Ion Propulsion System 

The Lightcraft ion propulsion system is used primarily for subsonic, low-performance maneuvers. These maneuvers generally include aerial taxiing, passenger retrieval functions, and low-observable covert operations. The maximum speed the Lightcraft can attain using this air breathing propulsion system is approximately 160 km/h.

The key to the success of the Lightcraft's ion drive system is the buoyancy of the Lightcraft. The heliox gas mixture that pressurizes the craft also provides approximately 16.5 kN of lift at 20 degrees Celsius. At that temperature, the ion propulsion unit needs only to provide an additional 8 kN of lift to hold the 24 kN vehicle in the air.

The pulsed ion propulsion system that is integrated into the Lightcraft differs from its predecessors in its lack of physical cathodes and accelerator grids. The craft emits pulsed 1 MeV electron beams, thereby becoming positively charged as it forms a negative ion cloud in the direction of travel. The electrons attach themselves to oxygen and water vapor molecules in the air, thereby forming negative ions. These ions are then attracted toward the ship through a large electric potential, and this momentum exchange with the atmosphere results in a distributed electrostatic thrust force on the Lightcraft hull.

In most cases the ion propulsion system must produce a vertical component of thrust because the vehicle is only partially buoyant. This vertical component is a result of charged air rushing over the top of the craft as it is accelerated downward through an electric potential. The wake left by this rushing air creates an "ion plasma cone," within which air is recirculated. In flight, this cone looks like a tail hanging off the bottom of the Lightcraft

The loading that the ion propulsion system induces on the disc is an important characteristic, just as it is in conventional rotorcraft. This disc loading is well within an acceptable range, however, as can be seen in the graph below ; it is an entire order of magnitude lower than the rotor loading of even standard ultralight helicopters.

a) Electron beam is ejected and begins to bloom.

b) Electron beam is fully developed and charges the surrounding air.

c) Negatively charged air mass is accelerated past the positively charged hull.

Fig. 7.1.4 Starting, cruising, and stopping maneuvers with free- body-diagrams

     


The ion propulsive drive is made possible by three essential processes. First, the onboard electron accelerators charge from the Lightcraft to the surrounding atmosphere. The negative charges then attach to oxygen and water vapor molecules, and are finally accelerated back toward the craft.

The electron beam is not ejected in a straight collimated path away from the ship. The beam disperses because of atmospheric scattering and Brehmsstrahlung effects, in which the electrons induce photon emission in atmospheric atoms, which then cause further ionization and electron emission. In practice, the electrons produce a bell-shaped ion cloud at an average distance of 5 m from the craft.

The transmitive efficiency of the beam is pressure-limited. The combined effects of virtual cathode instability, mode instability, and two-stream instability limit the performance of the electron accelerators. Of course, this pressure sensitivity, along with decreasing air density at higher altitudes, equates to an altitude limit. The Lightcraft has a conservative ion propulsion mode ceiling of approximately 10-15 km.

In free air, the charge cloud has a decay time of about 4 ms, which means in order for the cloud to produce sufficient thrust, it must be recharged frequently. In fact, the electron accelerators pulse between 10 Hz and 200 Hz in free air. Once the ion cloud is formed, it expands and is attracted toward the ship simultaneously, dragging much of the surrounding neutral air mass along with it. This process produces a low-pressure region in front of the craft, which translates into increased forward thrust. The force of this thrust can be calculated, and is given for three different cloud configurations in Table 7.2.1. The corresponding maximum lateral velocities are also given. For representations of these three ion cloud configurations.

Table 7.2.1
Ion thruster performance at sea level

Ion Cloud Geometry Azimuthal Spread (phi)
60-degree 90-degree
Off-Angle (30 deg.) 1359 N
30.65 m/s
2039 N
37.54 m/s
Dual-Off Angle (+/- 30 deg.) 1917 N
36.4 m/s
2878 N
44.6 m/s
Straight-Ahead (0 deg.) 1439 N
31.54 m/s
2158 N
38.62 m/s

The ion drive system shares components with many of the other systems aboard the Lightcraft. The superconductive can be used to store energy for the ion engine in stealth mode and the rectenna array can be used to gather low-power microwave energy for the engine. The electron accelerators that produce the ion cloud are also employed in the creation of the Space Plasma Shield, which requires 30 keV electrons. However, there are some main components that are specifically designed for the ion system.

The accelerators were developed specifically for the Lightcraft and are 33 times more powerful than required by the plasma shield. They are the most efficient 1-MeV, low-mass electron beam guns ever produced. These accelerators fire a beam of electrons away from the craft to create the ion cloud. There are 24 electron accelerators spaced radially about the Lightcraft, 12 pointed "upward" at 30 degrees, and the remaining 12 pointing "downward" at 30 degrees.

The thin film GaAs photovoltaic cells that cover the ventral surface of the craft can be used to collect either incident solar energy or beamed laser light (at 860 nm) and power the ion propulsion system in the inverted landing mode. The GaAs crystals that form these cells are grown on the inside surface of the outermost hull layer in order to protect them from the abusive space and atmospheric environment and allow them to be actively cooled by the Lightcraft closed-cycle heliox cooling system.

The GaAs array is approximately 25% efficient in the solar-powered mode, but as high as 60% efficient in the laser-powered mode. Of course, power densities in the laser mode are significantly higher than in the solar mode; active heliox cooling of the hull becomes extremely important under laser power. Table 7.4.1 gives power densities and maximum power outputs of the GaAs array under both solar and laser power in nominal conditions. Maximum thrusts are based on available energy and a thrust/power coupling coefficient of 30 kN/MW. Actual thrusts are also dependent on atmospheric conditions, equipment specifications, and other variables.

Table 7.4.1 Max thrust of Ion Propulsion System
(assumes coupling coefficient of 30 kN/MW)

Power Source Power Density (kW/m^2) Onboard Power (kW) Maximum
Thrust
(kN)
Laser 22.29 4200 1260
Solar 1.373 107.8 32.34

The outer hull surface of the Lightcraft is covered with electric and magnetic field sensors, which feed this critical data back to the Central Onboard Processor, to precisely control the electric potentials everywhere around the craft. This monitoring helps in the prevention of electric discharges and in the immediate repair of discharged areas of the ion cloud. When the potential between the ion cloud and the Lightcraft exceeds the breakdown threshold of the local atmosphere, an arc discharge, similar to atmospheric lightning, occurs. This discharge destabilizes ion cloud formation momentarily in regions near the arc, because the negative charge is attracted back to the positively charged hull. The computer senses this potential drop and compensates as much as possible. In the event of multiple discharges and subsequent attempts to repair the ion cloud, however, flight will become erratic.

The above sections have covered the general theory, application, and necessary components of the ion propulsion. Here, the emphasis is on specific usage of the ion propulsion engine. It is important to know not only the nominal course flight performance but also the limits to which ion propulsion technology can be pushed during maneuvers. The performance charecteristics for the Lightcraft; attitude control, flight maneuvering, and performance charecteristics are further elaborated below in three subcategories. The three subcategories are 1. vehicle orientation 2. thrust, power, and efficiency and 3. terminal velocities.

The Lightcraft is lens shaped, so only pitch and roll apply; there is no yaw, but rate of spin is an important parameter. (spin torque, yaw, is activated by the PDE mode in conjunction with azimuthal MHD forces) Which charges are ejected, and at what angle to the plane of symmetry, determines whether pitch or roll is produced as well as their respective rates of rotation.

Pitch is produced by ejecting charges either in the forward or rearward direction at an angle to the plane of symmetry. Ejecting a charge in front of the Lightcraft at a positive angle to the plane of symmetry produces a torque that pitches the vehicle counter-clockwise. Using the given force of 785 N at 30 degrees, with a torque arm of 7.12 m, results in a torque of 5589.2 Nm and a pitch of .288 rad/s.

Rolling the Lightcraft requires charge ejection from both lateral sides of the Lightcraft at right angles to the flight direction. A rolling maneuver is carried out if we assume magnitude of the charge, and ejection angle of +- 30 degrees used in the pitch maneuver, resulting in a pitch rate of .576 rad/s. To induce a 90 or a full 180 degree flip, the +- 30 degree clouds must be ejected, or for quicker rates the +- 60 degree clouds should be ejected.

The maximum feasible thrust that can be produced by the Lightcraft's ion drive is 14.6 kN. Maximum velocities using each of the previously described cloud configurations are given in Table 7.5.2. These values are based on aerodynamic drag and thrust calculations and test flight data.

Table 7.5.2 Maximum velocities for different ion cloud configurations

Configuration 60-Degree Cloud 90-Degree
Cloud
Off-Angle 30.65 m/s 37.54 m/s
Dual Off-Angle 36.40 m/s 44.60 m/s
Straight-Ahead 31.54 m/s 38.62 m/s

The Lightcraft routinely executes a number of low-performance maneuvers that use the ion propulsion system to evade detection by potential threats. These maneuvers include a low-noise, low-visibility silent running mode, microwave beam engagement procedure, and two methods for hiding among and within cloud cover.

By using the energy stored in the field of the main magnets, the Superconducting Magnetic Energy Storage (SMES) unit can power the ion drive for longer than 50 minutes at low thrust levels (see Table 7.6.1). This power source eliminates the possibility of detection of power beams and allows the Lightcraft to be autonomous and self-sufficient over a short range.

Table. 7.6.1 Duration of SMES unit

SMES Energy
(MJ)
Vertical Vel.
(m/s)
Vertical Thrust (kN) Thrust Power
(kW)
Efficiency (%) Time (min.)
100 3 5.4039 32.4234 50 51.4
100 3 9.7826 58.6956 50 28.0
100 3 11.2878 67.7223 50 24.6