That will give you a "simple" measurement of linear burn rate, but may
not have much relevance to the actual burn rate inside a combustion
chamber as most propellants have burn rates that vary with pressure.
The best way to measure the actual burn rate is to cast either a pair
of thin wires or optical fibers in the propellant and burn it in a
combustion chamber under pressure. You know the distance between the
two elements, and measure the time between when the first one breaks
and the second one breaks. You can "implant" multiple elements in a
single grain, across the radius or down the length. This is an
inexpensive method and can be quite accurate. It's a standard
methodology for measuring burn rates in propellants and explosives.
The DATAQ starter units will do digital measurements for about $25.
One could connect a 5v source to each digital input via a different
element wire. When the wire breaks the signal drops to zero. The
DATAQ starter units will time to 0.005 seconds or better (high-speed
add-on will time to about 70 microseconds!)
The DI-194RS is a serial port unit. I've got one of these, but no
longer have a laptop with a serial port. It costs $25. see:
The DI-148U is USB and costs $50. http://www.dataq.com/products/startkit/di148.htm
. Either of these could also be used with a load cell and associated
circuit to build a test stand to measure the thrust performance of an
engine. Richard Nakka has some good stuff about building test stands:
Thanks for the information. I think the DATAQ solution is more
reasonable for my computer/electronic circuit skills. I do have an
old laptop that has both a parallel and serial port. I found that the
DI-194RS is not now available at DATAQ. I would not be surprised to
find that they have discontinued it but are not willing to say so yet,
so I will probably have to shell out an extra $25 to get the DI-148U.
Setting it up to record load cell readings as well is a great idea,
then I guess I could calculate specific impulse and find out a bunch
of other interesting things about performance.
Depending on how small a measurement you want to make, the circuit can
be quite simple. You can also us a PC to make the measurement (and
start the burn). If you don't want to measure smaller than 1
millisecond (1/1000 second), you can buy oscillator modules where you
just hook them up to power and they start running. The circuit
requirements are: a time base (the oscillator), a counter (or counter
circuit), and the start/stop logic.
As I mentioned, using a PC (an old laptop would work well), you can
use the parallel port to start the igniter and measure the differences
between the burn-through times. You will want to use DOS, as Windows
is not a real-time operating system. Typically, a PC's clock will
only measure down to 55 milliseconds (55/1000 sec). There are
routines that will let you get millisecond resolution. The one I'm
most familiar with was written by a guy names Robert Graves at the
Univ. of Victoria in British Columbia. Can't find a google reference
to him (the article was written about 1995 or so), but did find other
references to millisecond timing on the PC.
If you don't know enough to design the circuit (it's very
straightforward), perhaps you can get someone at the
sci.electronics.design newsgroup to help. I don't have any way to
post a circuit diagram or pictures. There are lots of references on
the web for interfacing to a PC parallel port for measuring things
like switch closure and turning things on/off.
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.