[RocketsNW] Rockets Digest, Vol 10, Issue 13

[Paul Bogdanich]

"Graphite or glass composite pistons molded to match the inside of the 
glass body tube and then sealed with an O-ring in a simple sawn and filed 
groove can maintain 3 or 4 thousandths radial clearance, which is the 
recommended clearance."

Maybe so but this is where you get into the expansion rates of dissimilar 
materials and the heat and pressure of the ejection charge itself 
distending the body tube.  For systems with dissimilar materials (in my 
case quantum tube for the rocket body) I found that as rigid a piston as 
possible, slightly tapered (2 thousands per inch 5 thousandths a side 
maximum) at 10 thousands clearance at the tapered end is safer.  P.S.  I 
found out the hard way after following the "recommendations!"