of the adaptor module (close to the spacecraft's center of mass). Two forward-pointing thrusters at the same location, provided aft translation, and two thrusters located in the aft end of the adapter module provided forward thrust, which could be used to change the craft's orbit. The Gemini reentry module also had a separate Reentry Control System of sixteen thrusters located at the base of its nose, to provide rotational control during reentry. The Apollo Command Module had a set of twelve hypergolic thrusters for attitude control, and directional reentry control similar to Gemini. The Apollo Service Module and Lunar Module each had a set of sixteen R-4D hypergolic thrusters, grouped into external clusters of four, to provide both translation and attitude control. The clusters were located near the craft's average centers of mass, and were fired in pairs in opposite directions for attitude control. A pair of translation thrusters are located at the rear of the Soyuz spacecraft; the counter-acting thrusters are similarly paired in the middle of the spacecraft (near the center of mass) pointing outwards and forward. These act in pairs to prevent the spacecraft from rotating. The thrusters for the lateral directions are mounted close to the center of mass of the spacecraft, in pairs as well. Location of thrusters on spaceplanes The suborbital X-15 and a companion training aero-spacecraft, the NF-104 AST, both intended to travel to an altitude that rendered their aerodynamic control surfaces unusable, established a convention for locations for thrusters on winged vehicles not intended to dock in space; that is, those that only have attitude control thrusters. Those for pitch and yaw are located in the nose, forward of the cockpit, and replace a standard radar system. Those for roll are located at the wingtips. The X-20, which would have gone into orbit, continued this pattern. Unlike these, the Space Shuttle Orbiter had many more thrusters, which were required to control vehicle attitude in both orbital flight and during the early part of atmospheric entry, as well as carry out rendezvous and docking maneuvers in orbit. Shuttle thrusters were grouped in the nose of the vehicle and on each of the two aft Orbital Maneuvering System pods. No nozzles interrupted the heat shield on the underside of the craft; instead, the nose RCS nozzles which control positive pitch were mounted on the side of the vehicle, and were canted downward. The downward-facing