New magnetic rocket thruster concept could propel astronauts to Mars
A crewed mission to Mars may be more practical thanks to a new rocket concept developed by Fatima Ebrahimi, a physicist at the US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), that uses magnetic fields to generate thrust.
Over the past 64 years, there's been remarkable success with robotic satellites and probes, but these have been relatively small, with the heaviest being the ATV cargo ship weighing in at 44,738 lb (20,293 kg) fully loaded – and that one only went into low-Earth orbit. The largest deep space probe was the Cassini-Huygens mission to Saturn, which came in at a titchy 12,467 lb (5,655 kg).
This is because the greatest obstacle to humanity becoming a true spacefaring species is the engines used to propel spacecraft across the solar system and beyond. Chemical rockets can push out an impressive amount of thrust, but have very little specific impulse. That is, they can't fire for very long before they run out of propellant. Electric propulsion systems, like Hall thrusters, are the opposite. They only put out about as much thrust as the weight of a small coin, but they can burn for months as opposed to minutes, so they can (slowly) build up to great speeds.
Unfortunately, neither is very attractive for carrying astronauts to Mars aboard craft weighing tens, if not hundreds, of tons. One might give a fast start and the other a slow one, but they both mean a long, hazardous voyage of months, if not years. Both of these basic propulsion approaches have their advantages and disadvantages, but what is really needed, at least, in the short term, is one that combines properties of the two. Ideally, something that has higher thrust and more specific impulse.
The new Princeton concept works by using the same mechanism that helps to blast solar flares away from the Sun. These flares consist of charged atoms and particles called plasma, which are trapped inside powerful magnetic fields where complex interactions take place.
For propulsion systems, Ebrahimi is particularly interested in one type of interaction called magnetic reconnection, which is where magnetic fields in highly charged plasmas restructure themselves to converge, separate, and re-converge. As they do so, they generate large amounts of kinetic energy, thermal energy, and particle acceleration. It's a phenomenon not only seen on the Sun, but also in the Earth's atmosphere and inside Tokamak fusion reactors, like PPPL’s National Spherical Torus Experiment (NSTX).
In a very general way, the magnetic thruster is like the ion thrusters that are becoming increasingly common on spacecraft. These work by charging a propellant made up of heavy atoms like xenon and then accelerating them using an electrical field. For the new concept thruster, the magnetic fields do the accelerating.
So far, computer simulations by PPPL computers and the National Energy Research Scientific Computing Center at the Lawrence Berkeley National Laboratory in Berkeley, California, show that a magnetic reconnection thruster could produce exhaust velocities 10 times faster than that of current electric propulsion systems.
"Long-distance travel takes months or years because the specific impulse of chemical rocket engines is very low, so the craft takes a while to get up to speed," says Ebrahimi. "But if we make thrusters based on magnetic reconnection, then we could conceivably complete long-distance missions in a shorter period of time."
In addition to cutting down travel time, the new thruster concept is throttlable by fine-tuning the magnetic fields. In addition, the thrusters don't just shoot out plasma, but plasmoids, which are balls of plasma contained in magnetic bubbles, adding more power. Also, the thruster doesn't depend on heavy elements for propellant and can be loaded with lighter, cheaper ones.
"While other thrusters require heavy gas, made of atoms like xenon, in this concept you can use any type of gas you want," Ebrahimi says.
The research was published in the Journal of Plasma Physics.