M734 Fuze - Power Supply

Power Supply

The wind stream in flight provides both the mechanical power needed to arm the S&A and the electrical power needed for the fuze electronics. Figure 5 shows the system of components used in the M734 to capture and regulate air flow within the fuze and convert a portion of the air power to mechanical and electrical power before exiting the fuze.

• In this system, the Air Inlet directs the wind stream into a converging-diverging nozzle (Venturi tube) that limits the mass flow rate (choked flow).
• The axial flow then rams the center of a flat impeller (Figure 6) and flows radially through the blades. The curvature of the blades continuously redirects the flow and the net pressure on the concave surface area generates a torque that rotates a drive shaft.
• The drive shaft is permanently connected to an alternating current generator called the Turbine Alternator Assembly whose operating principles resemble an automobile belt-driven alternator miniaturized to provide 20 volts and withstand an acceleration of 20,000g.
• The drive shaft is also engaged with the S&A mechanism (Figure 4) at the time of launch, but disengages after a specific number of shaft revolutions. This action unscrews a Jackscrew locking device and enables the explosive train to snap into alignment, thereby completing the second step in mechanical arming.

Since arming is required to occur after a flight of 100 meters for three mortars that have a wide range in launch velocities, the RPM that releases the Jackscrew at the slowest launch velocity must increase in direct proportion to any increase in launch velocity. The turbine, however, will tend to spin faster than desired, so, to prevent early arming, three governors are used to reduce the spin:

• First, the number, size, and curvature of the turbine blades is designed to achieve arming at 100 meters or more for the lowest launch velocity of 45 m/s.
• Second, the Venturi tube is designed to limit the mass flow rate of air available to propel the turbine.
• Third, the tips of the turbine blades are undercut to introduce a flexibility that allows centrifugal force to bend the tips outward (Figure 7). This decreases the effective blade curvature, which lowers the surface pressure on the blade and the turbine spins slower than with an inflexible blade. By undercutting deep enough, the Jackscrew drive shaft rotation is slow enough at all launch velocities to assure arming is beyond the minimum 100 meters.

Once the air flows from the tips, the Air Outlet directs the exhaust into the atmosphere at an angle oblique to the external wind stream. The resulting turbulence degrades the accuracy of flight toward the target, so the exhaust is directed onto a vertical metal fin that guides the flow into the external wind stream.

A noteworthy point is that the performance of the Turbine Alternator is unaffected if the mortar shell encounters a tropical rainstorm while en route to the target.