MHD Propulsion System
The magnetohydrodynmaic (MHD) slipstream accelerator is used for Mach numbers greater than 2. MHD acceleration involves the conversion of electric power to kinetic energy. The conversion is accomplished by the interaction of air with the on-board intense magnetic fields.
The MHD accelerator is energized by beamed microwave power that is converted by the lightcraft into electric power. This conversion process is accomplished by two 35GHz rectifying antennas. The efficiency is about 85%. The antennas are configured to capture and utilize 5 to 7% of the 10 billion Watt microwave beam that is normally reflected and lost.
The MHD accelerator will start when the microwave power station in space beams a direct microwave link. The propulsion system is designed to accelerate the Lightcraft in flight directions either lateral or vertical to the beam (Fig 5.0.1). Also, the lightcraft is designed for travel both around the earth and into space. The microwave beam must be aligned with the lightcraft axis of symmetry with in a few degrees of accuracy. In transatmospheric flight, the lightcraft hull reaches temperatures that exceed 2700K. These temperatures could never be accommodated without ceramic materials.
Vertical and Lateral Flight Directions
The principle advantages of the MHD system are high engine efficiency in the hypersonic transatmospheric flight environment, high temperature, plasma compatibility, and savings in launch mass of expendable fuel.
The MHD slipstream accelerator is used for accelerating into orbit. An electric current is discharged through the air between the rim electrodes.
The action of the on-board magnetic fields and this electric discharge will accelerate the air downward using a Lorentz force. The air will create a force to accelerate the lightcraft in a direction of flight that is opposite of the Lorentz force.
The Lorentz force is always directed opposite that of the flight vector. The gaseous Air Spike fore-body of the lightcraft can serve as an effective hypersonic air inlet. To exploit the atmospheric environment to the maximum the best type of hypersonic engine is an air-breathing engine.
The MHD accelerator system includes the Air-Spike (hypersonic inlet), receiving antennas, 2 rectennas, 9 super conducting magnets, rim electrodes, electric power switching circuitry and an open-cycle rectenna cooling system. Flight propulsive forces received by the rectennas are circulated to the air by means of electromagnetic fields. The forces effectively lift on the lightcraft rim magnets. Then, the system is cooled by the open-cycle cooling system.
The 35GHz microwave beam had an atmospheric transmission limit of 4kW/cm2. The transmission will target the vehicle with an 18m diameter beam centered around the vehicle's axis of symmetry. As shown in Figure 5.2.2 two rectennas receive the microwave power beam from the satellite. The two 35GHz rectennas are 18m and are located in the lightcraft interior. The rectenna panel thickness is 2.143mm, and the reflecting back plane is spaced 1/4 wavelength behind the front surface. The rectenna is a tri-polarization array designed to assure 33% redundancy of the dipole antenna elements. A high packing density of dipoles per unit area is utilized about 21/cm2 incident. The rectenna can be programmed to reflect 10% to 100% of the incident microwave power beam on demand.
The rectenna is used to convert the microwave beam energy from the satellite. They convert the microwave beam into electric power. This electric power can be used by the lightcraft. The lightcraft will take the electric power to travel at subsonic speeds. An ignition circle is created around the lightcraft's periphery. The ignition circle is created by using the microwave energy to blast the air into plasma. The ionized air or plasma that is created is forced to the rim of electrodes.
There are two primary super-conducting magnets located at the lightcraft rim . Both are attached to the annular pressure vessel and provide the magnetic field for the MHD accelerator. Other superconductors of a smaller ring are limited electrically with the twin rim coils to form a superconductor magnetic energy storage (SMES) unit.
The superconducting magnets will catch the plasma as it expands. The plasma is caught by creating electromagnetic fields. These fields are created by the magnetic coils. Magnets create a magnetic nozzle that propels the plasma. The nozzle of the magnetic field generates a thrust. Plasma is propelled downward. This downward force will give an opposite but equal reaction propelling the lightcraft upwards.
Electrodes are placed all around the lightcraft's exterior. The air that is around the lightcraft is from forced ionized air from the rectennas. The electrodes help create an electric field in this ionized air. A current will jump from one electrode to the other. This jump will help the magnets with creating a magnetic field.
This energy is then refined into a direct current that is delivered to the rim electrodes of the MHD slipstream accelerators. Due to the extremely high power density of the rectenna array about 60MW/kg. The tri-polarization was selected over the dual polarization option in order to provide a 33% redundancy.
The microwave beam illuminates the lower surface of the lightcraft. A barrage of energy is created from the microwave beam. This energy will break up the air. To break up the air, the energy will take the molecules of the air and separate them. When the molecules are broken up, the molecules turn into plasma. The lightcraft takes the magnetic fields to create a false surface away from the vehicle. A false surface will help the plasma be whisked away. This plasma creation and movement will shove the lightcraft skyward. The air generated by the plasma helps the shock waves to be pushed away from the surface of the vehicle. The reduction in shock waves creates a smoother ride for the passengers of the Lightcraft.
One fundamental component of the Lightcraft's propulsion system is called an Air Spike. Originally proven at Mach 10 in late April 1995 at Rensselaer Polytechnic Institute, the Air Spike concept is used only at supersonic flight velocities.
The air spike system (see fig. 5.5.1) uses the parabolic microwave reflector, located on the top hull of the lightcraft, to reflect part of the microwave beam to a point ahead of the vehicle. An explosion occurs here and shock waves are created ahead of the lightcraft. The shock waves drive air out of the vehicle's path, and thereby greatly reducing drag and creating an inlet for the Lightcraft's MHD Fanjet.
The Lightcraft employs an active Thermal Management System (TMS) to reduce the excess heat generated in flight during various propulsion modes. This preserves the structural integrity of the ship's hull as well as maintaining acceptable temperature levels for human survivability. The TMS uses the breathable heliox mixture which inflates the craft to draw excess heat away from the rectennas, and converts it to steam to be jettisoned from the craft during heat critical maneuvers.
The MHD slipstream accelerator is capable of propelling the Lightcraft in both axial and lateral flight directions. The choice of which flight mode is often based on the position of the lightcraft with respect to the power station or the requirements of the desired maneuver.
The axial flight mode is certainly the most common when using the MHD engine. Axial flight is used exclusively for high G acceleration into space. In a region of dense atmosphere, however it may be desirable to use the MHD engine in lateral flight direction. The lateral flight direction will be used until the lightcraft climbs out of the dense atmosphere. Then switched to the axial flight mode. A hyper-jump, although usually done in a vertical (i.e. axial) direction, is sometimes made in an oblique or horizontal (i.e. lateral) direction. If the MHD engine is used for such a hyper-jump, the lateral flight mode is a necessity.
A key to being able to use the MHD accelerator in the lateral mode is the ability to create a wedge shaped Air Spike over the leading edge. The Air Spike is pulsed to reduce compressibility effects on drag along the effected length of the vehicle. The objective is to change the high drag around the shock patterns that form over the leading edges of the lightcraft into low drag oblique shock geometry. Due to the oblique shock wave, asymmetry over the lens shaped vehicle effects the vehicle when it is accelerating rapidly. The pulsing of the Air Spike greatly reduces vehicle drag and heat transfer. This makes axial flight mode more efficient and capable of higher acceleration performance.
Another concern in the lateral flight mode is the positioning of the power station. Specifically, if the axis of symmetry is not aimed directly at the station, as it is in the axial flight mode, the MHD engines can not function. The MHD engine will not be getting enough of the microwave beam. When traveling laterally, there are therefore two options. The first option is to travel a circular arc path. Hence, the satellite power source travels along the segment of the microwave beam. This makes the lightcraft fly along the radius of the circle that is created. At any point on the circular path, the lightcraft departs the arc and coast engine off until it pitches over to enter the next arc segment. The second option is to receive the microwave energy when the lightcraft is aligned with the energy source. This momentum accelerates the lightcraft using the MHD in lateral flight. The lightcraft can then coast at this new velocity. Then, the pulsed detonation engine (PDE) when running on internal SMES power does not require the beam. The CCFS can be used to change from the MHD lateral flight mode to the PDE mode. The PDE mode can be used to vector the lightcraft at any point into alignment for the next power pulse. At the next power pulse the internal power of the MHD accelerator is fixed again. This second option for lateral flight mode offers more flexibility than the first. The problem is that it is greatly restricted at lower flight speeds.