The Ion Propulsion System – What the… #!$@*!!

Four different ion thrusters. Source: Michigan Technological University (MIT)“That’s Star Trek stuff!”

… is pretty much the bemused reaction you’ll get if you allow yourself to answer casual questions about science over a drink with a non-physicist.  AB-SO-LUTE disbelief.  Your fault!  Shouldn’t have gone there…  Pretend you didn’t hear the question…  Especially if the answer is ion propulsion!  

Stick to the mundane.  (Yawn.)  Most people only smile at the idea of new technologies.  Some get angry because they think you’re having a joke.  Even though you know that it may sounds like SciFi, but it’s not.  It’s real science.

 

Newton again?

The propulsion of choice for science fiction writers has become the propulsion of choice for scientists and engineers at NASA.  The ion propulsion system’s efficient use of fuel and electrical power enable modern spacecraft to travel farther, faster and cheaper than any other propulsion technology currently available.

Despite their high-technology allure, ion thrusters depend on the same elementary Newtonian principle that conventional rocket engines do.

This is Newton’s Third Law:

Every action has an equal and opposite reaction.

 

Thrust

Thrust is the force that the thruster applies to the spacecraft.  Thrust is a reaction force that is described quantitatively by Newton’s second and third laws.  When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction on that system.

This force is applied on a surface in the direction perpendicular (or normal) to the physical surface under consideration.  Thrust is measured in Newton (N), which represents the amount of force needed to accelerate 1 kilogram of mass at the rate of 1 metre per second squared.

Despite its exotic build, an ion thruster relies on the same simple equation as any other rocket engine. The force F it produces equals the mass m of propellant moved times its acceleration a.

Thus,

\boldsymbol F = m \boldsymbol a.

 

A diagram showing how an equal thrust can result from two different methods. The captions read: "Large amount of propellant x Small speed of propellant" and "Small amount of propellant x Large speed of propellant."The Modern Ion Thruster

Modern ion thrusters can deliver up to 0.5 Newtons (0.1 pounds) of thrust, equivalent to the force you would feel by holding nine U.S. quarters in your hand.  To compensate for low thrust, the ion thruster must be operated for a long time for the spacecraft to reach its top speed.

Ion thrusters use inert gases for propellant, eliminating the risk of explosions associated with chemical propulsion.

The usual propellant for the majority of modern ions thrusters is xenon, which is chemically inert, colourless, odourless, and tasteless.  But other gases such as krypton and argon may be used.

 

The Early Development of Ion Thrusters

The first known published mention of ion propulsion was made in 1911 by the founding father of Russian cosmonautics, Konstatin Tsiolkovsky (1857-1935).  American rocket pioneer Robert H. Goddard (1882-1945) independently conceived of the idea of ion propulsion in 1906 and began his first laboratory experiments in 1916.

With the advent of the Space Age and the anticipated need for high-performance propulsion systems, work on developing working ion engines began in earnest.

But first…

What’s an Ion?

Ions

An ion is simply an atom or a molecule that is electrically charged.

Ionization is the process of electrically charging an atom or a molecule by adding or removing electrons.  Ions are positive (when they lose one or more electrons) or negative (when they gain one or more electrons).  A gas is considered ionised when some or all the atoms or molecules contained in it are converted into ions.

A diagram showing the formation of a Sodium atom ion.

 

Plasma

Plasma is an electrically neutral gas in which all positive and negative charges – from neutral atoms, negatively charged electrons, and positively charged ions – add up to zero.

Plasma exists everywhere in nature.  Often, it is designated as the fourth state of matter (the others are solid, liquid, and gas).  Plasma has some of the properties of a gas, but it is affected by electric and magnetic fields and is a good conductor of electricity.

Plasma is the building block for all types of electric propulsion, where electric and/or magnetic fields are used to push on the electrically charged ions and electrons to provide thrust.  Examples of plasmas seen every day are lightning and fluorescent light bulbs.

While NASA had been performing ground-based ion engine tests at the Lewis Research Center (LRC) in Cleveland, Ohio since 1959 (now known as the Glenn Research Center), it was quickly recognised that there were limitations to what could be done in a vacuum chamber.

 

Beam Neutralisation

One of the key steps in ion engine operation is beam neutralisation where electrons are injected into the departing ion stream to keep it and the engine electrically neutral.

Without it, the ion engine and the spacecraft using it will build up a negative charge that will divert the flow of positively charged ions from the engine negating the thrust it produces.

Several methods were developed and ground tested to neutralise the ion beam, although the possibility remained that electrons from the metallic walls of the vacuum test chamber or the thin plasma that filled the chamber during engine operation could be affecting the results.  The only definitive means of verifying the effectiveness of the beam neutralisation methods was to perform an experiment with an ion engine in the vacuum of space.

 

SERT

In summer 1961, the development of SERT (Space Electric Rocket Test) began under the direction of NASA’s Marshall Space Flight Centre in Huntsville, Alabama with the primary goal of field testing beam neutralisation.

By the end of 1961, management had been transferred to the LRC with continuous work on building and launching SERT I starting in January 1963 after many months of preliminary study.

 

Conventional Ion Thrusters

In a conventional ion thruster, electrons are generated by a hollow cathode, called the discharge cathode, located at the centre of the engine on the upstream end.  The electrons flow out of the discharge cathode and are attracted to the discharge chamber walls, which are charged to a high positive potential by the thruster’s power supply. 

An ion thruster does two simple things:

  • It creates charged particles – or ions,

  • It accelerates the particles opposite to the intended direction of travel.

Ion thrusters expose atoms (xenon, in the case of the GOCE satellite) to electrons, which knock electrons from the atoms, thus generating charged xenon ions.

Ions respond to magnetic and electric fields, and these ions are attracted to a positive grid at the back of the firing chamber.  The grid’s electric field accelerates the ions into a ghostly blue beam travelling at about 60,000 miles per hour (See top left in top image. Source: Michigan Technological University).

The last step is to neutralise this beam of ions. Otherwise they would be attracted back to the spacecraft’s positive surfaces and cancel out the thrust.

The electrons from the discharge cathode ionise the propellant by means of electron bombardment.

High-strength magnets are placed along the discharge chamber walls so that as electrons approach the walls, they are redirected into the discharge chamber by the magnetic fields.  By maximising the length of time that electrons and propellant atoms remain in the discharge chamber, the chance of ionisation is maximised, which makes the ionisation process as efficient as possible.

The propellant is injected from the downstream end of the thruster and flows toward the upstream end.  This injection method is preferred because it increases the time that the propellant remains in the chamber.

 

Electric Propulsion Systems

The ion propulsion system (IPS) consists of five main parts:

  • the power source,

  • the power processing unit (PPU),

  • the propellant management system (PMS),

  • the control computer,

  • the ion thruster.

Ion Thruster Operation: Step 1 - Electrons are emitted by the discharge hollow cathode, traverse the discharge chamber, and are collected by the anode walls. Step 2 - Propellant is injected from the plenum and travels toward the discharge cathode. Step 3 - Electrons impact the propellant atoms to create ions. Step 4 - Ions are pulled out of the discharge chamber by the ion optics. Step 5 - Electrons are injected into the beam for neutralization. Source: NASA
Ion Thruster Operation: Step 1 – Electrons are emitted by the discharge hollow cathode, traverse the discharge chamber, and are collected by the anode walls. Step 2 – Propellant is injected from the plenum and travels toward the discharge cathode. Step 3 – Electrons impact the propellant atoms to create ions. Step 4 – Ions are pulled out of the discharge chamber by the ion optics. Step 5 – Electrons are injected into the beam for neutralisation. Source: NASA

The IPS power source can be any source of electrical power, but solar and nuclear are the primary options.  A solar electric propulsion system (SEP) uses sunlight and solar cells for power generation.  A nuclear electric propulsion system (NEP) uses a nuclear heat source coupled to an electric generator.

The PPU converts the electrical power generated by the power source into the power required for each component of the ion thruster.  It generates the voltages required by the ion optics and discharge chamber and the high currents required for the hollow cathodes.

The PMS controls the propellant flow from the propellant tank to the thruster and hollow cathodes.  Modern PMS units have evolved to a level of sophisticated design that no longer requires moving parts.

The control computer controls and monitors system performance.

The ion thruster then processes the propellant and power to perform work.

The conventional method for ionising the propellant atoms in an ion thruster is called electron bombardment.  The majority of NASA’s research consists of electron bombardment ion thrusters.  When a high-energy electron (negative charge) collides with a propellant atom (neutral charge), a second electron is released, yielding two negative electrons and one positive ion.

An alternative method of ionization called electron cyclotron resonance (ECR) is also being researched at NASA.  This method uses high-frequency radiation (usually microwaves), coupled with a high magnetic field to heat the electrons in the propellant atoms, causing them to break free of the propellant atoms, creating plasma. Ions can then be extracted from this plasma.

 

How Can Tiny Ions Move an Entire Spacecraft?

An infographic explaining the idea behind the Ionic Micro-motor that powers Nanosatellites.Like other particles and despite their tiny sizes, ions have energy and momentum.

This infographic describes how particularly useful the technique is for micro-satellites.

In the reduced friction environment of the Earth’s outer atmosphere, this tiny momentum suffices to maintain a small spacecraft into orbit long enough to make it viable: farther, faster, cheaper.

The Momentum of an Ion

For example, a molecular ion with kinetic energy 2.0 x 10-19 J takes 3.0 x 10-3 s to travel 0.6 m.  What are the mass and magnitude of the momentum of the ion?

The ion has speed

v = \frac{0.6 m}{3.0 \times 10^{-3}} = 2.0 \times 10^2 ms^{-1}.

The kinetic energy of the ion is

E = \frac{1}{2} mv^2.

This gives the mass of the ion

m = \frac{2E}{v^2} = \frac{2 times 2.0 \times 10^{-19} J}{(2.0 \times 10^2 ms^{-1})} = 1.0 \times 10^{-23} kg.

As for the momentum magnitude of the ion, we now have

p = mv = (1.0 \times 10^{-23} kg) \times (2.0 \times 10^2 ms^{-1}) = 2.0 \times 10^{-21} kg ms^{-1}

Simples!

 

Space Probes and Satellites

Ion thrusters are currently used for station-keeping on communications satellites and for main propulsion on deep space probes.  Ion thrusters expel ions to create thrust and can provide higher spacecraft top speeds than any other rocket currently available.

Modern ion thrusters are capable of propelling a spacecraft up to 90,000 metres per second (over 200,000 miles per hour).

To put that into perspective, the now-decommissioned space shuttle was capable of a top speed of around 18,000 mph.  The trade-off for this high top speed is low thrust (or low acceleration).

 

Future Applications

NASA’s primary application of ion propulsion will be for main propulsion on long missions that are difficult or impossible to perform using other types of propulsion.

The Dawn spacecraft, scheduled for launch in May 2006, will use three NSTAR ion thrusters as main propulsion.  The mission will study Ceres and Vesta, two protoplanets located in the asteroid belt that exists between Mars and Jupiter.  Studying proto-planets – the first bodies formed in our Solar system – will afford researchers the possibility to gain valuable information about the stellar system’s early development.

The JIMO spacecraft will use an array of high-power ion thrusters as main propulsion.  JIMO will perform an extensive exploration of Jupiter’s icy moons Callisto, Ganymede, and Europa.  The spacecraft will investigate each moon’s composition, history, and potential for sustaining life.

Research in the area of ion propulsion continues to push the envelope of propulsion technology.  Advancements are being made that allow the thrusters to operate at higher power levels, higher speeds, and for longer durations.  PPU and PMS technologies are being developed that will allow NASA to build lighter and more compact systems while increasing reliability.

More and more companies are beginning to use satellites with electric propulsion to extend the operational life of satellites and reduce launch and operation costs.

A picture of my very own avatar. The Ionic Propulsion of a Physicist into Space. Image: NaturPhilosophieAs new power sources become available, higher power thrusters are developed that can deliver greater speed and more thrust.

Supporting technologies such as carbon-based ion optics and ECR discharges may greatly increase ion thruster operational life, enabling longer duration missions or high-power IPS operation.

These technologies will allow humankind to explore the farthest reaches of our Solar system.

But, we all knew that…  Didn’t we?

So…  About your average cocktail-party physicist

Play it safe.  Walk before you run.

Remember it’s hard for a physicist to attend cocktail parties…