Did anyone check how far engine flames reach?

[Sharpy]

I'm especially interested in 2.5m versions - Mainsail, Skipper, Twin Boar, Poodle.

I'm planning a miniature version of Claymore for Class C etc asteroids and I wanted to know how far to put the engine not to burn the asteroid.

[Sharpy]

Okay, I'll answer my question myself, as I got impatient and did some experimenting. I built a "floor" of wings, with a tower in the middle, with tanks and engines attached, facing the "floor" and fine-tuned the engine height for when the thing would take off, or refuse.

About all engines I tested (from Swivel to Mammoth - only Twin Boar defied the rule, for obvious reasons) act at a distance of about 4.5 lengths of the Modular Girder Segment XL  with very little variance - only Twin Boar required 7 segments from attachment node height, all the rest fit well within 5 from the engine attachment node (and most - except Mammoth - within the lower half of the 5) to the object they interact with. Interestingly, that's completely regardless of the flame animation - some will refuse to fly with the "floor" still far from the flame, while the larger ones burn clear across and fly just fine, moved a notch one way or another.

 

[Geschosskopf]

Of course, in real life, the asteroid would block the rocket at these (and most other) distances, so the whole idea of a tractor asteroid tug is rather gamey.  OTOH, if you push the asteroid instead of trying to pull it, then you avoid both problems.

Also note that KSP does not model radiant heat from flames at all, which is the only thing that allows any sort of tractor rocket to work in KSP.  If KSP modeled radiant heat from exhaust flames, then the central part of a tractor rocket would get too hot even though not directly in the path of the flames.

[pincushionman]

You can also angle your rockets out so they fire around the asteroid. Your separator can be significantly shorter that way.

This introduces cosine losses, yes, but cosine losses >> blocked exhaust.