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Comment by dvdkhlng | Hacker News Reader

I think the most obvious way to utilize a close-to-earth black hole is for space-flight. By utilizing the Oberth effect [1] it is possible to gain extra kinetic energy when firing engines very deep inside the black hole's gravity-well (while performing a sling-shot). The Penrose Process [2] may also allow for some kinetic energy boost.

Unfortunately both effects do not seem to offer the kind of multiple-orders-of-magnitude gain required to make interstellar travel practical.

[1] https://en.wikipedia.org/wiki/Oberth_effect

[2] https://en.wikipedia.org/wiki/Penrose_process

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I'm pretty sure I read a short story about this in an anthology called "Black Holes" by Jerry Pournell back when I was a youngster. In short, the black hole was a "gift" from some advanced race who dropped it into orbit of our star by braking it with Pluto (hence its erratic orbit) for the purpose of interstellar travel.

You can use a small black hole for mass transmutation of lighter elements (like all that surplus hydrogen just sitting idly in your local gas giants or star) into much more useful heavier elements (metals, carbon, oxygen, etc.):

https://www.orionsarm.com/eg-article/464790d2497de

The extreme temperatures and pressures in the accretion disk are basically used to fuse the lighter elements together into heavier ones, which are then pulled out by machinery in close orbit.

Machinery made of... plasma \o/

Because anything in close orbit is by definition plasma.

It doesn't have to be that close.

However if it is a primordial blackhole it is probably not much rotating... thus no much energy to get by the Penrose process.

I thought that just by letting masses getting sucked in while pulling ropes tied to alternators we could generate electricity... is it too naive? The amount of energy given by the fall into the blackhole would be superior than that used to bring the masses there in the first place.

I don't think that this would work: whoever is holding the alternator would also be pulled into the black hole while generating the electricity.

Maybe the "right" way to convert potential energy is via conversion to heat and black-body radiation in the accretion disk? Might be difficult to capture significant percentage of that energy, though. See also [1]

[1] https://arxiv.org/pdf/astro-ph/0307333v1.pdf

I would have started by an orbiting station (or even an orbiting ring-shaped station), staying at a safe distance from the horizon, from which the masses would be droped.

Is it still too unrealistic?

For the radiation energy, it sure makes sense! Moreover isn't it any hard radiations emitted when the hadrons' quarks are torn from each other on reaching the events horizon?

If your station is in orbit, then so is the mass you want to drop into the black hole. Orbital mechanics works quite differently to what you are used to from the surface of a planet.

But the black hole is very small as far as I know, maybe we could build an orbital ring around it, attach alternators to the orbital ring and then feed "ropes" to the alternator one one end and on the other end let them fall into the black hole.

By "ropes" I mean charged particles and by alternators I mean just very powerful electromagnets that can extract the energy of the charged particles falling into the black hole.

My point was that things attached to the outer core of an orbital ring are not in 0 G, but they feel the actual gravity at the particular height the orbital ring is orbiting -- on Earth if you would be sitting on an orbital ring situated at a height of say 300km, you would feel as though you were sitting on a 300km mountain; maybe on a primordial black hole you could build an orbital ring just a few km from the black hole and have spokes going down very close to the black hole (maybe active structures to overcome our current material strength limitations) and let charged particles fall into the black hole and extract their energy as they fall into the black hole.

Or maybe the black hole is small enough that a very crude electromagnets field could just encompass all of the black hole and it could very easily extract all that sweet energy of a charged particle falling into the black hole with an electromagnet an amateur could build in his garage.

Isn't there some way to stockpile the kinetic energy in several passes, instead of only one slingshot, then finally use all of that for a final throw?

Not really. The thing is, you're not so much stockpiling kinetic energy as you are using the mass of the body to amplify the force you apply. There is an optimal time to apply that force, which is when you are closest to the body you are orbiting.

So what may happen is if you have a low-thrust engine, you will do a burn at the optimal time, then stop and wait an orbit until you reach the optimal time again. But you're not "stockpiling" anything so much as you are just thrusting at the optimal time. And once you reach escape velocity you have to keep thrusting, there is no more opportunity to do another pass.

I can't recall which probe, but NASA has used the same body for multiply slingshots. But that's stealing orbital energy from the body you pass in the same direction it's orbiting.

I think the mass of the body only matters in how much momentum it has. If you fly by an asteroid you will deflect its course. You could convert the entire mass of earth into spaceships and slingshot them past Jupiter and it would barely register.

You could make multiple passes, if needed, gaining a + delta V each time by designing the orbit such that the spacecraft is on a transfer orbit back to PBH to make another pass. (If, as guelo mentioned, you did not at some point reach escape velocity of the solar system at that orbit.) It would be a very long wait between passes though...

Seems like a Jupiter gravity assist would always be much more practical.

The limit is the escape velocity of the orbit.

Not if you have time. And three bodies. If you've achieved escape velocity from the 2nd body (eg, Earth) but not the first body (eg, the Sun) you can keep swinging around the 1st body stealing speed off of the 2nd one each time.

Takes some careful orbits and a long time, but NASA does it all the time.

It would be very cool to have a binary black hole system somewhere in the solar-system. I.e. two black holes in a tight orbit around each other at very high orbital velocities. This could allow for very powerful slingshots to bring probes to interstellar speeds.

If you have the energy to get to black holes rotating around each other, you probably can build ships that don't need to slingshot. 'Cept perhaps for braking maneuvers.

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