These are the most powerful thrusters currently available. They are needed because many new satellites are becoming increasingly large and so need far larger forces to manoeuvre them. A commonly used bi-propellant mixture is monomethylhydrazine (CH3N2H3) as the fuel and dinitrogen tetroxide (N2O4) as the oxidant. This burns very hot and so has a high specific impulse of over 300 s. Another property of this propellant mixture is that it is ‘hypergolic’, that is it ignites on mixing. This means that the two substances have to be handled extremely carefully but also that there is no need for a complex ignition system. It also means that the two materials cannot accidentally build up in one place, leading to an explosion the next time the thruster is fired.
The monomethylhydrazine/dinitrogen tetroxide combination is also used in the space shuttle ‘orbital manoeuvring system’ and in the third stage of the Ariane V, this 115 kg motor can produce 2.7 tonnes of thrust and burn 9.8 tonnes of propellant in around 1000 s, some thruster!