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Figure 2. Rocket launch-induced noise and related vibration-induced response of structures. Numbers in circles are related to those effects of vibration response that need to be considered as potential problems during the operation of the shuttle. These include structure-borne wave transmission. Numbers in triangles are related to the acoustic radiation, such as airborne sound waves. Reproduced from Arenas and Margasahayam (2006, Figure 8).
into active noise control, which results in achieving sound reduction in real-time using a power source, and passive noise control, which incorporates sound reducing mea- sures into the original system design or retrofits them. Pas- sive treatments are most commonly used to mitigate rocket launch noise.
Water-based acoustic suppression systems are commonly used on launch pads (see Figure 3), where they offer typical noise reductions of 3-5 dB (depending on frequency) in the overall sound pressure level at most frequencies of interest (Krothapalli et al., 2003; Norum, 2004; Houston et al., 2015). Interestingly, the technology on which these suppression systems are based was originally developed to help subma- rines avoid detection. Naval engineers designed the exhaust of submarine engines to emit bubbles, which have the ability to absorb an amazing amount of sound. As sound waves en- counter the bubbles, the bubbles compress and convert the acoustic energy into heat, thereby shrouding both the noise emitted by the submarine, and incoming sonar waves.
In a water-based launch pad acoustic suppression system, water molecules sprayed into the air begin to vibrate on con- tact with a sound wave, converting the acoustic energy into heat. Additionally, any air bubbles present in the water will be compressed by sound waves, again converting the sound energy into heat energy. At the same time, below-deck (that is, under the launch pad) systems inject water into the ex-
Figure 3. Water from an acoustic suppression system test soaks the mobile launcher platform at the Kennedy Space Center Pad 39A with 350,000 gallons of water. Photo available at bit.ly/2P1W93e.
haust plume with the aim of reducing far-field noise by more rapid dispersion of the rocket exhaust (Allgood et al., 2014). Moreover, above-deck systems, so-called rainbirds, inject water around the top of the pad as well as into the plume (Houston et al., 2015) that, in addition to suppressing the noise, helps cool the launch pad and environs. Care must be taken, however, not to deluge the pad, degrade materials and structures (Pico et al., 2016), or adversely affect performance of the diffuser (Allgood et al., 2014), which is a device used during sea-level rocket tests to simulate the effects of altitude. Water injection helps to reduce the SAN (Norum, 2004), and the extent of the reduction depends on where in the plume the water is injected and how great the injection pressure is (Gely et al., 2000, 2005; Lambare, 2016). To achieve any sig- nificant noise reduction, the quantity of water injected must be at least three times the jet flow rate; for the new acoustic suppression system on the mobile launcher platform at the Kennedy Space Center (KSC) Pad 39A, this means that the water flow rate exceeds 900,000 gallons a minute at liftoff. See bit.ly/2o9FEa7 for a recent test of the water suppression system (using about 450,000 gallons of water) at the KSC Pad 39B, from which the SLS will launch.
A key component of any launch pad is a flame deflector (FD), which is a trench used to channel the rocket exhaust away from the pad (see Figure 4). Flame deflectors are gen- erally not specifically designed for acoustic purposes but nonetheless can have an important effect on the noise. Al- though the impingement of the plume on the FD generates noise that propagates away from the vehicle, the unsteadi- ness of the plume flowing along the FD emits the dominant noise that is directed toward the vehicle. As a consequence, factors such as trench cover and shape have a significant ef- fect on the ability of the deflector to reduce noise (Gely et
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