At about 400 feet agl on climb, the helicopter suffered a partial loss of engine power. The pilot decided to use the available power to carry out a landing on the airfield. At about 50 feet agl, the engine stopped and the aircraft was subsequently landed heavily, collapsing the skids. The engine had seized. An inspection revealed that the condition of the engine was consistent with that of having been operated without oil. There was no evidence of inflight oil loss or of any component failure which might have caused the loss. When the engine began to lose power the helicopter was climbing at an airspeed of 60 knots. When the engine stopped the helicopter no longer had sufficient performance to allow the successful completion of an auto-rotational approach.