The slingshot maneuver only works by taking momentum from the object you sligshot around. <br />If you had your black hole and you drop matter towards it, then you don't get any velocity out. The particles gain energy as they fall down the gravity well but then they lose that energy by climbing up out of the well again. <br />Think of it like a pendulum. The starting point of the particle is like the pendulum being in a high position. You let the particle fall towards the hole like you let the pendulum go. The pendulum reaches its lowest point but then has to move against gravity and so deccelerates.<br /><br />The slingshot maneuver is when the center of gravity of the system is moving. That way you can fire particle towards the object and it will accelerate. You have set the system up so that the massive object will be moving out of the particles way. That way the particle misses the object, now the massive object and particle are moving away from each other, this results in the gravitational field falling off faster than the inverse square law would suggest. The particle has therefore accelerated more than it has deccelerated thus gained energy. The total energy is the same as the massive object has lost energy.<br /><br />This maneuver is not possible with your black hole engine.<br /><br />Please go learn some newtonian mechanics, that way your creativity will not be wasted.