Chinese scientists appear to have validated a propellentless space propulsion technology previously branded as impossible. Based on earlier British research, it is averred that the EmDrive concept provides sustained thrust at low cost and weight, but this has yet to be accepted even as a workable theory by the wider propulsion community.

The EmDrive story started in 2001 when engineer Roger Shawyer set up Satellite Propulsion Research (SPR) to exploit his new concept in electrical propulsion. He was helped by a modest grant from the U.K.'s now defunct Trade and Industry Department.

Space propulsion relies on Newton's laws of motion: propellant is ejected backward at high velocity, and the craft is pushed forward with equal and opposite momentum. Even with high exhaust velocity, such as ion drives ejecting particles at 30 km per second (more than 62,000 mph), the mass of propellant is a limiting factor.

Shawyer's EmDrive does not have any exhaust. It consists of a tuned cavity shaped like a truncated cone into which resonating microwaves are channeled. Like other radiation, these exert a tiny pressure when reflected off a surface. According to Shawyer, the pressure exerted on the large end of the cavity is greater than the pressure on the small end, producing a net thrust.

This appears to be a violation the law of conservation of momentum. However, Shawyer says net thrust occurs because the microwaves have a group velocity (the velocity of a collection of electromagnetic waves) greater in one direction than the other and relativistic effects to modify the Newtonian mechanics. Shawyer compares the EmDrive to a laser gyroscope, which also looks like a closed system but is actually open and works thanks to relativistic effects.

Shawyer's analysis was challenged after the EmDrive was featured in a science magazine in 2006. John Costella, a researcher in relativistic electrodynamics, described the EmDrive as a fraud and argued that even with relativity there can be no net thrust. Shawyer built demonstration EmDrives to back his claims, including a 7-lb. version he said produced a thrust of 85 millinewtons (mN) with a 300-watt input. Skeptics, convinced of its impossibility, have not even tested the EmDrive.

But the controversy attracted the attention of China's Yang Juan, professor of propulsion theory and engineering of aeronautics and astronautics at the Northwestern Polytechnic University in Xian. Her team set out to explore the EmDrive independently. A 2008 theory paper by Yang and colleagues describes the EmDrive in terms of quantum theory and indicates net thrust is possible. A 2010 follow-up paper calculates a possible thrust of 456 mN from a 1-kw input, and states that the team was getting positive experimental results.

The latest paper, “Net Thrust Measurement of Propellentless Microwave Thruster,” is in the June edition of the journal Acta Physica Sinica published by the Chinese Academy of Sciences. Yang's team used a magnetron as a 2.45 GHz microwave source and produced a measured thrust of up to 720 mN from 2.5 kw of input power. On the surface, this appears to be a peer-reviewed validation of the science.

However, Yang says she is unable to answer questions from Aviation Week on her team's work at this time and is deferring until the end of the year for unknown reasons. Shawyer, meanwhile, has no doubts about its significance.

“The new paper independently proves the EmDrive theory by publishing thrust levels five times higher than SPR results, but with a similar specific thrust,” Shawyer tells Aviation Week.

The result, 720 mN, is just 2.5 oz. of thrust, but satellites often work with less. Boeing's advanced XIPS thruster, which fires out Xenon ions at high speed, achieves 165 mN of thrust from 4.5 lb. It weighs 35 lb., more than an equivalent EmDrive, and the propellant for prolonged operation can weigh much more.

XIPS and EmDrive can both run off solar electricity, but the EmDrive never runs out of propellant. Propellant to maintain satellite position is a major weight contribution; Shawyer suggests that the EmDrive could halve the cost of geostationary satellites.

There has been little interest in the EmDrive in the West so far, and Shawyer's government funding has ended. Boeing's Phantom Works, which has previously explored exotic forms of space propulsion, was said to be looking into it some years ago. Such work has evidently ceased. “Phantom Works is not working with Mr. Shawyer,” a Boeing representative says, adding that the company is no longer pursuing this avenue.

Still, the latest Chinese work may revive Western interest in the EmDrive as a viable, revolutionary technology.

“The EmDrive will give much higher performance, at lower cost, for many types of mission,” says Shawyer. “In an increasingly competitive, international industry, space companies will have to use EmDrive technology or go out of business.”