The Oberth effect is fascinating. It sounds like a violation of conservation of energy but it’s not. You can get multiples of thrust. An Oberth effect flyby of

The Oberth effect is fascinating. It sounds like a violation of conservation of energy but it’s not. You can get multiples of thrust. An Oberth effect flyby of a primordial black hole with a high impulse drive like nuclear pulse propulsion would be pretty spicy. It’s a feasible way to achieve speeds close to the speed of light. Acceleration would kill you though. If we did find such an object anywhere within a few light years of the sun it would be a way to send probes out at significant fractions of the speed of light.