Page 43 - Winter Issue 2018
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m ‘ ‘ W’ % Figure 3. Water from an acoustic suppression system test soaks the
‘°"A_u_ mobile launcher platform at the Kennedy Space Center Pad 39A with
%" &$.''°'m ®&'»f..."'5°"" 350,000 gallons afwater. Photo available at bit. ly/2P1 W93e.
Figure 2. Rocket launch—induced noise and related vibration—induced _ , , ,
response of structures. Numbers in circles are related to those effects haul“ Plume hath the mm ofreducmg fapfield Home by more
ofvibration response that need to be considered as potential problems 1'3P1d ‘h5Pe1'51°h 0f the rocket exhaust (Ahgood at 31-> 2014)-
during the operation of the shuttle. ‘These include structure—borne Moreover, above—deck systems, so-called rainbirds, inject
wave transmission. Numbers in triangles are related to the acoustic water around the top of the Pad as well as into the Plume
radiation, such as airborne sound waves. Reproduced from Arenas (Houston er al” 2015) that’ in addition to suppressing the
and Mmgamhayam @006’ figure 8)‘ noise, helps cool the launch pad and environs. Care must be
taken, however, not to deluge the pad, degrade materials and
into active noise control, which results in achieving sound structures(Pico et al., 2016), or adversely affect performance
reduction in real-time using a power source. and passive of the diffuser (Allgood et al., 2014), which is a device used
noise control, which incorporates sound reducing mea- during sea—level rocket tests to simulate the effects of altitude.
sures into the original system design or retrofits them. Pas- Water injection helps to reduce the SAN (Norum, 2004), and
sive treatments are most commonly used to mitigate rocket the extent of the reduction depends on where in the Plume
launch noise. the water is injected and how great the injection pressure is
Water-based acoustic suppression systems are commonly (Gely et 31-» 2000» 2005; L3-mha-‘er 2016)- T0 achieve my Sig‘
used on launch pads (see Figure 3), where they offer typical nificant noise reduction, the quantity of water injected must
noise reductions of 3_5 dB (depending on frequency) in the be at least three times the jet flow rate; for the new acoustic
overall sound pressure level at most frequencies of interest 5hPP1'e55i°h 5Y5t°m 0h the ‘h°hhe launcher Platform at the
(Krothapalli et al., 2003; Norum, 2004; Houston et al., 2015). Kehhedl’ SP3-Ce Center (KSC) Pad 39A» this means that the
Interestingly) the technology on which these suppression water flow rate exceeds 900,000 gallons a mmute_at liftoffi
systems are based was originally developed to help Subrna_ See for a recent test of the water suppression
rines avoid detection. Naval engineers designed the exhaust 5Y5t°m (“Sing ahhht 450»000 S3-h°h5 0f Water) at the KSC
of submarine engines to emit bubbles, which have the ability Pad 393» from which the SL5 Wh1h‘hhCh-
to absorb an amazing amount of sound. As sound waves en- A key Component of any launch Pad is a flame deflector
Counter‘ the bubhles’ the bubbles comprehs and convert the (FD), which is a trench used to channel the rocket exhaust
acoustic energy into heat, thereby shrouding both the noise away from the Pad (see Figure 4)_ Flame deflectors are gem
emitted by the submarine’ and incoming Sonar waves‘ erally not specifically designed for acoustic purposes but
In a water-based launch ad acoustic su ression s stem, nonetheless can have an im ortant effect on the noise. Al-
water molecules sprayed into the air begin to vibrate on con— though the impingement of the plume on the FD generates
tact with a sound wave, converting the acoustic energy into noise that propagates away from the vehicle, the unsteadi-
heat. Additionally, any air bubbles present in the water will ness of the plume flowing along the FD emits the dominant
be com ressed b sound waves, a ain convertin the sound noise that is directed toward the vehicle. As a conse uence,
P Y 3 S ‘l
energy into heat energy. At the same time, below-deck (that factors such as trench cover and shape have a significant ef-
is, under the launch pad) systems inject water into the ex- feet on the ability of the deflector to reduce noise (Gely et
Winter 2018 ] Acoustics Thday | 41

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