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| Fig. 1: Concept image of an electrically powered spacecraft being propelled through deep space. (Source: Wikimedia Commons) |
Traditional chemical powered rockets have existed for almost a century. They have been very successful in getting humans on the moon, getting payloads into space, etc. It has continuously inspired generations of scientific advancements and has pushed the human race to think outside of our planet. While there have been many improvements in the propulsion technology, the fundamentals of the technology have not changed much since its conception. The propulsion used on the Saturn V that sent Apollo to the moon is not that different from the propulsion used today by modern rockets. The biggest drawback of this technology is that the fuel takes up most of the mass of the rocket. The Apollo 7 mission launched with the Saturn 1-B rocket on the launch pad was 87% propellant by mass. [1] This also minimizes the payload mass that the rocket carries while increasing the cost per kilogram of the payload.
Therefore, scientists developed a powerful propulsion system that costs less, carries more, and uses less fuel. [2] This revolutionary technology is in-space electric propulsion. Fig. 1 shows an artist's concept of an electrically propelled spacecraft in deepspace.
Electric propulsion like chemical propulsion is based on Newton's third law to create thrust. The propellant is pushed out at large velocities and it in turn pushes back on the rocket to move it forward. But unlike chemical rockets where fuel is burned to create thrust, electric propulsion uses electric fields and ionized gases to push out ions which ultimately provides the thrust. Since there is no combustion involved in electric propulsion, it takes away the risk of explosion which is a large safety concern of chemical rockets. [3]
The downside of electric propulsion lies in the fact that pushing out ions in this manner only provides a small amount of thrust. The thrust lies on the order of 1 Newton. [4] Therefore, this technology cannot be used to lift off from earth's surface as it cannot escape its gravity. However, for space travel, electric propulsion is much more efficient and can accelerate the spacecraft to much larger velocities than with a chemical propellant. Although the thrust is low, electric propulsion engines can run continuously throughout its flight for long periods of time using only a small amount of fuel. For instance, to go to Mars with a traditional propulsion system, only about 2% of the initial mass of the rocket can be payload mass. However, an electric propulsion system starting from low earth orbit could transfer 70% of the initial mass as payload. [3] Deep Space 1, an electrically propelled spacecraft, ran for over 16000 hours and used less than 159 pounds of its propellant. [3] This continuous thrust time can accelerate the spacecraft to speeds of 90,000 meters per second (over 200,000 mph). In comparison, the Space Shuttles can reach speeds around 18,000 mph. [3]
Specific impulse Isp is defined as the propellant exhaust velocity divided by the gravitational acceleration constant g. [5] This is a common measure to gauge performance of propulsion systems. Chemical monopropellant thrusters have an Isp in the range 150-225 s while chemical bipropellant thrusters have it in the range 300-450 s. In comparison, electric propulsion systems such as ion thrusters have an Isp between 2500-3600 s and Hall thrusters have an Isp in the range 1500-2000 s. [5] This large difference in Isp is indicative of how much more powerful electric thrusters are.
Another advantage that the electric thruster has is that it can use solar energy to power itself. This means that it can continue to gain energy from an external source thus reducing the weight of the spacecraft by minimizing the propellant mass. It can reduce the amount of fuel, or propellant, needed by up to 90% compared to chemical propulsion systems, saving millions in launch costs while providing greater mission flexibility. [2]
Traditional chemical rocket engines have been around for a long time and will continue to stay around because it is the best system available to get payloads off earth's surface. However, due to it high fuel mass, low Isp, and low efficiency, it is not a good contender for spacecrafts that require deep space travel. Electrically propelled rockets are the most powerful system for such missions due to its high efficiency, low cost and its ability to collect energy from external sources like the sun. Exploring planets in our solar system and beyond, mining for resources in space, building colonies on mars, etc. are no longer visions that only science fiction fans hold but a reality that can be made possible because of efficient deep space propulsion technology like electric thrusters.
© Ashwyn Sam. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. W. Orloff and D. M. Harland, Apollo the Definitive Sourcebook (Springer, 2006), p. 584.
[2] Thermionics Quo Vadis?: An Assessment of the DTRA's Advanced Thermionics Research and Development Program (National Academy Press, Washington, D.C., 2001), Appendix C.
[3] S. Mazouffre, "Electric Propulsion for Satellites and Spacecraft: Established Technologies and Novel Approaches," Plasma Sources Sci. Technol. 25, 033002 (2016).
[4] R. G. Jahn and E. Y. Choueiri, "Electric Propulsion," Encyclopedia of Physical Science and Technology, 3rd ed. (Academic Press, 2001), pp. 125.
[5] D. M. Goebel and I. Katz, Fundamentals of Electric Propulsion: Ion and Hall Thrusters, (Wiley, 2008), p. 5.
