First test firing! No continuous ignition.

This is the first test of a rocket igniter (i.e. small rocket engine) running on compressed air and propane. Using compressed air as the oxidizer is notorious for causing difficulty in keeping a rocket engine lit, as you can see here where the rocket only ignites a few times (by the spark plug) and never stays lit for any length of time. Basically, it appears like the spark just ignites what's in the ignition chamber before it all goes out. I tried some nozzles slightly smaller and slightly larger, and one just very slightly smaller seems to work best. (I also tried increasing the spark frequency from 31Hz to 122Hz, which helped slightly.) All of them seem to work best after the chamber becomes warmer after more testing. I may try running it for longer to warm it up and see if it ever stays lit for an appreciable length of time. I'm just concerned about melting the plastic tubing which brings the propellants into the chamber if I get the chamber any warmer (yeah, I probably should've tried metal plumbing there...). I may try seeing if I can water-cool it a little.

http://youtube.com/7alGMFgz52s

I originally had a lot more trouble with the spark generator producing a whole bunch of EMF which caused the microcontroller to go crazy. The relay which I was using for isolation earlier wasn't helping anymore for some reason, so I switched to an LED/photoreceiver setup which worked pretty well then had its own problems. I separated the power supplies to separate AC circuits and then used a metal mesh to shield the non-sparker side of the test stand, and that seemed to do the trick, 95% of the time, at least (good enough for me, though I can do better). I think I'm slowly wearing out the microcontroller somehow.

If I'm ever able to get it working smoothly, I will make a parts list. I need suggestions about how to keep it lit.
Here's a picture of the chamber with the nozzle unscrewed:

Choked Flow! Mach Diamonds! Schlieren! (but just with compressed air)

Here are a couple "Schlieren" images I made (really just shadow graphs) using a DSLR camera without a lens, the bright video flash (just a bright white LED) on my cell phone, and compressed air going through the oxidizer orifice that will be used for my rocket igniter (I just drilled it totally free hand, no drill press... 1/16" diameter orifice, I think). You can see nice shock diamonds, especially with the last image (which was taken with less pressure drop, actually... I was running my air compressor in "blow down" mode because it's loud). I need to put my pressure gauge closer to the orifice (should be on the orifice side of my valve) to get a better reading of the pressure drop, but I think the first image is at about 100psi, with the next one being significantly less.

I simply projected the shadow of the end of the orifice directly on the DSLR image sensor. I held the orifice pretty close to the camera. To get more contrast, I could have held it further away (allowing the diffracted vs undiffracted light to diverge more, giving more contrast).

And here's what the orifice actually looks like, next to my breadboard that controls the spark generator (indirectly through a relay, ~31Hz PWM) and the valves, with three LEDs hooked up so I can test the code without the valves and stuff connected:

The purple thing is just the FTDI cable used for programming the ATMEGA chip and powering it with my laptop (in the field, it has its own 5V power supply). FTDI is optional, I'll probably remove it once everything is integrated.

I'll post more later.

Calculating vapor pressure of propane during test firing

I will be relying on the vapor pressure of a propane tank to maintain a certain feed pressure for my rocket igniter (which I will measure with a pressure gauge). But will the boiling/evaporating of the propane while it's being used lower the temperature enough that there's a significant reduction in feed pressure?

Using figures mostly from here:
http://en.wikipedia.org/wiki/Propane_(data_page)

propane heat of vaporization:
356kJ/kg (or 356J/gram)

propane liquid molar heat capacity:
98.36J/(mol*K)

propane molar mass:
44.1 gram/mol

propane liquid heat capacity:
98.36J/(mol*K)/(44.1 gram/mol)
=2.23J/(gram*Kelvin)

Thus a 4 second firing consuming .5 grams of propane a second for a total of 2 grams will cool the liquid propane in the tank by:
2grams*356J/gram=712Joules
which will change the temperature of half a liter (roughly 500 grams) of propane by:

712J/(500grams*2.23J/(gram*Kelvin))
=
.639 degree (in K or C)
that's roughly 1 degree F (or Rankine)


It's quite possible that the whole tank won't equalize in temperature, but even so, it does mean there's not a big enough drop in total temperature to lower the overall vapor pressure by a considerable amount over the length of the burn.


Here's a chart of the vapor pressure of propane (and other substances) as it changes with temperature:

Wolframalpha.com says that propane at 77 F has a vapor pressure of about 138.1 psi, and at 76 F, it's 136.1 psi. So, if we're operating the tank at 77 F, we'll get roughly a 2 psi change in feed pressure over the length of the burn, assuming 500 grams of propane in the tank, a feed rate of .5 grams of propane per second and a 4 second burn length. That's low enough that we don't care.

BTW, since we'll be wanting to operate at closer to 80psi for the propane tank, that means we need it to be at 41 Fahrenheit (5 C), which means an ice bath or something like that.
http://www.wolframalpha.com/input/?i=propane+vapor+pressure+at+5+Celsius
Still need to figure this out, but I will be doing some experiments with flow-rates and orifice sizes (I'll be drilling them my own).

Another update, now in 2012

Well, it's been a couple months since the last update. HOWEVER, I now have renewed my subscription to my local hackerspace, acquired the block of brass (for free), have very nearly acquired the coil-on-plug (for free, just have to pick it up), and have some favors I can call on for my local machinist. I also bought a decent CAD program, Alibre Design Personal, which makes certain things a lot easier (like putting threads in my drawing and making a 2D diagram).

And now, I think the valves need 12V and 24V, so that's three power supplies I need (5V for Arduino... could be from laptop, though I may want to be better isolated, 12V for one valve, and 24V for the other, and 12V for the coil-on-plug)

Still need:
*the right taps for my spark plug and my orifices (if I don't just drill them) or my existing adapters
*correct gender for the propane tank adapter
*orifices (again, if I don't drill them)
*propane tank (may have one lying around)

Semi-optional:
*filters
*usb opto-isolator (or, just run it from a cheap PC or semi-broken laptop).
*digital pressure and temperature probes

I've also been thinking about making the throat and nozzle replaceable, perhaps allowing me to 3d-print different nozzles (in steel) for testing different shapes in subscale.

Also been thinking of using two, three, or four servos to do some experiments with vectored thrust, ala V2 and Redstone, etc on a subscale. I've got 2 servos already.

I have to make sure I can safe the system by just turning the power off on everything. I'll have everything hooked up to a power strip I can just switch off or unplug everything if something goes bad. Also, I want a big, red "fire" button. ;)

Fire durations will necessarily be very short, one the order of a few seconds, starting with just two seconds.

I just got a nice digital SLR (not video-capable, though), so better pictures will be up shortly.

EDIT: Most of the stuff I got: