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070331-001 - Launch Sound Analysis
Date 31st March 2007 6:30am - 7:45am
Tested by GK and PK
Location Sydney, Australia
Test Procedure Launch
References N/A

Aim

  •  To analyse the different elements of a launch using the sound recorded during launch.

Experiment Setup

An 18.75L rocket (Acceleron II) was set up on the launch pad and pressurised to 120psi. A DV camera was used to film the launch and record the sound.

Setting up Acceleron on the pad.
Frame from video showing the launch.

Results


Big water rocket 18.75 Liters - booster

Video containing the flight

The sound file was imported into a video editing suite and the sound waveform was extracted.

Launch Sound Analysis, showing the different features of  water rocket noise during take-off.

Conclusions / Analysis

We had a close look at the sound produced by the rocket, as much more accurate measurements can be made on the timing of events on the rocket. (refer to diagram)

  • You can clearly see the click the rocket makes at is leaves the launch tube. With that you can accurately measure for how long the launch tube is effective, and get the rocket's initial acceleration.
  • You can also measure exactly how long the water phase is when comparing your rocket to simulators. There is a distinctive transition to air pulse.

    You can also measure the length of the air pulse to a certain degree but as the pressure tails off in the rocket it is hard to determine where the air pulse finishes.
  • When we zoomed in on the start of the air pulse there was a distinctive set of regular spaced pulses, with fairly high amplitude. The camera easily picked these up at a range of perhaps 20 meters. From the waveform we measured the frequency to be 297 Hz ever so slightly increasing over time, as their amplitude decreased before being swamped with noise. They are clearly visible for about 20 ms (see diagram).
  • We do not know what these represent, whether they are perhaps some sort of sonic shockwaves, as the air accelerates close to mach 1, or the resonant frequency of the rocket or nozzle.
  • Since all objects have a resonant frequency, we tried just blowing across the nozzle of the rocket and recording the sound. We couldn't quite get a pure note, but when we removed the nozzle we got quite a nice pure note. The resonant frequency of the rocket segment in this case was: 122Hz which isn't close to the frequency we observed. The waveform shape in this case was also very different - more sinusoidal rather than the sharp pulses. This is why we are leaning to some form of shock waves rather than just resonance.
  • Thinking that perhaps this was a unique case we reviewed the audio from a number of launches and other rockets and similar pulses were present (in both stereo channels) although in this example these were the clearest.
  • Since the audio was gathered from the ground there is likely to be a Doppler shift in the audio frequency also, but since we don't have a clear measurement how far away from the rocket we were, it may be difficult to accurately predict the expected Doppler shift.

We don't know if these pulses at the start of the air pulse and their frequency are significant, but it may turn out to be important when designing CD nozzles.



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