Rockets and sparks?

In my decade of using and building J-tube rockets, I have never seen any sign of a spark exiting the top of a flue pipe or chimney.

And I have stressed tested all the units by, for example, adding lots of dry pine to the test burns. LOTS!

Have you ever seen sparks exit your flue pipe? (I ask because someone I know is concerned about setting their thatched roof on fire if they install a rocket).

More specifically, let's limit things to an 8 inch system with a single drum heat radiator, with an elbow out the bottom side of the barrel into the flue pipe. I.e. no bell. A bigger brother version the one in the picture below.

In your opinion, what is the potential for sparks to exit the flue of an 8 inch version of the pictured 6 inch unit?

Hi David;
I ran our 8" J tube for seven years in our greenhouse and never witnessed any sparks leaving the stack.
Nothing but steam or heat shimmers  out the chimney.

I can’t imagine sparks coming out of one, you can always put a screen around it, just like spark arrestors  on a chainsaw. On a regular wood stove the screens get plugged up often from creosote, but I can’t imagine this happening on a rmh