The acoustic module is an electronic device that allows us to record the sound atmosphere under the rocket’s fairing during the flight, which means measuring the most relevant acoustic frequencies. The noise under the nose is the consequence of the rocket’s progression through the air and is influenced by the shape of the fairing and the rocket’s speed. Our goal is to determine the acoustic atmosphere so we know if we can have a freight in the nose without having it damaged by the acoustic vibrations.
The module is constituted of an electronic unit and two microphones and is able to make measurements during the flight.
We place two microphones under the rocket’s nose. The first one is at its centre and the other one is oriented to the outside of it, and is integrated into the rocket thanks to a small hole in the nose. The data go through a low pass analogical filter that retains the frequencies under 40 kHz, which are the ones that could potentially damage a payload.
After the filter, we proceed to an A/D conversion before stocking the data on a SD card. Once we get this card back, we can make further analysis of the data.
However, we considered an eventual loss of the data that are on the SD card, and to prevent a total loss of the recording from happening, we decided to use telemetry to have a direct transmission of the data. Yet, the telemetry system’s capabilities are limited and have to be shared with other parts of the program, such as the video. For that specific reason, we only use telemetry to send the average value and the standard deviation of the frequencies, on a 8 ms basis.
In order to get a first estimation of the results that we would obtain, we realise two experiences in laboratories. These two experiences are performed before the end of the realisation of the module and use consequently other types of way of measures.
The first experience is intended to define the transfer function of the fairing: we try to know which acoustic frequencies are absorbed and which are amplified.
To do so, we expose the fairing of the rocket to a white noise broadcasted by headphones, meaning that the sound is the same everywhere in the room et that it contains every acoustic frequencies (toward 10 kHz) at equal proportion. We place a microphone in the fairing and another in the room. The difference between amplitudes of each sound allows us to determine the transfer function of the fairing.
The second experience consists in placing the fairing in a wind tunnel in order to simulate the flight of the rocket. We place two microphones in the same configuration that during the real flight. This experience will allow us to get a fist estimation of the acoustic environment under the fairing. However, the wind tunnel that we have access to can only reach 50m/s (corresponding to approximately 180 km/h), so results will be insufficient and the real flight remain essential to get a complete estimation of the acoustic environment under the fairing.
The acoustic module will allow to determine the acoustic environment in the fairing of SERA-3, essential data for this place to satisfy requirements enforced by the payload. Moreover, experiences realised on laboratories will allow to better understand the generation of this acoustic environment. They will give the opportunity to open to new directions of improvement of fairings for next PERSEUS projects.
The totality of this module and experiences have been realised by the CLC (Centrale Lyon Cosmos).
*the fairing used for the experiences is the one from SELENA, an old PERSEUS rocket, which presents physical and geometrical characteristics very close from SERA-3’s fairing.