If the Kerostart turbine spits fire during the start, reduce the pump start voltage! The distance depends on the tube diameter verses the tailpipe dia as cool air needs to pass around and through the tube. As a general rule, as I said 25-30mm is a safe distance with the temperatures as a concern.
Obviously if the starter slips, the ECU battery is weak or the pump starting voltage is too high the turbine will spit flame, as long as it is being blown down the tube and not getting the tube blue I'm happy enough.
Open duct setups suck in my mind, some get away with it by using a cover/deflector. Minimum need is a decent front cone

Dw