Experimental Investigation of Starting Process for a Variable Geometry Air Inlet operating from Mach 2 to Mach 8

A lot of key issues have to be addressed for developing the dual mode ramjet technology. But the main difficulty is to design a propulsion stream tube, which allows in the same time the constraints for obtaining a good performance at high flight Mach number (supersonic combustion) and those for ensuring a stable operating at low flight Mach number (subsonic combustion). If the overall Mach number range is limited (Mach 4-Mach 8), a completely fixed geometry can be defined for the propulsion stream tube with relatively good possible performance in the whole flight envelope. But, even if this limited Mach number range could be sufficient for missiles application, it is clear that airbreathing propulsion can not have any interest for reusable space launcher application if it is not able to power the launcher from Mach 2- to Mach 10+. This extension of the Mach number range requires some variation of the propulsion stream tube geometry to obtain acceptable performances. Indeed for higher Mach numbers, it will be necessary to use a high contraction ratio for the inlet in order to limit the supersonic Mach number at the entrance of the combustion chamber and a less and less diverging combustion chamber as Mach number will increase. At contrary, in order to extend the flight envelope to lower Mach numbers, the inlet contraction ratio must be reduced (with or without reduction of the capture area) to avoid a too large cowl spillage and the corresponding additive drag section of the combustion chamber must be open. Several approaches can be considered to fulfil this required adaptation of the propulsion stream tube geometry. One of the simplest somutions consists in obtaining the combined variation of the inlet and the combustion chamber geometry by a simple translation of the inlet/combustion chamber cowl along an axis close to horizontal axis of the vehicle. Thanks to this translation, it is possible to adapt the contraction ratio to the flight conditions in the Mach number range from 2 to 8 and then to obtain good performance at higher Mach numbers without penalty for the low Mach number regime. Such an air inlet has been designed and tested in ITAM blown-down and hot-shot windtunnels in the Mach number range from 2 to 8. The modularity of the model and its extensive instrumentation allowed acquiring a very valuable data base contributing to the knowledge of key parameters determining inlet performance and building a performance model. As the considered air inlet is mainly an internal compression one, a particular effort has been made to understand the self-starting limits and provide some improvement by using a porous bleed located on the compression ramp. Moreover, the effect of windtunnel test section starting on the controlled starting of the air inlet has been evaluated by comparing results obtained in blow-down windtunnel (long starting of windtunnel) and in hotshot windtunnel (short starting) and by using a specific starting device in blow-down windtunnel to quickly open the air inlet only when the flow is fully established in the test section.