Questions for a Near-Future story

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  • #1
Drakkith
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Main Question or Discussion Point

Okay, I've been thinking of writing a story and I have a few questions.

First, the idea is that a cargo ship on the way back to Earth from the outer solar system is sabotaged or has an accident, leading to a loss of communication with the rest of the solar system and, more importantly, their fusion reactor has been damaged and they cannot accelerate or decelerate. And to make matters worse, I was considering having them be on a collision course with Earth unless they can get their ship working again. (If feasible)

Ship specifications:
Some type of ion engine to provide propulsion.
Fusion Reactor for power.
Spinning ring where the crew live/work. No cryogenics or anything like that.
Designed to haul large, bulky cargo. At least 1000 tons worth. Cannot enter the atmosphere of a planet.


Now, specific questions.

1. What kind of minimum mass would a ship like this have?

2. What kind of velocity could they realistically reach given ship mass, engine, and fuel constraints? Would 0.01c be feasible?

3. Without primary power, what kind of problems would they have to deal with? I've thought of things like shortage of fresh water, ship temperature regulation, oxygen generation, and difficulty repairing the ship. What else?

That's it for now. If anyone has any suggestions please feel free to comment.
 

Answers and Replies

  • #2
Vanadium 50
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.01c means you need to convert 0.01% of the ship's mass to energy - half when speeding up and half when slowing down. This is not a rocket - this is running on the ground, so it is hugely optimistic - this works out to something like 8 billion BTUs/ton.
 
  • #3
Ibix
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Friction will spin down the living ring and spin up the rest.

It's awful dark out there in the outer system.

Earth is a small target to hit by accident. Maybe a line about the saboteurs picking the time for the attack carefully?

I don't know how practical Bussard ramscoops are, but it might help with your fuel requirements.
 
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  • #4
Ryan_m_b
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Not sure if this scenario counts as near future but I'll do my best to help.

1. What kind of minimum mass would a ship like this have?
A fusion reactor is going to weigh a lot. The ones on the ground that we've been experimenting with might weigh a few tonnes but one in space is going to have to dissipate a hell of a lot of heat meaning you'll need to carry a lot of radiators. No idea what kind of number would be feasible for a mass I'm afraid. But given that the ISS masses 500 tonnes perhaps as a ballpark figure go for ten times higher. At least to give something to work with.

2. What kind of velocity could they realistically reach given ship mass, engine, and fuel constraints? Would 0.01c be feasible?
This is VERY back of the envelope, not to mention hand waving away the technobabble but using the rocket equation:

Given a mass of 6000 tonnes (5000 craft plus 1000 cargo), a target speed of 0.01c and a specific impulse of 100,000 (near magic in other words but you did say fusion drive) you'd need ~128,000 tonnes of fuel. But that's only to speed up, to slow down you'd need a total of ~2,700,000 tonnes of fuel.

Even if you did have that much fuel your thrust would probably be so low that you wouldn't get to 0.01c before halfway point.

3. Without primary power, what kind of problems would they have to deal with? I've thought of things like shortage of fresh water, ship temperature regulation, oxygen generation, and difficulty repairing the ship. What else?
Heat is the main one. That ship is going to get dealthly cold, not right away but probably in an inconvenient amount of time. Regarding things you haven't thought of it all depends on what emergency power does. If you don't have any then darkness, a lack of computers and no space toilets spring to mind.

3That's it for now. If anyone has any suggestions please feel free to comment.
It seems likely that in a setting where intrasystem is so routine that manned cargo vessels from the outer system are economical that tracking said vessels will be common. It probably wouldn't take too long for a system of satellites to detect that a ship had shut down and was drifting, slowly cooling. A rescue craft would probably be sent out ASAP.
 
  • #5
Drakkith
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Awesome. Thanks guys.
 
  • #6
Borek
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you'd need ~128,000 tonnes of fuel. But that's only to speed up, to slow down you'd need a total of ~2,700,000 tonnes of fuel.
20 times more? And not just twice?
 
  • #7
Bandersnatch
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20 times more? And not just twice?
Since you need 128 000 tonnes to decelerate a 6000 tonne ship, during the acceleration phase your ship will need to carry all that additional dead weigth.
 
  • #8
Borek
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Obvious. Somehow I got confused.
 
  • #9
Ryan_m_b
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Obvious. Somehow I got confused.
The rocket equation is easily forgettable. It's so intuitive to think "it takes X fuel to go Y speed therefore 2X to go 2Y" forgetting the mass of the extra fuel.

Incidentally if anyone wants to experiment with the equation but like me your maths is rusty here's a handy online calculator: http://www.quantumg.net/rocketeq.html
 
  • #10
Borek
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It was a little bit more convoluted - I was first confused by misunderstanding what you wrote, and that put me on the wrong approach to the problem.
 
  • #11
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Given how long you've been around here, you probably already know about this site, but just in case you don't, I think this will be helpful: Project RHO
 
  • #12
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I'm thinking that in a future where spaceships are becoming ubiquitous there would be massive defenses in place around Earth to stop someone from using a spaceship as a massive bomb like that. Expect nuclear missiles and gigawatts of coherent UV-light to be delivered to you if your vector transects Earth and your speed is deemed too high.
 
  • #13
Ryan_m_b
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I'm thinking that in a future where spaceships are becoming ubiquitous there would be massive defenses in place around Earth to stop someone from using a spaceship as a massive bomb like that. Expect nuclear missiles and gigawatts of coherent UV-light to be delivered to you if your vector transects Earth and your speed is deemed too high.
Enter stage left a nation state with a grudge against their neighbour and a cyberwarfare division and suddenly those "defences" start to look a little bit ominous.
 
  • #14
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The same line of thinking could be applied to the current US nuclear arsenal, is someone going to hack that anytime soon?
 
  • #15
Ryan_m_b
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The same line of thinking could be applied to the current US nuclear arsenal, is someone going to hack that anytime soon?
The US arsenal is not computer controlled unlike a hypothetical network of weaponised satellites. Regardless another concern is the polices turmoil this defence network would create. It's one thing for a nation to have nuclear weapons (and nations developing those programs is a huge political issue) but it's quite another to think of surrounding the earth in giggawatt lasers. What nation controls these? Is it international? Do different nations have their own to force a balance of power?

All interesting topics for fiction to explore.
 
  • #16
Borek
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Build the lasers on the dark side of the Moon.
 
  • #17
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Write the story, not get bogged down by trivial details! Unless your story ends with hundreds of facts
 
  • #18
Borek
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not get bogged down by trivial details!
Well... I hate it when the author doesn't treat me seriously. Obvious holes insulting my intelligence are what makes me put the book aside.
 
  • #19
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Well... I hate it when the author doesn't treat me seriously. Obvious holes insulting my intelligence are what makes me put the book aside.

If I were reading my chemistry textbook I would completely concede to your statement but if I'm watching Star Wars I dont expect scientific accuracy. Which is analogous to when watching batman you don't turn off the tv because he accelerates faster than the woman who falls out.
 
  • #20
Borek
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If I were reading my chemistry textbook I would completely concede to your statement but if I'm watching Star Wars I dont expect scientific accuracy. Which is analogous to when watching batman you don't turn off the tv because he accelerates faster than the woman who falls out.
I never said you have to switch YOUR TV off, but let me use my remote my way :tongue2:
 
  • #21
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In my opinion, if anything can happen, nothing is interesting. The writer must use a reasonably rigid framework to construct his/her world, otherwise it isn't SF, it's a comic book.

Then of course, there is no rational way to prove that "my" level of realism to suspend disbelief is correct while both the comic book fans and the people who only read biographies are wrong. To each his own...:)
 
  • #22
Borek
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In my opinion, if anything can happen, nothing is interesting. The writer must use a reasonably rigid framework to construct his/her world, otherwise it isn't SF, it's a comic book.
Bolding mine. Exactly my point. I never stated it has to be superrealistic. It can't be an insult to logic.

Sure, everyone has their own point where the story starts to be stupid. I have no problems with fake physics as long as it tries to be consistent and the rules don't change to fit what the author needs. If something was not possible at the beginning, it is not possible near the end (unless the book is about how the rules change for some reason :wink:).
 

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