I am new to rocket engines. I was looking at an image of RS-25 engine's MCC and found these acoustic cavities, and I want to know how they work. Can somebody please explain their working principle? Thank you. the image


1 Answer 1


They are provided to help damp out combustion instabilities.

The main injector uses cooled baffle elements, developed at Glenn in the 1960s to control pressure waves that could destroy the engine. Pressure waves in the space shuttle main engine combustion chamber are also controlled by acoustic cavities. Testing by Glenn engineers determined the most effective size and location of these cavities, which act somewhat like cavities in acoustic ceilings.


You can read more on the theory of how they work in this book Liquid Rocket Engine Combustion Instability starting at page 384.

It starts out

Acoustic cavities such as Helmholtz and quarter-wave resonators were successfully used as damping devices for the suppression of combustion oscillations. resonator consists of a small volume connected with the combustion chamber through an orifice. If the dimensions of the various resonator elements are small in comparison to the wavelength of the oscillation, the gas motion behavior in the resonator is analogous to a mass-spring-dashpot system.

Acoustic cavity design is also discussed in the NASA monograph Liquid Rocket Engine Combustion Stabilization Devices. It includes this figure showing some other applications.

enter image description here

  • $\begingroup$ So, some of the liquid fuel from the cooling jacket is directed to the MCC through the acoustic cavities to provide suppression of combustion oscillations, and then burned with MCC propellants? $\endgroup$ Jul 11, 2019 at 7:11
  • 1
    $\begingroup$ There is a small flow of hydrogen that passes through the acoustic cavities and then into the main chamber, but I think that's mostly for cooling. The gas in the cavities interacts with the gas in the MCC to provide damping. There's a cutaway view in this answer: space.stackexchange.com/a/28142/6944 $\endgroup$ Jul 11, 2019 at 11:54
  • $\begingroup$ I get the idea. Thanks for the information. $\endgroup$ Jul 11, 2019 at 13:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.