Propellant generator for generating first and second propellants from at least a lean reactant and an excess reactant comprises: (a) a reactor in fluid communication with a lean reactant delivery line and an excess reactant delivery line, for reacting all of the lean reactant and all but a base amt. of excess reactant into a mixt. including a first propellant, water and the base amt. of excess reactant; (b) an electrochemical separator having an entry chamber that receives the mixt., and a membrane and electrode assembly adjacent the entry chamber that electrochemically separates and pressurises the base amt. of excess reactant from the mixt. into an excess reactant discharge chamber of the separator that is in fluid communication with the excess reactant delivery line; (c) a current measurer in electrical connection with the membrane and electrode assembly, for measuring the amt. of excess reactant in the mixt. by measuring the current consumed in the electrode assembly by the electrochemical removal of the excess reactant from the mixt., the magnitude of the current being proportionate to the amt. of excess reactant separated from the mixt. by the separator; (d) a phase separator in fluid communication with the electrochemical separator that receives the water and first propellant from the separator, for sepg. the first propellant from the water and for delivering the first propellant into a first propellant storage chamber; (e) an electrolyser in fluid communication with the phase separator that receives the water from the phase separator, for electrolysing the water into diatomic hydrogen gas and the second propellant diatomic oxygen gas, for delivering the second propellant oxygen into a second propellant storage chamber, and for delivering the hydrogen to the excess reactant delivery line; and (f) a lean reactor flow controller in electrical connection with the current measurer and with a lean reactant flow control valve on the lean reactant delivery line that controls flow rate of the lean reactant into the reactor, for adjusting the lean reactant flow control valve in response to measurements of the current measured so that the lean reactant flow controller automatically controls the proportions of first propellant, water and excess reactant leaving the reactor by adjusting the lean reactor flow control valve to alter the rate of flow of lean reactant into the reactor whenever the current measurement is not the same magnitude as a desired current consumed by the electrochemical sepn. of the base amt. of excess reactant.    Also claimed is a method of generating first and second propellants from at least a lean reactant.    USE -   Generation of propellants for powering rockets and/or I.C. engines.    ADVANTAGE -   Propellant generator produces fuel and oxidation the surface of the planet Mars using atmos. carbon dioxide, required for the exploration of outer space.