Mars Excursion Module

The Mars excursion module (MEM) is similar in design to the lunar module used for the Apollo missions.  The most significant difference is size.  The MEM is larger, since it must accommodate three astronauts on the surface for two months. Consequently, the life support requirements are much greater.  

The MEM uses plug nozzles rather than the more traditional bell nozzles, because of their superior thrust-to-weight ratio.  The descent engines are powered by a mixture of fluorine/oxygen and methane and produce 140,000 pounds of thrust.  The ascent engines on the Russian craft are powered by the same propellants, but the engines on the American craft are powered by a mixture of nitrogen tetroxide and hydrazine.  Both engines produce approximately 35,000 pounds of thrust.