Sizing of scramjet vehicles

Department of Mechanics and Aeronautics University of Rome La Sapienza Via Eudossiana 18, Rome 00184, Italy

Abstract

The current European project LAPCAT II has the ambitious goal to define a conceptual vehicle capable of achieving the antipodal range Brussels-Sydney (~18,000 km) in about 2 h at Mach number Ma = 8. At this high speed, the requirement of high lift to drag (L/D) ratio is critical to high performance, because of high skin friction and wave drag: in fact, as the Mach number increases, the L/D ratio decreases. The design of the vehicle architecture (shape and propulsion system) is, as a consequence, crucial to achieve a reasonably high L/D. In this work, critical parameters for the preliminary sizing of a hypersonic airbreathing airliner have been identified. In particular, for a given Technology Readiness Level (TRL) and mission requirements, a solution space of possible vehicle architectures at cruise have been obtained. In this work, the Gross Weight at Take-Off (TOGW) was deliberately discarded as a constraint, based on previous studies by Czysz and Vanderkerkhove [1]. Typically, limiting from the beginning, the TOGW leads to a vicious spiral where weight and propulsion system requirements keep growing, eventually denying convergence. In designing passenger airliners, in fact, it is the payload that is assumed fixed from the start, not the total weight. In order to screen the solutions found, requirements for taking-off (TO) and landing as well as the trajectory have been accounted for. A consistent solution has finally been obtained by imposing typical airliner constraints: emergency take-off and landing. These constraints enable singling out a realistic design from the broad family of vehicles capable of performing the given mission. This vehicle has been obtained by integrating not only aerodynamics, trajectory, and airliner constraints, but also by integrating the propulsion system, the trimming devices and by doing some adjustments to the conceptual vehicle shape (i. e., spatular nose). Thus, the final vehicle is the result of many iterations in the design space, until performance, trajectory, propulsion systems, and airport constraints are successfully met.