Plasma is the fourth state of matter (others being solid, liquid and gas), it is mostly associated with ionised gas. Typically plasma is witnessed here on Earth as lightning in thunderstorms, or the glow within neon signs and even our sun which is essentially a giant ball of plasma. The primary type of rocket engines in use at the moment are chemical propulsion engines. To traverse space more rapidly and more efficiently many space agencies have developed and are developing various plasma propulsion engines.
What is a Plasma Propulsion Engine?
A plasma propulsion engine is a type of engine that uses electric propulsion (electrical energy to move the spacecraft), the force generated gains its energy from quasi-neutral plasma (quasi-neutral meaning the particles have the same number of both positive and negative charges). It is also essential to mention ion thruster engines, which generate thrust from an ion current being passed through a plasma source and accelerated at a high velocity.
Plasma propulsion engines are better suited to long-distance space journeys as they provide less thrust than chemical propulsion engines but can do so for a much more extended period. Plasma propulsion engines can be fuelled with common gasses such as carbon dioxide, meaning fuelling them is rather inexpensive.
Advantages of Plasma Propulsion Engines
High Specific Impulse
Plasma propulsion engines have a high specific impulse value when compared to the majority other types of rocket propulsion engines. Specific impulse is the total change in momentum delivered per unit of propellant consumed. So in layman's terms, plasma propulsion engines are more fuel efficient. The high specific impulse value of plasma propulsion engines means that plasma powered rockets can reach higher velocities in space than chemical rockets, in fact, NASA scientist and ex-astronaut Franklin Chang-Diaz is currently developing the VASIMR engine, and he boasts that his engine has the capability of reaching Mars in 39 days, travelling at speeds of up to 34 miles per second.
Certain variations of plasma propulsion engines can be fuelled with a significant number of different gases and combinations of gases. Argon, Helium and even Carbon Dioxide can be used to create the plasma; almost anything could be used for refuelling on faraway worlds in the depths of space.
The chemical rockets currently in use have to spend all their fuel at once; this is not the case with plasma propulsion engines. This means that plasma rockets can change speed and direction mid spaceflight.
Disadvantages of Plasma Propulsion Engines
Generating Enough Electricity to Turn Gases into Plasma
Turning gases into plasma requires a substantial amount of electricity. In Diaz's VASIMR plasma propulsion engine the power necessary amounts to that of several nuclear reactors. This is a problem because the nuclear reactors add a significant mass to the rocket's payload. Also if something was to go wrong and the nuclear reactors exploded the fallout would be devastating.
Reliability of the Plasma Propulsion Engine
Plasma propulsion engines gradually break overtime, this is because as the plasma hits its container walls, it damages them. So when the engine is run for a specified amount of time it will break, so it is possible that a plasma propulsion engine would not even be able to make a round trip to Mars, getting stuck halfway between Earth and Mars would be disastrous. However there is currently work ongoing with self-repairing walls for plasma propulsion engines, so we do not have to rule out the use of plasma rockets just yet.
Plasma Propulsion Engines have Low Thrust
Plasma propulsion engines only generate a maximum of two pounds of thrust. The low thrust means that it is only financially viable to send light payloads using plasma rockets. Low thrust is only a problem when launching from planets so to counter the issue of low thrust chemical propulsion systems can be used and then during spaceflight plasma propulsion engines can take over.
Types of Plasma Propulsion Engines
Helicon Double Layer Thrusters
Helicon double layer thrusters are a comparatively straightforward design made out of widespread materials and can be run off of inexpensive gases. Miniature versions of these engines are ideal for small corrective manoeuvres in space, great for satellites in orbit around Earth.
So how do Helicon Double Layer Thrusters work? Plasma is created within the engine from an antenna enveloping around a chamber of gas; energy emits from the antennae and couples with the gas finishing the process. When plasma is exposed to a magnetic field electromagnetic waves are present within it, these waves are also known as Helicon waves. The magnetic field forces the plasma out of the engine creating thrust.
Magnetoplasmadynamic Thrusters and Pulsed Inductive Thrusters
Magnetoplasmadynamic thrusters work due to the Lorentz force. The Lorentz force is a force that results from the interaction between a magnetic field and an electric current; this force is enough to thrust an object in space. In Magnetoplasmadynamic thrusters, the electrical current is present within the plasma, and a magnetic field is used to accelerate the plasma out of the thruster.
Within Pulsed inductive thrusters, the Lorentz force is used to generate thrust also. The difference between this type of plasma propulsion engine and Magnetoplasmadynamic thrusters is that they do not use an electrode so do not erode. Instead, the magnetic field is hastily varied causing the plasma to accelerate.
Stationary Plasma Thrusters/Hall Effect Thrusters
Stationary Plasma Thrusters or Hall Effect Thrusters use a static magnetic field that is perpendicular to an electric field. Where the plasma emits from the engine, there is an area of high electron density; this accelerates the plasma. The Soviet Union initially produced this type of plasma propulsion engine in the 1970s.
Variable Specific Impulse Magnetoplasma Rocket (VASIMR)
The Variable Specific Impulse Magnetoplasma Rocket or VASIMR for short is a type of plasma propulsion engine that is undergoing development by Ad Astra Rocket Company; ex-astronaut Dr. Franklin Chang-Diaz is leading the project. The VASIMR is set to be fitted to the International Space Station; this revolutionary plasma propulsion engine is predicted to shorten travel time from Earth to Mars from six months to just thirty-nine days and from Earth to Jupiter from six years to fourteen months.
The aspect of the VASIMR that sets it apart from the other plasma propulsion engines is that the engine fires within a vacuum chamber. This allows it to consume very little fuel and generate enough thrust to explore the solar system. The VASIMR uses radio waves to ionise a propellant into a plasma. A magnetic field is then used to accelerate the plasma out of the engine.
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