Laser propulsion

Laser propulsion is a form of Beam-powered propulsion where the energy source is a remote (usually ground-based) laser system. This form of propulsion differs from a conventional chemical rocket where both energy and reaction mass come from the solid or liquid propellants carried on board the vehicle.

The concept of laser propelled vehicles was first introduced by Arthur Kantrowitz in 1972.

Laser propulsion systems may transfer momentum to a spacecraft in two different ways. The first way is that photon radiation pressure drives the momentum transfer, the principle behind the propulsion of solar sails and laser sails. The laser elevator, a variation on the original laser sail idea, uses mirrors at the spacecraft and near the ground laser to repeatedly reflect the photons back and forth and extract orders of magnitude more of the available photon momentum. High spacecraft velocities may be possible with laser elevators because the system is not limited by the rocket equation. A second way of driving momentum transfer to a spacecraft, used in the devices described below, is using the laser to help expel mass from the spacecraft as in a conventional rocket. The second class of propulsion systems are fundamentally limited in their final spacecraft velocities by the rocket equation.

Forms
There are several forms of laser propulsion.

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. This propulsion method is currently being investigated by several reasearch groups, including Dr. Andrew Pakhomov and his research group at UAH. 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.

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.

CW plasma propulsion
A continuous laser beam focused in a flowing stream of gas creates a stable laser sustained plasma which heats the gas; the hot gas is then expanded through a conventional nozzle to produce thrust. Because the plasma does not touch the walls of the engine, very high gas temperatures are possible, as in gas core nuclear thermal propulsion. However, to achieve high specific impulse, the propellant must have low molecular weight; hydrogen is usually assumed for actual use, at specific impulses around 1000 seconds. CW plasma propulsion has the disadvantage that the laser beam must be precisely focused into the absorption chamber, either through a window or by using a specially-shaped nozzle. CW plasma thruster experiments were performed in the 1970's and 1980's, primarily by Dr. Dennis Keefer of UTSI and Prof. Herman Krier of the University of Illinois at Urbana-Champaign

Heat Exchanger (HX) Thruster
The laser beam heats a solid heat exchanger, which in turn heats an inert liquid propellant, converting it to hot gas which is exhausted through a conventional nozzle. This is similar in principle to nuclear thermal and solar thermal propulsion. Using a large flat heat exchanger allows the laser beam to shine directly on the heat exchanger without focusing optics on the vehicle. The HX thruster has the advantage of working equally well with any laser wavelength and both CW and pulsed lasers, and of having an efficiency approaching 100%. The HX thruster is limited by the heat exchanger material and by radiative losses to relatively low gas temperatures, typically 1000 - 2000 C, but with hydrogen propellant, that provides sufficient specific impulse (600 - 800 seconds) to allow single stage vehicles to reach low Earth orbit. The HX laser thruster concept was developed by Jordin Kare in 1991; a similar microwave thermal propulsion concept was developed independently by Kevin Parkin at Cal Tech in 2001.

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.