Chuffing, also called chugging, is a characteristic, low-frequency pulsating noise associated with the irregular burning of fuel in a rocket engine. Chuffing is the lowest in frequency of three main types of combustion instability that may arise in rocket engines, the other two being buzzing and screaming. It involves fluctuations of 10–400 hertz and is caused by pressure interactions between the propellant feed system, if not the entire vehicle, and the combustion chamber.
Chuffing stems mainly from the elastic nature of the feed systems and structures of vehicles or the imposition of propulsion forces upon the vehicle. Chuffing of an engine or thrust chamber assembly can occur in a test facility, especially with low chamber pressure engines (100 to 500 psi), because of propellant bump cavitation, gas entrapment in propellant flow, tank pressurization control fluctuations, and vibration of engine supports and propellant lines. It can be caused by resonances in the engine feed system (such as oscillating bellows inducing a periodic flow fluctuation) or a coupling of structural and feed system frequencies.
When both the vehicle structure and the propellant liquid in the feed system have about the same natural frequency, then force coupling can occur, not only to maintain, but also to amplify oscillations. Propellant flow rate disturbances, usually at 10 to 50 Hz, give rise to low-frequency longitudinal combustion instability, producing a longitudinal motion of vibration in the vehicle. This vehicle flight instability phenomenon has also been called pogo instability or the pogo effect because it is similar to motion of a pogo stick. Pogo instabilities can occur in the propellant feed lines of large vehicles such as space launch vehicles or ballistic missiles.