Hypersonic Waveriders
Summary
The waverider is the fastest wing of all, the wing for Mach 6 and beyond. The concept of waverider has evolved over the years. Nonwieler, 1963, devised the concept of waverider as a lifting system generated from streamlines behind a known shock wave.
Kuchemann, 1978, in his fundamental text on aircraft design saw the waverider a technical possibility by assuming a constant value of the product between propulsive efficiency and aerodynamic efficiency.
Using the Breguet range formula, Kuchemann concluded that a non-stop trans-atmospheric flight at Mach 8 to the farthest point on Earth is possible.
In light of recent research on supersonic transport vehicle, this idea seems unfeasible. However, the current interest in waverider lyes in its possible replacement of the launch systems for the space craft (new generation of spacecraft) and for Aero-Assisted Orbital Transfer Vehicles (AOTV).
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Figure 1: Conically-derived waveriders
From a theoretical point of view a waverider is a conically-derived lifting system, that is a system that is always behind the shock wave generated by itself (Fig. 1). More practically, the waverider is a wing, an aircraft, and a propulsion system at the same time (Fig. 2) , therefore it requires a multi-disciplinary integrated design.
Preliminary studies addressed, among other issues, the aerothermodynamic heating, the thermal and structural fatigue, the aeroacoustics, the propulsion system, the type of fuel.
Figure 2: Hypersonic airplane based on waverider theory
Air-Breathing Engines
The propulsion system for hypersonic speeds is a scramjet (= supersonic combustion ram jet), that is a system without moving parts. (A recent review of scramjet technology is available in Curran-Heiser-Pratt, 1996.)
In the concept design illustrated in Fig. 2 the scramjet is placed roughly at the
center of the waverider, so that the forebody acts as a dynamic compressor for the
engine, the aft body acts as an expansion nozzle.
From a review of existing studies:
At M = 25 aerodynamic efficiency is estimated at about 3÷4; peak skin
temperatures at the nose are of the order of 2000 K (higher in the inlet and the
nozzle); acoustic noise level at 170÷180 dB.
Lower values of the quantities reported above are predicted for a M = 10
trans-atmospheric plane: for example the maximum skin temperature is of the order of
1300 K.
The Lockheed SR-71A Black Bird holds at least 3 absolute records, achieved at
speeds above M = 3 (among which the fastest eastbound transatlantic flight in 1 hour
and 55 minutes, 1976). No longer flying, this aircraft reached top speeds of the
order of M = 3.5. NASA’s experimental aircraft X-15 reached M = 6.7 on solid rocket
propulsion in 1967. NASA’s new X-43 HyperX is set to reach M = 7 and beyond on
air-breathing propulsion (with boosted launch), thus making a practical demonstration
of the viability of scramjet engines.
Most of the programs to date are focusing on aircraft-launched systems, which would
be further boosted by a rocket to speeds high enough to ignite the scramjet (at least
M = 4-5).
Another idea that is being analyzed is the intermittent hypersonic scramjet. An
aircraft with such a system would boost its speed to M = 10 into transatmospheric
flight, shut off its engines to coast back into atmosphere and restart again for a
number of times. This roller coasting could also limit the effect of
aero-thermodynamic heating.
A propulsion system that would work from rest to hypersonic Mach numbers remains a
dream not to be fulfilled for a long time. Research programs are ongoing in many
parts of the world (USA, Europe, Australia, Japan).
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Selected References
Prentice Hall International, 3rd edition, 1998.