Spool valve is a very common and important item in fluid power control applications for controlling flow direction, pressure and discharge. The control is exercised on the force driving the spool in a manner so as to displace it to change the opening at the valve port through which the flow takes place. With increasing demand on high-speed and high-precision control, some critical sizing, in the vicinity of the valve port, is approaching the limit of most up-to-date manufacturing capability. On the other hand, the theoretical design and performance analysis techniques are based on simplified, one-dimensional, fluid dynamic treatment. Of course, such analyses were backed by sophisticated and rigorous experimentation. A single-stage spool valve driven directly by a linear force motor [Jones (1997)] is an example of an open-loop valve. Such a valve, also called the direct-drive valve or DDV, requires substantial electro-magnetic power from the motor, the construction of which has become possible with the advent of high power-density magnetic materials. Prior to this, the two-stage valve construction evolved, where the force, producing a torque as an output from the motor made of low power-density magnetic material, is boosted by means of a hydraulic amplifier to drive the spool. The major components of the forces that need to be overcome by the spool valve working in tandem with the actuator are the flow forces, steady and transient. The sense and magnitude of these forces depend on the actuator load and construction of both the valve and the actuator.