Experiments were performed with single, sonic, helium jets at downstream angles of 15° and 30° relative to the free stream to determine their mixing, penetration and total pressure loss when injected into a supersonic air cross flow. From this information, the performance of these jets as fuel injectors in a supersonic combustion ramjet (scramjet) combustion chamber was estimated. Both injection angle jets were made flush to the wind tunnel wall. The jets were injected into a Mach 3 free stream with a total pressure of 6.5 atm, a total temperature of 283 K and a Reynolds Number of 52.5x10⁶ /m. The flow field of each injection angle was documented at jet expansion ratios of one and five. Spark schlieren and nanoshadowgraph methods were used to visualize each flowfield. At axial stations 20, 40, and 90 jet diameters downstream of each jet, continuous vertical profiles of flow quantities were made. Profiles were taken at seven lateral stations including the jet centerline at each axial station. Spacing between the lateral stations was one jet diameter. This data yielded profiles of helium concentration, Mach number, static temperature, static pressure, density, flow speed, mass flux, total pressure, and total temperature. The different injection schemes were then compared on the basis of helium mass fraction decay, the distance required to reach the stochiometric H₂-air concentration and total pressure loss. For all cases except the 15° jet with an expansion ratio of one, large eddies were observed to penetrate into the free stream. These eddies were believed to significantly enhance large scale mixing. The jet cores of the underexpanded jets had bifurcated 20 jet diameters downstream of the injection point, but had re-united by the 40 diameter station. Wandering of the jet core about the geometric centerline was observed for all cases. The decay rates increased rapidly with the jet to free stream dynamic pressure ratio until about 1.5 where the decay rate leveled off. This indicated that there was no significant increase in mixing from increasing the dynamic pressure ratio of the present jets past 1.5. The decay rate of the present 30°, matched pressure case was about 16 percent greater than that of a normal jet at similar dynamic pressure and expansion ratios. These results were reflected in the distances required to reach the stochiometric H₂-air concentration. The 15° jet with an expansion ratio of one had the lowest total pressure loss. It was concluded that injection at low downstream angle shows promise for application to scramjet fuel injection and merits further study. / Master of Science