Two kinds of limit
A chemical engine is limited by the energy in its propellant: the exhaust can only be as fast as the chemistry allows. An electric thruster is limited by electrical power: the exhaust can be much faster, but accelerating a lot of mass that fast needs more power than a spacecraft has. The result is a trade between efficiency and thrust.
How a Hall thruster works
Gas flows into a ring-shaped channel. A radial magnetic field traps electrons, which circle around the channel in what is called the Hall current. These electrons ionise the gas, and the electric field across the channel accelerates the ions out at tens of kilometres per second. A separate cathode releases electrons into the beam so that the spacecraft stays electrically neutral.
When to use which
| Chemical, e.g. 100 N methane/oxygen | Hall thruster | |
|---|---|---|
| Thrust | newtons to meganewtons | millinewtons, roughly 50–70 mN per kW |
| Specific impulse | 300–460 s | about 1,500–3,000 s |
| Burn time | seconds to minutes | months to years |
| Typical job | launch, landing, fast manoeuvres | orbit raising, station keeping, deep space |