What is the role of the hydraulic power unit (HPU) in a steam catapult system?

• A. It generates steam to power the catapult
• B. It pressurizes hydraulic fluid to operate the catapult piston and stores energy for launch
• C. It provides electrical power to flight deck systems
• D. It cushions the aircraft during launch

Answer: (B)

In a steam catapult, the hydraulic power unit is the system that converts steam energy into hydraulic energy and stores it for a rapid, controlled launch. A turbine-driven pump pressurizes hydraulic fluid and fills accumulators, so there’s a ready supply of high-pressure fluid. When a launch is commanded, valves release that fluid to the catapult piston, driving the shuttle along the track and providing the strong, precise push needed to accelerate the aircraft.

That’s why the function described is the best answer: it captures both the act of pressurizing the hydraulic fluid to operate the catapult piston and the energy storage aspect for the launch. The other options describe roles handled by different systems—generating steam, providing electrical power, or cushioning during launch.