Editing Rocket engine (section)

====Screeching====
A high frequency oscillation in chamber pressure above 1000 [[Hertz]], sometimes called screaming or squealing. The most immediately damaging, and the hardest to control. It is due to acoustics within the combustion chamber that often couples to the chemical combustion processes that are the primary drivers of the energy release, and can lead to unstable resonant "screeching" that commonly leads to catastrophic failure due to thinning of the insulating thermal boundary layer. Acoustic oscillations can be excited by thermal processes, such as the flow of hot air through a pipe or combustion in a chamber. Specifically, standing acoustic waves inside a chamber can be intensified if combustion occurs more intensely in regions where the pressure of the acoustic wave is maximal.<ref name=strutt1896>
{{cite book|author=John W. Strutt|title=The Theory of Sound &ndash; Volume 2|edition=2nd|publisher=Macmillan (reprinted by Dover Publications in 1945)|date=1896|page=226}} According to Lord Rayleigh's criterion for thermoacoustic processes, "If heat be given to the air at the moment of greatest condensation, or be taken from it at the moment of greatest rarefaction, the vibration is encouraged. On the other hand, if heat be given at the moment of greatest rarefaction, or abstracted at the moment of greatest condensation, the vibration is discouraged."</ref><ref>Lord Rayleigh (1878) "The explanation of certain acoustical phenomena" (namely, the [[Rijke tube]]) ''Nature'', vol. 18, pages 319–321.</ref><ref>E. C. Fernandes and M. V. Heitor, "Unsteady flames and the Rayleigh criterion" in {{cite book|editor=F. Culick|editor2=M. V. Heitor|editor3=J. H. Whitelaw|title=Unsteady Combustion|edition=1st|publisher=Kluwer Academic Publishers|date=1996|page=4|isbn=0-7923-3888-X|url=https://books.google.com/books?id=Je_hG6UfnogC&pg=PA1}}</ref><ref name=sutton1975>
{{cite book |author=G.P. Sutton |author2=D.M. Ross |name-list-style=amp |title=Rocket Propulsion Elements: An Introduction to the Engineering of Rockets |edition=4th |url=https://archive.org/details/rocketpropulsion0000sutt/page/258/mode/2up |publisher=Wiley Interscience |date=1975 |isbn=0-471-83836-5 }} See Chapter 8, Section 6 and especially Section 7, re combustion instability.</ref>

Such effects are very difficult to predict analytically during the design process, and have usually been addressed by expensive, time-consuming and extensive testing, combined with trial and error remedial correction measures.

Screeching is often dealt with by detailed changes to injectors, changes in the propellant chemistry, vaporising the propellant before injection or use of [[Helmholtz damper]]s within the combustion chambers to change the resonant modes of the chamber.{{citation needed|date=April 2018}}

Testing for the possibility of screeching is sometimes done by exploding small explosive charges outside the combustion chamber with a tube set tangentially to the combustion chamber near the injectors to determine the engine's [[impulse response]] and then evaluating the time response of the chamber pressure- a fast recovery indicates a stable system.