Static Tests - Bottle Rocket Motor Static Tests

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Date: 2005-10-22


Six different motors were assembled, five on Rich Wolter's bottle rocket tooling, all using pyrotube casings.

Two use BP propellant, constructed as usual with clay nozzles and bulkheads, one charged with the usual "contaminated meal", the other of the meal batch in the previous rocket tests. The other four were nozzleless, one end-burning salicylate whistle, one each of core-burning salicylate and benzoate whistle, and finally one core-burning of variegated streamer.

A short length of blackmatch was used to ignite each motor.

The motors were placed in my 1/4" test stand, and the thrust produced measured with a 500 g mechanical diet scale I purchased from Big W for $7. As usual baudline, xanim and mplayer were used to extract data from the videos.


Quite valuable and suprising results.

Unfortunately the "new meal" test video did not work, so that test will need to be repeated. The other five were recorded at 30 fps and played back frame-by-frame to recover thrust curve data. The under-dampened mechanics of the scale and relatively low frame rate making the results difficult to extract, but usable data was recovered none the less. For the price and simplicity of the experiment I can't complain, the 5 kg load cell I purchased on eBay is still sitting in my junk box, I just walked into Big W today and grabbed this scale for pocket change.

The salicylate core-burner was the winner in terms of peak thrust. It pegged the scale at over 700 g of thrust sustained for about 70-100 ms! That is truely terrifying performance from a 1/4" motor, I still can't believe the figure but you can clearly see the stand is hard-down against the stop, the pointer having gone all the way around between frames. Its end-burning thrust was also quite good, at 20-25 g. The preview picture is an animated GIF of the frame-by-frame play back of its core-burning phase, note the huge bounce after transition to end-burning, the rocket and scale are completely off the ground (I'll tape stand together next time).

The end-burning salicylate whistle also produced 20-25 g of thrust for its 2.5 second burn. Its pitch decaying from 5 kHz to 2.8 kHz in a fairly smooth curve. The charge length was 25 mm, giving a burn-rate figure of 10 mm/s. Its standard classification would be a high 1/4A0.24-2.5.

The salicylate rocket matched this performance quite closely in its 1.7 second end-burning phase, falling from 4.1 to 2.9 kHz in that period. The frequency difference at burn-out was probably because of the different resonator length due to different placement of the clay end-plug. The whistle upper-stage motor in a previous test was essentially identical except for length of the grain above the core. Comparing the two gives some dynamic data for that test from the doppler shift (ignoring squint and a lot of other variables), suggesting an initial seperation velocity of 0.33 Mach or about 430 km/hr, a more trustable figure can be extracted from beyond the dog-leg in the curve; 34 m/s or 125 km/hr. The extremely high figure is perhaps due to lengthening of the effective resonator length due to aerodynamic pressure changes at the tail-end of the moving motor casing which settled down after the motor reached a terminal velocity during end-burn. The second figure isn't too insane considering these rockets achieve apogees exceeding 30-50 metres in under 2 seconds with a power spike of only 200 ms, so most of that 2 seconds is spent coasting to a stop.

The benzoate whistle decayed from 4.5 to 3.2 kHz in 3.5 seconds during its end-burning phase. Its core-burning phase lasted 350 ms. The benzoate propellant is quite noticabably slower and less powerful, with a peak thrust of 200-250 g sustained only very briefly. The end-buring phase produced less than 5 g of thrust.

The BP motor that was videoed successfully also put out a peak of 200-250 g of thrust in its 350 ms long core-burning phase. Its 3 second end-burning phase putting out less than 5 g of thrust. Its overall performance quite similar to that of the benzoate whistle motor.

The Variegated Streamer motor produced a peak of 30 g of thrust in its 700 ms core-burning phase, and virtually nothing in the 6.5 seconds of end-burning. It would have been quite attractive at night and might make a pretty silver gerb.

Note the wall ablation in the salicylate motor tubes, they actually feel (and mass) lighter than unused tubes. The benzoate tube on the other hand is coated with black soot and is not significantly burnt away at all. The salicylate whistle is probably a bit over-oxidised.

Data reduction continues, I hope to produce a graph comparing the motors shortly.


title type size
Contaminated BP Core-Burner Video video/x-msvideo 4.811 Mbytes
Variegated Streamer Core-Burner Video video/x-msvideo 5.423 Mbytes
Salicylate Whistle End-Burner Video video/x-msvideo 4.269 Mbytes
Benzoate Whistle Core-Burner Video video/x-msvideo 4.750 Mbytes
Salicylate Whistle Core-Burner Video video/x-msvideo 4.752 Mbytes
Benzoate Whistle Core-Burner Spectrogram image/jpeg 72.106 kbytes
Salicylate Whistle End-Burner Spectrogram image/jpeg 67.306 kbytes
Salicylate Whistle Core-Burner Spectrogram image/jpeg 74.860 kbytes
Pre-Test Picture image/jpeg 43.697 kbytes
After Test Picture image/jpeg 33.744 kbytes
Nozzle-end of Tubes image/jpeg 32.834 kbytes