Space Forum / Shuttle / April 2008

"POGO is a closed-loop oscillation involving longitudinal
i structural vibrations and pressure oscillations within the
liquid rocket propulsion system. The theory of such oscillations
is now well understood and are probably best described
by Rubin."

The above definition is found in the introduction of the following paper:

  VOL. 7, NO. 12, DECEMBER 1970 J. SPACECRAFT 1407
  Simulation of Saturn V S-II Stage Propellant Feedline Dynamics
  R. S. RYAN,* L. A. KIEFLING^ AND H. J. BUCHANANJ
  NASA Marshall Space Flight Center, Huntsville, Ala.
  AND
  W. A. JARVINEN
  Northrop Corporation, Huntsville, Ala.
  Oscillations involving the propellant feedlines, engines,
  and longitudinal structural modes of the Saturn V S-II
  stage are investigated, using an electronic analog computer.
  The methods used to develop mathematical models and some
  problems encountered are described. The effect of a nonlinear
  gas bubble (cavitation) at the pump inlet is studied. The
  simulation was able to match several characteristics of the
  flight, including stability variations with NPSH, amplitude
  sensitivity, and nonlinear waveforms. The cause of the limit
  cycle which occurred on two flights was not found. The need
  exists for accurate and complete test data, especially for
  structural damping and local and engine dynamics.

So, the classic definition of POGO only involves liquid fueled rocket
engines.

Here is an abstract talking about solid rocket motors.

  Instabilities and pressure oscillations in solid rocket
  motors*1
  Yves Fabignon, , a, Jöel Dupaysa, Gérard Avalona, Francois
  Vuillotb, Nicolas Lupoglazoffb, Grégoire Casalisc and Michel
  Prévostc a Office National d'Etudes et de Recherches
  Aérospatiales (ONERA), Fundamental&Applied Energetics
  Department, 29 Avenue de la Division Leclerc, F-92322,
  Châtillon Cedex, France b Office National d'Etudes et de
  Recherches Aérospatiales (ONERA), CFD&Aeroacoustics Department,
  29 Avenue de la Division Leclerc, F-92322, Châtillon Cedex,
  France c Office National d'Etudes et de Recherches
  Aérospatiales (ONERA), Aerodynamics&Energetics Modeling
  Department, 29 Avenue de la Division Leclerc, F-92322,
  Châtillon Cedex, France

  Abstract
  The purpose of this paper is to give an overview of the main
  results obtained on instabilities and pressure oscillations
  in segmented solid rocket motors. A major part of this work
  was carried out in the framework of the ASSM and POP R&T
  programs supported by the French national space agency CNES
  during the last decade. ASSM is related to Aerodynamics of
  Segmented Solid Motors and POP for Pressure Oscillations
  Program for the Ariane 5 solid booster (P230). Due to the use
  of segmented technology for the P230 motor and the possible
  acoustic oscillations inside the motor chamber, anticipated
  at the beginning of the programs and confirmed later on static
  firing tests, the main scientific objective of the ASSM program
  was oriented towards the comprehension and the modeling of the
  vortex shedding phenomena that are supposed to be responsible
  of the pressure and thrust oscillations observed in the P230.
  POP program was started in order to obtain an experimental and
  numerical data base using subscale tests of 1/15th of the P230.
  After the description of the instabilities observed in the P230
  solid rocket booster, the scientific approach of the ASSM
  program is detailed insisting on the validation of numerical
  tools in order to predict oscillation frequencies and amplitudes.
  The logic of work regarding POP program is also presented. The
  main section of this paper provides an overview of different
  results obtained in ASSM and POP programs to understand the
  mechanisms driving to the instabilities in solid rocket chamber.
  The most important recent result, inside ASSM and POP programs,
  was the discovery of the parietal vortex shedding and the role
  of aluminum combustion on instabilities. Together, these two
  mechanisms seem to be an important potential source of
  instabilities and provide a new vision of the P230 stability.

So there you go.  Not terribly easy to sort through, but it's pretty clear
that the mechanisms which create POGO are different than the mechanisms that
create thrust oscillations in solid rocket boosters.

Jeff