Orifice sizes, redo CAD drawing for right dimensions, Day 4

Well, I didn't get the chance to calculate the orifice size last night, but I have done it today.

The formula for subsonic mass flow rate through an orifice is:

http://en.wikipedia.org/wiki/Orifice_plate#Calculation_of_expansion_factor
mdot is mass flowrate (kg/s)
C is the orifice flow coefficient (unitless), .61-.9 (we pick .7)
A₂ is cross-section area of orifice (m^2), ((.0265 in)^2) * pi = 1.42334131*10^-6 m^2
ρ₁ is upstream gas density (kg/m^3), 7.2 kg/m^3
P₁ is upstream gas pressure (Pa or N/m^2), 90 psi = 620528 Pa
P₂ is downstream gas density (Pa or N/m^2), 60 psi = 413685 Pa
k is the ratio of specific heats (dimensionless), 1.4 (for air at room temp)
http://www.google.com/search?q=.7*(.0265in)^2*pi*sqrt(2*7.2kg/m^3*90psi*(1.4/(1.4-1))*((2/3)^(2/1.4)-(2/3)^((2.4)/1.4))) = 0.00137946227 kg / s
So, the mass flowrate for air will be about 1.4 grams per second. A little more than that will be the total, since we have to add propane (which is going to be ~4% by volume). I should add that this is with using one of my existing orifices for the air. I may choose to use a larger (existing) orifice for air and use this smaller orifice for propane, but honestly I'm too tired right now to calculate any further tonight.

I also realized today that my chamber will be about 16.5 mm in diameter (not 18mm), since the recommended drill size for a 18mm diameter with 1.5mm threads tap is 16.5mm (or 21/23") according to this: http://www.carbidedepot.com/formulas-tap-metric.htm

That forced me to redo my CAD drawing. I also decided to make it not as thick to go with a cuboid of 1.5"x1.5"x2.75" instead of 2"x2"x3" in order to save a little on material costs. This will allow me to use bronze for about the same price if that turns out to be easier to drill. Bronze reported works better than stainless with liquid oxygen, though of course we're nowhere near liquid oxygen right now. You can't tell much difference in this picture, though. I've also started to learn how to program OpenSCAD, which is a CAD program which doesn't have a WYSIWYG interface, but instead allows you to model with a kind of programming language. This would make it pretty easy to change parameters automatically or with a formula, which may be handy in the future, though it's sort of limited, now. Reminds me of POV-Ray, which I used to use a lot "back in the day."

This weekend, I will try to nail down the orifice size for the propane side and trade my orifice options. I'll also take pictures of the orifices and possibly (if I have time) try making a mock-up of the rocket igniter using wood (allowing me to actually use the tools I will need to use for the metal one, allowing me to check clearances, etc).