Whats the deal with mainsails and jumbo tanks overheating?

[Merinsan]

It's an issue with jumbo tanks themselves. I've had atomic engines overheat and explode on me when stuck to them.

I eventually solved THAT by putting the engines on pylons, with little fuel tanks between girder segment and engine. It also means I can add more engines for shorter burns.

I've done the same thing with the nukes. I was doing a transfer burn to Duna and watching on the map screen, suddenly there is a bang and the Gs drop to 0.

From now on, I just put the small tank on the bottom of them by default - problem solved.

[wilt57]

If you have to throttle your engine back anything less that around 90% for any appreciable amount of time, rebuild your rocket. You have too much thrust. There is nothing wrong with having too little TWR to reach terminal velocity and/or keep it there for any appreciable amount of time. If you are looking to launch payloads most efficiently, do not focus on lowering your D-V spent, focus on raising your payload fraction. The largest portion of mass that reduces payload fraction is fuel. Reduce the fuel needed, raise the payload fraction, lower the fuel cost (even though cost means nothing right now). Balancing gravity losses with the mass of the extra fuel and extra engines to reduce those loses is the key, IMO. I'll gladly take more gravity loses if my rocket still gets to space burning less fuel.

[Kerba Fett]

I'm surprised that nobody has ever released a "heatsink" part to place between the engine and fuel tank to address this issue. While functionally identical, a little finned spacer seems more logical way to deal with overheats than a little fuel tank under a big fuel tank.

[Wayfare]

However fuel will only be transferred in the stack, not sideways so if you put anyting on the side of you stack you need fuel lines to transfer fuel.

Not always. The MOdular Mission System's propulsion modules use nuclear engines mounted onto Jumbo fuel tanks with radial engine bodies, and the fuel flows to them without fuel lines just fine. On the other hand, this won't work with many other radially mounted parts, most notably fuel tanks. Yeah, I know. KSP's fuel logic is a little opaque

[littlebattler]

Not always. The MOdular Mission System's propulsion modules use nuclear engines mounted onto Jumbo fuel tanks with radial engine bodies, and the fuel flows to them without fuel lines just fine. On the other hand, this won't work with many other radially mounted parts, most notably fuel tanks. Yeah, I know. KSP's fuel logic is a little opaque

just FYI, other things that automatically transfer fuel are the inline ram intake fuselage and engine attachment pylons (abusable bendy hinges). Wings may or may not do it, I don't remember.

[Kenobi McCormick]

I can not use Mechjeb even if I wanted to ... which I don't ... to launch things because it will put the thrust to 100% and blow up my rocket. .

Wrong. Mechjeb can and will prevent overheats just fine. It will ride the fine line between explode and not exploding, giving you the most performance you can get without overheating the engine to the point of failure.