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An experimental assessment was conducted to identify the sensitivity of self-pulsation injection and spray characteristics to throttling of propellant simulants in a liquid rocket swirl-coaxial injector element. A set of test conditions were scaled to preserve flow similitude from a single case of spontaneous self-pulsation, wherein the inner LOX post was configured with no recess. The natural dynamics of pressure-swirl injection were examined to assess possible influence on the overall stability characteristics of self-pulsating swirlcoaxial injection. Pressure fluctuations in both the liquid and gas manifolds, as well as external acoustic signatures in the injector far-field, were compared with time-resolved pattern recognition of spray dynamics via backlit high speed cinematography. While discrete coupling in frequency was identified between the liquid manifold, acoustic radiance, and strong spray field fluctuations during self-pulsation, spectra in the liquid manifold was dominated by low frequency activity in all cases. Low frequency structures had slight, but persistent, influence in the spray field even when high frequency self-pulsation was encountered; however, these low frequency effects were not found to be explicitly coupled to manifold fluctuations. LOX post resonance is thought to have been excited by self-pulsation in several test cases. NomenclatureA = injector cross-sectional area a = temporal amplitude coefficient f = frequency K = autocorrelation matrix = mass flow rate N = mode number P = pressure SPL = sound pressure level, P ref = 20 μPa t = time U = velocity ṽ = pixel intensity fluctuation Z = acoustic impedance z = array of pixel intensity values, i.e. image Δ = differential ζ = hydro-resistance Λ = dimensionless depth of liquid sheet penetration into gas stream κ = wavelength λ = eigenvalue, i.e. modal energy ρ = density υ = eigenvector Φ = momentum flux ratio φ = phase angle 2 ϕ = proper orthogonal mode, i.e. POM ω = angular frequency subscripts c = convective i= temporal index j = temporal index g = bulk gas l = bulk liquid pk-pk = peak-to-peak r = image sequence index superscripts ' = fluctuation

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