Editing Rocket engine (section)

===Combustion instabilities===
The combustion may display undesired instabilities, of sudden or periodic nature. The pressure in the injection chamber may increase until the propellant flow through the injector plate decreases; a moment later the pressure drops and the flow increases, injecting more propellant in the combustion chamber which burns a moment later, and again increases the chamber pressure, repeating the cycle. This may lead to high-amplitude pressure oscillations, often in ultrasonic range, which may damage the motor. Oscillations of ±200&nbsp;psi at 25&nbsp;kHz were the cause of failures of early versions of the [[LGM-25C Titan II|Titan II]] missile second stage engines. The other failure mode is a [[deflagration to detonation transition]]; the supersonic [[Longitudinal wave|pressure wave]] formed in the combustion chamber may destroy the engine.<ref name="titan2">{{cite book|author=David K. Stumpf|title=Titian II: A History of a Cold War Missile Program|publisher=University of Arkansas Press|date=2000|isbn=1-55728-601-9}}</ref>

Combustion instability was also a problem during [[SM-65 Atlas|Atlas]] development. The Rocketdyne engines used in the Atlas family were found to suffer from this effect in several static firing tests, and three missile launches exploded on the pad due to rough combustion in the booster engines. In most cases, it occurred while attempting to start the engines with a "dry start" method whereby the igniter mechanism would be activated prior to propellant injection. During the process of man-rating Atlas for [[Project Mercury]], solving combustion instability was a high priority, and the final two Mercury flights sported an upgraded propulsion system with baffled injectors and a hypergolic igniter.

The problem affecting Atlas vehicles was mainly the so-called "racetrack" phenomenon, where burning propellant would swirl around in a circle at faster and faster speeds, eventually producing vibration strong enough to rupture the engine, leading to complete destruction of the rocket. It was eventually solved by adding several baffles around the injector face to break up swirling propellant.

More significantly, combustion instability was a problem with the Saturn [[F-1 (rocket engine)|F-1 engines]]. Some of the early units tested exploded during static firing, which led to the addition of injector baffles.

In the Soviet space program, combustion instability also proved a problem on some rocket engines, including the RD-107 engine used in the R-7 family and the RD-216 used in the R-14 family, and several failures of these vehicles occurred before the problem was solved. Soviet engineering and manufacturing processes never satisfactorily resolved combustion instability in larger RP-1/LOX engines, so the RD-171 engine used to power the Zenit family still used four smaller thrust chambers fed by a common engine mechanism.

The combustion instabilities can be provoked by remains of cleaning solvents in the engine (e.g. the first attempted launch of a Titan II in 1962), reflected shock wave, initial instability after ignition, explosion near the nozzle that reflects into the combustion chamber, and many more factors. In stable engine designs the oscillations are quickly suppressed; in unstable designs they persist for prolonged periods. Oscillation suppressors are commonly used.

Three different types of combustion instabilities occur:

====Chugging====
A low frequency oscillation in chamber pressure below 200 [[Hertz]]. Usually it is caused by pressure variations in feed lines due to variations in acceleration of the vehicle, when rocket engines are building up thrust, are shut down or are being throttled.<ref name=sutton1975/>{{rp|261}}<ref name="HuzelAndHuang"/>{{rp|146}}

Chugging can cause a worsening feedback loop, as cyclic variation in thrust causes longitudinal vibrations to travel up the rocket, causing the fuel lines to vibrate, which in turn do not deliver propellant smoothly into the engines. This phenomenon is known as "[[pogo oscillation]]s" or "pogo", named after the [[pogo stick]].<ref name="sutton1975" />{{rp|258}}

In the worst case, this may result in damage to the payload or vehicle. Chugging can be minimised by using several methods, such as installing energy-absorbing devices on feed lines.<ref name=sutton1975/>{{rp|259}} Chugging may cause Screeching.<ref name="HuzelAndHuang"/>{{rp|146}}

====Buzzing====
An intermediate frequency oscillation in chamber pressure between 200 and 1000 [[Hertz]]. Usually caused due to insufficient pressure drop across the injectors.<ref name=sutton1975/>{{rp|261}} It generally is mostly annoying, rather than being damaging.

Buzzing is known to have adverse effects on engine performance and reliability, primarily as it causes [[material fatigue]].<ref name="HuzelAndHuang" />{{rp|147}} In extreme cases combustion can end up being forced backwards through the injectors – this can cause explosions with monopropellants.{{citation needed|date=April 2018}} Buzzing may cause Screeching.<ref name="sutton1975" />{{rp|261}}

====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.