Plasma Rockets, non-thermal processes deliver faster results!



A rocket engine is a can of gas at high pressure.  When the gas escapes it has a high velocity.  The gas particles travel in one direction with momentum  of the mass of the escaping gas particles times their velocity.  The net momentum remains zero because of  conservation of momentum, so the rocket (can) must travel in the opposite direction with equal momentum.  If the ratio of mass of the escaping gas to the rocket mass is constant the velocity of the rocket will be proportional to the velocity of the escaping gas. However, in space there is no drag or air resistance so after a very short period the rocket has achieved an initial velocity, so the escaping gas momentum increases the velocity on top of the initial velocity. This is referred to as integration with respect to time. So the velocity will increase linearly with time, or with a constant acceleration. The momentum per second is called the thrust.  So the longer the rocket is running the faster the rocket travels.  The limit will eventually be the speed of light. Einstein showed that as the rocket goes faster it appears to have a higher mass that limits it increase in speed so that it will never reach the speed of light.

rocket 1

To make the rocket go faster we need to get the gas ejected to go faster.  The speed of the gas in determined by its temperature. The hotter the gas the faster the particles travel on average.  This is actually the definition of temperature.  So when we say the temperature is 30 degrees we are defining the average speed of the air particles.  This is called the equilibrium speed, so what about when the wind blows. When the wind blows we have a non-equilibrium flux of air.  The wind cannot blow from every angle at the same time.  So if we could get the gas leaving the rocket to go in one direction then we have a non-equilibrium gas speed. Why do we need it to be non-equilibrium, well, because there is a limit to how hot we can make the gas, it is the melting point of our rocket can.

The speed of an air molecule at 30 degrees centigrade is an incredible 500m/s or a 1000 miles per hour. That’s right on a calm day in summer the average speed of the air around us is 1000 miles an hour, and we do not feel it.  That is because it is in equilibrium, for every air molecule hitting the outside of a surface, another one is hitting the inside in the opposite direction.  So if we could remove all the air from the inside of a house then we would see the result of the hurricane winds on the outside. It would crush the house in seconds.

But, back to rockets, we now know that we want to make the gas go fast but not by an equilibrium process such as a thermal process, we want a non-equilibrium process. This is because it will not heat the rocket just shoot the gas out the back.  One way to create a non-equilibrium process is to create a plasma in the gas.  We do this be knocking off electrons from atoms to produce free ions and free electrons.  The electrons are heated by an electrical field, the heating in only in the direction of the electric field so it is non-equilibrium.  Once the plasma is formed we can accelerate the ions to a 1000 Volts and direct them out the back of the rocket.  An ion traveling through an electric field of 1000 Volts can reach a speed equivalent to a temperature of 10 Million degrees.

This trick is not just useful for rockets but non-equilibrium processes are a key reason why plasmas are becoming so important in many industrial applications.

So let us look at a typical plasma rocket.  The first part of the rocket ionizes the gas.  In this part we heat electrons in an electrical field, an RF field is used to make the electrons go back and forth rather than just leave the vessel and hit the wall.  This stops too much heat being lost in wall collisions. The electrons bump into gas atoms and knock off more electrons, a process called ionization. We build up a high density of electrons and ions. The electrons are hot but the ions are cold.


The plasma then flows into a second region where another RF electric field is designed to heat the ions.   Magnetic fields are used to help confine the plasma and make sure it travels where we want it to go. The ion heating is directional, so it only heats ions in one direction and is non-equilibrium.  One last issue is that if we only eject ions then a flow of positive charge leaves the spacecraft.  This leaves behind a residue of electrons or negative charge and the spacecraft will charge up negative.  This might appear not to be a problem but space is not really empty and particles will be attracted to the spacecraft at higher and higher speeds and will damage it. So we have to neutralise the ion beam by also creating a beam of electrons so that the net current is zero.  The momentum of the electrons is very small relative to the ions, so it does not affect the thrust.

What does it look like. the next picture is a low pressure magnetic plasma.

The plasma is a circular ring in this photo and reflects the shape of the magnet field which is created by permanents magnets. We do see the beam here.

The next photo shows the beam from a small thruster for cubesats.

This photo is from the California-based satellite propulsion startup Phase Four targeting the emerging market for constellations of cubesats and small satellites which unveiled test results of its electric radio frequency thrusters that exponentially outperformed a previous iteration tested earlier this year.

The plasma thrusters cannot be run in ordinary atmosphere and must be tested inside a large vacuum chamber.  The thrust from this type of plasma is used to control and position small satellites in space and is quickly replacing solid fuel thrusters because of the increased efficiency.

Satellites flying with plasma propulsion would supply electricity from solar panels to the thrusters that would power a magnetic field and with radio waves to ionize xenon and force the heated plasma out of a nozzle to produce thrust. Xenon is used because it is easy to onize and is a very heavy ion.

A recent Global Electric Propulsion System Market Research Report 2018 to 2023 shows that the market for plasma engines is growing rapidly.

Worldwide Electric Propulsion System Industry Analysis By Segments
Key Players Busek Co. ., SITAEL, Accion Systems ., Aerospace Corporation and Bellatrix Aerospace
Types Gridded Ion Engine (GIE), Hall Effect Thruster (HET), High Efficiency Multistage Plasma Thruster (HEMPT) and Pulsed Plasma Thruster (PPT)
Applications Nano Satellite and Microsatellite
Countries China, Japan, USA, India, Europe and South East Asia

Do Inquiry Before Purchasing Report Here:

Clearly plasma propulsion has a bright future, and I would be happy to answer any questions or hear your thoughts in the comment section below. Please also consider signing up for further posts on plasma technology.
Mike B Hopkins




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