Pulsed Detonation Engine (PDE)

The Pulsed Detonation Engine (PDE) is a high thrust, air breathing propulsion system, powered by pulsed microwave or laser energy in the 0.3 mm - 10 mm wavelength range. The beamed energy first passes unhindered through the transparent upper hull, before being reflected off the off-axis parabolic rectennas and focused out through the lightcraft's perimeter (Figure 1). The beam then passes through the hull to a focal point 1 meter away, where the surrounding air is detonated into a high-pressure plasma. This plasma expands outward rapidly as it cools. The result is an inertial force applied to the lightcraft opposite to the direction of the expelled air. After each pulse, air rushes back to the focal point and is refreshed for a subsequent pulse (Figure 1). The energy is pulsed rapidly (hundreds to thousands of times per second, depending on the required thrust level) to provide continuing quasi-steady thrust. The frequency of these pulses is normally restricted to the sub-audible (below 20Hz) or super-audible (above 20,000 Hz) range so as to remain silent for covert operations. Directional control is achieved via thrust vectoring by way of the two superconducting rim magnets. The strength of the magnets is varied to force the expelled plasma either up or down, depending on the desired flight path, or torquing maneuver. This vertical vectoring, coupled with the ability to pulse separate sections of the hull perimeter at a time, allows for complete freedom of movement.

Figure 1
Cross-sectional view of Lightcraft showing PDE thruster.

Figure 2
Lateral Air-spike Mode

While operating in the lateral flight PDE mode, the lightcraft can also employ an air-spike via the central (or off-axis) parabolic rectenna just as when operating in the MHD accelerator mode. The air-spike allows the PDE to achieve higher speeds more rapidly, because of the greatly reduced aerodynamic drag provided by the air-spike. The lateral air-spike mode creates a plasma "wedge" ahead of the lightcraft when flying in a lateral direction (Figure 2). The PDE focal length is extended out in the direction of flight roughly 10 meters, depending on Mach number. Then rapid pulsing at this point causes a series of cylindrical blast waves that form together producing a wedge of hot air out in front of the Lightcraft's leading edge, thereby streamlines the blunt leading edge. With the lateral air-spike off, maximum flight speeds are limited to Mach 2 at sea-level altitudes. This process is shown in Figure 2.

The PDE is the most versatile propulsion system of the three employed on the lightcraft. It is capable of producing flight speeds from 0 to Mach 2.0 without the lateral air spike (and up to Mach 6 while employing the air spike) and can function effectively from sea level up to about 20 Km in altitude. One of the most important functions it serves is as the initial propulsive unit responsible for rapidly accelerating (up to 300 Gs) the lightcraft to Mach 2.0. From here the MHD fanjet engine can function effectively and complete the boost to orbit. The PDE's ability to perform from a standstill allows for effective evasive maneuvers such as "blink outs" in hostile environments when the crew might not be situated for a full boost to orbit. A "blink out" is simply a rapid acceleration (greater than 20 Gs) that is unable to be followed by the human eye. The result is simply conveyed as the illusion of the craft disappearing.

The design of the PDE creates the necessity for the thin film silicon carbide hull to be transparent to a variety of laser and microwave wavelengths of energy employed by the various lightcraft propulsion systems. Without the transparent hull the energy source could not be reflected and focused properly by the rectennas. The PDE is relatively simple in overall structure and requires only a few major components for its operation. The first important component is the off-axis parabolic rectenna, which in the PDE mode do not extract any energy from the beam. This rectenna contains a tripolarization arrangement of solid state silicon carbide, integrated microelectronic circuits (diodes, transistors, etc.) that are shut off so that 100% of the incident microwave beam is reflected and focuses out to the proper air detonation distance.

The second component crucial to the success of the PDE is the pair of superconducting magnetic coils that encircle and attach to the outer edges of the toroid hull (see Figure 1). These coils provide the applied magnetic fields that enable thrust vectoring to maneuver the ship under PDE power. The expanding plasma follows the maximum gradient of the magnetic field lines as they diverge away from the hull. Therefore the magnetic coils provide the capability of both a magnetic nozzle and thrust vectoring for the PDE. The SMES unit magnets provide redundancy in the system to safeguard against short term orbital power losses (i.e "hiccups" or temporary glitches in power beam delivery).

The final system component needed for steady and efficient operation is not actually a part of the PDE but the Lightcraft's hull cooling system. This recirculated heliox coolant system also allows the hull to withstand the elevated air temperatures produced behind the air-spike at hypersonic flight speeds. Pulsed plasma created by the PDE engine can reach as high as 10,000 K.

The maneuvering capabilities of the PDE are a direct result of the magnetic nozzle created by the pair of superconducting magnets at the rim, and the thrust vectoring derived by altering the "shape" of this magnetic nozzle. As the magnetic field is increased in either the top or bottom magnetic coil the nozzle is distorted down or up respectively. For example, if the electric current is increased in the top coil, its magnetic field is stronger than that of the bottom coil, and the magnetic nozzle would slope downward and the lightcraft would pitch upward.(See Figure 3)

Figure 3
PDE Thrust Directed Downward to Lift up on the Lightcraft Rim

Although the magnetic nozzle can influence the pitch of the aircraft, the direction of thrust vectoring is controlled by the combination of the magnetic nozzle shape and the azimuthal location of the active PDE thrusters around the Lightcraft rim. If the rear facing portion of the PDE thrusters were pulsed then the craft would move forward in a lateral flight mode (Figure 4). This combination of vectoring allows for lateral movement in any desired direction perpendicular to the microwave power beam without any mechanical control surfaces.

Figure 4
PDE Thrust aligned with place of Lightcraft symmetry.

The Lightcraft is powered by microwave radiation beamed from an orbital power station with super accurate pointing and tracking capabilities. With the cooperative lightcraft "target". The spin axis of the lightcraft must be aligned to within 1 or 2 degrees of the incident microwave beam whenever line of sight allows successful beam transmission. The lightcraft's reflective rectennas can support the use of several wavelengths of microwave power depending on the weather, the propulsion mode being operated in, and the craft's location in the atmosphere. Viable wavelengths range from 0.3 mm to 10 mm. Shorter wavelengths have a higher breakdown intensity and this intensity decreases for all wavelengths as the craft gains altitude.