The X3, a new ion thruster that could one day propel humans beyond Earth, was successfully tested a few months ago and is one design that could be selected by NASA as a component of propulsion system for future Mars missions.
NASA’s new X3 thruster, which is being developed by researchers at the University of Michigan in collaboration with the agency and the US Air Force, has broken records in recent test. It’s hoped that the technology could be used to ferry humans to Mars. The X3 is a type of Hall thruster, a design that uses a stream of ions to propel a spacecraft.
Plasma is expelled to generate thrust, producing far greater speeds than are possible with chemical propulsion rockets, according to NASA. A chemical rocket tops out at around five kilometers per second (1.86 miles/sec), while a Hall thruster can reach speeds of up to 40 kilometers per second (25 miles/sec). This kind of increase is particularly relevant to long-distance space travel, like a prospective voyage to Mars.
In fact, project team leaders project that ion propulsion technology such as this could take humans to the Red Planet within the next 20 years. Ion engines are also more efficient than their chemical-powered counterparts, requiring much less propellant to transport a similar amount of crew and equipment over large distances. Alec Gallimore, the project lead, stated that ionic propulsion can go around ten times farther using a similar amount of fuel in an interview with Space.com.
There are of course many other forms of deep-space travel on the table. The flaw of chemical-based designs is the need to bring the chemical fuel with them into space, which adds more mass that needs more fuel to lift into space, and so on. A Bussard ramjet, which is a type of fusion rocket, collects diffuse hydrogen in space with a huge scoop, which means, since its fuel is picked up en route, that it could approach light speed.
Sci-fi fans would recognize faster-than-light theoretical forms like the warp drive. General relativity stipulates that nothing can travel faster than the speed of light in the universe. However, if we could compact and expand the fabric of spacetime ahead of and behind us, respectively, we could technically be moving faster than the speed of light. However, the scientific consensus so far is that we’re just nowhere near this kind of technology. Recent tests demonstrated that the X3 thruster can operate at over 100kW of power, generating 5.4 Newtons of thrust — the highest of any ionic plasma thruster to date. It also broke records for maximum power output and operating current. The technology is apparently on track to take humans to Mars sometime in the next twenty years. However, it’s not without its limitations.
Compared to chemical rockets, the ionic alternative is capable of a very small amount of thrust. This means that it would have to operate for a very long time to reach the same level of acceleration as a chemical system, and as a result it’s not currently suitable for the launch process.
However, the engineers are currently attempting to mitigate these issues with the very much extraordinary X3 design. Multiple channels of plasma are being used rather than just one, but the current challenge is producing an engine that’s sufficiently powerful as well as being relatively compact. While most Hall thrusters can be picked up and carried around a lab with relative ease, the X3 needs to be moved with a crane.
In 2018, the team will continue to put the X3 through its paces with a test that will see it run continuously for 100 hours. A very tough and reliable shielding system is also being developed that would prevent plasma from damaging the walls of the thruster, allowing it to operate for even longer, perhaps even several years at a time.
How It Works
Electrons are generated by a hollow cathode (negative electrode) at the downstream end of the thruster. The anode (positive electrode) or "channel" is charged to a high potential by the thruster's power supply. The electrons are attracted to the channel walls and accelerate in the upstream direction. As the electrons move toward the channel, they encounter a magnetic field produced by the thruster's powerful electromagnets. This high-strength magnetic field traps the electrons, causing them to form into a circling ring at the downstream end of the thruster channel. The Hall thruster gets its name from this flow of electrons, called the Hall current.
The propellant, which consists of a inert gas such as xenon or krypton at low pressure, is injected into the thruster's channel. Since Hall thrusters use inert gas for propellant, there is no risk of explosion as there is with chemical rockets. Some of the trapped electrons in the channel collide with the propellant atoms, creating ions. When the propellant ions are generated, they experience the electric field produced between the channel (positive) and the ring of electrons (negative) and accelerate out of the thruster, creating an ion beam. The thrust is generated from the force that the ions impart to the electron cloud. This force is transferred to the magnetic field, which, in turn, is transmitted to the magnetic circuit of the thruster. The electrons are highly mobile and attracted to the ions in the beam, causing an equal amount of electrons and ions to leave the thruster at the same time. This enables the thruster to remain overall electrically neutral.
X3 is one of three prototype engines that could be selected by NASA to power future manned missions to Mars. Scientists estimate that such a human mission to the Red Planet will require a propulsion system operating at least 200kW. Given that X3 features the largest throttling capability of any Hall thruster to date, seven firing configurations and power levels ranging from 2 to 200 kW, it could be the best choice to become a fundamental component of spacecraft carrying astronauts beyond Earth.