Another 54mm Static test

[Ray]

This weekend we did another static test, this time it was a 54mm 4 grain motor. Total impulse was around 1600 NS or a moderate size "K" motor. Grain configuration was 3" length, 0.5" core standard bates grain. Total weight of propellent was around 700g, with a calculated (not delivered) ISP of about 220 seconds.

No video of the burn this time, nobody had a camera...

Visual observation of the burn was the only thing we had. It seemed like the motor burned for longer than it should, about 4-5 seconds, should have been around 3 second. Internal pressure seemed low, there was a pretty good tail off of the motor, smoky and sparky at the end. A pretty good indication of low chamber pressure.

Possible causes for the long burn.

Low outside temp, only about 5 degrees Farenheit. For some reason this has a much more profound impact on burn time than one would expect. According to the experts this usually only accounts for 10-15% longer burns...so its not likely it was only this.

The formula I have calls for 200 micron AP, and I use 400 micron AP. This slows things down a bit. In the 38mm testing I have done, this appears to account for about another 10% in burn time. I'll have some 200 micron AP in the near future and I'll cast another set of grains to see if my 10% estimate is accurate. This combined with low temps could account for most of the lengthened burn time.

During the mixing of this batch, there were several people milling about. One of them knocked over the cup holding the iron oxide...Iron is a burn rated accellerator. We got *most* of it back in the cup, but naturally, not all. We didn't remeasure the qty. The formula is pretty sensitive to the amount of iron in the mix. This could easily account for the remaining time.

So, lessons learned. Test on warmer days...my chestnuts were the size of peanuts...Use 200 micron AP, make sure I have accurate measurments of the Iron I use, and get some more sophisticated test equipment, my eyes just don't cut it.

Test equipment for the near future include a thrust test stand. We currently have one capable of around 150 lbs max thrust. We need to increase this limit. A pressure transducer to measure chamber pressure, it needs to be in the 3000 psi range, 5000 psi may be better. Software (pby labview custom application) to log the data and plot it.

Not a bad test, its fun to watch a motor like this burn! Next motor will be a 6 grain 54mm. Grains will be 4" long 0.75" core for a total of about 1400g of propellent. That'll put us firmly in the "L" catagory for total impulse.