Does anyone know the optmimal distance between the turbine exhaust and the start of the jet tube? I am building a PJP Jets 1/6th scale Hawk with a Simjet 75 N turbine?

Optimum distance varies with turbine and install. Best bet is to make a thrust tube un which the location can easily be adjusted, and to work out best thrust/temp ratings by trial and error. Will cost about 2 liters of fuel and half an hour work, but will set you up with the distance that works best your specific plane.

Best regards
Hank

Thanks Miniflyer, I will try that.

what is it the 1200?

keep about 3 fingers of space between pipe and end of exhaust

Is that the end of the exhaust, or the end of the exhaust cone? A 7N is a 17.5lb thrust turbine.