Thermoacoustic instability in liquid rocket engines can have potentially catastrophic consequences. Active control based on feedback promises improved stability margin over a wider range of operating conditions than passive control. In the present work, the thermoacoustic characteristics of a trim adjustment active control system based on variable volume tunable Helmholtz resonator are investigated in open loop. The effect of variation of parameters for a ducted resonator in relation to a duct alone is brought out for the fundamental and first harmonic mode in a closed-open duct using a mode tracking technique. Thermoacoustic instability of the first harmonic is predicted for lean mixtures. Combustion delay has an optimum with regard to the combustion stability of the flame in a duct. Introduction of the resonator stabilizes the fundamental mode completely with respect to flame position. Increasing the volume of the resonator is destabilizing for both fundamental and harmonic modes in fuel rich mixtures. The resonator has maximum effect in controlling instability when placed near the antinode of the standing wave pattern and when its characteristic frequency matches the modal frequency. Volume variable resonators are seen to be effective for control of thermoacoustic instability in open loop. © 2018, Journal of Aerospace Technology and Management. All rights reserved.