A water jet assembly and method of use is provided comprising a tank with cutting fluid, fuel and oxidizer lines, and a tank discharge lines. In operation, the tank is filled with oxidizer; the oxidizer line is closed and cutting fluid is supplied compressing the oxidizer. When the fluid reaches a level, the fluid line is closed and fuel is injected. A spark generator ignites the fuel/oxidizer mixture thereby raising the tank pressure. As the pressure rises, a low pressure valve simultaneously closes at a prescribed level. The vent line and a discharge to a nozzle are opened thereby, forming a gas bubble. When the bubble reaches a desired size and pressure drops below a level, the vent closes, allowing combustion expansion to force fluid through the nozzle to form a cutting jet.