Axisymmetric Bi-propellant Air Augmented Rocket Testing with Annular Cavity Mixing Enhancement

Author

Allen A. C. Capatina, California Polytechnic State University, San Luis ObispoFollow

DOI: https://doi.org/10.15368/theses.2015.146
Available at: https://digitalcommons.calpoly.edu/theses/1493

Date of Award

10-2015

Degree Name

MS in Aerospace Engineering

Department/Program

Aerospace Engineering

Advisor

Dianne Deturris

Abstract

Performance characterization was undertaken for an air augmented rocket mixing duct with annular cavity configurations intended to produce thrust augmentation. Three mixing duct geometries and a fully annular cavity at the exit of the nozzle were tested to enable thrust comparisons. The rocket engine used liquid ethanol and gaseous oxygen, and was instrumented with sensors to output total thrust, mixing duct thrust, combustion chamber pressure, and propellant differential pressures across Venturi flow measurement tubes.

The rocket engine was tested to thrust maximum, with three different mixing ducts, three major combustion pressure sets, and a nozzle exit plane annular cavity (a grooved ring). The combustion pressures tested were , , and allowing for a nozzle pressure ratio range of relative to ambient pressure. The mixture ratio was fuel rich throughout all tests. The engine operated very consistently throughout all the tests performed; however, pressure losses in the feed system prevented higher combustion pressures from being tested.

Three mixing ducts of the same outer diameter were tested. The short and diverging ducts were the same length and the long duct was long. The short and long ducts created positive mixing duct thrust and the diverging duct created negative mixing duct thrust. The long duct case did show better performance than the no duct case when the total thrust was divided by combustion pressure and nozzle throat area. The long duct always created several times more mixing duct thrust than either the short or diverging ducts, but none of the mixing ducts created positive overall thrust augmentation in the over expanded cases tested. The mixing duct thrusts ranged between and . As the combustion pressures were increased, getting closer the nozzle’s optimal expansion, the mixing duct thrusts started converging indicating a difference between nozzle operation at over expanded and under expanded.

The annular cavity had a noticeable effect on the thrust of the engine and the appearance of the plume. The total thrust of the system was decreased by a maximum of and the plume was more sharply defined when the annular cavity was attached. Better mixing between the primary (engine exhaust) flow and the secondary (ambient air) flow was promoted by the annular cavity because it increased the shear layer’s turbulence and the increased turbulence reduced thrust. The greater mixing also allowed for secondary combustion which made the plumes more sharply defined. The annular cavity was also seen to enhance the mixing duct thrusts for all three mixing ducts.