Kiwi: Successes and Data

Before discussing the Kiwi project, I want to mention that our blog has a new, modern format. If you're reading this on the RPL website, you can check it out at uscrpl.blogspot.com!

January 26 was a very successful and productive day for RPL. We were in the full swing of setup by 0930 hours, and conducted the firings at approximately 1300, 1500, and 1600. The first firing was with our normal 72/10 propellant, the second with oxamide, and the third with melamine. See the previous post for an explanation of why we wanted to investigate these additives.

Integration was mostly trouble-free; the data acquisition setup, by our DAQ chiefs John Rising and Tim Ellis, was among the smoothest RPL has ever had. Video and photography capturing of the firings was superb, with two cameras close to the motor behind protective stands, a GoPro overlooking the motor, multiple keychain cameras all over the place, and many handheld cameras in the blockhouse and bunker. Some of the videos are truly astonishing, none more so than the one from a keychain camera taped to the igniter dowel on the third firing. Watch for that in our highlights video, which will be posted soon.

The filament-wound carbon fiber motor case stayed in good condition throughout the firings, except for a small bit of sealant damage at the head end. The case has now been fired four times, double the total of RPL's previous record holder, Trunnion. The insert and leads of the head-end ignition system showed little degradation.


Now for the data analysis. Our DAQ equipment consisted of an S-beam load cell, a pressure transducer, and a thermocouple. We used a new method for mounting the pressure transducer -- instead of screwing it directly into the upper bulkhead, we put it at the end of a grease-filled tube, in a successful attempt to keep it relatively cool and avoid damaging it. The thermocouple was attached near the pressure transducer to verify that it remained cool. In actuality, the transducer never got above 25 degrees C, so we will use this method on future tests.


Clockwise from top left on the below graphs, we have the time histories of the total thrust of the motor, the pressure at the head end, the thrust coefficient (a property of the nozzle geometry), c-star (a measure of the propellant chemistry), and specific impulse (the overall "efficiency" of the motor). Each graph contains one curve for each propellant formulation, with "Billy Mays" referring to oxamide and "Big Mel" referring to melamine. Due to switching to a different pressure transducer shortly before the test, the tops of the pressure curves went above the transducer's limit, so the curves in those intervals have been inferred based on the thrust data.


The performance of each propellant was close to our calculated optimum values. The 72/10 propellant produced 150 pounds of thrust and fired for about 4.5 seconds. The oxamide and melamine grains produced lower thrust, about 100 pounds, but fired for 6 to 7 seconds. Since they fired longer, their total impulse is about the same as that of the 72/10, meaning that the burning rate suppressants do not significantly lessen the energy release of the propellant. In fact, the slower-burning propellants would have produced even better performance had we used a nozzle with a smaller throat in order to bring their chamber pressure up by about a hundred psi to our usual level.

If you have any questions about the graphs or the propellants, please post in the comment section.

Kiwi will be back in late February for another series of static firings, for which we have already started preparing hardware. We'll try different concentrations of oxamide, different throat designs, and maybe even an aerospike!

In other news, our 8" and 6" diameter two-stage rocket, Texas Two Step, is deep into construction. The nosecone is complete, as are many miscellaneous bits of hardware. We are currently working on the nozzles and fins. Daily updates are being posted at twitter.com/uscrpl!