Fig. 1: Representational Image of Electric Propulsion

Humans can walk on land in the absence of automobiles, we can swim in water without boats, but without planes and rockets we cannot fly. Leaving the ground defies the Earth's gravitational pull and therefore requires a certain amount of work that a body needs to do to propel it forward. This driving force is known as 'impulse'. If we consider a rocket, for example, when it takes off, chemical reactions occur in its combustion chamber, producing large amounts of trapped energy that explodes at high speed through a nozzle and exerts force on the atmosphere. The atmosphere reacts according to Newton's third law to push the rocket upward. Space exploration in the 1900s was more of a government affair, while the ^21st century is seeing increasing private sector participation as innovations emerge in the aerospace sector from companies such as SpaceX, Boeing, Virgin Galactic, The Spaceship Company (TSC) etc. opted as a viable front for open research. There are many reasons for such high costs, such as thousands of tons of fuel, state-of-the-art electronics and control systems, precise manufacturing of large parts, and the failure to reuse launch vehicles. In this article we discuss a technology that could very well replace conventional chemical rocket propulsion and in what aspects it is better and what its competition lacks.

Preview – Rocket Propulsion

So what exactly is the term propulsion? We immediately imagine a big cloud of exhaust coming out of a rocket, but why do we need that boost? The answer is: achieve acceleration. Acceleration can be used to move a body from rest, change a body's velocity, or overcome various drag (retardation) forces. Rocket propulsion is a subcategory of jet propulsion in which propulsion is achieved by ejecting a stored mass (propellant) at high speeds.

Conventionally, rocket propulsion is achieved by chemical combustion. In these chemical rocket propulsion systems, there are two separate tanks containing the fuel and oxidizer. These are fed into a combustion chamber at high pressures. At this high pressure, the mixture ignites and heats the gas to extremely high temperatures, causing the gas to expand. This is passed through a nozzle where all this pressure energy is converted into kinetic energy causing it to exit at extremely high speeds (up to 5,000 meters/sec). Chemical propulsion systems are classified into solid-propellant chemical rocket propulsion engines, liquid-propellant chemical rocket propulsion, and hybrid rocket engines. Solid rocket engines are the most primitive system where a hollow cylinder of solid fuel is ignited to heat and expand gas in the hollow region (similar to rockets used in fireworks). The most complicated and advanced rocket propulsion system is the liquid rocket propulsion system, where the liquid forms of fuel and oxidants are controlled by engines, by controlling the power, the rate of combustion is controlled, thus giving us control of thrust. The hybrid rocket engine is a combination of solid and liquid system where a liquid oxidizer and a solid fuel are used.

Fig. 2: Schematic image of the liquid rocket engine

Figure 3: Schematic image of the solid rocket engine

Although chemical propulsion systems are known for producing large amounts of thrust, they run out quickly. Therefore, to carry out a required mission, a lot of fuel is consumed (thousands of tons). This is where the concept of electric propulsion comes in, which greatly reduces the relative consumption of propellant, while also providing high exhaust speeds.

Introduction

Electric Propulsion – Introduction

Fig. 4: Plasma Globe

Figure 5: Sun

First we need to understand a concept called specific drive (i _sp ). It is the total thrust per unit weight of propellant. It is directly related to the efficiency of the propellant. In can be considered synonymous with a car's mileage. If less weight can provide more thrust at a given time (high specific impulse), then less fuel will be consumed. In chemical propulsion, we attempt to extract the internal energy of a propellant through combustion and breaking bonds (which are mostly limited or not fully extracted). But what if we could take a thruster and provide external energy with no theoretical limit? The higher the energy content, the more work can be done. This is the basic objective of electric propulsion.

Electric propulsion, as the name suggests, uses electrical energy (external source) to heat and eject the propellant. Furthermore, when a gas is placed in a strong electric field, its positive and negative elements split creating a charge pool (where the positive charge is equal to the negative charge). This new physical state of the gas is called 'Plasma' and the process is called 'Ionization'. It is fluidic like a gas, but it reacts to electric and magnetic fields (as it contains charged particles) and can be manipulated by these fields. Thus, the plasma generated by the electric field is accelerated through a magnetic field and exhausted at extremely high speeds (up to 100,000 m/s). The greater the electrical energy supplied, the greater the current density, causing a stronger magnetic field and therefore a faster exhaust velocity. The concept of electric propulsion was first conceived by the father of Liquid Propulsion, “Robert Goddard” in 1906 in the USA, although its development began in the late 19th century. ^century by NASA and ESA. Today, advanced plasma thruster concepts are being researched to enable deep space missions.

The Plasma Thruster

Now that we've looked at the science behind electric propulsion, let's delve into the engineering part of designing a basic plasma thruster. The application of this principle in the manufacture of a tangible product can be done in more than one way. But what every system requires are these four basic subsystems

1. An energy source (solar energy, nuclear energy, or exotic matter)

2. A conversion system (to convert source energy into electrical energy)

3. A propellant system (to store, measure and deliver propellant)

4. A propulsive device (where electrical energy is converted into kinetic energy)

Propellant selection is very important. It must be an inert gas so as not to react with the nozzle atmosphere. It should preferably be monatomic since extra energy is required to break the intermolecular bond. The most common propellants used in electric thrusters are Argon, Neon, Xenon, Helium etc.

Plasma thrusters can be distinguished based on the method by which the plasma is accelerated as electrothermal, electrostatic and electromagnetic. The electrothermal method involves heating the gas through electroresistive heaters and causes thermodynamic expansion (Resistojets). The electrostatic method consists of accelerating the plasma through the interaction of electrostatic fields. The electromagnetic method consists of accelerating the gas through the interaction of the electric and magnetic field generated by the plasma. The most common types of thrusters seen are MPDT (Magneto Plasma Dynamic Thrusters), Hall Thrusters, Helicon Thrusters, Resistojets and Ion Thrusters. Let's look at the construction of one of them, the MPDT.

Fig. 6: Image showing the MPD thruster

Figure 7: Image of the Hall thruster

Magneto Plasma Dynamic Thruster (MPDT)

Figure 8: MPDT nozzle model diagram

MPDT is a type of electromagnetic thruster that consists of a coaxial electrode design. The cathode rests inside a hollow anode. The anode and cathode are fixed using an insulating material to ensure that there is no electrical contact. The propellant gas is introduced upstream of the nozzle and passed through the electrode opening. Electrical energy is normally stored in a bank of capacitors. The spar is created between the electrodes by an ignition circuit similar to that used in a Tesla coil and is maintained by the capacitor bank so that the voltage is sufficient to maintain a plasma state (called Breakthrough Potential). Electrons flow in a radial direction from the cathode to the anode, simultaneously creating a magnetic field according to the right-hand rule. The magnetic field accelerates the downstream plasma, creating thrust. This is called an eigenfield drive. If an external magnetic field is applied to stabilize the exhaust flow, it is called an applied field thruster. Plasma reaches extremely high temperatures (6,000 to 10,000 Kelvin). To withstand this temperature, a material with a high melting point must be used as the cathode (such as tungsten), as it is also a conductor. The anode can be thicker to improve heat dissipation and a highly electropositive element must be used (such as copper). MPDT efficiencies can reach up to 55% and specific impulse magnitudes can be 10 times greater than those of chemical rockets.

Figure 9: Cross-sectional diagram of the MPDT nozzle model

Conclusion

Plasma is the most abundant state of matter found in the universe and can be manipulated in many ways. Even on Earth we witness plasmas in the form of lightning, auroras, even small sparks when a plug is not properly placed in an outlet, etc. It is a state of matter known for its abundance of energy and therefore it makes sense to use it. for high power applications.

Propulsion in the presence of atmosphere and in a vacuum are quite different concepts. When the atmosphere is present, the thrust is obtained by the action-reaction pair of the force applied by the exhaust on it and, therefore, higher values ​​of thrust are required here. But in a vacuum there is no atmosphere. Here thrust is achieved by conservation of momentum as the propellant's changing momentum drives the payload forward, and therefore higher exhaust velocities are required here. Therefore, it makes sense to use electric thrusters in a vacuum.

The impact of Specific Impulse: As a unit weight of propellant can provide more energy over a period of time in the electric booster compared to the chemical rocket, less fuel would be required to accomplish the task. For example, consider an asteroid rendezvous mission to carry a 500 kg payload. A chemical propellant with an exhaust velocity of 3,000 m/s and I _sp of 306 seconds will require 2,150 kg of propellant. On the other hand, an electric propellant with an exhaust velocity of 30,000 m/s and I _sp of 3,060 seconds would require only 90 kg of propellant.

Research is intense in the electric propulsion department. It is a futuristic concept and could enter a larger market very soon. Recently, researchers developed an engine that can produce thrust without any propellant, using quantum plasma. The most current engines include Microwave Thrusters that do not require electrodes to generate plasma. The current popular electric motor is the VASIMR (Variable Specific Impulse Magnetoplasma Rocket), which uses RF (radio frequency) couplers to generate and heat plasma according to the required thrust and mission-specific thrust parameters. It can adapt to the mission at hand and provide longer life and reliability than other thrusters. They can be powered by solar panels or nuclear energy. These technologies will give us access to greater depths of our solar system and eliminate the need for complicated mechanisms to store and deliver propellant in the case of chemical rockets. Electric Propulsion has the ability to see humans progress to an interstellar crossing civilization in the distant future (where science will catch up with science fiction).

Figure 10: Image showing the schematic of the VASIMR mechanism