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Usually during a rocket launch on Earth, lots of water is poured to decrease the sound (deluge discussed in Wikipedia's Sound suppression system) and some other methods are used.

What I am looking for is how can you decrease the sound the rocket itself is producing. Not what methods are used to block the sound.

How can you make the terrestrial launch vehicle itself launch more quietly?

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    $\begingroup$ related: "acoustic efficiency," How much of the energy of a rocket ends up as a noise? and Why are rockets so loud $\endgroup$
    – Erin Anne
    Feb 27 at 7:18
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    $\begingroup$ Launch it from the Moon? $\endgroup$
    – uhoh
    Feb 27 at 8:59
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    $\begingroup$ @uhoh Yes, if you launch it from the moon it would be quiet because there is a vacuum. In this question I was looking for answers about launching from Earth. $\endgroup$
    – The Rocket fan
    Feb 27 at 9:10
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    $\begingroup$ Turn down the throttle a bit! $\endgroup$
    – Jacob Krall
    Feb 28 at 2:26
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    $\begingroup$ Fix the muffler. $\endgroup$
    – Mark
    Feb 28 at 4:30

3 Answers 3

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According to https://www.mdpi.com/2076-3417/12/10/4874, jet noise in aircraft is due to shear “turbulence pulsation formed by the rapid mixing of the high-speed air jet from the nozzle and the surrounding medium“ The central core of the exhaust has high velocity which rapidly decreases when you move radially towards the ambient air. High bypass engines reduce this turbulence by reducing the gradient stepwise from core exhaust to bypass to ambient. enter image description hereRockets have a zero bypass ratio, but a similar effect could be achieved by injecting water around the periphery of the nozzle cluster. The above sited paper describes water injection used for noise suppression in jet engines.

This paper https://www.hypersonic-cfd.com/research/projects/2020/Multi-jetNozzleInteraction.html looks at multijet nozzle interaction. They say “ flows span from subsonic to supersonic Mach numbers and consist of a multitude of shocks and expansion waves … simulation of these flows is an exceptionally challenging task.” Understatement? enter image description here

Water injection for temperature control and increasing ISP was suggested in Has combustion chamber water injection been used in RP1 rocket engine design?. It would be elegant if noise supression could be added to the list of potential advantages of water injection.

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    $\begingroup$ obviously building the water deluge system into the rocket rather than the launch pad would increase the size and mass of the rocket by a very large factor, necessitating the rocket to become even larger to fit the larger engines and fuel tanks to compensate, requiring even more water for the water deluge system, ad infinitum. $\endgroup$
    – jwenting
    Feb 28 at 6:56
  • $\begingroup$ @jwenting ... Yes, an on board deluge system is obviously a silly idea. Water injection is not. It has been used in military and civilian aircraft as well as high performance automobiles. en.wikipedia.org/wiki/Water_injection_(engine) $\endgroup$
    – Woody
    Feb 28 at 16:25
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The main way to reduce the noise generated would be to reduce the chamber pressure of the rocket engines and/or increase the size of the engine bell.

Sadly both greatly reduce surface level thrust, so they aren't suitable for first stage engines (both are used for efficiency reasons in vacuum optimised engines, not for reasons of noise suppression).

SpaceX is referring to its launch pads as "stage 0" btw, so technically you could consider them part of the rocket, a reusable (hopefully, N1 explosion notwithstanding) that stays behind when the rest of the rocket goes off into the wild blue yonder and hopefully to space.

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  • $\begingroup$ Would a small, but longer bell have any effect? $\endgroup$
    – mustermax
    Feb 28 at 9:30
  • $\begingroup$ @mustermax no, as the velocity of the escaping gasses is what's causing the sound. And you want that velocity as high as you can get it to maximise thrust, as low as you can to minimise sound. Which are contradictory requirements $\endgroup$
    – jwenting
    Feb 28 at 10:10
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    $\begingroup$ I don't see how the third paragraph is relevant to the question. $\endgroup$
    – Erin Anne
    Feb 28 at 10:46
  • $\begingroup$ @ErinAnne it redefines what is meant with "a rocket" to include the launch infrastructure. Which redefinition can change the sound signature as well $\endgroup$
    – jwenting
    Feb 28 at 11:44
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    $\begingroup$ "it redefines what is meant with "a rocket" to include the launch infrastructure." No, this will never fly :) $\endgroup$
    – Organic Marble
    Feb 28 at 13:53
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A large proportion of rocket noise is due to resonance between fluid behavior and natural frequencies of the combustion chamber. This could be reduced by stiffening the chamber, but since the chamber is very large, that would add a lot of weight. The generation of rocket noise, which is quite different from jet noise, is nicely described in Sixty years of launch vehicle acoustics Proc. Mtgs. Acoust. 31, 040004 (2017); https://doi.org/10.1121/2.0000704 by Caroline Parsons Lubert

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  • $\begingroup$ from Section 3.2 of that paper: "Rocket launches generate a significant amount of acoustic energy, with the primary source of rocket noise being the high jet exhaust velocity required to boost the launch vehicle during takeoff. Shock waves are formed by the collision of the supersonic exhaust with the ambient air, and the acoustic intensity of these waves depends primarily upon both the size of the rocket, and its exhaust velocity ... It is generally agreed [12] that there are three types of supersonic (rocket engine) jet noise" How different are rocket and jet noise then? $\endgroup$
    – Erin Anne
    Mar 1 at 2:19
  • $\begingroup$ Can you cite an authority that supports your first sentence? Temporary downvote until the assertion is supported. Both the paper you cite and the one here space.stackexchange.com/questions/17929/… disagree. $\endgroup$
    – Organic Marble
    Mar 1 at 2:38
  • $\begingroup$ Perhaps you are thinking of combustion instabilities, which have names like "screaming" and "chugging", and are indeed resonance phenomena. $\endgroup$
    – Organic Marble
    Mar 1 at 2:50
  • $\begingroup$ Thank you for these comments. I agree that I overstated my case, but I was interpreting the question in terms of the take-off noise from space launchers, which I still maintain is radically different from rocket propelled missiles. My sourse is the ancient monograph "Theory of combustion instability in liquid propellant rocket motors" by L Crocco, SI Cheng - 1956 -available on the internet at apps.dtic.mil> Because space launchers are multistage, they are, at takeoff, almost 100% propulsion unit, and this is a huge hollow chamber with combustion events inside it that can be unstable...... $\endgroup$
    – Philip Roe
    Mar 2 at 19:16
  • $\begingroup$ ........., known as screaming and chugging, as you say. I was hoping to find a more recent reference, but I did not choose very well. $\endgroup$
    – Philip Roe
    Mar 2 at 19:22

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