Stability and control of the Lightcraft in transatmospheric flight is essential in both lateral and axial flight modes. This section will present a basic overview of the Lightcraft's flight dynamics, stability, and control in subsonic, supersonic, and hypersonic environments.
INTRODUCTION TO FLIGHT DYNAMICS
The Lightcraft aeroshell is merely a lenticular disc with no traditional control surfaces to direct or stabilize its flight path. The absence of fins, and any other surfaces jutting out from the smooth lenticular surface is a direct consequence of design requirements for the space plasma shield. In the subsonic ion propulsion mode such surface irregularities would trigger massive corona discharges off these edges and prevent the envelope from charging up for flight; in space, the hull would not be able to reach the 200 million volts needed to reflect solar proton storms. Attachment of any physical control surfaces to the thin pressurized hull of the lightcraft would be extremely difficult. Such fins and stabilizing surfaces would compromise structural integrity during the high Mach number maneuvers for which the craft is designed. Due to the lack of control surfaces the lightcraft must instead rely on active thrust vectoring from its Ion, PDE, and MHD propulsion systems.
The real time interaction of lift, drag, and vehicle moments of inertia determine the flight dynamics of a vehicle. The response of the Lightcraft lightcraft to flight control inputs is modeled by a series of equations stored within its control computers. The Lightcraft's flight characteristics set it apart from all conventional aircraft, primarily in the following areas: Drag on the vehicle can be reduced at will, by using air-spike technology. The craft can accelerate at 200 Gs or more. The craft is able to fly in both vertical and lateral directions. The lenticular disc hull of the Lightcraft is unstable in lateral flight.
The Lightcraft lightcraft is dynamically unstable in several flight modes, and is designed for outstanding maneuverability in subsonic as well as hyper-energetic flight regimes. With the aid of its 3 flight computers, and active thrust vectoring, the Lightcraft enjoys stable and controlled flight throughout its transatmospheric design envelope.
SUBSONIC FLIGHT REGIME
The Lightcraft's ion propulsion system enables flight speeds up to 44 m/sec, whereas the pulse detonation engine (PDE) can quickly push the lightcraft supersonic. During calm atmospheric conditions at low subsonic velocities, the ion propulsion mode does not require gyroscopic stabilization. Hence the crew can remain on the bridge on the observation deck. This means that the lightcraft is capable of performing the low altitude flight maneuvers necessary for rescue or reconnaissance missions.
STABILITY AND CONTROL (ION)
The ion thrusters use electromagnetic fields to vector the engine exhaust. The lightcraft can be programmed to fly in any direction, without necessarily having to first pitch or roll. High winds are one of the adverse weather conditions most often faced by the lightcraft under the ion thrust mode. With partial buoyancy at sea level, the 20 m craft can be blown around by high wind gusts. Uncompensated, each gust can tilt or flip the craft, or throw it off course. The ion engines are capable of compensating for gusts up to 100 mph. Once the craft's sensors detect even a minute unintended change in either the crafts attitude or lateral position, the ion propulsion unit is used to compensate, by projecting the ion clouds at the necessary angles to maintain the crafts previous tilt and position.
ADVERSE FLIGHT CONDITIONS
Adverse flight conditions, whether natural or artificial, have always caused a variety of threats to flying vehicles. The lightcraft is no exception. The lightcraft faces difficulties from 2 forms of adverse conditions, those natural and those created by humans. Most commonly faced are the natural adverse conditions, such as rain, high winds, lightning storms, or poor visibility. Light rain and mists cause few problems for lightcraft operations in the ion propulsion mode when energized by a low power microwave beam. Beamed laser power, however, is not a viable option due to the extreme absorption and scattering loses. Heavy rains are to be strictly avoided. The cumulus-nimbus clouds that produce them may be impenetrable to the power beam, necessitating the use of on-board SMES power for short time periods. Unlike light rain, lightning storms are a major threat to the craft. The high positive charge on the surface of the craft during the ion propulsion mode causes it to attract lightning strikes. Besides the potential structural damage associated with lightning strikes, such a discharge could also temporarily kill thrust on a large surface of the lightcraft, causing it to suddenly dip. There is little protection against this threat except to avoid areas of active electrical storms. Fair weather cumulus clouds, rather than presenting an adverse condition, can actually be used to advantage by the lightcraft when in ion propulsion mode. By depositing negative charge into the base of the clouds, the lightcraft can actually reduce the power necessary for it to maintain flight or hover. (Such clouds do cause a problem for hyperjumps in that they must be evaporated, at the cost of a high-energy expenditure, before a hyperjump can be performed using the PDE and MHD engines.)
The lightcraft does have an alternative to its ion-propulsion mode for torquing, to maintain level flight, or to tilt the craft. By using the magnetic dipole created by the pair of rim super-conducting magnets, the lightcraft is able to pitch or roll inside the Earth's magnetic field, producing torque comparable to that of its ion thrusters. The magnitude of torque available is primarily dependent on the angle between the B-field and the craft's magnetic dipole.
SUPERSONIC FLIGHT REGIME
When accelerating beyond 100 mph through mach 1 in lateral flight using the PDE thrusters the lightcraft must be rotating in order to have intrinsic stability. In order for a spinning disc to maintain stable lateral flight, the pitch and roll rates and linear velocities must return to equilibrium when disturbed. This does not mean that the angle of attack or position of the lightcraft will stay at the same value.
PITCH AND ROLL RATES (PDE)
Theory application and component parts of the Pulsed Detonation Engine (PDE) were discussed earlier. A short quick review of the main concepts is needed to understand how the PDE thrusters are used for pitch, roll, and rotation. The PDE thrusters operate by a high- energy microwave beam transmitted from an orbital power station. This beam must be precisely aligned with the lightcraft axis (i.e. rectennas) in order for this engine to function properly. For stealth the microwave beam is pulsed at either sub-audible or at super- audible repetition frequencies. When this beam is received upon the rectennas, it is reflected to focus just outside of the lightcraft rim, where it triggers electrical air breakdown. Focal intensities are sufficient to produce detonations of up to 30 atmospheric pressure. This conducting air plasma is then vectored in every desired direction by application of electromagnetic fields emanating from the rim superconducting magnets. These thrust-vectored pulses are barely visible to the unaided eye, since they each last only about 1 ms. The PDE's main function as an air-breathing engine is to rapidly push the lightcraft through Mach 1, jumping it into supersonic speeds (a hyperjump). To initiate an axial hyperjump the PDE thrusters sequentially fire all around the lightcraft rim, creating a uniform exhaust flow, which is vectored down. The plasma cloud can just as easily be vectored up, creating a downward thrust. In a maximum performance maneuver the Lightcraft can jump a few kilometers in 1-2 seconds. Aside from rapid accelerations in the axial and lateral directions, the PDE can be used to hover, pitch, roll, and spin the lightcraft. A pitch up or rolling maneuver necessitates producing a torque in the desired direction. The pulses are quick and discrete; hence, the lightcraft can sharply torque to most any attitude in a milli-second. To torque the lightcraft in the desired direction, the PDE exhaust is vectored by manipulating the electric currents carried in the upper and lower rim superconducting magnets. This distorts the magnetic fields producing a variable geometry magnetic nozzle to direct the expanding air plasma as necessary. To pitch counterclockwise, the upper superconducting magnet is held at a higher current than the lower magnet; this vectors the plasma exhaust down. And pitches the lightcraft down. The opposite coil currents produce a clockwise torque on the lightcraft. Finally, the PDE thrusters can be used to induce lightcraft rotation. By detonating the thrusters in sequence around the lightcraft, oblique, rotating, detonation wave fronts form around the lightcraft rim. Gradually the spiralling air around the rim expands and the friction forces transfer angular momentum to the lightcraft, causing it to spin. To accomplish MHD spin augmentation, the rectennas extract some fraction of the incident microwave beam pulses for conversion into electric pulses, which is delivered to the rim electrodes.
HYPERSONIC FLIGHT REGIME
The most fundamental aspect of the lightcraft, which makes transatmospheric flight to space possible, is the air spike linked to an annular MHD slipstream accelerator. A central laser beam is used to create this air spike. The air spike is formed when energy from the focused laser beam causes air to be radial driven out of the vehicle's path and pushed into the annular inlet located outside the rim of the lightcraft. In the process fore-body drag, and heat transfer are greatly reduced; also the propulsive efficiency of the MHD accelerator is increased. The electrical energy required by the MHD engine comes from two rectifying antennas located aboard the lightcraft. These 35 GHz rectenna arrays can deliver up to 9 GW of electric power to the engine. The biggest advantage of the air spike technology is that the lightcraft can be streamlined without a mass penalty, which is an extremely important issue for transatmospheric vehicles.
RE-ENTRY AND AEROBREAKING METHODS
The major mission parameter that defines the Lightcraft Lightcraft geometry is its capability for transatmospheric flight. The vehicle is designed to transport its crew safely around the Earth, as well as, between the lunar surface and Earth. This section deals with the re-entry and aerobraking legs of these journeys. As it comes upon the Earth's atmosphere, the vehicle reorients itself to achieve the proper angle of incidence. Because of the tremendous speed at which the craft enters the atmosphere, it must either decelerate under great G loads or find an alternative method that will allow it to gradually reduce speed. The Lightcraft uses 3 different approaches in resolving this difficulty.
The principle of aero-braking was first put to practical use by the early space exploration efforts. The space capsules of the era had a parabolic under-side, similar to that of the lightcraft, which acted as a heat shield. Upon re-entry, the capsule was orientated to create a strong normal shock wave across the bottom hemispherical surface of the craft. This "foot-print" generated a tremendous amount of drag, and thus decelerated the vehicle rapidly. However, the lunar Lightcraft's re-entry velocity can be up to twice that of the Apollo space capsules. Aero-braking alone does not allow the lightcraft sufficient flexibility for a safe recovery. Three alternative options are to use a different flight profile, flight magneto-hydrodynamics, or a powered re-entry.
The first solution is a flight profile known as the "skip trajectory". Any vehicle with adequate lift that re-enters the atmosphere may use one or more passes through the atmosphere in order to reduce its initial kinetic energy. After the initial lost of velocity, the lightcraft may use the lift generated by its body to "skip" out of the atmosphere. Once outside of the atmosphere, the vehicle will gradually lose lift and drop back into the outer atmosphere. By repeating such a maneuver, the craft may safely re-enter the atmosphere without paying the penalty of great G loads.
One of the drawbacks associated with aero-braking is the rapid heat build up on the re-entry shield. Hypervelocity air comes in direct contact with the outer hull, the friction between the two causes the temperature of the hull to rise rapidly. This results in a construction of a heat resistant hull or a need for a heat-dissipater with great capacity, both of which pays dearly in weight penalty. Kartrowitz first demonstrated the concept of flight magneto-aerodynamics in the 1950's, discovering that a magnetic field has a great effect upon the re-entry bow shock. A magnetic field interacts with hypersonic air plasma surrounding a re-entry vehicle to push the shock wave away from it. The Lightcraft Lightcraft takes the full advantage of this principle upon its re-entry. By charging the two superconducting magnets along its rim, a powerful 2 Tesla magnetic field forms around the vehicle. This field pushes the oblique shock away from the hull. This prevents the heated hypersonic air molecules from coming into direct contact with the craft, and thereby alleviates the re-entry heating problem.
In sub-orbital flights around the Earth for the surveillance/rescue mission, the Lightcraft may require a maximum performance decent to low altitude hover. In this "powered re-entry" mode the lightcraft enters the atmosphere with lateral flight orientation with the Air Spike and MHD slipstream accelerators energized.