At a Brazil-based lab, a hypersonic shock tunnel is linked to two pulsed infrared lasers with peak powers reaching the gigawatt range – the highest power laser propulsion experiments performed to date.
Leik Myrabo is an aerospace engineering professor at Rensselaer Polytechnic Institute who has demonstrated the feasibility of using ground-based lasers to propel objects into orbit; possibly reducing orbit-flight costs by a factor of 1000. Small scale tests were done to a 233 feet or so.
Myrabo reportedly has made more than 140 test flights using small prototypes. He isn’t the only one exploring this field, either. Five years ago, NASA joined Tim Blackwell, a researcher at the Center for Applied Optics at the University of Alabama in Huntsville, in using laser propulsion to power a small model airplane. Researchers at the University of Tokyo have used a laser to propel a tiny airplane and detailed their findings in the journal Applied Physics Letters in 2002.
Engine and Inlet Experiments Underway
Scale cross-sections of various LightCraft engine and inlet geometries are currently undergoing tests in a hypersonic wind tunnel in South America at Mach 7 to 10. These laser propulsion experiments are aimed at confirming physics-based computer models for: a) Directed Energy AirSpike (DEAS) inlets; and, b) full-scale pulsed detonation engine segments-examining interactions of expanding plasma blast waves with inlet flows and thruster surfaces. Tests thus far have shown excellent conformity.
Flight Dynamics & Control Laws Developed
Working with graduate students at Rensselaer Polytechnic Institute (RPI) in New York state, Dr. Myrabo and his team developed and modeled a comprehensive set of Flight Dynamics and Control (FDL) laws for the LightCraft, calibrated against 16 historic lightcraft flights at White Sands Missile Range, NM. Subsequent computer simulations have confirmed that complete dynamic control of a full-size LightCraft along a launch trajectory into low Earth orbit is feasible. Aerodynamics and propulsion data bases now being collected in both low-speed and hypersonic wind tunnel experiments will be used to upgrade subroutines in the FDL code.
A ground based laser is the power source that propels the Lightcraft into orbit. Lightcraft can deliver payloads into space for a fraction of the cost of traditional rockets because most of the engine stays on the ground, thereby unburdening the craft from having to lift the energy source for its propulsion system.
The back side of the craft is a large, highly polished parabolic mirror that is designed to capture the laser beam projected at it from the ground. The mirror focuses the beam, rapidly heating the air to 5 TIMES the temperature of the sun, creating a blast wave out the back that pushes the vehicle upward. As the beam is rapidly pulsed, the vehicle is continuously propelled forward, on its way to orbit.
Types of Laser Propulsion
Pulsed plasma propulsion
A high energy pulse focused in a gas or on a solid surface surrounded by gas produces breakdown of the gas (usually air). This causes an expanding shock wave which absorbs laser energy at the shock front (a laser sustained detonation wave or LSD wave); expansion of the hot plasma behind the shock front during and after the pulse transmits momentum to the craft. Pulsed plasma propulsion using air as the working fluid is the simplest form of air-breathing laser propulsion. The record-breaking Lightcraft, developed by Leik Myrabo of RPI (Rensselaer Polytechnic Institute) and Frank Mead, works on this principle.
Laser electric propulsion
A general class of propulsion techniques in which the laser beam power is converted to electricity, which then powers some type of electric propulsion thruster. Usually, laser electric propulsion is considered as a competitor to solar electric or nuclear electric propulsion for low-thrust propulsion in space. However, Leik Myrabo has proposed high-thrust laser electric propulsion, using magnetohydrodynamics to convert laser energy to electricity and to electrically accelerate air around a vehicle for thrust.
Ablative laser propulsion
Ablative Laser Propulsion (ALP) is a form of beam-powered propulsion in which an external pulsed laser is used to burn off a plasma plume from a solid metal propellant, thus producing thrust. The measured specific impulse of small ALP setups is very high at about 5000 s (49 kN·s/kg), and unlike the lightcraft developed by Leik Myrabo which uses air as the propellant, ALP can be used in space.
Material is directly removed from a solid or liquid surface at high velocities by laser ablation by a pulsed laser. Depending on the laser flux and pulse duration, the material can be simply heated and evaporated, or converted to plasma. Ablative propulsion will work in air or vacuum. Specific impulse values from 200 seconds to several thousand seconds are possible by choosing the propellant and laser pulse characteristics. Variations of ablative propulsion include double-pulse propulsion in which one laser pulse ablates material and a second laser pulse further heats the ablated gas, laser micropropulsion in which a small laser onboard a spacecraft ablates very small amounts of propellant for attitude control or maneuvering, and space debris removal, in which the laser ablates material from debris particles in low Earth orbit, changing their orbits and causing them to reenter
ALP is being developed by Professor Andrew Pakhomov at the University of Alabama in Huntsville of the UAH Laser Propulsion Group.
Keith Henson has a plan for large scale use of laser propulsion to boost space based solar power.
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