article Fall 2016

Moving Spacecraft with Microwaves

By Stephen Eick

Designing methods of launching an object beyond the gripping claws of Earth’s gravity is one of the most challenging problems an engineer can expect to tackle. Even more challenging is creating new methods of propulsion that push physics beyond its edge while opening new doors for the future of humanity. Right now, a thruster is being developed that, if usable, will provide a truly revolutionary method of propulsion both on Earth and far beyond.

Earlier this year, NASA’s MESSENGER probe crashed into its test subject, Mercury. This wasn’t a freak accident, however. Gravity is persistent and pervasive; MESSENGER was out of propellant, or fuel, and was unable to adjust its orbit around Mercury. It was slowly tugged closer and closer to the planet until it crashed. Propellant is a necessity for present-day space flight because of Newton’s Third Law: for every action, there must be an opposite but equal reaction.

As a demonstration, plop a stationary spacecraft in an empty spot of space. The denizens of the spacecraft want to go somewhere more interesting, so an engine is fired. Firing an engine in a classic chemical rocket is simply the process of extracting energy from fuel by burning, then speedily chucking it from the spacecraft in the direction opposite the destination. The problem is only a finite amount of material exists to eject, and once that runs out, the spacecraft is unable to maneuver until more fuel is provided. This creates a predicament because space is relatively devoid of matter; rocket fuel doesn’t just appear (unless you have Nibbler on-board and pack a dark matter engine under the hood).

Robert Goddard, the father of rocketry, incubated initial ideas of alternative thrusters using electric fields early in the twentieth century, but these ideas only lived in laboratories until the ion thruster was demonstrated in the 1960s. The key differences these thrusters possess from classical thrusters are the usage of heavier elements such as xenon for the fuel and electric fields to expel the fuel at high velocities. However, this engine fails to reduce the dependency on chemical propellant. Additionally, this style of engine, while extremely efficient, is unable to compete with the classical chemical thruster due to the chemical thruster’s unadulterated power.

In 2001, an engineer named Roger Shawyer introduced a theory that, if reproducible, could provide direct conversion of microwave energy to thrust without the need for propellant. To create the thruster, first find a magnetron, a device that generates microwaves in applications such as the microwave oven. Microwaves from the magnetron would be pumped through a waveguide, a structure that directs waves down a specific path. The waveguide would be designed to possess resonance, a phenomenon where the amplitude of waves increases at a specific frequency. The microwaves would then enter a tapered cavity. According to Shawyer’s theory, microwaves at opposite ends of the cavity should experience differing velocities. The surfaces of the cavity would be subjected to radiation pressure, which is a force experienced by an object exposed to electromagnetic radiation (in this case, microwave radiation). However, according to the theory, the differing velocities would create a difference in force magnitudes at opposite ends of the waveguide assembly. By designing an assembly to optimize the resonance properties, one can amplify this difference in forces to produce a usable thrust force.

General responses to the operational theory of the RF resonant cavity thruster have varied from hostile to immensely hopeful. The standard argument against realistic operation note that classical propulsion methods require force to be imposed on the ejected mass of the propellant, which isn’t happening because no mass is being ejected. Additionally, critics point out that an output force cannot exist since the cavity is a closed system and will only strain the device walls (it should be noted that, according to Shawyer, by taking Einstein’s Special Theory of Relativity into account, this effect is eliminated).

However, a slowly-growing pile of evidence appears to support the thruster’s operation. The theory received seven positive independent reviews from notable aerospace and electrical institutions. Shawyer created a company to develop a functional thruster, which has produced both experimental and demonstrative versions of the thruster, dubbed the EmDrive. Researchers from China, NASA Eagleworks, and, most recently, Dresden have all reported thrusts from their initial tests of the concept (it should be noted that each group created slightly different versions of the thruster). NASA was the first to allegedly observe positive force measurements when testing the thruster in a hard vacuum, which is one of the most critical developments to date because the vacuum eliminates the error that can be introduced by atmospheric matter while testing the system. At this point in time, more testing is underway to eliminate additional potential sources of error.

“Humanity could harness the free energy of suns to move through interstellar space…”

If this theory translates to practice, a number of existence-altering shifts will occur. A device that creates thrust without propellant will enable objects in orbit to remain in orbit for the complete lifetime of the on-board equipment. When equipped with this thruster, the International Space Station would no longer require boosts from visiting capsules; the ISS’s solar panels would capture the necessary energy to provide orbit-preserving thrust. Humanity could harness the free energy of suns to move through interstellar space without requiring expensive, heavy, and dangerous propellant. By providing 9.81 m/s2 of acceleration per kilogram of mass with the thruster, objects on Earth would be able to enter a controlled state of levitation independent of surfaces. While it’s still just a theoretical device, a true electromagnetic thruster would change the destiny of humanity, all while using technology found in devices that warm Hot Pockets.

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