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: