it could be even better if you used the smallest available tank directly on the engine, but I haven't tried this.

[RocketPilot573]

Yea this has been around for a while, just don't go to 100% throttle and you should be fine.

[PaleoGamer]

Huh. Didn't know that. I was trying to figure out what was wrong with my design and when I figured out the correlation it was a big WTF? moment.

OK, I guess I'll leave the X200 on the bottom for now. Thanks for the feedback.

[UmbralRaptor]

It's a bug in the heat transfer system -- if an object's center of mass is far enough away, there's no heatflow, even though the parts are in contact. The orange tank is the only stock part that I'm aware of that's large enough for this to be an issue.

[Temstar]

Unity's heat transfer from part to part depends on the distance between COM of the parts. There's a maximum distance for part to part heat transfer. Because the orange tank is so long it's COM is far away enough from COM of the engine that the engine cannot transfer heat to the tank, hence why hot engines (mainsail, LV-N) directly connected to orange tank tends to overheat.

The trick is to stick a grey tank underneath. If your only need about an orange tank worth of fuel then use the smallest 800L pancake tank.

[Finnerty]

I think its the structure of the big orange tank being weaker vs heat from the large rocket than the other 'apparently' all-metallic tanks.

[Joe_Bender]

I use a cat food can fuel tank between the orange tank and my main motor.

I also use struts to secure the two tanks together.

I place four struts from the cat food tank top to the orange tank bottom because you will get part wobble when you launch.

[triffid_hunter]

yep, stick an X200-8 between orange and mainsail, and you'll be able to run it at max throttle until you run out of fuel

[Jokurr]

Interesting, I never noticed this phenomenon. I've always just kept my throttle below 85% when using a mainsail.