Author: George Boyt* – Sumner County Middle College High School (Gallatin, TN USA)
First Reviewer: Jonathan Kenigson, Ph.D., FRSA (Athanasian Hall, Cambridge LTD, UK)
If you have ever watched a video of a rocket launch, right as the rocket lifts off and an immense cloud envelops the launch pad, you have likely heard a faint crackling noise over the roar of the rocket. While this might seem to be a recording artifact, it is a phenomenon that gives some insight into one of the many problems of rocket science. When a rocket launches it releases a massive amount of energy. This energy is produced by fuel mixing with oxidizer to produce exhaust. This exhaust travels through a rocket nozzle to accelerate the rocket. Initially, when this exhaust leaves a rocket nozzle, every part of the exhaust is moving in the same direction at about the same speed. Pockets of the high-speed exhaust at the edge of the plume can break off and rapidly decelerate due to interactions with low-speed air. This creates pressure waves in the air. If the pressure wave is not very energetic it sounds like a thunderous roar. If the pressure wave exceeds the pressure of the atmosphere, it becomes a shock wave, and the resulting roar sounds clipped and crackly.
Although the rocket is far above the exhaust plume where much of the noise is generated, it is not immune to this noise. This noise is incredibly damaging to ground service equipment, rockets, and rocket payloads. It creates vibrations and subjects the launchpad to repeated stress cycles, which can lead to premature failure of the launchpad or even failure of the rocket. During liftoff, much of the sound can reflect from the ground back at the rocket. This liftoff environment is the highest-stress environment that a rocket usually experiences. Rockets must be incredibly sturdy to survive the vibrations trying to tear them apart. Any reduction in the noise level can significantly reduce the maximum forces the rocket has to resist. Noise mitigation strategies can allow for lighter, cheaper rockets.
To realize these cost savings, many techniques have been developed to reduce the noise created by rockets at liftoff. Water sound suppression systems are one such method. If rocket exhaust is surrounded by water, it can significantly reduce the sound level. It is not very practical to build launchpads over water, so deluge systems are used. Water is sprayed into the rocket exhaust at ignition. The exhaust becomes surrounded by an air water mixture. This mixture is full of bubbles of air. When these bubbles collapse, energy is absorbed, reducing the total sound energy transmitted to the launchpad, the rocket, and the rocket’s payload. This has been used since the 1960s in the US. This does have the drawback of launch service equipment having to be proof to the insane amount of water used.
It is hard to overstate the amount of water needed to create an effective sound dampening air water mix. NASA’s Pad 39B’s launch deluge system has a peak flow rate of 1.1 million gallons per minute during a rocket launch, the equivalent of two Olympic sized swimming pools in a minute. This deluge system is the source of the large white cloud at launch that you can see during many rocket launches. Another technique to reduce the sound transmitted to a rocket during a launch is elevating the entire launch pad above a large pit. This eliminates a large amount of the reflections back at the rocket. This also reduces the amount of ground equipment in the way of the rocket exhaust to be damaged. This technique is used at Roscosmos’s Baikonur Site 1 in Kazakhstan. The next time you watch a rocket launch, be sure to listen to the sounds you hear. It might just be informative.