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.

本周，在丹佛举行的第45届联合推进会议与展览上，研究人员、科学家和工程师们将讨论航天器和商用飞行器推进系统的最新进展。 
 

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.

研讨的热点之一是束能推进，即利用定向能量射束，加热航天器推进剂或者向其发动机传输电流。通过取消火箭自身的能源，束能推进有望使航天器发射更廉价和更可靠。

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.

麻省理工学院航空航天系的研究人员Alexander Bruccoleri说，在常规化学推进系统中，大部分能量贮藏在火箭的燃料中，这些燃料占有火箭的主要重量。此外，化学推进系统被加热升温后而超过火箭自身部分材料的熔点。在8月3日的会议上，他发明了一种测试束量系统的比较方法。

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.

束能概念是在二十世纪七十年代末，由NASA艾姆斯研究中心和加州理工学院提出的。Bruccoleri 说，该概念利用激光作为热交换器，携带能量并使热流从喷嘴喷出。目前研究人员正在开发陆基激光器系统，利用该系统加热燃料（例如氢）达到易于控制的温度。在相同的温度下，氢分子的加速是水分子的2倍，可获得更好的喷出速度。利用光束作为外部能量源可以减轻火箭自身携带的系统重量和质量，为科学研究扩大有效载荷空间，并提供更大的推力。

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