Page 44 - Winter Issue 2018
P. 44

From Sputnik to Spaoex
. V   '- +3 ate ii’ V Theoretical Work and
e 1 V’ T‘ Scale-Nlodel Experiments
 ‘ Noise mitigation techniques have been fairly successful and
, ' a >5 in some cases decrease peak acoustic levels by up to 5 dB.
t h - Feet  However, to achieve further reductions, much greater under-
— ,  ‘-1  .. t'-.,‘..;..,1 - - - - ~
= , _ V _  5,; _.. t__ .  e I t standing of the mechanisms by which rocket launch noise is

e e  ‘T7: " ‘V  t if ll   l “'55 generated and propagated is necessary. Although the impor-

 t  Kt ‘  .' ‘  ‘*  tance of acoustic loading in causing structural failure has been
,.- [Ell  T ‘ l .._ 1 "1 known for 60 years (e.g., Hess et al., 1957), only relatively re-
”" er.-7" '”;:,tee"e:':7."  . ee ‘ ‘I 7*” ““' cently have significant advances in sensors, data acquisition,
W ' "  ' ' ‘T’ J and processing techniques, along with huge improvements in
.. ‘T  7’ numerical simulation ability, allowed the measurement and
‘ J prediction of launch noise with any degree of accuracy.
The main issue in accurately predicting rocket launch noise
a H e is determining the relationship between the aerodynamic
Figure 4' The Ammes hmmh PM “t the Wuhops Fhght Fuelhty’ Vlr’ characteristics of the flow and the spatial characteristics of
ginia. The rocket is attached to the pad by the rocket launch mount, . . . .
. . . . . . the sound field. Most rocket noise models are semiempiri-
which is the large white rmg that can be seen in the middle of the t
figure. The (square) rocket flame trench exit is shown at right. cal and based on the elassle NASA SP'8072 methodology
(Eldred and Jones, 1971) in which the rocket plume is the
primary noise source throughout launch. A major problem
al., 2005; Pico et al., 2016). Pl'e1imiI131’Y Work (Tsutsllmi 9t with this method is that it is not consistent with Lighthill’s
al., 2009) has indicated the possibility of substantial noise (1952, 1954) generally accepted iet noise theory because the
reduction, particularly if the initial inclination of the FD is initial approach to the aerodynamics was far too sirnplistic_
steep and it is covered. The longer the trench, the greater the Nevertheless) modified versions of the N As A model con.
noise reduction (GCIY et 3-1-9 2000)- tinue to be used. Revisions typically focus on improving the
Rocket nozzle configuration and shape also impact launch estimate of the lalhlhal eole length (Valhleh 2001) °l °h
noise emission (Humphrey, 1957; Viswanathan et al., 2012), alhehdlhg the aeoustle efllelehey by a factor that slghlheahh
and tailored nozzles can provide a reduction in the direc- ll’ lmploves the ht to the expellmehtal data whlle aeeohhtlhg
tional noise by Providing ti lew_sPeed layer around the out_ for the launch pad and FD geometry as well as for shielding
side of the primary jet that partially blocks sound transmis- (pletklh et al" 2009)‘
sion. This layer can be further modified by the use of wedges, Other empirical or semiempirical methods based on histori-
Pail'S 0fVaI1€S, and fl3PS(ViSW3I13th3Il Ct 91-) 2012)- Finally, cal data, engineering judgment, and/or acoustic measure-
flat concrete (reflecting) surfaces are predominant on launch ments have also been used extensively (Arenas and Mar.
pads, and recent studies have indicated that the inclusion of gasahayam, 2006; Fukuda et al., 2009). For example, data
perforations in these surfaces is effective at reducing noise were recently collected by the Japan Aerospace Exploration
(Natafajafl and Veflkata-l<l'iShIlaI1. 2015)- Agency (IAXA) during two static-firing tests of a solid rocket
To control the vibration levels on launch structures, their meter‘ The data were then cotntlpared  the results of the
. . . classical NASA SP-8072 empirical prediction method and a
dynamic characteristics need to be thoroughly understood, t _ t t t
and e eieni fleet‘ t emetm t er teeent week has feeueed on this computational fluid dynamics (CFD) calculation (Herting et
. . al, 1971). The former overestimated the sound pressure level
(Caimi and Margasahayam, 1997; Margasahayam et al., t _ t _ t
. . at certain angles from the let axis, although the prediction
2002). Whereas previous pad configurations have been de-

. . . . . . at other angles was reasonable. The CFD model was effec-
signed based on reducing liftoff peak acoustic load, Caimi e t dt t f b th th d t tt Id t
and Margasahayam’s (1997) workindicates that the duration wefitor Pm Chou 0 0 6 near‘ an at‘ e acoustlc
of plume impingement is a far more damaging and crucial Pm es‘
design parameter. However, it should be noted that the feasi- Once the acoustic load generated by liftoff has been pre-
bility of utilizing such modifications in practical launch pad dicted, it is then used to predict internal vibration responses
design still remains to be determined. of the vehicle, its payload, and the launch pad. Due to the
42 | Acoustics Today | Winter 2018

   42   43   44   45   46