I just finished the most recent season of The Expanse – my current favourite Sci-Fi series. Unlike most of my other go-to Sci-Fi, The Expanse’s narrative is (thus far) mainly contained to our own Solar System. In Star Trek, ships fly about the galaxy at Faster-Than-Light speeds giving mention to the many light years (or parsecs *cough* Star Wars) travelled to say nothing of sublight journeys within solar systems themselves. The distances between stars is huge. But, for current-day Earthling technology, our Solar System itself is still overwhelmingly enormous. It takes years to get anywhere.
In The Expanse, ships use a fictional sublight propulsion called The Epstein Drive to travel quickly through the Solar System at significant fractions of light speed. We’re not nearly there yet, but we are getting closer with the announcement of a new theoretical sublight propulsion. It won’t be an Epstein drive, but it may come to be known as the Ebrahimi Drive – an engine inspired by fusion reactors and the incredible power of solar Coronal Mass Ejections.
Flip and Burn
Rocket engines have been the backbone of space exploration lifting humans to the Moon, rovers to Mars, and sending probes outside the Solar System. However, for all their blast-offy awesomeness, they are inherently inefficient and bulky. You can only get so much energy out of rocket fuel. As a result, most of your entire spacecraft is a giant fuel tank. The mass of a rocket destined for Mars could be as much as 78% fuel. To reduce weight, we need more efficient engines.
Measurement of engine efficiency is called “specific impulse”, expressed as how many seconds a given mass of propellant can accelerate itself in Earth gravity. For example, if I have a pound of fuel, how many seconds can that pound of fuel accelerate itself before it’s exhausted? The more seconds that fuel burns, the more efficient your engine. Specific impulse can also be expressed as the velocity of an engine’s exhaust thrust (the stuff flying out the back of it) relative to the rocket itself. One of the most efficient rocket engines ever built is the RS-25 – the main engine on the Space Shuttle – which featured a specific impulse of 453 seconds and exhaust speeds of 4.4 km/s – which seems pretty fast!
If we want to push the boundaries of human space exploration, we need to out perform even the most efficient rocket engines. The next generation of space propulsion came in the form of ion drives. Ion engines use electromagnetic fields to accelerate charged particles – ions – which are then exhausted from the spaceship accelerating you in the desired direction. Like Newton said, equal and opposite reaction. If you shoot stuff one way, you go the other way. That doesn’t need to be rocket fuel, it could just be ionized gases.
The Hall-effect Thruster is an ion engine design that has been successfully deployed on spacecraft including the current SpaceX Starlink satellites. In contrast to rockets, Hall Thrusters can achieve exhaust speeds of 10 – 80km/s and specific impulse of 1000-8000 seconds. However, while a huge leap in efficiency, these engines operate on small scales producing little overall thrust of only a few Newtons force (a Newton is the force needed to accelerate 1kg at one meter per second each second). Ion thrusters are therefore ideal for small robotic spacecraft and satellites, but another design is needed for larger payloads.
This is where the new engine comes into play – not an ion thruster but a plasma thruster – Fatima Ebrahimi’s design. The plasma thruster shares similar characteristics to the ion thruster in that it too uses electric fields and charged particles. Gases of electrically charged particles are also known as plasma – considered a fourth state of matter. Hot plasma makes up 99% of the visible Universe churning away in stars like the Sun which is itself a giant ball of plasma. In dramatic outbursts called Coronal Mass Ejections (CME’s) the Sun will sometimes launch billions of tons of that plasma off into space.
The physical mechanism catalyzing CMEs is called magnetic reconnection. On the Sun’s surface, plasma is often channeled along magnetic fields creating enormous loops or “prominences” several times larger than Earth. The lines of the field twist and strain under the magnetic energy until they snap, like a rubber band, and reconnect with other field lines. The reconnection converts magnetic energy into kinetic energy and heat and dramatically accelerates massive quantities of plasma out into space at hundreds or even thousands of kilometers per second.
Ebrahimi’s plasma thruster creates similar magnetic reconnections we see in the Sun’s corona. Rather than a steady stream of accelerated particles like an ion engine, think of this design like mini CMEs going off every few milliseconds creating individual bubbles of plasma called “plasmoids.” These plasmoids are exhausted to create thrust. A simulated Ebrahimi engine reached specific impulse of 50,000 seconds with exhaust speeds of up to 500km/s! Much higher efficiency than current ion engine designs. The force generated is also much higher than ion thrusters – up to 100 Newtons.
Another huge advantage of the plasma thruster – it can run on almost any gas. Ion engines like the Hall Thruster launch with a limited supply of gas like Xenon which is ionized to create thrust. The plasma thruster’s magnetic reconnection process is more important to the total thrust than the type or mass of the gas used to generate plasmoids. So, your spacecraft could literally refuel out in space using gases found in rocks and asteroids and then continue on its journey.
“High thrust electromagnetic propulsion of tens of thousand of seconds is needed to explore the solar system beyond the Moon and Mars”-Fatima Ebrahimi
Stars in Bottles
Ebrahimi’s plasma drive concept was inspired by her work as a principal research physicist at the Princeton Plasma Physics Laboratory (PPPL). while observing plasmoids within the PPPL National Spherical Torus Experiment (NSTX) fusion reactor. Currently, all power generating nuclear reactors on Earth are fission reactors which split the atoms of heavy elements like Uranium to liberate energy. Fusion reactors are the opposite – fusing lighter elements together replicating the nuclear cores of stars. There are definite advantages to fusion power over fission. Fission reactors create radioactive nuclear waste in the form of exhausted fuel rods which must be stored safely for thousands of years and the Uranium fuel itself must be mined.
Fusion reactors could run essentially on Hydrogen liberated from water – a nearly inexhaustible fuel source – and do not create waste products that need to be buried. The challenge for fusion reactor designs is containing super heated plasma. Plasma within a fusion reactor can reach a hundred millions degrees and power is required to both heat the plasma and generate powerful magnetic fields to contain the reaction. Net energy positive reactions have been rare. Reactors like NSTX create high velocity plasmoids, through magnetic reconnection, which Ebrahimi observed travelling within the reactor at speeds upwards of 20km/s. She considered how the plasmoids could be deployed within a space engine design leading to her research.
NSTX has developed components and scientific data for ITER (International Thermonuclear Experimental Reactor), the world’s largest fusion reactor which is currently under construction in France. A collaboration of 35 nations, ITER is one of the most complex engineering projects ever undertaken. The reactor’s mission is to generate a sustained 500MW reaction (enough to power a city) from 50MW input power by 2035.
In terms of space propulsion, Ebrahimi says the next step will be to build a prototype plasma engine taking her design from simulation to reality. The physics of stars may power our future world, bring us to other worlds, and perhaps deliver us to the stars themselves.
Feature Image: Artist’s rendition of a plasma thruster equipped future space craft c. ITER
More to Explore:
Ebrahimi, F. (2020). An Alfvenic reconnecting plasmoid thruster. Journal of Plasma Physics, 86(6), 905860614. doi:10.1017/S0022377820001476 (Original Research Article. Open Access)