Course: Advanced Aerodynamics
Introduction
Physics is the study of matter and energy and the relationship between them. One of the most important principle of physics is the concept of motion. Motion is simply the act of moving or the change of position of an element with respect to a reference point within a defined time interval. This physical phenomenon was thoroughly studied by the great Sir Isaac Newton. He established three important laws of motion among which the third law state that for every action in nature there is an equal and opposite reaction (NASA, 2015). This concept is particularly true in the field of aircraft and spacecraft propulsion. Aircraft and spacecraft propulsion systems are machines that produce thrust to accelerate the aircraft or spacecraft often forward or upward (NASA, 2015). Applying the basic principles of Newton’s third law to spacecraft design prompts the realization that rocket engine thrust is used as a reaction force to weight in order to launch the spacecraft. The quest for engine efficiency has led spacecraft manufacturers to thoroughly explore both chemical propulsion systems and electrical propulsion systems. How do chemical propulsion systems compare with electrical propulsion systems?
Chemical propulsion systems essentially generate thrust through a combustion reaction of fuel and oxidizers that create a high-temperature, high-pressure mixture of combustion products. The high-temperature combustion gases reach supersonic speeds while exiting the rocket nozzle and propel the spacecraft upward as it progressively attain the orbit velocity of 7.57 km/s and the escape velocity of 11 km/s (Anderson, 2016 P 769 -787). In contrast to violent chemical reactions, electrical propulsion systems generate thrust through the use of electrical power to accelerate low-molecular weight gazes by different electrical and or magnetic means (ESA, n.d.)
Chemical and electrical propulsion systems have been evaluated through multiple research studies and textbooks. In the textbook titled “Fundamentals of Electrical Propulsion: Ion and Hall Thrusters”, Goebel and Kats indicate that both systems use the basic principle of accelerating mass and ejecting it from the vehicle to propel the spacecraft. But the mass ejected from electrical thrusters are energetic charged particles. This affects the system’s performance and new methods are used to determine parameters such as specific impulse and efficiency (Goebel & Kats, 2008).
Methodology
This case-study was conducted through documentation analysis and a mathematical evaluation of scientific laws, theories, and equations. Textbooks and research studies were used to determine specific scientific phenomena and physical quantities that are essential to chemical and electrical propulsion systems. Scientific theories and equations governing these physical quantities were then analyzed and compared. Data tables and computed graphical data was used to support a numerical analysis. A matrix analysis was then used to evaluate results and findings.
The first edition of Goebel and Kats’ textbook titled: Fundamentals of Electrical Propulsion: Ion and Hall thrusters, the eight edition of Anderson J.D.’s textbook titled: Introduction to flight, and the ninth edition of Sutton and Biblarz’s textbook titled: Rocket Propulsion Elements were used to determined that thrust, specific impulse and propulsive efficiency are essential to an analysis of chemical and electrical propulsion systems. These Textbooks were also used to define systems schematics, scientific theories and equations for thrust, specific impulse and propulsive efficiency. NASA and ESA website articles on chemical and electrical propulsion were used to support, verify, and validate information contained in the textbooks from a practical standpoint. NASA and ESA website articles were also used to evaluate chemical and electrical propulsion systems technologies and their uses. Martinez-Sanchez and Pollard research overview on electrical propulsion system was referenced to define a schematic of electrical propulsion systems. The aforementioned materials were used to evaluate experimental and real-life applications of the systems. Data tables characterizing each system were used to compare both systems.
Results and Findings
Physical theories, quantities and characteristics of chemical and propulsion system are specified based on materials published within he first edition of Goebel and Kats’ textbook titled: Fundamentals of Electrical Propulsion: Ion and Hall thrusters, the eight edition of Anderson J.D.’s textbook titled: Introduction to flight, and the ninth edition of Sutton and Biblarz’s textbook titled: Rocket Propulsion Elements.
Due to the lack of public data, the above data table comparing physical quantities characterizing real-life spacecraft chemical and electrical propulsion systems was based on data made public by the European Space Agency (ESA) on its website for its Smart-1 Spacecraft.
Schematics illustrating chemical and electrical propulsion system are shown below in figure 1.
Evaluation and Discussion
There are multiple configurations of chemical propulsion systems and various type of electrical propulsion systems. Chemical propulsion systems can generally be characterized by the propellant type: the use of liquid propellants or solid propellants. Electrical propulsion systems can generally be characterized by the thruster type: the use of an electron-ion thruster, a magnetoplasmadynamic thruster or an arc-jet thruster. But the fundamental operational principle of each system does not vary by propellant type or thruster type. Comparing spacecraft chemical propulsion systems to Spacecraft electrical propulsion systems requires a matrix approach.
Table 1 shows that the mass flow rate of fuel and oxidizer and the combustion process is essential to chemical reaction-based propulsion. Contrary to chemical systems, table 1 shows that it is the charge of the propellant and the total electrical power into the system that is essential to electric propulsion. This is supported by the real-life applications data displayed in table 2. The mass of propellant consumed by the chemical system is 5350 kg for a thrust time of 0.24 hrs. In the meantime, the electrical system consumes only 80 kg for a thrust time of 5000 hrs. This indicates that spacecraft electrical propulsion systems are ideal for deep-space exploration since they require years of travel. Table 2 also shows that electric system generates a much lower thrust over a short time of period with a significantly higher specific impulse. This indicate that electrical propulsion systems are ideal for trajectory correction maneuvers in space as the lower thrust they generate is better suited for such maneuvers that typically requires changes in velocities of about 10 m/s to 20 m/s. Nevertheless, a spacecraft propulsion system must reach the average escape velocity of 11 km/s that is required to escape earth’s gravitational forces. This requires a system that can generate a very high thrust over a short period of time. This functional requirement is easily achievable by chemical propulsion systems.
Technological development in the field of aerospace and the critical need for space exploration favors the use of electrical propulsion system. The matrix displayed in table 3 is a determination of the most suitable spacecraft propulsion system by mission capabilities or functional requirement. It shows that the development of electrical propulsion system will allow humans to challenge the frontiers of our solar system and perhaps even our galaxy due to suitability for missions requiring years and decades of space travel. It is important to note that the development of computerized control system also renders the use of electrical propulsion system intriguing because of their capability of being operable with other spacecraft systems. Electrical propulsion systems can be equipped for example with solar panels, electro-magnetic panels, actuators, sensors, and generators that may serve for example as components of an alternate energy source system for a deep space exploration mission.
Conclusion
Spacecraft chemical propulsion systems essentially generate thrust through a combustion reaction of fuel and oxidizers. Spacecraft Electrical propulsion systems generate thrust through the use of electrical power. Despite the high amount of thrust generated by Spacecraft chemical propulsion systems, thrust is only generated for a limited amount of time. Contrary to spacecraft chemical systems, spacecraft electrical propulsion system generates a much lower thrust over an extensive amount of time. This makes spacecraft chemical systems ideal for rocket launches while spacecraft electrical systems are ideal for deep space exploration and trajectory correction maneuvers.
References
NASA. (2015, May 5). Newton’s Third Law Applied to Aerodynamics. Retrieved from
NASA. (2015, May 5). Beginner’s guide to propulsion. Retrieved from
Anderson, J.D. (2016). Introduction to Flight. New-York, NY: McGraw-Hill Education
ESA. (n.d.). What is Electrical Propulsion? Retrieved from
EASA. (2019 September, 1) Electric Spacecraft Propulsion. Retrieved from
Goebel, D., & Katz, I. (2008). Fundamentals of electric propulsion ion and Hall thrusters.
Hoboken, N.J: Wiley.
Sutton, G. P., & Biblarz, O. (2016). Rocket propulsion elements. Retrieved from https://ebookcentral.proquest.com
Martinez-Sanchez, M., Pollard, J.E., (1998, September-October). Spacecraft Electric
Propulsion-Overview. Journal of Propulsion and Power, Volume 14, No. 5. doi:
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