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).

CAD drawing of simple rocket igniter, Day 3

So, here is a CAD drawing of my rocket igniter. The small grid spaces are one millimeter on a side. It will be machined out of a block of metal (most likely stainless, maybe steel or brass) using a drill press. I don't know how I'm going to do the nozzle, yet, though.

The top will be threaded for my 18 mm spark plug, and the two holes, one on each side, will be threaded for my orifices (the size of which I haven't calculated, yet... should finish that up tonight). I may also just drill the orifices directly in the metal, though I'll still have to thread holes for attaching the propellant hoses to them.

First sparks, Day 2 (continued)

Well, I went to my hackerspace TC Maker at the HackFactory (http://www.tcmaker.org/blog/hack-factory/), and got a small group of people brainstorming for me about how to drive the spark plug. Turns out there was a semi-broken neon sign in the corner. We hooked up the spark plug (in series to the rest of the sign that wasn't broken), and we got first sparks:

Here's the video:

Well, I accomplished all my goals for today!

For tomorrow: calculate orifice size and make CAD model of igniter.

L*, Characteristic Chamber Length, Day 2

One of the first things you need to do for sizing your rocket chamber is to find the characteristic chamber length (L*) for the propellant combination you've chosen. A list of values can be found here: http://www.braeunig.us/space/propuls.htm#engine
Excluding hydrogen peroxide (which needs a catalyst bed) combinations, the longest L* is 127 cm (typical values are between 50 and 100 cm, or so I've heard). Based on anecdotal searching around the internet, I get the impression that using gaseous propellants means your necessary L* is lower. I've heard values of L* around 80 cm (~31.5") being used for propane/air (though I think that's conservative), so let's go with that as a minimum.

Given a certain throat area and chamber cross section area, L* (L star) helps you find how long your chamber needs to be:
http://www.braeunig.us/space/propuls.htm#engine

L* is characteristic length
V_c is chamber volume
A_t is throat area.
So, we can re-write. that as:
L*=A_c·L_c/A_t
where: A_c is chamber cross-section area, which is fixed for us at ~pi·(18mm/2)^2 = ~2.545cm^2 = ~.3944 in^2
and
L_c is chamber length, which cannot be more than travel length of our drill press at 2.5 inches minus length of the spark plug into the chamber which is .75 inches or so, giving a max of about 1.75". Ideally, we'd want a little less than that so we can drill the nozzle straight through with the drill press, so ideally we'd want about 1.25" for L_c, though that's a little too short, as we will find out. (I still assume L_c is 1.75" in the rest of the section, here.)

We can rearrange the above to get:
A_t=A_c · L_c/L* = .3944 in^2·1.75 in/31.5 in = ~.02191 in^2
pi·r^2=area, thus 2·sqrt(area/pi)=diameter, so: 2·sqrt(.02191 in^2/pi)=.1670 inches, or a little less than 11/64" (but more than 5/32") for the throat diameter. This fits in with my previous Back-of-the-Envelope calculations, which gave me a throat diameter of about 1/8" (the older BotE calculations were based on conservative assumptions on the steady-state capability of the air compressor I have access to, which is something like 5 SCFM at 90psi).

So, I want to make my nozzle throat no wider than 11/64" diameter and use the full travel-length for my drill press. I think I'll go with 1/8" throat for a nearly 100% margin in the throat area (I can always bore it out larger). That's a pretty small rocket, but as long as it's flamey and makes mach diamonds and doesn't leave me broke or dead, I'm happy. :)

Introduction, Day 1

I am Chris. I live in Minnesota, have a B.S. degree in Physics, and am trying to introduce myself to the field of rocketry and aerospace. To do this, I am planning to first build the simplest (safest?) bipropellant rocket engine I can muster. It is really more of a rocket igniter, but the only real design goal is to achieve ignition and supersonic flow (i.e. see mach diamonds). I figured the easiest way to get started is to set the bar low enough that I can confidently expect to finish it soon and within my available budget but still gain experience with most of the systems needed for a more complicated rocket engine. I don't want to deal with liquid oxidizers right off the bat, for instance.

It will be small with thrust in the range of single digits pounds of thrust, chamber pressure of somewhere between 25 and 100 psi, will fit in a cube about 2 or 3 inches on a side, and will use gaseous propane for fuel and compressed air for oxidizer. It will not be cooled, but I expect it will probably run cooler than other engines which use a more concentrated oxidizer.

I plan to make it out of stainless steel hopefully only with a drill press as far as power tools go (I have access to better tools, but I do not yet have experience with them). It will be ignited by a sparkplug which is as large in diameter as the combustion chamber (about 18 mm). I will try to update this blog each day to try to keep making progress until I am finished. Today, I bought the spark plug, got a tap for the spark plug on order (with no commitment to buying it) and called the metal stock company to ask about stock. Tomorrow, I hope to get sparks from it.