Umair Gaines
Electric propulsion for spacecraft was first introduced in 1911 by Konstantin Tsiolkovsky, and the world's first electrothermal rocket engine was created in the early 1930s by Valentin Glushko. While electric thrusters use much less propellant than chemical rockets and can provide thrust for longer, they provide lower thrust due to limited electrical power. Electric propulsion is also ineffective inside an atmosphere, as the thrust and efficiency attainable are drastically worse than in a vacuum. Therefore, while electric propulsion is possible and widely used on spacecraft, it is not currently possible to use an electric rocket to launch a spacecraft from the surface of a planet.
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What You'll Learn
Electric propulsion requires multiple steps, unlike chemical rockets
Electric propulsion, unlike chemical rockets, requires multiple steps. Chemical rockets impart energy to combustion products directly, while electrical systems require several steps. For instance, electrically powered propulsion with a nuclear reactor was considered for the interstellar Project Daedalus in 1973, but the approach was rejected due to its thrust profile and the weight of the equipment needed to convert nuclear energy into electricity.
Electrothermal engines, which entered use in the USSR in 1971, are another example of electric propulsion requiring multiple steps. These engines use hydrazine as a propellant and accelerate ions using the Lorentz force or electromagnetic fields. However, they require a lot of energy, and according to the rocket power equation, thrust is inversely proportional to exhaust velocity for a given power.
Another type of electric engine is the resistojet and arcjet, which work similarly to nuclear-thermal engines by heating an inert propellant with external power. While these engines can theoretically reach higher exhaust velocities than nuclear-thermal engines, they often result in lower thrust.
Electric propulsion technology, such as the popular Ion Engine used for deep space probes, is also ineffective inside an atmosphere. The thrust and efficiency attainable are much worse than in a vacuum. However, despite these multiple steps, electric propulsion offers several advantages over chemical rockets. Electric thrusters typically use much less propellant and can provide thrust for longer periods, making them ideal for deep-space missions.
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Electric rockets produce very low thrust
Electric propulsion, or the use of electricity to generate thrust and modify the velocity of a spacecraft, was first introduced in 1911 by Konstantin Tsiolkovsky. Despite being a widely used technology on spacecraft today, electric propulsion produces very low thrust compared to conventional chemical thrusters. This is because electric propulsion is limited by the available electrical power on board the spacecraft.
The performance of electrothermal systems in terms of specific impulse (Isp) is 500 to ~1000 seconds, which exceeds that of cold gas thrusters, monopropellant rockets, and even most bipropellant rockets. However, according to the rocket power equation, for a given power, thrust is inversely proportional to exhaust velocity. Therefore, while electric propulsion requires much less propellant than chemical rockets, the thrust is much weaker. For example, the thrust from a plasma engine is equivalent to the weight of a sheet of paper resting on your palm.
The low thrust of electric propulsion systems becomes a problem when trying to lift a vehicle from a planet's surface, as the propulsion may not offset the gravitational force. This is why electric propulsion is not suitable for launch from Earth, and is instead used for station-keeping maneuvers, orbit raising, and interorbital transfer. However, electric propulsion can be incredibly effective for deep-space missions due to its constant acceleration, which results in speeds that ultimately surpass those of chemical rockets.
Despite the drawbacks of low thrust, electric propulsion has many advantages. Electric thrusters use less propellant, are more mass efficient, and are not limited in energy like chemical propulsion systems. The development of more powerful electrical power sources for spacecraft will likely make electric propulsion an even more attractive alternative to chemical propulsion in the future.
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Electric propulsion is ineffective inside an atmosphere
Firstly, electric propulsion requires several steps to impart energy to the combustion products, whereas chemical rockets can do this directly. This means that electric propulsion cannot provide enough thrust to lift a vehicle from a planet's surface. The thrust force of an electrostatic thruster is so weak that it cannot lift even a paperclip off the ground.
Secondly, electric propulsion requires a lot of energy. For a given power, thrust is inversely proportional to exhaust velocity, meaning that a large amount of energy is required to achieve high exhaust velocities. This is a significant challenge, as the weight of the equipment needed to generate sufficient electricity can become a burden, reducing the overall performance of the vehicle.
Thirdly, electric propulsion is ineffective in an atmosphere because the thrust and efficiency attainable are drastically reduced compared to in a vacuum. This means that the already limited thrust force of electric propulsion systems is further diminished when used within an atmosphere.
Finally, while nuclear reactors can provide the energy density required for effective electric propulsion, they present additional challenges. These include the need for large, possibly red-hot radiators to evacuate waste heat and the difficulty of achieving high enough temperatures without melting the reactor.
In conclusion, electric propulsion is ineffective inside an atmosphere due to limited thrust force, high energy requirements, reduced efficiency in an atmosphere, and the challenges associated with nuclear power sources.
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Electrothermal engines require a lot of energy
Electrothermal engines are a type of electric propulsion system that uses electrostatic or electromagnetic fields to accelerate mass to high speeds, generating thrust and modifying the velocity of a spacecraft in orbit. They are powered by ionized xenon gas, which produces very low thrust compared to their solid or liquid-fueled counterparts. However, they use minimal propellant, resulting in longer-lasting performance.
Electrothermal engines require a significant amount of energy due to the relationship between power and thrust. According to the rocket power equation, for a given power level, thrust is inversely proportional to exhaust velocity. This means that as exhaust velocity increases, the thrust decreases, and vice versa. As electrothermal engines can theoretically achieve higher exhaust velocities than nuclear-thermal engines, they often have lower thrust.
The challenge of achieving sufficient thrust with electrothermal engines is further exacerbated by the need to evacuate waste heat. While electrothermal engines can operate at higher temperatures, they require large, potentially red-hot radiators to remove waste heat from the nuclear generator. This adds to the overall complexity and weight of the system.
Additionally, electrothermal engines face the challenge of converting nuclear energy into electricity efficiently. In the case of the interstellar Project Daedalus, the weight of the equipment needed for this conversion was a significant drawback, leading to small acceleration gains that would have taken a century to achieve the desired speed. This highlights the energy-intensive nature of electrothermal propulsion and the challenges in balancing weight and acceleration.
Despite these challenges, electrothermal engines have been used successfully in satellite technology, where their advantages, such as lower propellant consumption and longer thrust duration, outweigh the need for high initial thrust. However, for applications requiring rapid acceleration and substantial payload capacity, such as launching a rocket from Earth, the current limitations of electrothermal engines in terms of energy requirements and thrust make them less feasible.
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Electric propulsion is relatively unproven
Electric propulsion, also known as spacecraft electric propulsion, is a technique that uses electrostatic or electromagnetic fields to accelerate mass to high speeds, generating thrust to modify the velocity of a spacecraft in orbit. The first electric rocket engine was created in the early 1930s by Valentin Glushko at the Soviet Gas Dynamics Laboratory (GDL). However, the concept of electric propulsion was first introduced in 1911 by Konstantin Tsiolkovsky, with earlier notes by Robert Goddard in his personal notebook. Despite this long history, electric propulsion remains relatively unproven, especially when compared to traditional chemical rockets.
The main challenge with electric propulsion is the limited electric power available, resulting in much weaker thrust compared to chemical rockets. While electric thrusters use less propellant due to their higher specific impulse, the lower thrust makes it difficult to lift a vehicle from a planet's surface. Electric propulsion is more suitable for low-thrust manoeuvres near a planet, where the thrust can be applied for a long interval. This makes electric propulsion ideal for deep-space missions and satellite applications.
Electrospray propulsion, a type of electric rocket technology, has been pushed from theory to reality by researchers at MIT, including Paulo Lozano, Natalya Brikner, and Louis Perna. Electrospray thruster chips are comparable to Hall thrusters and gridded ion engines in terms of thrust density, but they are far weaker than chemical rockets. Electrospray propulsion chips have the potential to reach thrust densities up to 10,000 times higher with improved manufacturing techniques, but they are still in the testing phase on two satellites in space.
Another challenge with electric propulsion is the energy requirement. The rocket power equation states that for a given power, thrust is inversely proportional to exhaust velocity. This means that electrothermal engines, which require a lot of energy, may have lower thrust despite achieving higher exhaust velocities. Additionally, the weight of the equipment needed to convert nuclear energy into electricity can offset the benefits of using electric propulsion.
While electric propulsion has been used successfully for decades by American and Russian satellites, it is still considered relatively unproven, especially for launching rockets from the Earth's surface. The current electric propulsion technology is almost completely ineffective inside an atmosphere, as the thrust and efficiency are drastically reduced compared to in a vacuum. Further research and development are needed to fully realize the potential of electric propulsion and overcome the challenges of low thrust and energy requirements.
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Frequently asked questions
Electric rockets are possible and have been used since the 1960s. However, they are not suitable for launching from Earth due to their low thrust.
Electric rockets use less propellant than chemical rockets because they have a higher exhaust speed. This means they can last much longer.
Electric rockets provide lower thrust compared to chemical rockets due to the limited electrical power available. This means they cannot provide enough thrust to launch a rocket from Earth.
