Rocket Tuning - 9.5 mm Core-Burning Rockets

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Date: 2006-04-11


Two 2 oz (9.5 mm ID) rockets, built to flight test the new 7:3:1 batch.

Rich Wolter 2 oz standard tooling. Skylighter tube. Standard wax-treated cat litter nozzle, no bulkhead plug however. Blackmatch fuses, run all the way up the core, with a small wad of paper towel at the nozzle to retain them. Fairly short stick for a 2 oz rocket, a standard ~ 270 mm bamboo kebab stick, fixed in place with hot-melt glue.

One rocket had a 1/4" Salicylate end-burning whistle as a payload. Friction-fitted into the top of the 2 oz motor casing, with a little paper to tighten the fit. Some blackmatch and a pinch of loose 7:3:1 propellant to pass fire. The payload whistle had no stabilisation system and was not intended as a true second stage, mearly garniture/apogee detection.

The other had the exposed top of the propellant grain covered with a piece of pasted paper.


Suprisingly good.

I was expecting a rather violent CATO (based on the results of the burn-rate test), but both rockets performed well. The sticks should be a little longer, there was a touch of instability, especially the second rocket, but it wasn't very aerodynamic or mechanically stable. I'll simply use longer sticks or more nose weight (i.e. a payload) next time.

Sight-to-sound delay is easily observed in the video with the whistle payload, suggesting quite high apogee. Both rockets exhibited a 500 ms core-burning phase and a 2.5 second end-burning delay phase.

The Salicylate whistle burnt for 3.3 seconds, a little longer than expected. Its tone dropping from 5 kHz to 2.4 kHz in that time. It was constructed using my new 1/4" whistle jig that Rich Wolter custom made for me (the 1/4" Micro-Guy's Activity Center). It may have been loaded with a little more propellant than previous whistles because of the different geometry of the whistle jig.

Why these rockets didn't cato is now something of a problem. It suggests I need to add gas-production metrics to my testing of new propellant batches. Burn-rate alone is probably sufficient for repeatability across otherwise identical composition batches, but even closely related BP variants appear to differ in gas production (and burn temperature) to a sufficient extent to prevent burn-rate alone being a reliable metric to predict performance in rockets.


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Bare Rocket Video video/x-msvideo 1.221 Mbytes
Rocket with Whistle Video video/x-msvideo 2.629 Mbytes
Pre-Test Picture image/jpeg 41.311 kbytes