Ion thruster

An ion engine test

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.

The first Ion Thrusters were developed by Harold R. Kaufman, working for NASA in the 1960s, and were based on the Duoplasmatron.

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:

Other forms of high-efficiency electric thruster have also been proposed; see spacecraft propulsion.

General design

In the simplest design, an electrostatic ion thruster, a gas like Argon or Mercury vapor is ionized by exposure to electrons provided by a cathode filament. The ions are accelerated by passing them through highly-charged grids. 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 I_sp, is very high).

Energy usage

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 ^[1]).

Therefore, the power supply of the vehicle ultimately limits the exhaust velocity an ion thruster exhaust beam of a given mass can reach. It can be calculated using kinetic energy formula, if the electrical energy efficiency of the specific thruster to be used.

For an extreme example, an ion thruster using a particle accelerator can be designed to achieve an exhaust velocity approaching the speed of light. This could provide an ion propulsion specific impulse approaching 30,000,000 seconds. But this would inevitably result in a very low thrust. This is because increasing the ion exhaust velocity 10 times requires that 100 times more electrical energy. A one-thousand-fold increase in ion exhaust velocity likewise requires the expenditure of 1,000,000 times more energy to accelerate the ions. Therefore one millionth of the amount of propellant can be accelerated per second, and even though it is travelling 1000 times faster, negligible thrust would be producted. Scaling up the power supply would not help; once the vast majority of the vehicle's dry mass consists of power generation equipment, hardly any further gains in performance can be obtained.

The exhaust velocity attained by ions when they are accelerated inside of an electric field can be calculated using the following equation.

V_{e}={\sqrt {2{Q \over M}V_{a}}}

Thrust

In practice, with currently practical energy sources of perhaps a few tens of kilowatts, and given a not untypical I_sp of 3000 seconds (30 kN·s/kg), ion thrusters give only extremely modest forces (often 100ths , or 10 tenths of a newton). Large Ion propulsion engines require large & massive electric power sources. Ion engines typicaly provide space craft acceleration rates of from 10 ^ -5 G to 10 ^ -3 . This is from .0001 m/sec^2 to to .01 m/sec^2. An acceleration rate of 10 ^/4 G is .001 m/sec^2.

The amount of ion engine thrust that can be generated using a given amount of available electric power tends to decrease, as the operating exchaust velocity, & specific impulse of the ion rocket engine is increased. It is therefore necessary to optimize the exchaust velocity, and the thrust that is generated by a specific ion propulsion system, for a space vehicle of a given mass. .

Lifespan

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 gain a useful velocity.

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.

Missions

Of all the electric thrusters, ion thrusters have been the most seriously considered commercially and academically in the quest for interplanetary missions and orbit raising maneuvers. Ion thrusters are seen as the best solution for these missions as they require very high ΔV (the overall change in velocity, taken as a single value) that can be built up over long periods of time.

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 energy and magnetic fields. The ionization is achieved through a process called Electron Cyclotron Resonance (ECR). In ECR, a uniform magnetic field is applied to a chamber holding xenon gas. The small number of free electrons present in the neutral gas orbit around the magnetic field lines at a fixed frequency called the cyclotron frequency. Microwave radiation is applied that is carefully tuned to this frequency, supplying energy to the electrons, which then ionize more xenon atoms through collisions. This process is a highly efficient means of creating a plasma in low density gases. 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 I_sp values of less than 3,000 s (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 70 cents per tonne per kilometer of altitude ($1/(short ton·mile)).

The Japanese Space Agency's Hayabusa, which successfully rendezvoused with the asteroid “Itokawa” and remained in close proximity for many months to collect samples and information, is powered by two xenon Ion Engines.

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. Furthermore, ion propulsion is the most commonly used method of sub light propulsion for the great majority of the starships in the fictional Star Wars universe.

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 (computer game) (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 pulses visibly at a low frequency, causing some fans to theorize that the fusion reactor powering the ion engines operates using flux compression for pulsed operation (as with reversed field pinch and z-pinch reactors on Earth).

References

^ The energy computed from the rocket equation

External links