Earlier this summer, NASA took a "sound ride" on a small-scale SLS rocket, which gave engineers an idea of how the acoustics of NASA's massive new launch platform will work. While we might not think about it, those sound waves and vibrations are things engineers need to think about before launching a new rocket.
Acoustic considerations of a rocket launch go beyond the noise we hear when the engines start to fire. The sound power of a launching rocket is the result of the mechanical energy of the rocket's exhaust plume, which is defined by the thrust of the rocket times its exit velocity independent of the fuel and oxidizer being used. The end result is a lot of noise.
That noise is spread across a range of frequencies, but not evenly. Engineers at NASA's Stennis Space Centre, the agency’s rocket engine test site in Mississippi, have determined that the acoustic energy of a launch is concentrated in the low to mid frequency range, exactly the range that can knock down buildings or hurt people standing nearby.
That's because sound waves are physical waves, the result of back and forth vibration of the particles that make up the medium through which the sound wave is moving. In the case of a rocket launch, the danger comes from the sound waves creating air pressure as the molecules are pushed aside so the sound wave can pass, or creating harmonics that can vibrate the rocket or nearby objects to failing points.
It's something engineers had to address when building NASA's iconic Vehicle Assembly Building. Just three miles from the launch pads where Saturn V rockets would leave the Earth, the agency had to consider what the acoustic pressure would do to the nearby structure. The solution was to use insulated aluminum panels fastened to steel girders to cushion the VAB and anything inside from the sound pressures of a Saturn V launch.
Unsurprisingly, the highest sound power levels ever recorded at Stennis was during testing for the Saturn I-C stage, the first stage of the Saturn V that used five F-1 engines to generate 7.5 million pounds of thrust. One test registered about 204 decibels, while more recent rockets, which generate between 100,000 and 650,000 pounds of thrust, typically generate around 195 decibels at launch.
But the SLS, which is still on track to launch in 2017, will be bigger than the Saturn V, generating more power at launch and having a correspondingly high sound power profile.
But it's not just buildings engineers have to think about with the next big rocket. "The noise the engines and boosters generate is so great that it can impact the rocket, and the crew, during liftoff," said Jeremy Kenny, an acoustics engineer at NASA's Marshall Space Flight Center. "We have to ensure we have the proper suppression system to basically turn that noise down to a safe level."
Scale tests of the SLS are helping engineers understand what the sound power of this rocket will be so they can take the appropriate safety measures. This includes sound suppression systems, which typically use large volumes of water to dampen sound waves; the water increases resistance on a moving sound wave, slowing it down.
It might not look like much, but this kind of small scale testing is exactly what's going to help NASA regain a heavy lift launch capability that won't destroy buildings, spacecraft, or injure our nation's astronauts.