Rocket Science — Basics — Part 5 — Electric Engines🚀🧑‍🚀

Rocket Engines are of different types. We will go by latest and futuristic to currently used chemical engines to get super excited. In this blog post, we will look into Electric engines.

We all knew that from high school chemistry, there are three main states of matter - Solid, liquid and gas. But there is one more state that can be very useful in modern rocket engines. Can you guess it 🤔?

Rocket Cartoon

The answer is Plasma. Let’s look at a hydrogen atom. Hydrogen atom is very simple with one proton and one electron. Proton is positively charged and electron is negatively charged. So, it is balanced and called as neutral.

Creation of hydrogen Cation

Now, we can pass very high temperature or excited photons or electric power or electro magnetic field to the gas, it can dethrone electron from its orbit. Now, it is called as ion (cation). So, plasma is a mixture of free electrons and ions. The process is called as Ionization. Remember from the high school, we say that electric current is nothing but flow of electrons. This ionization allows plasma to conduct electricity and respond to electromagnetic fields.

Different states of matter

Plasma can be created by using various techniques:

1. Heating a gas to high temperatures results in energy levels being raised and causes strong collisions between the atoms and that leads to stripping of electrons from atoms leading to plasma.
2. By passing electric current
3. By passing high energy photons like ultra violet rays
4. By passing High-intensity lasers

Plasma can exist in various temperatures and densities from neon lights to Aurora.

Aurora Courtesy: PBS North Carolina

Most of us would have wondered and even wanted (🙋‍♂️) to see Northern Lights, also called as ‘aurora borealis’ that happens in sky in northern hemisphere. The solar particles from the sun is usually deflected by Earth’s magnetosphere (the magnetic field surrounding the planet). But in the hemispheres the magnetic field converges and acts like a funnel for the charged particles from the sun interact with oxygen and nitrogen atoms and makes them to go to excited state. When they come to normal state they release those energies in the form of stunning light of different colors based on the different levels of altitude that the interactions happen.

Plasma Propulsion Engines:

Liquid Rocket Engine

Let’s recall how normal liquid rocket engines work. We have a fuel tank and an oxidizer tank and these are combined in the combustion chamber to produce a very hot gas. Then these gases are expelled through a nozzle to generate thrust. The action is the expulsion of gas from the rocket, and the reaction is the thrust that pushes the rocket forward which we call it as Newton’s third law. This is the thrust that propels the rocket, which can be controlled by regulating the flow of fuel and oxidizer into the combustion chamber. You can ask me question like it is working and why we need another type of engine🤔?

The answer is these type of engine’s exhaust velocity (speed of the exhaust gases as they exit the rocket engine) depends on the amount of heat generated during the combustion process.

Here comes the Electric engines. There are various types of electric engines like electrostatic, electromagnetic and electrothermal engines. We will look into Electrostatic engines also called as Plasma Thruster or Ion Thruster. In these type of engines, we first pass high amount of current to a gas like Xenon and thus creating plasma (Ionization) in a container. Then, we provide an electric potential to top and bottom of the container, say positive charge on top and negative charge at bottom. This creates an electric field and the ions (which are positively charged) and the electrons (which are negatively charged) move in opposite directions.

Plasma Thruster Schematic diagram Courtesy: University of Michigan

Now, we will make the negative charged plate (cathode) like a mesh. The ions move towards the cathode as they get more speed when they move closer and closer and they pass through the screen at high velocity. Ions are also having some mass which is order of magnitude higher than that of electrons. So, mass moving with velocity is nothing but Thrust (mass * velocity). We can get as much as 50,000 m/s as exhaust velocity which is way higher than chemical (both solid and liquid) propellent. Now, you can ask me a question like exhaust velocity is very high for plasma (electric) engines, why can’t we replace entire chemical propellants?? 🤔

The answer is ‘mass flow rate’ is very very small and we need a huge amount of electricity. So, it will be hard for the rocket using electric engine to be used for launch. These type of engines can be very well used if you need a satellite that needs power for a long period of time in orbit that needs some booster to mitigate the atmospheric drag and even moving the satellite say from LEO (Low Earth Orbit) to MEO (Medium Earth Orbit) etc., It can even be used for say interplanetary missions like travelling to Mars. This type of electric rocket engine is called as ‘Electrostatic Propulsion Engines’.

Ion Thruster Courtesy: Space Techie

We can use magnetic field also with electric field to make the ions and electrons move in one direction which can yield some more efficiency mostly used for long range missions and they are called as ‘Electromagnetic Propulsion Engines’.

The last type is ‘Electrothermal Propulsion Engines’ — the propellant is heated using electrical energy (via resistors or electric arcs) before being expelled through a nozzle. This can provide exhaust velocities up to 5,000 m/s.

In the upcoming posts, we will look into nuclear energy based engines.

Thanks for reading!!!