# How can black holes be used for daredevil gravity slingshot sport?

This is set in the same universe as my Andromeda galactic war question (same tech level). In this question, I had somewhat purposely labelled the galactic core as "off-limits" (black holes and other nasty things).

However, young fighter-jocks use this space for training and recreation - so much so that it's a broadcast sport.

Black holes can be used as gravity slingshots, and this has turned into an event in itself where pilots attempt to out-do each other in terms of exit speeds from the slingshot before manually shooting at a target.

In order for this to be credible in the story, I'll need some help:

• How much acceleration could you get from using a black hole as a gravity slingshot?
• What might happen if you got slightly too close during the attempt?

Assume that pilots have tech that helps them withstand g-forces, but it has limits - manually shooting at a target on exit demonstrates the ability to continue fighting after extreme-G manoeuvres.

Black holes being used here are Intermediate in size (lets pick one of 5,000 solar masses)

As before, there's no time dilation effects in this fiction - it's lots of gravity and lots of speed.

• It's not a full answer, and I'm not up to writing a full answer at the moment, but keep well in mind that black holes aren't magic. You can treat them as point gravity sources, which makes some things easier (no need to mess around with an actual radius, for example, beyond knowing the event horizon radius) but they have no special gravitational effects or properties beyond those of a different object of a similar mass. They are just a lot more compact than your run-of-the-mill gravity source. – a CVn Mar 13 '17 at 20:54
• @MichaelKjörling you can orbit them at the speed of light though, so in practice they will be quite different – PyRulez Mar 13 '17 at 23:45

If the black hole is stationary the net change to your speed is 0. That is to say, once you are as far away from the black hole as you started, you'll be moving at the same speed you started at. We use planets that are already travelling in the general direction we want to go. See Gravity_assist. During a close pass you could certainly pick up some extraordinary speed before shedding it as you move away from the center of gravity.

EDIT: Loren Pechtel pointed out the Oberth Effect, which I don't believe is usually what is meant by a "gravity slingshot", but may certainly be relevant to the story idea. Note, this just means your have a greater exit speed from the gravity well than by using the same thrust in empty space. Your maximum speed will still occur around the closest point of your pass by the black hole.

As for what happens when you get close, well first there's frequently a lot of matter in the accretion disk that is getting sucked in pretty fast. If you get close the gravity on the side of you that is closer to the black hole is actually greater than the gravity on the opposite side, resulting in a stretching force. This force is enough to tear anything apart before you even reach the event horizon. In other words it's never the event horizon that really kills you. See Spaghettification

• Can you get a gravity assist of a stationary thing if you activate your thrusters while near the black hole? – PyRulez Mar 13 '17 at 23:47
• @PyRulez No, but you do get the benefit of the Oberth effect if you do your burn deep in a gravity well. – Loren Pechtel Mar 14 '17 at 2:13

If the black hole is spinning, you can still get a speed boost.

I see two spots of trouble.

One is the tidal stress if you get too close (also known as spaghettification).

The other problem is when will you come out. Time travels slower the closer to the black hole you get. You could come out decades or centuries after you went in. Imagine coming out and bragging about your skill at piloting 400 year old tech.

• Huh? Time slows for you but that doesn't mean you fly any slower, you just perceive the pass by the black hole taking less time than it actually did. – Loren Pechtel Mar 14 '17 at 2:19

There's no reason for the guns.

Put some targets in orbit about the black hole. Closer in targets are worth more, targets that require a greater deflection are worth more. The pilots may manually maneuver at will on the way in, they are not allowed to use their engines after periapsis. Their score is the point value of the targets divided by a factor based on how close to the target they got.