Ion thrusters have two major considerations however. The first is that they take a lot of energy, both to ionize the materials, but most especially to accelerate the ions to the extremely high speeds. Exhaust speeds of 30km/s are not uncommon. This compares well with the 3-4.5km/s for chemical rockets in terms of propellent usage. However, since energy consumption is proportional to exhaust velocity squared and the thrust per kg of fuel is only proportional to exhaust speed, the overall thrust for a given power supplied by an energy source is inversely proportional to exhaust speed. Thus, in practice, it turns out that with currently practical energy sources masses and vehicles, only extremely modest accelerations are feasible, typically of order a milligee.

The second consideration is the relatively short life of the thruster. The ions often hit the grids on their way through the engine, which leads to the decay 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 acceleration.

Of all the electric thrusters, ion engines have been the most seriously considered commercially and academically in the West. (In the Russian block Hall Effect thrusters have been used for decades for station keeping). Ion engines are best used for missions requiring very high ΔV (the overall change in velocity, taken as a single value), interplanetary missions, for example.

NASA has developed an ion engine called NSTAR for use in their interplanetary missions. This engine 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.

In 2003 NASA ground-tested a new version of their ion engine called High Power Electric Propulsion, or HiPEP. The HiPEP engine differs from earlier ion engines because the xenon ions are produced using a combination of microwaves and spinning magnets. Previously the electrons required were provided by a cathode. Using microwaves significantly reduces the wear and tear on the engine by avoiding any contact between the speeding ions and the electron source.

The European Space Agency's satellite Smart 1 also uses an ion thruster engine.

Most other electric spacecraft engine designs are based on the same principles, but attempt to avoid the problems with grids with a combination of other electric or magnetic fields.