1.5 Intake starting/unstarting

1.4 Streamline tracing: wavecatching, morphing

Hypersonic Vehicles - Past, Present and Future Developments

The design technique of replacing known streamline sheets garnered from simple flows, by solid surfaces to generate aerodynamic shapes, known as "streamline tracing" has been applied to wing-body shapes [20]. The resulting airplane shapes are named "waveriders." The objective in waverider design has been to generate airplane-like shapes that produce high ratios of airplane lift-to-

"wavecatching," is applied to intake flowpath design [4, 7, 13, 14, 21–26] to generate intake surfaces. The objective, in this case, is to generate intake surfaces that capture, support and contain internal flows that have a high performance as supersonic/hypersonic air intakes. Both waverider and wavecatcher applications rest on the fluid-mechanical principle of replacing impervious streamline sheets by solid wall surfaces. In both applications the design starts with the selection of a freestream capture area cross-sectional shape. The shape is projected, as a closed trace, onto the leading shock wave of a prescribed simple flow. In the case of wavecatchers, the trace becomes the leading edge of the intake and the shock wave or Mach wave covers the leading edge of the intake at the design condition. All the mass flow passing through the trace is captured into the intake. The streamtube extending downstream from every point of the closed trace forms the shape of the intake's flowpath. By selecting a suitable shape for the entry flow trace, much design flexibility is available in integrating the resulting engine flowpath with the airframe shape and the intake's exit flow shape to suit the combustor. Pre-selecting the simple flow determines the internal flow as well as the intake performance. The

drag force. The same streamline tracing technique, in this case called

technique is equally applicable to planar and axial flows as well as to nonsymmetric flows. The basic wavecatcher technique, when applied to symmetric flows, produces geometrically similar flowpath cross sections. Various methods of morphing can be applied to gradually change the flowpath shape if the entry and exit cross sections are not to be geometrically similar [7, 11, 26]. Two very important extra virtues of the wavecatcher design method are that it produces flow paths with swept leading edges, much like a sugar scoop where, at the design Mach number, the leading shock is everywhere attached to the leading edge; there is no flow spillage. However, at off-design conditions, such a swept leading edge does permit overboard mass spillage during intake flow starting, making otherwise nonstartable intakes startable. Experimental results on wavecatcher intake shapes, based on Busemann flow, were presented in [4]. Using streamline tracing methodology, based on the flow in a straight conical duct, the notion of selecting portions of the axisymmetric versions of internal flow was used also in [21–23, 27]. The significant virtues of wavecatcher intakes has been utilized in many subsequent intake studies [10, 13, 21–23, 28–30]. The technique of streamline tracing, to produce modular flowpaths of arbitrary cross sectional shape (wavecatching), results in the following attractive aspects: (a) the total mass flow is divided

between individual modules, reducing the mass flow demand of test facilities, both wind tunnels and flight test, (b) thrust vector control is easier to implement with flow in individual modules, (c) highly swept module leading edges make mass flow spillage possible for intake flow starting, (d) module freestream capture shapes are easily integrated with airframe shapes, (e) modules can be raised off the airframe surface so as not to ingest the fore-body boundary layer, (f) properly designed

Wavecatching and morphing techniques for modular Busemann intakes will be

modules are self-startable.

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discussed in Section 7; module startability in Section 8.

For spontaneous ignition and supersonic combustion, the Mach number at combustor entry should be about one-third of the flight Mach number. In a flight Mach number range 4–25, the intake cross-sectional area must decrease by a factor of 5–20. Such a highly convergent duct can support two distinctly different flow configurations at any given supersonic flight Mach number. One flow type produces a bow shock in front of the intake that diverts much flow overboard and, in this case, the intake flow is subsonic with unacceptably low performance. This is termed "subcritical" or "unstarted" flow. The other possible flow configuration has no bow shock, no overboard spillage and is supersonic throughout. This "supercritical," or "started" flow, is required for efficient scramjet engine operation. Attainment of supercritical flow in high contraction ratio intakes present a problem in that the intake flow will not assume the started flow state spontaneously under steady flight conditions. Starting requires that the near-normal bow shock, in front of the unstarted intake, moves downstream into the intake to be "swallowed" and that a stable hypersonic/supersonic flow is established throughout the converging portion of the intake. Spontaneous starting will not occur in intakes whose exit-to-entry area ratio is below 0.6. Unfortunately, startable intakes with exit-to-entry area ratios at or above this value do not produce enough compression to be useful as scramjet intakes. Methods of intake flow starting must be found and implemented for high contraction intake flowpaths. Intake starting is not open to design compromises; it is a critical, non-negotiable requirement that presents challenges and places severe conditions on intake design. Since startability is determined, to a limited extent, by flight Mach number, there is some design flexibility in choosing the start Mach number. Various methods of promoting intake flow starting have been explored in [9, 11, 18, 25, 27, 31–37]. Section 8 presents an analytical approach to the design of spontaneously starting, modular Busemann intakes.

Unstarting of started flow is also a concern in that flight at an extreme angle of attack or at combustor overpressure conditions can cause the intake to regurgitate a stopping shock and the intake flow to revert from a started condition to a condition of unstart. Such an event must be prevented since it is followed by a catastrophic loss of thrust. A review of research progress on detection and control of unstart mechanisms of hypersonic inlets is described in [27, 34].
