Aero-Astro Magazine Highlight

Future of rocket engines is electrifying

By Paulo Lozano

Paulo Lozano, of Aero-Astro's Space Propulsion Lab, is researching non-traditional configurations for Hall-effect plasma thrusters and their ability to propel spacecraft. (William Litant)

Many of us can pinpoint the exact moment when we decided what to do with our lives: during a trip, a conversation, reading a book, and so on. I was eight years old when my parents took me to the only operational planetarium in Mexico. Suddenly, I was confronted with physical processes that form stars, nebulae and galaxies, planets, and life. I was deeply affected by all these concepts. At that moment I knew I wanted to do something related to space science and exploration, something that would complement my already existing passion for airplanes and rockets. I decided to be a pilot, an engineer, a physicist, and a rocket scientist. Today, I have the privilege to say I am all four, and working at MIT’s Department of Aeronautics and Astronautics allows me to move closer towards my lifelong dreams.

The most effective engines possible

Philosopher Karl Popper said “All life is problem solving.” This is the way we operate in our search to understand better the world we live in and to provide alternatives to improve the standard of living in the world. The problems I deal with in Aero-Astro’s Space Propulsion Laboratory have to do with how we make the most effective rocket engine possible to carry valuable payload to different locations in space. For years, chemical-based rocket engines have dominated this technological area. However, there is a severe limitation; the thrust force they produce for every gram of propellant they use, or specific impulse, is somewhat low. This is the reason why in a mission that requires large changes in velocity, for instance a mission to explore the moons of Jupiter, most of the vehicle mass is propellant, leaving little room for payload. An elegant solution to this problem is to replace chemical reaction propulsion with direct propellant acceleration of electrically charged gas. Through this method, it is possible to increase the specific impulse of space thrusters 1000 percent, or even more. Propellant consumption becomes very efficient, reducing considerably its contribution to the total vehicle mass. Electric propulsion thrusters’ sole limitation is the availability of onboard power. Since, in most cases, power comes from solar arrays, the thrust levels are small compared to chemical rockets. On the other hand, they can operate continuously for days or months, delivering a modest but constant acceleration that eventually makes the spacecraft move at large velocities — velocities practically unachievable for some missions using chemical engines.

At SPL, we are constantly looking for exciting ways to improve the state-of-the-art in space propulsion. For example, we are researching non-traditional configurations for Hall-effect plasma thrusters, which use magnetic fields to confine electrons that ionize gas through inelastic collisions, while ions are accelerated using electrostatic fields. As part of our undergraduate curriculum, some of our students are building a version of this engine for an eventual mission to the moon. Hands-on learning cannot be better than this! The engine uses permanent magnets and a conical geometry that departs significantly from traditional designs.

Interesting as plasma thrusters are, the focus of my research for the last few years has been on miniaturizing electric propulsion technologies. There are many reasons why this is important. Think in terms of the extraordinary achievements in micro- and nano-manufacturing that have boosted the electronics industry to a prominent place in the world economy. With today’s techniques, we are miniaturizing communications, attitude, payload, control, power, and other subsystems of spacecraft. There is no apparent reason why a fully functional satellite the size of a hand-held MP3 player could not be built today — only missing from the equation is a comparably-sized propulsion subsystem that would allow the small object to perform for an extended period of time just as well as its heavyweight satellite cousins. Very small satellites, or clusters of them, could fill a niche market in communications, remote sensing, exploration, security, and other applications at a fraction of the launching and manufacturing costs of current platforms.

A conical shell plasma plume fires from a student-built space thruster in an Aero-Astro Space Propulsion Laboratory vacuum chamber. As electric thrusters' power comes from solar arrays, they can operate continuously for months, attaining velocities practically unachievable for missions using
chemically-fueled engines.

Power from the tip of a needle

The propulsion technology I propose is based on the extraction and acceleration of ions from liquid surfaces, specifically from the liquid menisci located at the tip of micron-sized sharp needles. This is possible since the electric field on the surface of the liquid is amplified enormously to billions of volts per meter after the meniscus deforms into a stable cone-like structure. Ions are extracted through a quantum mechanics process and accelerate quickly to practically the full energy applied by the power supply, therefore operating at very high efficiencies. Each one of these ion emitting tips produces a thrust force of less than the weight of a human hair, whereas typical low power plasma engines push with a force several thousand times greater than this. Such a low thrust is not necessarily a weak point; in fact there are missions that require forces of that order to compensate for orbital perturbations, like high-altitude atmospheric drag or solar pressure. If higher thrusts are required, clusters of ion emitting tips working in parallel could be used. Since the individual size of these emitters is very small, they can be distributed on a surface in very tight arrangements. For example, the amount of thrust per unit area in such an array when spacing the emitters by one-third of a millimeter approximates that of a larger plasma thruster, and it becomes much larger for smaller separations. We use microfabricating techniques to produce arrays like this on different materials, like silicon and porous metals, and different geometries. Every component can be machined at the smallest of scales, thus producing a truly integrated propulsion subsystem that would scale with the rest of the spacecraft.

As an add-on to my research, we discovered that ion beams from these emitters are of such good quality that they can be used in other applications. For instance, we can use them as a source of monoenergetic, high-brightness negative ions for microscopy, lithography, implantation, mass spectrometry and others. As it turns out, these little liquid menisci provide a lot of material to do research on, experimentally and theoretically. I feel lucky to have encountered this topic on my way, and I look forward to the next big challenge in space propulsion.

Paulo Lozano is the Charles Stark Draper Assistant Professor of Aeronautics and Astronautics in the MIT Aeronautics and Astronautics Department. He holds an Ingeniero Fisico Industrial from ITESM, Mexico (1993); M.Sc. from CINVESTAV, Mexico (1996); and an S.M. (1998) and Ph.D. (2002) from MIT. His interests include electric propulsion, electrosprays, thruster physics, electrochemical microfabrication, engine health monitoring, and space mission design. He may be reached at plozano@mit.edu