Robots that use impulsive mechanisms to achieve high-speed and high-powered motion are becoming more common and better understood, but control of these systems remains relatively rudimentary. Among robots that use spring actuation to generate motion, robot actuation and mechanisms are usually not controlled intentionally in order to achieve variation in the system's behavior, or they are controlled only roughly via adjustments made to the amount of energy stored in the mechanism. We describe the development, construction, and test of an impulsive catapult mechanism whose design is inspired by the grasshopper leg and for which extensive variation in the projectile trajectory is achieved by force control of the actuator that restrains the spring. As a step toward future controlled jumping robots, we give a detailed model of this system, validate this model experimentally, and explain how the actuator dynamics are critical to our ability to vary the system's trajectory using this approach. This work represents a novel approach to the control of spring actuated robots and illustrates how they can be controlled even under highly limiting actuator constraints.