Grain Geometry:
Number of Grains: 6
Outer Diameter: 1.384"
Core Diameter: .52"
Web: .432"
Grain Length: 2.38" each (14.28" total)
Nozzle Throat Diameter: .34375"
Kn: 418, 479, 446
Propellant Weight: .9834 pounds
Casing: 1.75" OD x 1.62" ID x 18"
Nozzle: Graphite 1.25" long
Closure: Steel .75" long
Retention: Snap ring on closure; Steel washer/ snap ring on nozzle
The night before I had recalibrated my test stand and amplifier for the lower thrust motor, this should give better
resolution on the smaller, long burn motor. It was a blustery, cool day, but I decided to go for it anyway and
loaded my car with all the required equipment. Once set up, I had my video camera rolling... 5,4,3,2,1 and ignition.
Click Here for a short video of
the test. (1.15 MB Divx file)
Here is the motor at full thrust.
The motor burn was just about exactly as expected, perhaps just a touch longer burning though. If you watch the video, you can see some ejected paper from the casting tubes towards the end of the burn, and the 3 corresponding pressure spikes in the data below.
Here is the thrust/time curve from the test. The high resolution of the load cell amplifier really shows all the minor fluctuations, but the overall curve is very much what was expected.
Data from the test:
Burn Time: 3.434 sec./ 2.95 sec.
Burn Rate: .1464"/sec.
Total Impulse: 99.51 pound seconds
Isp: 101.2
I probably should add a little iron oxide, or other burn rate modifier to speed up the burn a little. As is, this motor would do a fine job of propelling a rocket in the 5 pound liftoff weight range. But I think I'll increase the length slightly to increase the average thrust somewhat. I'll also have to rethink the multi layered casting tubes I used, I really don't want them being ejected at the tail end of the burn.
The fairly low Isp is indicative of really two things.
First, any smaller motor loses performance because a larger portion of the propellant is used up in the initial thrust build up, and again at thrust tail off compared to a larger motor.
Second is the graphite nozzle; It just isn't possible/practical to get the expansion ratio out of graphite, so the thrust coefficient suffers. I think there are other factors as well, the graphite likely causes more absorption of heat, decreasing performance to a degree. And the graphite isn't as smooth as steel, causing more nozzle friction loses.