Since ramjet propulsion eliminates the need for oxidizers, it provides an alternative to classic rocket engines for missiles and projectiles. However, to capture free-stream air, ramjet engines require large supersonic air intakes, which can be an operational nuisance. A possible
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Since ramjet propulsion eliminates the need for oxidizers, it provides an alternative to classic rocket engines for missiles and projectiles. However, to capture free-stream air, ramjet engines require large supersonic air intakes, which can be an operational nuisance. A possible alternative is to make the supersonic intakes flush with the vehicle body by “submerging” them into the side of the fuselage: a Submerged Supersonic Intake (SSI). Due to supersonic expansion, submerging will incur some aerodynamic losses, which will lead to reduced ramjet engine performance. To assess the feasibility of SSI's, finding the magnitude of SSI performance losses compared to regular supersonic intakes is necessary. The performance losses involved with submerging a supersonic intake can be found by finding the upper limit of the SSI operational domain, which this report aims to do in two ways. The first is to investigate performance losses by an inviscid study into SSI performance characteristics and the thrust coefficients of ramjets equipped with SSI’s, to find the theoretical maximum to SSI performance. The second way is to carry out an experimental campaign to validate an existing viscous analytical model: the Submerged Ramjet Intake Model (SRIM), which may later be used in optimizing SSI’s, to include viscosity into the analysis. The experimental campaign focused on measuring the total- and static pressure, mass flow and Mach number, as well as visualizing the flow in the SSI models. Two different SSI geometries were produced to conduct experiments in the ST-15 wind tunnel at Delft University of Technology. Measurements of static pressure and schlieren visualization were successfully performed. Partical Image Velocimetry (PIV) was planned, but turned out to be impossible due to hardware limitations. To compensate for the lack of velocity measurements from the canceled PIV measurements, a method was devised to derive the Mach number in the SSI model internal channel from the schlieren images, from which mass flow and total pressure could be computed.
The experimental results were used to validate the SRIM. Overall, the SRIM showed good agreement with the experimental results, with differences of $5\%$ at the highest for the most important intake characteristics of mass flow and total pressure recovery. Although some discrepancies and uncertainties remained, which were studied and used suggest future experiments and improvements to the SRIM. An inviscid study was also performed. Using a routine to compute optimal inviscid intake performance, the maximum attainable total pressure ratios of regular and submerged intakes were computed. It was found that with increasing Mach number and expansion angle, SSI performance quickly deteriorates. More important, however, is the effect of submergence on the ramjet engine thrust capabilities. An inviscid thrust coefficient analysis was conducted which showed that the reduced total pressure ratio of submerged supersonic intakes has limited effects on thrust coefficient. However, it became apparent that reduced capture area due to the supersonic expansion has a much larger influence on the thrust coefficient when comparing submerged and regular intakes of similar size. This report provides two methods of computing the consequences of submerging a supersonic intake in terms of performance. An inviscid method was presented to use in early stages of missile design, and an existing quick analytical model with viscous capabilities was validated so that it may be used in optimization schemes for more detailed analysis. Using these tools missile designers can assess whether the operational gains of submerging a supersonic intake outweigh its losses.
