SoftwareAerodynamic Optimization of the Aeropropulsive System of a Ramjet Powered Missile
Supersonic inlets are major components of air breathing propulsion systems. Their role is to provide the engine with a sufficient mass flow of energetic and homogenous air. Inlets significantly contribute to the global lift, drag, mass and stealth of the missile. Reliable prediction of inlet performance is a keypoint for the design of an effective air-breathing engine. In order to limit cost and time, a design methodology for Supersonic inlets mixing intensive CFD for design and wind-tunnel testing for identification over the flight domain is now industrially used at MBDA France.
Fig.1 - View of the generic ramjet powered missile testcase
In recent years, optimization algorithms have become sufficiently powerful and robust to be used in an industrial context. Global stochastic methods such as Genetic Algorithms proved their ability to and an optimum even on complex and discontinuous design spaces. For example, a missile design must satisfy aerodynamic performance for several flight conditions of the mission. The final choice is a compromise between these performance and other criteria such as structures, stealth, guidance... Pareto Fronts given by multiobjective algorithms are crucial tools in order to find the best compromise.
Fig.2 - Pressure repartition on the missile
The generic missile test-case is representative of a small ramjet-powered missile piloted in bank-to-turn. It has a cylindrical fuselage equipped with wings and fins. Since the flight is mainly conducted at positive angle-of-attack, one inlet is located under the fuselage. The inlet is on-design, i.e., with the shock wave formed on the ramp focusing on the cowl lip, at M = 2:2.
Although the final performance of the missile can be characterized only by simulating complete trajectories involving modelization of aerodynamics, thrust and guidance, characteristic flight points must be used during the design phase. In our case, the flight conditions for the ramjet system are the acceleration point where an important total pressure recovery and mass flow are needed at low Mach number and low angle of- attack, the cruise point where the fuel consumption has to be minimized and the manoeuvre point where the total pressure recovery of the inlet has to be sufficient even at a high angle-of-attack.
The automated loop used in this study links together a multiobjective optimization algorithm with computes the balance of a generic missile for three flight conditions: acceleration, manoeuvre and cruise. This computation is made using three data tables: a ramjet modelization with theoretical equations, internal aerodynamic coefficients and external aerodynamic coefficients. Since the inlet shape varies, the internal data are automatically computed with the 2ES2D CFD tool which predicts for each flight condition of the model the mass flow rate, total pressure recovery and the additive drag and lift of the considered inlet shape. One of the main point of this study is that the performance to be optimized is a result of two different and non-linear evaluation tools. The best missile performance do not necessary correspond to the best isolated inlet shape but to a design leading to the best propulsive performance for the inlet/ramjet aero-propulsive system on a missile design having specific external aerodynamics and mass.
Fig.3 - View of the multi-domain structured mesh
Three isolated inlets optimization were performed at M = 1:9 , M = 2:2 and M = 2:8 with the objective to find the best total pressure recovery. The Pareto Front is composed of 133 designs. Good levels of performance are achieved. Table 13 presents remarkable designs obtained during this study with their respective performance. The main purpose of the inlet design was to find a shape satisfying the whole mission. One can see that only multiobjective optimizations reach such designs.
Fig.4 - Shape and flow of some of the best compromise designs
References
[1] A. Gaiddon - Vehicle and Propulsion Dept. - MBDA France - F-92323 Chatillon, France
[2] D. D. Knight y - Dept. of Mechanical and Aerospace Engineering - Rutgers - The State University of New Jersey - 98 Brett Road, Piscataway, NJ 08854-8058