Cold flow testing of laboratory-scale hybrid rocket engine test apparatus

A cold-flow experimental investigation is performed on the Ryerson University lab-scale hybrid rocket engine test apparatus, in order to gain a further understanding of transient phenomena affecting the engine’s hot test firing results to date. The hot test firing data was characterized primarily by noticeable thrust oscillation magnitudes at low frequency being measured by the test stand’s thrust-measuring load cell, relative to somewhat lower magnitude low-frequency pressure oscillations being measured by a head-end pressure transducer. The present investigation allows for the evaluation of the fluid-structure interaction behaviour of the rocket engine’s combustion chamber and upstream oxidizer feed-line/injection apparatus (along with the surrounding test stand structure). Pressurized air at a moderate temperature acts as the working fluid (rather than hot gas arising from combustion), passing through the internal flow system, and exiting at the engine’s exhaust nozzle. Cold flow tests are conducted at three different flow-regulating orifice-plate conditions upstream of the head-end injector: 1) unchoked, 2) marginally choked and, 3) choked, in order to potentially observe any trends in that regard, as tied to feed-system stability/instability. The cold flow test results, from the experimental time-dependent measurement of pressure, thrust and axial acceleration, in turn undergo FFT analyses to help identify any frequency-dependent trends in regard to transient behaviour. Hammer tests are conducted to further establish the relevant lower frequency natural modes of structural vibration of the test apparatus with the engine in position The potential applicability of Karabeyoglu’s well-known thermal lag-combustion-gasdynamic predictive model (for estimating a characteristic frequency), which captures to some degree the intrinsic low frequency combustion-based instability behaviour of hybrid rocket engines, is considered for the present test engine setup. There are some promising comparisons in terms of relevant frequencies of mechanisms in the 20 Hz range, mechanisms that might be coupling to produce a noticeably augmented oscillation condition (as was observed in the hot firing thrust measurements).