The force from a typical ion thruster is roughly equivalent to the weight of a few sheets of office paper. Yet, this small force is crucial to the performance and success of the most daring and cutting edge space missions today.
For example, the types of ion thrusters that have flown or are flying today on missions such as the Gravity field and Ocean Circulation earth Explorer (GOCE), Small Missions for Advanced Research in Technology No. 1 (SMART-1) and Dawn all have thrusts in the 1 to 100 milli-Newton range. This is equivalent to a weight range of approximately 0.1 to 10 grammes.
If you consider that the average sheet of A4 office paper weighs about 10 grammes then ion propulsion devices such as these are not exerting any real force by Earth standards.
It is this very idea though that makes them so useful and often critical to satellite missions in space; because in space weight is not as important as here on Earth. In addition, fine control of satellites and probes, for long periods, is starting to replace quick and powerful chemical burns for orbit change that have been used previously.
Most people have a picture of what a space engine looks like; it is either the kind of engine seen on a rocket, launching a payload into the sky in a fiery frenzy; or it is something out of science fiction, a device with a blue glow that propels spacecraft at unfathomable speeds across the universe.
Though some individuals recognise that there are varying types of thrusters, for example, used in space on satellites or on astronaut EVA suits, not many outside of the thruster community know how many types there are, how they are actually used and in particular just how versatile ion propulsion can be.
There are, of course, many types of ion propulsion these days all with a specific niche that they have been designed to fill. Here are some examples:
- Resistor jets and colloid thrusters
- Field-emission electric propulsion (FEEP)
- Ferroelectric thrusters and pulsed plasma thrusters
- Gridded ion thrusters – Kaufman, microwave or radio-frequency
- Magneto-plasma-dynamic thrusters
- Hall-Effect thrusters
How does a force so small become crucial to cutting edge missions?
The answer to this question lies in the nature of how ions are made and used. Typically, an ion thruster is a device in which a low temperature singly-charged plasma is formed using a gas like Xenon or Argon and application of electromagnetic fields. The ions are accelerated by high voltages once produced and then ‘neutralised’ using a special device conveniently called a ‘neutraliser’ that injects electrons into the ion beam.
Even in the more simplistic devices, creation of a plasma allows for a number of parameters and characteristics to be varied to optimise the device for a mission. This may be the range of gas flow used; it could be range of electric fields applied.
Typically each device can be characterised to fit into a performance envelope that allows quite a wide range of thrust levels to be achieved without degrading the performance of the engine.
A common characteristic is that a lot of the devices can be set on a low thrust level or a high thrust level if needed with the difference being much larger in thrust range than a turbo-fan jet engine used on an aeroplane.
Turbo-jets idle at around 35 to 40% of full thrust; an ion thruster can idle as low as 5% of full thrust as in the case of the GOCE T5 ion engine.
It is this variability of performance parameters and control of the thrust range that sets ion thrusters apart form other types of space propulsion. Yet, most ion thrusters are designed to operate at nominal levels for long durations or to be operated only for a few hours each day for station-keeping.
Has any ion thruster been used to its full potential?
There is currently only one mission that I am familiar with that utilises the full capability and promise of an ion thruster. It is the European Space Agencies GOCE mission, one that I was heavily involved with, principally in the design and development of the thruster and its control.
Unlike most ion thruster missions, and in fact most space missions, measuring the gravitation field of the Earth to the resolution desired, meant taking the risk of actually flying a spacecraft in the upper atmosphere.
This meant that a propulsion system had to be developed to counteract the drag on the spacecraft so that it could maintain its altitude and perform measurements. In other words, the thrusters had to literally stop the spacecraft falling out of the sky.
The thrust range specified was from 1 to 20 milli-Newton, or approximately one tenth to 2 sheets of A4 paper. If you were to rip off a corner of a sheet, that would be the lowest thrust; if you were to place 2 sheets on top of each other that would be the highest thrust.
However the specification did not stop there. The thrusters and propulsion system were designed to be able to vary the thrust at micro-Newton resolution. The actual ratio of the highest thrust level to resolution of that thrust level was just less than 2000:1.
If you consider your average motor car and its speed range in miles per hour, say 0 to 160 miles per hour, this would be like having a cruise control system that could keep you at 160 miles an hour with a variation less than 0.1 miles an hour. That is quite a control system by any standards.
Even then, there is an extra factor in that this control is achieved at a rate of 100 times per second, which arguably makes the propulsion system the fastest and most intricate system flying today.
It is not hard to see then that ion thrusters may be the vital tool to realise more intricate cutting- edge space missions as the nature of the process of ion production is such that devices can be built to meet very demanding specifications.
Yet, ion propulsion has not really been exploited to its full potential on every space mission it has been used on, mainly because it is typically tailored to each mission.
This tailoring reflects the demand; no-one truly needs a multi-functional thruster yet more projects demand a greater level of performance in lots of different aspects. Ion propulsion is seen as a major enabler for space missions to come but it is constantly pushed at the limit.
Perhaps this is a limit that is unsustainable for development. Hopefully this will mean that missions begin to be developed with thrusters in mind so that a successful marriage of performance, complexity, functionality and cost can be achieved, otherwise the technology may not be chosen due to bad experience with implementing it.