The landing propulsion system is housed on the SOM and consists of 4
Kaiser-Marquardt REA 20-4 engines which will gently bring the SOM to
the surface of Europa after being separated from the OOM at the
specified altitude. These engines are capable of producing 556 N of
thrust apiece (for a total of 2224 N) and can be throttled for
precision 
needs. The engines burn hydrazine alone with the
aid of catalyst beds built into the motor. While descending to the
surface, the SOM will also make use of the ACS propulsion system that
was described in the above section. Both the ACS and the landing
propulsion systems will be fed from the same fuel tank (teardrop-tank)
during the descent phase; each having its own regulated system,
however. The teardrop-shaped tank is composed of a helium tank below
the fuel tank and both are supported and bound together by the
composite outer liner. The fuel tank also makes use of diaphragm
isolation technology similar to the other tanks described earlier.
A monopropellant system was chosen for the lander because of its simplicity, high thrust levels, reliability, and compatibility of fuel with the attitude control propulsion system. Also, this system will require less fuel precautionary devices (and thus less massive) than that of a bipropellant system. The propellant mass that will be used by the lander propulsion system is calculated at 712.8 kg of hydrazine and this figure includes a 20% fuel margin. The REA 20-4's were chosen for this leg of the mission due in part to their successful heritage and also of the mission because of their compact size and the amount of power which they can deliver upon demand.