Going towards the black hole after the collision in orbit around Mann's planet

I've played a couple of orbital mechanics simulators, and the huge amount of fuel/energy/acceleration it takes to get a craft heading on a collision course with a central gravitational body (ie the sun, or in this case Gargantua) is way way way more than would be produced by an errant collision in the orbit around a planet orbiting said body.

Just to get out of the orbit of that planet would take a ton of energy and a lot of time, even then a fraction of that needed to fall towards the central body. A simple orbital collision just isn't going to do anything like any of that.

I wish they'd found a different way to have them heading towards the black hole. For instance, if they'd headed for the warp hole or the third habitable planet, gone into hypersleep and awoke months or years later to find a minor navigational error had them heading towards it.