When DARPA put money behind the controversial EmDrive in 2018, it looked like a big gamble. Many physicists had dismissed the revolutionary space drive as simply fake science. Now its EmDrive project is greenlit for Phase 2, DARPA told Popular Mechanics in February this year. Meanwhile, other teams are hoping to reach a final demonstration of the technology later this year.
"This is a technology which could transform space travel and see craft lifting silently off from launchpads and reaching beyond the solar system," says Mike McCulloch, a lecturer in geomatics at the University of Plymouth, U.K., and leader behind DARPA’s EmDrive project. “We can also get an unmanned probe to Proxima Centauri in a (long) human lifetime, 90 years.”
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But DARPA is tempering that idealistic vision.
“Theoretical model-based predictions of performance have led to new thruster designs, and these new designs may help inform future development and testing activities,” a DARPA spokesman told Popular Mechanics.
With two ongoing studies rigorously testing the EmDrive’s “impossibility,” the controversial drive that’s hung around astro-engineering circles for more than two decades is only months away from its do-or-die moment.
The Drive Continues
The original EmDrive, built by Roger Shawyer in 1998, is a sealed copper tube (pictured above) wider at one end than the other. According to Shawyer, if you bounce microwaves around inside the tube, they exert more force in one direction than the other, creating a net thrust without the need for any propellant.
According to conventional physics, this shouldn’t happen. Put simply, closed systems like the proposed EmDrive should not generate thrust. But several research groups, including NASA’s Eagleworks (formally known as the Advanced Physics Propulsion Laboratory, set up to explore new technologies) and a team at Xi’an in China, tried it and got the same result, a small-but-distinct net force.
Critics believe that the experimental results must be wrong while McCulloch believes we just don't understand the physics yet.
McCulloch has developed a theory of Quantized Inertia (QI), which explains the effect and how it could help with human space travel. McCulloch has spent much of the past 18 months honing this theory and checking how its predictions match results in the laboratory.
Jose Luis Perez Diaz in Madrid, Spain, and Martin Tajmar in Dresden, Germany, are carrying out the experimental side of the project. Tajmar confirms that he plans to publish two papers in February 2021, one on the “normal” microwave EmDrives and the other on the laser-based EmDrives. On the experimental side, Tajmar is still working on eliminating every possible source of error.
“We are still improving our balances and testing continues, “ says Tajmar. “In particular we are working on further reduction of magnetic field interactions with the environment, which was the major side-effect that we discovered in previous testing.”
When asked whether he might have an alternative explanation for the apparent thrust seen in previous tests, Tajmar only says to “Wait for the papers…”
Tales From the Loop
One of McCulloch’s predictions is that thrust can be achieved with light as well as microwaves. Diaz has carried out experiments in Madrid testing this hypothesis by using an asymmetric loop of optical fiber wound 2,000 times, which he calls a Photon Loop.
As with the microwave version, the asymmetry is what matters. A racing car experiences different forces depending on how sharp a bend it goes around, and according to QI, a similar effect occurs with light going around bends of different curvature. Diaz found that passing laser pulses through the Photon Loop produces enough net force to move it.
After Diaz constructed and tested this apparatus, he sent it to Tajmar’s laboratory in Dresden for verification and precise measurement of the thrust, which is being carried out this summer. McCulloch says that the thrust appears to be between one and four micronewtons—exactly the amount his theory predicts.
The advantage of this apparatus is that fiber optics are much cheaper and easier to set up and test than copper cavities requiring precise machining. Once the design is perfected, it should be possible for physics labs around the world to make their own Photon Loops and see the effect for themselves.
The big challenge is accurately measuring such a low level of thrust as an experimental error can skew results dramatically. McCulloch believes this can be resolved in the next phase. By feeding the thrust equations into optimization software, McCulloch has designed a drive thousands of times more efficient than Shawyer’s original. This design has a series of spikes at one end—McCulloch says they nicknamed it the Bart Drive after Bart Simpson’s spiky hair.
The current design is calculated to provide a thrust of about .012 Newtons (equal to the weight of a paperclip, or a raisin) for one kilowatt of power, similar to the drives used to maneuver satellites. But McCulloch believes they can do several times better at least, and that the experiments will prove this.
“The Bart Drive should provide 0.012 N/kW, comparable to industry ion drives already but without propellant, which makes them heavy, and with a limited lifespan,” says McCulloch. “If we can provide 0.1 N/kW [about 8 times more than existing drives] without needing propellant then the satellite companies will be very interested, and my hope is that this is the threshold that will make the technology take off.”
A Physics Violation
A working EmDrive which produces more thrust than existing space drives would be revolutionary, but McCullouch’s QI theory is not accepted physics—not even close.
The sticking point is the law of conservation of momentum, which says that inside a closed system momentum remains constant. This is why you cannot actually pull yourself up by your booststaps. This result has remained solid for the past century and is accepted by the vast majority of scientists.
This is why most critics don't take the EmDrive seriously, and some mock it with comments about ‘magic space unicorns’ providing thrust, or ‘EmDrive, more like BS Drive.’ The weight of science is on their side, and it will take something pretty dramatic to displace it.
Brice Cassenti, an expert in advanced propulsion systems at the University of Connecticut, goes with the general view that the EmDrive does not seem plausible because it violates the law of conservation of momentum.
“Only the electromagnetic waves emitted by the conical antenna can provide a momentum change, that can provide a force, and the force is several orders of magnitude too small,” Cassenti told Popular Mechanics. He believes the EmDrive’s enduring popularity with DARPA is because of the promise it offers.
“Many enthusiastic individuals want to believe it is a method that can be used to escape the constraints of known physical principles on space propulsion systems and open up humanity to voyages to the stars,” says Cassenti.
Cassenti believes it is important to explore new and challenging concepts, but given the infinite variety on offer, he prefers those with the backing of known physics.
“In my opinion, it will always be better to invest engineering efforts in existing avenues that satisfy known physical principles and gradually build up science in new avenues for engineers to explore,” says Cassenti.
Everyone Can Play
Whether real or not, DARPA isn’t the only one working on this technology.
McCulloch is also talking to a consortium in California who are creating their own Horizon Drive experiment. This group includes the University of Southern California and an organization, which he is only allowed to describe as a ‘major aerospace company.’ Their goal is to demonstrate the effect “viscerally” so that, unlike micro-thrust demonstration, anyone who sees it will believe. McCulloch says that results should be released this month.
Even the U.S. Navy has been dabbling in this area. One of their researchers, Salvatore Cesar Pais, recently took out a patent for a space drive with a conical resonant cavity and microwave emitters remarkably similar to the EmDrive. The Navy declined to discuss this work.
There are other players, too. Chinese researchers from Northwestern Polytechnical University in Xi’an were the earliest to take an interest in Shawyer’s work in the early 2000s, publishing a number of scientific papers outlining their apparently rapid progress. There was a flurry of media indicating they were preparing to test their drive in space in 2016, but everything has been silent since.
The current phase of the DARPA project will continue until May 2021, and it’s possible that the project will fizzle out unless there are tangible results. After two decades, the final verdict on the EmDrive is quickly approaching.