Lightcraft

 v1.1 
 15 sep 99 
 greg goebel (gvgoebel@yahoo.com) 
 public domain

Since the beginning of space exploration, there have been efforts to build launch systems that would greatly reduce the cost of putting payloads into space. So far, none of these systems has been put into operation, but they have been the focus of a great deal of ingenuity.

One of the most interesting of these proposed launch systems is the lightcraft, a spacecraft blasted into orbit on a beam of laser light or microwave energy. This article describes the lightcraft concept and outlines current progress in the field.

Contents List:-

  1. LASER LIGHTCRAFT

  2. MICROWAVE LIGHTCRAFT

  3. LASER LIGHTCRAFT IN TEST

  4. COMMENTS, SOURCES, & REVISION HISTORY

1. LASER LIGHTCRAFT

The concept of the lightcraft owes much to one man, Leik Myrabo, an associate professor of mechanical engineering at the Rensselaer Polytechnic Institute (RPI) in Troy, New York.

During the 1980s, Myrabo's interest in using a powerful laser to boost spacecraft into orbit progressed until by 1990 he and his colleagues at RPI were investigating concepts in detail, with backing from the US National Aeronautics & Space Administration (NASA) and Strategic Defense Initiative Organization (SDIO).

According to Myrabo, such a laser-boosted launch system offers much lower launch costs than a chemical rocket system. Since the spacecraft obtains most of its propulsive energy from external sources, a larger part of the spacecraft can be devoted to payload instead of fuel. As the spacecraft doesn't incorporate a large high-thrust rocket motor, it also is more reliable.

Myrabo's initial concepts were based on lightcraft propelled by a large laser. Early experiments envisioned ground-based lasers, but long-range concepts considered the use of orbiting lasers, built as part of a solar power satellite (SPS) system.

The first designs for laser lightcraft defined a cone-shaped spacecraft, surrounded near its base by a ring shroud. Lightcraft designs propelled by a ground-based laser have a spindle-shaped baseplate, while those propelled by an orbiting laser have a flatter baseplate. In either case, the lightcraft carries a relatively small fuel load of hydrogen, as well as typical spacecraft systems such as control electronics, communications, attitude-control thrusters, and so on.

Lightcraft operating scenarios varied, but in a representative scenario using a ground-based laser, a lightcraft begins its flight by being catapulted into the air by a compressed-air charge. Laser light is shined onto its bottom, and the reflective ring shroud focuses the light to below the tip of the baseplate spindle.

The focused energy is so intense that the hot air ionizes, creating a plasma finger that explodes outward and blasts the lightcraft upward at very high acceleration until the lightcraft reaches about Mach 1.

At this speed, air is flowing over the conical front of the spacecraft, compressing the airflow and directing it into the ring shroud. The lightcraft then begins operating as a type of ramjet engine, with the conical spacecraft front acting as the ramjet centerbody, and the ring shroud acting as the ramjet duct.

The configuration of the shroud is then adjusted to focus the laser light onto "igniters" underneath the ring that convert the air flowing into the shroud into plasma, providing thrust for the lightcraft. As the lightcraft accelerates to Mach 11, the ramjet operates in various "pulsed detonation" modes, in which thrust is obtained through a train of plasma explosions. During this flight phase, the airflow through the ramjet duct shifts from subsonic to supersonic, with the lightcraft then operating as a "supersonic combustion ramjet" or "scramjet".

Above Mach 11, the lightcraft uses magnetic fields to compress the intake air, and injects hydrogen into the plasma flow to maintain thrust. Power for magnetic field generation is obtained directly from the intense plasma flow over the lightcraft through "magneto-hydrodynamic" (MHD) principles.

MHD electrical power generation is conceptually simple, though difficult to achieve in practice because of the extreme nature of hot plasmas. A plasma is an ionized gas, consisting of free electrons and positive ions. If this plasma flows through a magnetic field, the electrons and positive ions are driven apart, creating an electrical potential difference.

If an electrical circuit connections are placed at the edges of the plasma stream, electrons flow as a current through the circuit's load and then recombine with the positive ions on the other side of the circuit. The lightcraft MHD system generates large amounts of electric power, enough to produce strong magnetic fields to control the raging plasma flow.

By the time the lightcraft reaches an altitude where there is not enough atmosphere to sustain an air-breathing engine, it has a speed of Mach 25. It then uses a laser-powered rocket engine, with hydrogen exhaust heated by the laser beam back on the ground, for final orbital injection.

Scenarios envisioning the use of an orbital laser were similar, except that of course the light was shined down on top of the lightcraft. The design was modified accordingly to allow reflection of the beam through the ring duct to a center point below the baseplate.

The lightcraft designs implied the development of extremely precise and high-quality mirrors. If more than the smallest fraction of the intense laser light were absorbed by the lightcraft rather than reflected, the lightcraft would be incinerated immediately.

Myrabo and his colleagues understood that the lightcraft was a long-range concept, and focused their research on basic physics and technology, hopefully leading up to small-scale demonstrations. They managed to show that a lightcraft system could produce thrust, but the US Strategic Defense Organization abandoned work on a 100 megawatt (MW) laser system that was needed for practical operation.

That left the laser option a dead end for the short run. Myrabo was undiscouraged, however, and joined with Yuri Raizer of the Russian Academy of Sciences to pursue another option: microwave power.

2. MICROWAVE LIGHTCRAFT

High power microwave (HPM) transmitter arrays are much more practical than high power lasers at present, and so HPM seemed like a better option for the short term. Myrabo and Raizer obtained funding from the US Space Studies Institute (SSI) to conduct studies. Their concepts for HPM lightcraft had much in common with those devised for the laser lightcraft.

HPM lightcraft development concepts focused on a ground-based microwave transmitter array, rather than a more ambitious microwave satellite power system. Early concepts for HPM lightcraft were basically an evolution of the laser lightcraft concept. The major difference was that instead of using reflectors to manipulate laser power, the HPM lightcraft uses an antenna array to transform microwave energy into propulsive power.

In this concept, the bottom surface of the HPM lightcraft is embedded with a large number of vertically-oriented wires, each acting as a receiving dipole, and tuned to the HPM transmitter wavelength. This "super-igniter array" absorbs the microwave energy and generates enough heat to ionize the atmosphere underneath, creating a plasma blast to drive the spacecraft upward.

Much like the laser lightcraft, at higher speeds the microwave energy sustains pulsejet operation through the HPM lightcraft's ring duct. At high altitudes, an auxiliary orbiting microwave beam satellite heats hydrogen to provide pure rocket propulsion. An alternate scenario envisioned a self-contained chemical rocket for propulsion out of the atmosphere.

In the SSI studies, Myrabo envisioned launching a network of very small "microsats" with microwave energy. Large numbers of microsats would be put in orbit to construct orbiting communications, earth resources mapping, weather observation, or military reconnaissance networks.

HPM microsat concepts envisioned a spacecraft weighing only 15 kilograms unfueled, including a 3 kilogram payload. Hydrogen fuel capacity was 15 kilograms of hydrogen fuel at launch, for a total launch mass of 30 kilograms.

The microsat is built with lightweight carbon composites, with a silicon carbide overcoat to protect the spacecraft from high temperatures. Subsystems include a 75 watt solar panel, rechargeable battery, communications and guidance electronics, magneto-optic data storage, and attitude control and pointing system.

The ground-based HPM array required to launch this microsat is 550 meters in diameter and operates at 220 GHz, a band where the atmosphere is mostly transparent to microwave energy. Beam power is 30 MW, with a maximum range of 500 to 800 kilometers.

HPM design concepts didn't stop there, however. Later work was more radical. Advanced HPM lightcraft were disk-shaped, resembling classical flying saucers, though in most concepts they flew top-first, rather than sideways like a frisbee.

Scenarios for such advanced HPM lightcraft involve energy provided by orbiting power satellites. The lightcraft converts the microwave energy into useful power with "rectifying antennas", or "rectennas", and has a ring of superconducting magnets underneath its rim that generate intense magnetic fields to control air and plasma flow, eliminating the need for a ring duct. Electrodes are also arranged around the edge of the disk to obtain MHD power from plasma flow.

The lightcraft overcomes the air resistance that would seem unavoidable in a top-first flight configuration by focusing some of the microwave energy above the spacecraft. The plasma generated acts as an "aerospike", creating a shockwave cone to divert airflow around the lightcraft.

Myrabo and his group have considered alternate scenarios for such HPM lightcraft, envisioning them for high-speed air transport.

Such advanced microwave lightcraft concepts were speculative, and Myrabo conceded that they would not be available for decades. However, their saucer-like configuration led UFO enthusiasts to suggest that the US government might actually be testing such vehicles at the present time.

3. LASER LIGHTCRAFT IN TEST

Despite all this effort, Myrabo's lightcraft remained mostly paper studies, with small-scale physics experiments conducted to demonstrate the feasibility of various features of the scheme.

However, in 1996, Myrabo once again obtained US military funding to actually fly small lightcraft models, working in conjunction with Franklin B. Mead of the US Air Force Research Laboratory. The Air Force sponsored small-scale tests of lightcraft propulsion at White Sands Missile Range, using a US Army 10-kilowatt pulsed carbon-dioxide laser. The tests were performed on beautifully machined solid aluminum models, ranging from 10 to 15 centimeters across and weighing about 50 grams.

Each laser lightcraft model resembled an acorn with a cone grafted onto its base. A model was spun up with a nitrogen gas jet before launch to keep it stable in flight, and then kilojoule laser pulses, pulsed at a rate of 28 times per second, were fired on its bottom shield. The shield focused the light to ionize the air at the bottom of the lightcraft into a plasma, blasting the model upward.

Initial flights were performed using a guide wire. The first free flight was performed in November 1997, and was followed by others that successively pushed the lightcraft higher and higher, though the limit was about 30 meters because of range safety problems.

The research team has obtained funding for experiments with a more potent laser that offers 100 kilowatts (kW) of power for more aggressive lightcraft test flights. The mid-term goal of the lightcraft effort is to put a 1 kilogram lightcraft prototype into orbit by 2004, using a custom-built one MW laser.

4. COMMENTS, SOURCES, & REVISION HISTORY

My appreciation of the ingenuity of lightcraft is tempered by caution instilled by decades of listening to grand schemes of "spaceniks" that never happen. However, Myrabo and his group have established a certain credibility through their persistence and practical efforts. Lightcraft might be a long shot, but they are one that inspires a certain fascination and a little excitement.

Sources for this document include:-

  1. "Laser To Lift Lightcraft Into Space", by Bill Siuru, MECHANICAL ENGINEERING, September 1990, 54:57.
  2. "Microwave Launches Of Small Payloads", by Leik N. Myrabo, SSI UPDATE, November-December 1994.
  3. "Fly By Microwaves", by Gregory T. Pope, POPULAR MECHANICS, September 1995.
  4. "Mini Driver", by Leonard David, SMITHSONIAN AIR & SPACE, April-May 1998, 15.
  5. "Highways Of Light", by Leik N. Myrabo, SCIENTIFIC AMERICAN PRESENTS: THE FUTURE OF SPACE EXPLORATION, spring 1999, 66:67.
Greg Goebel (gvgoebel@yahoo.com)
http://vectorsite.tripod.com/index.html

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