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Black Hole Starships

Anaximander

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I came across an article on arxiv (Click for PDF) about starships powered by small Schwarzchild black holes. It's pretty interesting, and I figured it could be useful for anyone planning some far-future hard sci-fi. Just thought I'd drop it here for your reading pleasure.
 

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I wish them all the luck in the world with figuring out how to scrunch all the mass of an object inside its Schwarzchild radius.
 

zerospark

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It wouldn't be that big a deal for a K2 civilization.
 

zerospark

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Nobody said it would be easy for us ;)

Lots of physically allowable things are still showstoppers in terms of engineering challenges. Doesn't mean it's not fun to talk about :)
 

Anaximander

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I wish them all the luck in the world with figuring out how to scrunch all the mass of an object inside its Schwarzchild radius.
They propose going with the lesser-known method of firing a whole load of lasers into a point. If you can get enough energy into a small space, it'll collapse. They say themselves, though, that it's beyond our current engineering capabilities. We can do the calculations to show that it'll probably work, but we can't build it, mostly because it would require a spherically converging laser with a mass of around a billion tons, floating in space, powered by a solar cell panel of several square kilometres, built from asteroid materials by an army of robots. So it could be done... just not yet. Still, it's been put forward as valid and actual scientists who know what they're talking about and are qualified to say such things have said that it should work. For hard sci-fi, that's plenty. So, we can at least write about it with confidence while the engineers spend a century or three working out how to build the damn thing.
 

Hallen

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The problem I see is the mass of the darned thing. Whereas today, we make engines for vehicles that weigh much less than the overall vehicle weight, the blackhole engine is a step backwards in that regard since the engine weighs as much, if not more, than the vehicle. You have to accelerate a lot more mass because of it. Yeah, I know you'll get virtually unlimited power out of it as long as you keep feeding it, but its propulsion method still seems dicey. I think the paper said it would take 10 years to accelerate to near light speed? That seems like pretty slow acceleration to me. Can you even maintain 1g acceleration?

Sounds like a great idea for stationary power plants, but not so much for ships.
 

Julie Worth

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I think the paper said it would take 10 years to accelerate to near light speed? That seems like pretty slow acceleration to me. Can you even maintain 1g acceleration?

1g is an excellent acceleration, as you can make a reality show about the ship's crew with no special effects. And it's effective at getting you places. In six months you're already at one half light speed. Of course, you can never get to light speed itself, at least relative to the television audience back home, but inside the ship is a different story. At 1g, your ship's gauges show you to continue to accelerate without limit. After a year you have apparently exceeded light speed and continue to accelerate. Before long you've zipped through the entire galaxy. Of course, back home civilization has died and a new intelligent species has arisen, but by dialated ship time, it's only been a few years.
 

Anaximander

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At 1g, your ship's gauges show you to continue to accelerate without limit. After a year you have apparently exceeded light speed and continue to accelerate. Before long you've zipped through the entire galaxy.
Not quite. Nothing can ever exceed the speed of light, in any frame of reference. Einstein said so. That means that from inside the ship, you'd still be below lightspeed - it'd just be 99.99-with more nines than you can count-%. And technically, you wouldn't notice how far ahead in time Earth was, because the light from it would be having to catch up with you, so it'd look like Earth had slowed down. Then when you stop, it goes back to normal speed, and when you fly back towards it, you'd have hundreds of years of messages arriving in a few weeks, and if you had a good enough telescope you could watch Earth in super-speeded-up time. Interestingly, even if you were flying towards Earth at nearly lightspeed, and they sent another ship towards you at a similar speed, neither of you could observe the other to be faster than light. Relativity alters the apparent time from each observer's perspective so that nothing ever exceeds lightspeed in any reference frame.
 
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Julie Worth

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Not quite. Nothing can ever exceed the speed of light, in any frame of reference. Einstein said so. That means that from inside the ship, you'd still be below lightspeed - it'd just be 99.99-with more nines than you can count-%. And technically, you wouldn't notice how far ahead in time Earth was, because the light from it would be having to catch up with you, so it'd look like Earth had slowed down. Then when you stop, it goes back to normal speed, and when you fly back towards it, you'd have hundreds of years of messages arriving in a few weeks, and if you had a good enough telescope you could watch Earth in super-speeded-up time. Interestingly, even if you were flying towards Earth at nearly lightspeed, and they sent another ship towards you at a similar speed, neither of you could observe the other to be faster than light. Relativity alters the apparent time from each observer's perspective so that nothing ever exceeds lightspeed in any reference frame.

Keep in mind that ship time is radically different than earth time. Inside the ship, nothing would change in any measurable way, but due to time dilation, you would be able to (apparently) travel faster than light, at least by the distances associated with your star charts. You would be able to cross the entire galaxy in just a few years, ship time, while to observers on earth, this will take you a hundred thousand years. There's nothing mysterious about this--it's just an extension of the classic twin paradox.

Unfortunately, even an anti-matter drive is not sufficient to achieve this velocity. You'd have to fly through interstellar clouds and scoop up fuel, or use some as-yet unknown energy source. But, assuming you can find a source of fuel, there's no limit to the apparent velocity you can achieve when time is measured by the ship's clock and distance by your star charts.
 
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The numbers from Project Valkyrie give a peak velocity of 92% c. A sufficiently robust amat drive could top out at some impressive numbers, although 92% c is still "only" a Lorentz factor of 2.55, so not quite the crazy time-stretch you'd get at even higher speeds.

A black hole engine would show similar performance (potentially even better than amat). The problem being again that you run up against the problems with mass ratio (most of the ship's mass would have to be fuel), and extracting fuel from the ISM as per a ramscoop is not looking feasible.

The dark matter engine is an intriguing possibility but it relies on several unproven assumptions.
 

Lhun

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The numbers from Project Valkyrie give a peak velocity of 92% c. A sufficiently robust amat drive could top out at some impressive numbers, although 92% c is still "only" a Lorentz factor of 2.55, so not quite the crazy time-stretch you'd get at even higher speeds.
Remember that there is no real maximum speed for a given drive. The maximum speed depends on the drive to payload ratio. Doubling the drive size will give you 50% more total thrust (assuming negligible payload, adjust for real number). The function diverges, so given a big enough rocket even a chemical drive will get you to 0,99c
An orion drive however is not a sensible choice anyway. Odds are that you can build an IC-fusion drive if you can build an orion drive, so just use that instead.
 

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The .92c number is given the Valkyrie's fuel load and mass ratio, along with ability to decelerate at the destination. Even that's a high end, given that .92c would be a mass ratio of 22.
 

Lhun

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22 isn't very high. Iirc, the space shuttle has a ratio that's about the same.
 

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Pretty high for a single-stage interstellar vehicle. Comparing it to a multi-stage rocket that only has to go to LEO isn't really the same thing.

For reference, the Space Shuttle is around 15-16. It took the Saturn V to get ~23. Asking that of a starship is a bit of a stretch.
 
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Lhun

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Hardly. I don't see how one could reasonably expect an interstellar vehicle to require a smaller drive than a mere orbital (or inter-orbit) shuttle.
 

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It's not about size, it's about mass ratio given the engine performance. A staged vehicle can support a much higher mass ratio than a dedicated single-stage craft because it can discard "used" mass once fuel is gone.

Building a bigger engine doesn't fix that, you just scale the whole problem up. You can only get so much delta-v and thrust even from an amat rocket.
 

Lhun

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It's not about size, it's about mass ratio given the engine performance. A staged vehicle can support a much higher mass ratio than a dedicated single-stage craft because it can discard "used" mass once fuel is gone.
The mass of a fuel tank is hardly relevant. You discard it for a multistage rocket of course, but more because it gets in the way than because of the mass.
Not even an issue then either, because you can build an interstellar craft easily staged as well, if that should prove beneficial for the design and fuel of choice.
Building a bigger engine doesn't fix that, you just scale the whole problem up. You can only get so much delta-v and thrust even from an amat rocket.
Size of an engine, in spaceflight terms is the size of the fuel tank. The engine itself doesn't really matter, only the total dV you get out of your fuel, because the practical limits on engine thrust are much lower than the limits on maximum carried fuel. E.g. it's not terribly hard to build a satellite booster engine that gets the payload to escape velocity in a few minutes, but the payload would be toast, and there's no point to doing that anyway, it doesn't save fuel, and time isn't critical.
 

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The mass of a fuel tank is hardly relevant. You discard it for a multistage rocket of course, but more because it gets in the way than because of the mass.

Every gram of mass matters in a universe without magic propulsion systems. Anything that relies on rocketry will be beholden to Tsiolkovsky's equations. A bigger fuel proportion is mass that has to be accelerated, and ship mass that is required to hold the fuel - and a multi-stage rocket is carrying engines along with that, not just fuel tanks.

A mass ratio much above 15-20 for a single-stage vehicle is probably not feasible, and that puts limits on what even a magical torch drive could accomplish in terms of peak velocity.

Not even an issue then either, because you can build an interstellar craft easily staged as well, if that should prove beneficial for the design and fuel of choice.
You can't stage an interstellar craft if you want it to stop at the other end of the trip. Project Valkyrie was looking at what it would take to get a manned mission to another solar system and back. You have to carry the means of deceleration and the entire return trip on board.

That puts a pretty hard limit on the performance of even a perfect space drive. Again, this is all in Tsiolkovsky's rocketry equation. Valkyrie has a speed "limit" because of the mass and the properties of the drive that limit total delta-v.
 
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Lhun

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Every gram of mass matters in a universe without magic propulsion systems. Anything that relies on rocketry will be beholden to Tsiolkovsky's equations.
Sure, every gram counts, but to take a real world example again, the Space Shuttle external tank has a ratio of about 300. Far less than the whole vehicle. Discarding it is done because it's possible, and saves a little fuel, not because it's absolutely necessary.
A bigger fuel proportion is mass that has to be accelerated, and ship mass that is required to hold the fuel.
Yes, but as i said, the function for total dV diverges. Adding more fuel runs into diminishing returns, but the returns are always positive. The question of top speed is one of economy, not of physics, because sooner or later it's just not useful to increase the fuel by a huge percentage to gain a tiny bit more top speed, even though it's possible.
A mass ratio much above 15-20 for a single-stage vehicle is probably not feasible, and that puts limits on what even a magical torch drive could accomplish in terms of peak velocity.
That depends completely on the design. An orion drive or IC fusion drive ship can have pretty much any fuel/payload ratio you want. It doesn't even make sense to talk about "stages" with those drives.
You can't stage an interstellar craft if you want it to stop at the other end of the trip.
Yes you can. Can't imagine where you'd get the idea that it's not possible.
That puts a pretty hard limit on the performance of even a perfect space drive. Again, this is all in Tsiolkovsky's rocketry equation. Valkyrie has a speed "limit" because of the mass and the properties of the drive that limit total delta-v.
There is no limit to maximum dV for any drive. Tsiolkovsky's function is logarithmic (thus it diverges).
 

zerospark

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Sure, every gram counts, but to take a real world example again, the Space Shuttle external tank has a ratio of about 300. Far less than the whole vehicle. Discarding it is done because it's possible, and saves a little fuel, not because it's absolutely necessary.

Because it couldn't get into orbit without discarding the external tanks and boosters. If it could, you wouldn't need the fuel tank and the boosters to get into orbit. And that's just to LEO. You needed the whole Saturn V just to launch the Apollo capsule to the moon.

Yes, but as i said, the function for total dV diverges. Adding more fuel runs into diminishing returns, but the returns are always positive. The question of top speed is one of economy, not of physics, because sooner or later it's just not useful to increase the fuel by a huge percentage to gain a tiny bit more top speed, even though it's possible.

Of course there's no physical limit besides the light-speed asymptote, but when you're discussing practical designs like Valkyrie, suddenly the practicalities of engineering and cost are relevant. Considering my original post was about the practical limits in the first place, I'm not sure why you feel the need to make this point.

That depends completely on the design. An orion drive or IC fusion drive ship can have pretty much any fuel/payload ratio you want. It doesn't even make sense to talk about "stages" with those drives.

Even a torch drive still has to obey the rules, even if it does have some obscene values for delta-v. It still has to carry fuel, and it still has limits on realistic mass ratio.

Yes you can. Can't imagine where you'd get the idea that it's not possible.

And it's all added mass that has to be accelerated, which makes your problem even worse.

There is no limit to maximum dV for any drive. Tsiolkovsky's function is logarithmic (thus it diverges).

I'm not sure why you're arguing on-paper points when the whole matter that started your reply was about a quasi-realistic rocket with real, known properties that have been calculated.

If you're that sure of your points, then I'd suggest emailing Winchell Chung and telling him his page is wrong.
 

Lhun

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Because it couldn't get into orbit without discarding the external tanks and boosters. If it could, you wouldn't need the fuel tank and the boosters to get into orbit. And that's just to LEO. You needed the whole Saturn V just to launch the Apollo capsule to the moon.
So? The question wasn't whether the fuel in the external tank is necessary, but whether discarding it is necessary. Which is more convenient than necessary.
Of course there's no physical limit besides the light-speed asymptote, but when you're discussing practical designs like Valkyrie, suddenly the practicalities of engineering and cost are relevant. Considering my original post was about the practical limits in the first place, I'm not sure why you feel the need to make this point.
Because i pointed it out several posts above and you appeared to argue the opposite.
Even a torch drive still has to obey the rules, even if it does have some obscene values for delta-v. It still has to carry fuel, and it still has limits on realistic mass ratio.
Sure, never said anything else. But what a realistic mass ratio is, is an economical question not a technical one.
And it's all added mass that has to be accelerated, which makes your problem even worse.
Not sure how this comment relates at all to the point in question. (i.e. staged interstellar ships)
I'm not sure why you're arguing on-paper points when the whole matter that started your reply was about a quasi-realistic rocket with real, known properties that have been calculated.
No, i'm pretty sure the question in the OP was regarding realistic concepts, not just concepts that have already been fleshed out an calculated with specific parameters in mind.
For example (again) i don't see a reason to expect an interstellar ship to have a better payload/fuel ratio than even a shuttle in any part of the original context for the discussion.
If you're that sure of your points, then I'd suggest emailing Winchell Chung and telling him his page is wrong.
No need for that, Project Rho is right on almost everything on the page, it does not however say the same things you posted in this thread.
 

zerospark

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No, i'm pretty sure the question in the OP was regarding realistic concepts, not just concepts that have already been fleshed out an calculated with specific parameters in mind.

You quoted me in response to a point about Project Valkyrie, and contested the value I gave as a peak velocity (which came straight from Project Rho).

I made the point that the drive limits were based on practical values for Tsiolkovsky's rocket equation, and at no point did I ever suggest that there was a physical limit to the rocket velocity aside from the light-speed asymptote.

No need for that, Project Rho is right on almost everything on the page, it does not however say the same things you posted in this thread.

Given that my argument was verbatim from that page, I don't see where the conflict lies.