An ion thruster, one of several types of spacecraft propulsion, uses beams of ions for propulsion. The precise method for accelerating the ions may vary, but all designs take advantage of the high charge-to-mass ratio of ions to accelerate them to very high velocities. Ion thrusters are therefore able to achieve high specific impulse, reducing the amount of reaction mass required but increasing the amount of power required compared to chemical rockets. Ion thrusters can deliver one order of magnitude greater fuel efficiency than traditional liquid fuel rocket engines, but are generally constrained to very low thrusts by the available power.
Types of ion thruster
There are many types of ion thruster currently in development; some are currently in use, while others have not yet been installed in spacecraft. Some of the types of ion thruster are:
- Electrostatic ion thrusters
- Hall effect thrusters
- Field Emission Electric Propulsion
- Pulsed inductive thruster
Other forms of high-efficiency electric thruster have also been proposed; see spacecraft propulsion.
In the simplest design, an electrostatic ion thruster, ions are accelerated by passing them through highly-charged grids (similar in concept to a vacuum tube). Electrons are also fired into the ion beam downstream of the grids as the positively charged ions leave the thruster. This keeps the spacecraft and the thruster beams neutral electrically. The acceleration uses up very little reaction mass (i.e., the specific impulse, or Isp, is very high).
A major consideration is the amount of energy or power required to run the thruster, partly to ionize the materials, but most especially to accelerate the ions to the extremely high speeds required to have any useful effect. Exhaust speeds of 30 km/s are not uncommon, which is far faster than the 3-4.5 km/s for chemical rockets, and makes for notably low propellant usage.
With ion thrusters, most of the energy is lost in the high speed exhaust and this affects the thrust levels. It turns out that the overall thrust obtained from a given amount of energy is inversely proportional to exhaust speed (since energy consumption per kilogram of propellant is proportional to exhaust velocity squared, but the thrust per kilogram of propellant is only proportional to exhaust speed ).
Therefore, the specific power of the power source of an ion thruster determines what exhaust velocity an ion thruster exhaust beam of a given mass can be accelerated up to, and it can be calculated using kinetic energy formula, and the electrical energy conversion efficiency of the specific thruster to be used.
An ion thruster using a particle accelerator can achieve an exhaust velocity approaching the speed of light. This can provide an ion propulsion specific impulse approaching 30,000,000 seconds for an ion engine; however such a high speed would be essentially useless, as the thruster would give negligible thrust for any plausible power supply.
The exhaust velocity attained by ions when they are calculated inside of an electric field can be calculated using the following equation. Ve = (2 * Q/M *Va) ^1/2
In practice, with currently practical energy sources of perhaps a few tens of kilowatts, and given a not untypical Isp of 3000 seconds (30 kN·s/kg), ion thrusters give only extremely modest forces (often tenths of a newton). With the weight of the energy sources and vehicle of hundreds of kilogram, the accelerations are typically in the milligee range.
Since thrust goes down with higher specific impulse, as specific impulse increases, a mission takes longer to achieve and this can incur additional costs. Since many missions are attempting to minimize costs, some of which increase with the length of the mission, an optimum specific impulse can be calculated.
Given the low thrust, the life of the thruster becomes important. Ion thrusters have to be kept running a large part of the time to allow the milligee acceleration to build up into something meaningful. In the simplest ion thruster design, an electrostatic ion thruster, the ions often hit the grids, which leads to erosion of the grids and their eventual failure. Smaller grids lower the chance of these accidental collisions, but decrease the amount of charge they can handle, and thus lower the thrust.
Of all the electric thrusters, ion thrusters have been the most seriously considered commercially and academically in the quest for interplanetary missions. Ion thrusters are seen as the best solution for these missions as interplanetary trajectories require very high ΔV (the overall change in velocity, taken as a single value) that can be built up over long periods of time (years).
The Hall effect thruster is a type of ion thruster that has been used for decades for station keeping by the Soviet-Union and is now also applied in the West: the European Space Agency's satellite Smart 1 uses it.
NASA has developed an ion thruster called NSTAR for use in their interplanetary missions. This thruster was tested in the highly successful space probe Deep Space 1. Hughes has developed the XIPS (Xenon Ion Propulsion System) for performing stationkeeping on geosynchronous satellites. These are electrostatic ion thrusters and work by a different principle than Hall effect thrusters.
In 2003 NASA ground-tested a new version of their ion thruster called High Power Electric Propulsion, or HiPEP. The HiPEP thruster differs from earlier ion thrusters because the xenon ions are produced using a combination of microwave and magnetic fields to oscillate electrons in the propellant atoms, causing the electrons to break free of the propellant atoms, leaving them as positive ions. This process is called Electron Cyclotron Resonance (ECR). Previously the electrons required were provided by a hollow cathode.
Other fuels have been considered for use with ion thrusters. Research has been invested in fullerenes for this purpose, specifically C60 (buckminsterfullerene), due in part to its large electron-impact cross section. This property gives the potential for ion thrusters with higher efficiency than current Xenon-based designs at Isp values of less than 3,000 lbf·s/lb (29 kN·s/kg).
JP Aerospace has been working to build an orbital airship, which uses a combination of a balloon and ion thrusters to achieve orbit without any use of conventional rockets, for roughly one dollar per short ton per mile of altitude ($0.70/(t·km)).
Ion thrusters in fiction
- Film creator and director George Lucas seems to have some confidence in ion thrusters: in the Star Wars movies, the technologically sophisticated Empire's TIE Fighters get their name from the Twin Ion Engines used for propulsion.
- Arthur C. Clarke's 1949 short story Breaking Strain features a cargo ship with an "ion drive" powered by "Atomic motors".
- In Star Trek, The engineer of the USS Enterprise, Scotty, says: "Captain, they're using an ion drive on that ship! I bet they could teach us a thing or two".
- In Freelancer (2002 PC video game), the Liberty faction uses ion engines and the electrostatic grid is clearly visible (albeit a bit large). The ships are powered by a fusion reactor. The engine exhaust is visible (relatively) low frequency energy pulses and some fans theorize this could be achieved with a form of electromagnetic flux compression to ramp up the power output of the reactor.
- Spacecraft propulsion
- Nuclear electric rocket
- Hall effect thruster
- Field Emission Electric Propulsion
- Pulsed inductive thruster