The ‘Pogo’ effect
Named after the children’s toy comprising a co-axially sprung vertical tube with two foot rests that is used to ‘boing’ up and down.
‘Pogo’ effect occurs when the dynamic characteristics of the propellant fuel feed system resonates due to the forcing frequency generated by the thrust oscillations during the combustion>expansion within the rocket engine.
Creating an ever increasing vibration (amplitude) resonant condition that causes fuel connections to loosen and tubes to either fatigue or fail.
Initially ‘Pogo" suppression was achieved by isolating the fuel feed system from the rocket structure during R7 development. 1000c/s
Modifying the combustion chamber could also modify the resonant frequency.
Modes of rocket chamber oscillation frequency that can cause these resonant conditions are related to one or more of the natural acoustic parameters of the chamber and can be separated into one of the following:
This is seen in small rockets with large length to diameter ratios. Or can be found in long thin tubes connecting the main LOX or LH tanks with the tubo pump.
More usually encountered with larger liquid propellant rocket engines.
First harmonic of the fundamental tangential frequency of the ‘Atlas’ sustainer engine is of the order of 1100c/s(Hz).
Resonant interaction between the gas phases of the combustion and the liquid phases close to the chamber wall and induction system created a ‘buzz’ frequency at 550c/s in the Atlas booster. That was observed both during static tests and in flight.
Unstable, longitudinal (up and down) oscillations induced in a launch system, mainly due to fuel sloshing and combustion induced engine vibration.
1958 Soviet R7 rocket failure.
After the explosion of the first stage boosters of two R7 development rockets on the
23rd September and 12th October 1958. An analogue model was developed to identify the forcing frequency inherent in the combustion process and the possibility of exitation of a natural frequency of the rocket structure.
The resonant condition analysis looked at the loop:
Oscillations in the fluid flow fed to the combustion helped amplify the combustion pressure pulse frequency amplitude. Which if left undamped would continue to increase until structural or pipeline failure occurred.
Flight pressure recording histories identified the cause to be due to a combustion resonance frequency of between 9 and 15 Hz exciting the main longitudinal structure frequency of the rocket . Leading to structural failure. Oxidiser feed line oscillations over the range of 9-15 Hz had the effect of producing combustion pressure pulse amplitudes of +4.5 bar at the point when the rocket exploded.
The solution was to introduce fluid pulse accumulators or dampers into the oxider and fuel feed lines prior to the pump which fed them into the combustion chamber.
So removing the closed loop linking between combustion pulse and structure feedback
The resonant loop conditions set up between the combustion forcing pressure pulse frequency and the natural axial elastic frequency of the rocket structure. Known as the ' Pogo Effect'
Also has a link with water rocket bottle flight dynamics and water jet pulse propagation .
hese signatures can be seen superimposed on the pressure curves for the 2L coke and 1L Badoit static bottle blow-down tests. Check out the links below.
Eliminating the Pogo effect of the Nasa Space Shuttle
Was achieved by using helium gas charged accumulators in the liquid gas feeds to the turbo-pump used to feed the combustion chamber.
Problems with Pogo effect on Apollo 6
History of eliminating Pogo from Gemini to Titan IV B
Liquid Sloshing Dynamics
Theory and Applications
Raouf A. Ibrahim
Wayne State University, Michigan
Coke 2L 8bar 1500Hz pressure pulsation
This site was created on the 15th April 2003
©John Gwynn and sons2003
You're welcome to reproduce any material on this site for educational or other non commercial purposes
as long as you give us proper credit by referring to "The Water-Rocket Explorer" http://waterocket.explorer.free.fr.