paaiyan said: Ideally, once the projectile reaches the tip of the barrel, the pressure should be at its maximum point and should then immediately begin to contract due to cooling Not really. The projectile will continue to accelerate after the pressure in the gun has peaked. The spud doesn't start ...

Ya know, you can buy a hydrogen / oxygen generator from Estes.... Or, do it the way I think the Estes system does, couple volts from some C batteries, steel (zinc plated?) electrodes, sodium ascorbate or sodium citrate as the electrolyte (and base). <a href="http://www.estesrockets.com/products.php?...