The Reaction Control System (RCS) [1] uses hypergolic fuels [2] which are highly toxic. Such fuels are desirable in this application because they combust whenev

The Reaction Control System (RCS) [1] uses hypergolic fuels [2] which are highly toxic. Such fuels are desirable in this application because they combust whenever the fuel and oxidizer are brought in contact. There's no need to an ignition system, which reduces the chance for failure. [1] https://en.wikipedia.org/wiki/Reaction_control_system https://en.wikipedia.org/wiki/Reaction_control_system [2] https://en.wikipedia.org/wiki/Hypergolic_propellant https://en.wikipedia.org/wiki/Hypergolic_propellant