At the altitude of the North Site, What is Mach 1 Speed?

My 'Plan B' on a Pro38 J285, using an altitude predictor sims
at 636.6 MPH. / 2779 ft.

It is a strong Bird.

Should a vent hole be drilled?

Should the vent hole be drilled above or below the piston?

What size hole?

Any other considerations?

Thanks Mike

This kind of snuck up on me. Learning J and K is thrilling 😯

Mach varies with a number of environmental factors... air density, temps, etc. I always use 750mph as a round figure. There is a calculator at http://www.grc.nasa.gov/WWW/K-12/airplane/mach.html

Most altimeter manufacturers suggest setting mach delays at about 0.8 mach and higher. I tend to always set them, just-in-case.

Holes... I don't think I have a single rocket w/o vent holes. I typically use a 1/16" or 3/32" hole and I place it where it can't be blocked. I usually go above the piston. If the piston does creep up, the air will still bleed out.

As you go higher and faster, don't forget that you transition thru mach TWICE -- once busting through, and the other time slowing down. Keep the 0.8 mach scenario in mind up above.

Bottom line - my advice is to always vent, always use shear pins (a whole 'nother topic), and "go long" on the mach delay. If you think you need 3 seconds, do 6 or 8. At the end of the day, mach is no big thing if your rocket is robust. M2 is where the real boogeymen come out 😯

0.8 of Mach

Thanks....Holes drilled.

Based on the data from my Cirrus Dart, mach is 730mph when 4000 feet above the north site, and 745mph when near ground level. This is of course only true in similar weather conditions, about 85F and dry (not to mention the air pressure).