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I'm writing a very nontechnical science fantasy with a star-ship (literally a giant spacecraft toting around a star) in stable orbit around a black hole. It uses one of the "stellar engines" mentioned here

Eventually, the spacecraft has to be able to break free, but I'm not sure how. I'm currently favoring a scenario where the spacecraft pushes the star towards the black hole, but would this generate enough force to escape? I'm also considering having the star go nova, but would it be possible to survive that? What would be the best solution?

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    $\begingroup$ I'm no scientist to say one way or the other (I know there are several around here who are), but I would think that if you have the ability to manipulate stars, black holes aren't going to be as big a problem as they are for us. $\endgroup$
    – Frostfyre
    Commented Mar 26, 2015 at 17:16
  • $\begingroup$ @Frostfyre has a two very good points. You can look for someone with the "science-based" badge in their profile. Also, you're spaceship is more than likely impossibly strong, and that allows you to do a lot of things normal spaceships/planets can't. $\endgroup$
    – PipperChip
    Commented Mar 26, 2015 at 17:32

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Just let go of the star and fly away.

There isn't anything magical about the gravity of a black hole. A ship can orbit a black hole in exactly the same way it orbits a planet or star. Star can orbit black holes, and we have actually observed this as well as black holes orbiting stars.

The biggest problem is going to be carrying the star with your ship. I don't know how it is your ship is toting around a star in the first place, but it's likely the most significant portion of the mass for your star-ship. Because your star-ship is in a stable orbit already, if your ship releases the star, the star will continue to orbit the black hole and your ship can simply fly away.

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Considering this is a technologically-light science fantasy, I'm going to give the answer in broad strokes.

Newton and Dat Third Law.

"For every action there is an equal and opposite reaction."

This supports your "throw the star into the black hole" proposal. If the mass of the ship, the mass of the star, the energy of the throw, and the force of gravity from the black hole are all right, this will work. Your other ideas, such as the star going nova, are still taking advantage of this. It's just that the particles of the exploding star are imparting the momentum to the ship, not the throwing of the star itself.

The good news is that your proposed mechanisms for escaping an orbit work, and are more-or-less how rockets actually do it in real life!

Going Nova!?

Giant ships like these proposed ones that seem to be made of impossibly strong materials. Since you have an impossibly strong space ship, why wouldn't it be able to survive novas? Survivability increased dramatically if the ship is able to distribute the force from the nova-explosion evenly. (It already needs to deal with the force of a solar wind from the star, so I would assume the design accounts for this.)

The hows of doing this are really beyond what our earth-based materials and economies can do. Ships of such sizes are hitting the fantasy part of sci-fi hard, but that's nothing to be ashamed about. I suggest not going into much detail, but presenting the plan in generalities rather than specifics will be more palatable to the scientifically inclined than mentioning specifics.

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You can orbit a black hole without being trapped by it. Heck, we, here on Earth, are orbiting a black hole and are not permanently trapped by it (the Supermassive at the center of the Milky Way).

The point at which you cannot re-escape from a black hole's gravity well by normal means is the Event Horizon. The distance from the singularity at which not even light has sufficient velocity to escape. The reason you can't do this is because (thanks to the theory of relativity) you would require an infinite amount of energy to even reach the speed of light...much less exceed it.

So, as long as you stay away from the Event Horizon, you can escape the black hole's gravity well the same exact way you escape from anything else's gravity well. Accelerate.

If you have somehow survived passing through a black hole's event horizon (Tidal forces would have ripped you into bits first) then unless you have a physics-defying FTL drive (or some way to neutralize gravity), you're screwed. Detonating a star releases a ton of energy, yes. But to exceed the speed of light (escape velocity from within the event horizon), you need more than infinite energy. A supernova does not provide this. Nothing does. All you would accomplish is to feed the star to the black hole, making the black hole bigger, more massive, and increasing its gravity well, putting the event horizon even further from you.

If you have an FTL or anti-gravity drive of some sort, you can probably still get away, since you can move faster than light (or ignore gravity) already. But, again, the insane tidal forces of a black hole at close range would probably have killed you.

But then again...why the heck would you go inside a black hole's event horizon? Just orbit at a safe distance.

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  • $\begingroup$ For a large enough black hole, tidal force at the event horizon are not a problem at all. For a 1 million solar mass BH, tidal stress is about 0.01 meters per second^2 per meter. When considering BHs, try this calculator -- xaonon.dyndns.org/hawking -- the BH at the center of our galaxy is 4.3 million solar masses $\endgroup$
    – Gary Walker
    Commented Mar 27, 2015 at 9:29
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It's just a matter of achieving escape velocity and that reminds me of a Blake's 7 episode (Orbit, series 4 episode 11) where they have to reduce mass for the engines to able achieve it or they crash. They are stripping the ship of components and ejecting them until one of the characters realises the mass loss required is just about the mass of his colleague and considers ejecting him before he discovers another way. I think that was a clever (and tense) take on the scenario that may give inspiration.

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  • $\begingroup$ Decent answer, just remember to not regularly cite tv shows else no on e will accept your answers ;) $\endgroup$
    – Jax
    Commented Mar 26, 2015 at 18:10
  • $\begingroup$ Really? Avon answers with a reference to Blake's 7? ;) $\endgroup$
    – IchabodE
    Commented Mar 26, 2015 at 23:32
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This is a complete non-issue.

Any vessel that mounts engines capable of moving a star has plenty of power to escape. The black hole poses no hazard unless they get close enough that the tides become a factor or if they collide with it's accretion disk.

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