An early model of a laser-propelled lightcraft. Credit: Rensselaer Polytechnic Institute

This week at the 45th Joint Propulsion Conference and Exhibit in Denver, researchers, scientists, and engineers will discuss the latest advances in propulsion systems for spacecraft and commercial aircraft.

One topic being examined is beam-energy propulsion--using a beam of energy directed at a spacecraft either to heat up its propellant or to deliver electricity to its engine. By removing the energy source from the rocket itself, beam-energy propulsion has the potential to make launching spacecraft cheaper and more reliable.

In conventional chemical propulsion, massive amounts of energy are stored in a rocket's fuel, which makes up a significant amount of its weight. In addition, chemical systems are heated to temperatures above the melting point of some materials in the rocket itself, says Alexander Bruccoleri, a researcher in the aeronautics and astronautics department at MIT, who recently received his master's from the Space Propulsion Lab. Bruccoleri presented a paper at the conference on August 3 on a comparison metric he invented to test beam-energy systems.

Beam energy was dreamt up in the late 1970s by NASA Ames Research Center and the California Institute of Technology. "The idea was to use lasers as a heat exchanger--take the energy and make a hot fluid that can expand out of the nozzle," Bruccoleri says. Now researchers are exploring ground-based lasers systems that heat fuels such as hydrogen to a temperature that is easier to manage. "The hydrogen molecules can be accelerated twice as fast as water molecules with the same temperature, providing better exhaust velocity--the thrust you get for the rate at which you are burning the propellant," says Bruccoleri. Using light as an external power source can alleviate the weight and mass of having an onboard system, leaving room for scientific payloads, for example, and it provides more propulsive power.

Leik Myrabo, an associate professor of mechanical, aerospace and nuclear engineering a Rensselaer Polytechnic Institute in Troy, NY, says the last three to five years have brought these systems closer to reality because energy-beaming technology like laser beams and millimeter wavelength systems have dropped in cost. Myrabo, founder of Lightcraft Technologies, has demonstrated that one can propel a small "lightcraft" 71 meters in the air by using pulses of light that heat the propellant. He currently has a five-year grant from the US Air Force to explore laser propulsion to launch satellites for extremely low cost at high reliability, and is conducting tests in Brazil in collaboration with that country's air force.

While Myrabo says that such systems could be a reality in 5 to 10 years, others are skeptical. Kevin Johnson, a space exploration and spacecraft propulsion manager at Lockheed Martin Space Systems in Denver, for example, expresses concern about the potential for atmospheric interference with the beam. Greg McAllister, a senior staff propulsion engineer also at Lockheed Martin, agrees and says that an energy source powerful enough to propel a rocket could also burn it up. (McAllister is presenting a paper at the conference on testing the pulse throttle thrusters used for the Mars Phoenix mission.)

Johnson says that while the system could generate enough power from a ground-based station and reduce costs, it is "20-plus years" from being feasible.

A test flight of a lightcraft using pulses of light conducted by Myrabo in 2000 at White Sands Missile Range in New Mexico. Credit: RPI