Hypersonic Glide Vehicles
Hypersonic glide vehicles are launched to a high altitude on a rocket or other boost vehicle and then glide to their destinations at hypersonic speeds. Hypersonic glide vehicles have been studied since the 1930s, and extensively studied beginning in the mid-1940s. By the 1950's, balancing range and speed, hypersonic glide vehicles had emerged as the best choice over competing skip, skip-glide and ballistic trajectories for unpowered flight from above or near above the atmosphere.
A hypersonic glide vehicle achieves range by exchanging its kinetic energy, plus a small amount of potential energy, for distance. The kinetic energy is used to overcome the aerodynamic drag forces encountered during the course of the flight. Thus the aeroballistic glide concept differs from the conventional notion of gliding flight in that, for the latter case, the main source of energy is the potential energy, and altitude is exchanged for distance rather than velocity.
A particular factor in making those determinations was heat from aerodynamic heating, primarily the heat from atmospheric friction. For example, while skip trajectories may be more efficient in converting the kinetic energy of such vehicles into range, the increased heat and other aerodynamic loads required to achieve those efficiencies are greater than most modern materials can handle. Heat load primarily comprises instantaneous heat loading, the maximum rate of heating at any time during an entire glide trajectory and, often more importantly, the total heat load, or integrated heat load, over an entire glide trajectory.
Hypersonic glide vehicles can achieve their significant efficiencies within the limits of modern materials, but not without large amounts of heat shielding, significantly reducing available passenger and cargo space and weight carrying capacity, and often not without additional mechanically complex heat removing systems.
Such heat removing systems typically attempt to radiate the heat away from the vehicle and delay heat from entering the vehicle. Once heat enters the vehicle, additional internal components are needed to manage that internal heat. The more integrated heat load can be reduced, not only can the need for heat shielding be reduced, but the need for elaborate heat removing systems eliminated.
Heat load can, of course, be reduced by reducing glide speed, such as by having the glide vehicle perform a slow spiral to return to earth. Reducing glide speed, however, reduces range and, more importantly, removes a primary reason for a hypersonic glide vehicle, time to destination. The prior art has investigated a variety of different trajectories, or trajectory paths, for reducing heat load with minimum loss of both range and time to destination.
Hypersonic glide trajectory studies usually begin with a so-called maximum lift-to-drag ratio (L/D) glide where the angle of attack of a glide vehicle is held at an angle providing a maximum lift-to-drag ratio and can be shown to result in both minimum drag and maximum range.
Maximum L/D glide trajectories result, however, in higher speeds and higher instantaneous heat rates. A modified glide trajectory with constant speed limits the maximum speed, but reduces range. The balance between speed and range can, of course, be adjusted by periodically trading speed for altitude by alternatively increasing and decreasing angle of attack. Increasing altitude helps preserve range, but also increases drag (and resulting integrated heat load) and time to destination.
Trading speed for altitude can occur naturally by so-called phugoid motion. A phugoid, pronounced "f{hacek over (e)}w-g d," motion is an oscillating aircraft motion where the aircraft pitches up and climbs, and then pitches down and descends, accompanied by speeding up and slowing down as it goes "uphill" and "downhill." Phugoid motion alternately trades kinetic for potential energy and back again. Phugoid motion is generally something sought to be avoided, or at least reduced.
A particular problem with such tradeoffs is integrated heat load, the total heat load over time. If some of the heat cannot be dissipated during glide, integrated heat load can be more critical than maximum instantaneous heat rate.
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