Page 40 - Winter2018

Caroline P. Lubert
Address:
Department of Mathematics & Statistics James Madison University MSC 1911 Harrisonburg, Virginia 22807 USA
Email:
Lubertcp@jmu.edu
From Sputnik to SpaceX®: 60 Years of Rocket Launch Acoustics
The field of rocket launch noise is 60 years old and has a lot to celebrate!
In the Beginning
At 7.28 pm (GMT) on October 4, 1957, the Soviet Union launched a 58-cm-di- ameter polished metal sphere into an elliptical Earth orbit at 29,000 kilometers per hour (kph), 800 km above the Earth’s surface. The satellite was equipped with two pairs of external radio antennae that broadcast pulses from a one-watt, 3.5-kg radio-transmitting unit inside the sphere. Signals were emitted on two frequen- cies, 20.005 MHz and 40.002 MHz, spending 0.3 second at each (see bit.ly/2fSYJNl to hear these transmissions). The successful launch of Sputnik took the world by surprise at a time when the USSR and the United States were both desperately seeking to establish dominance over the new frontier, space.
Sputnik 1, where sputnik means “something that is traveling (the satellite) with a traveler (the Earth),” was the world’s first artificial satellite. However, the language of rockets, the launch vehicles for satellites, goes back centuries. According to the Merriam-Webster Dictionary New Edition (2016), the first known use of the word rocket was in 1611. Deriving from the Italian word “rocchetta” meaning distaff, a rod used in spinning or weaving, it is defined in the Merriam-Webster Diction- ary New Edition (2016) as “a jet engine that operates on the same principle as the firework rocket, consists essentially of a combustion chamber and an exhaust nozzle, carries either liquid or solid propellants which provide the fuel and oxygen needed for combustion and thus make the engine independent of the oxygen of the air, and is used especially for the propulsion of a missile (such as a bomb or shell) or a ve- hicle (such as an airplane).” Although we think of rockets as relatively new inven- tions, the first device to use the basic principles of rocket propulsion was probably a wooden bird, invented somewhere around 400 BC and propelled by escaping steam using the law of action-reaction (see bit.ly/2BNOi7F).
During the early days of the Space Race, the risk of rockets exploding during or just after liftoff was high and the technical focus was on improving rocket reliability. As the prospect of launch failure diminished (Martinez-Val and Perez, 2009), attention turned to other aspects of the liftoff process. At the time, very little was known about the acoustics of rocket launches, and at 300 m from the launch pad, peak levels could reach as much as 200 dB during liftoff (as far as 3 km from the launch pad, peaks of 140 dB, which is the threshold of pain in humans, were being observed). Such high acoustic loads are a source of vibration, and vibroacoustic interaction can critically affect correct operation of a rocket and its environs. Thus it was quickly realized that even a small decrease in noise could result in a significant reduction in both risk and cost, and the discipline of Launch Vehicle Acoustics was born.
And Then Came Sputnik...
Many important breakthroughs in rocket design were made between the 16th and 19th centuries. For example, the concept of a vehicle with multiple stages that
38 | Acoustics Today | Winter 2018 | volume 14, issue 4 ©2018 Acoustical Society of America. All rights reserved. https://doi.org/10.1121/AT.2018.14.4.40