4 ms·

Not to take away from your post, because I largely agree, but the Epstein drive might as well be magic. In no realistic universe are there large battle cruisers

Not to take away from your post, because I largely agree, but the Epstein drive might as well be magic. In no realistic universe are there large battle cruisers zipping around the solar system under constant 1G (or even Mars’ third-G) acceleration for months at a time. But aside from that and the whole aliens technology thing the plot revolves around, it is very realistic. They certainly get a lot closer than anybody else.

saberdancer 6y ago
Physics is realistic, engineering not so much? :)
ivalm 6y ago
I am not sure how unreasonable is prolonged 1g. You can’t do it for months (since if it’s all in one direction you will start getting relativistic, one month of 1g gets you to almost 0.1c) but at medium speeds it isn’t too bad. Suppose you have a megaton ship moving at 100km/s with 10 m/s^2 acceleration Power = m v a = 1e9 x 1e5 x 1e1 = 1e15 W While petawatt is a lot, a megaton ship is also huge. Epstein drive is a fusion drive of sort, the easiest aneutronic reaction would be B+p -> C + 16 MeV So 2.5e-12 Joules per fusion Which means you need to burn 1e15 J/s /(2.5e-12 J/atom)*(2e-26 kg/atom) = 8 kg/s Suppose your engine runs at 10% efficiency, it means you are powering a megaton ship on less than a 100kg/s of material. So if your megaton ship starts as 10% fuel you have ~115 days of continuous thrust.
- garmaine 6y ago
  You're assuming way too high an efficiency. A real fusion rocket would fall far, far short of that. 2% efficiency would be generous. Googling fusion rocket mass efficiencies shows estimates of around 1 ton / MW, so you're not too far from the mark. But that gets to what the Epstein drive basically is, in-universe: a 100% efficient fusion drive. No physical laws being broken, technically, just the nuclear rocket equivalent of a perpetual motion machine. They do in fact go around at 1G (or 1/3 G) thrust for months at a time in the books and show, accelerating full speed for half the trip, then turn around and decelerate for the other half, for trips to/from the outer solar system. In the inner solar system it's only a few days to weeks to get anywhere. This lets them operate in artificial gravity without spinning the spacecraft, which is convenient for filming (although it's in the source novels too).
  - ivalm 6y ago
    My point is that even if it is 10% efficient, which is only 5 times more than your estimate， you have 4 months. Even if it is 2% efficient, you get ALMOST a month. Re: accelerating for many months at 1G is definitely bad physics, not only are you relativistic, you start getting into the kind of speeds where a minor meteorites will wipeout your ship. 1 gram meteorite with a relative velocity of 0.5c has 1.4e13 J of kinetic energy = 3.5 kton of TNT, so a small nuke.
    - garmaine 6y ago
      yeah the speed they travel through the inner solar system is pretty ludicrous, especially when you consider how much space junk is generated by all that activity.
  - TwoBit 6y ago
    I think comparing future technologies with today's Googke searches is unrealistic.
    - garmaine 6y ago
      Exploratory engineering does put firm limits on future capabilities. There are many possible steam engine designs, and I'm sure those we use today were unimagined in the 1820's. Yet none are more efficient than an ideal Carnot steam engine. The same principle applies here.
      - Symmetry 6y ago
        Yes, and for aneutronic fusion reactions the maximum drive efficiency is really high. Bremsstrahlung x-rays are still a problem in terms of heat dissipation but shouldn't cut into efficiency too much. EDIT: Well, I should say that the propellant mass/delta-v is really high and that most of the fusion energy will can end up as kinetic energy of the propellant and ship. But the vast majority of that kinetic energy will be the propellant rather than the ship. To maximize ship kinetic energy over fuel energy you want the propellant to exit the ship at the same velocity the ship is traveling which is very far from what you see in an Epstein drive. But fusion provides tons of energy so that isn't going to be a figure of merit we care about.
magicalhippo 6y ago
> the Epstein drive might as well be magic For me, good sci-fi does just that, ie limit the magic to one thing or technology, and use that to explore something interesting. Same with Altered Carbon. What would happen to society if we could swap bodies but keep our minds? That one piece of technology ("the stack") is magic, but from it follows lots of interesting exploration of the possible impacts on society etc. What I hate is when they try to explain the magic. Don't do that. It never works. Just assume it's there, someone discovered it, invented it whatever.
- TeMPOraL 6y ago
  But you have to try and explain the magic. If you didn't, your universe would break. Depending on the postulated mechanisms of action, the principles behind the magical invention will have vastly different impact on societies. For example, it matters how is it the ships can casually sustain a 1G burn for a week at a time. If it's efficient nuclear fusion, then the ships get bulky, energy is no longer a problem in the society at large, and the exhaust of said engine is a weapon of mass destruction. If it's subspace field shenanigans, then aforementioned impacts don't necessarily apply, but there is an open question of how small such subspace field combobulators can get, and what else could you do with them. Technologies have second-order consequences, and societies shape to accommodate them. Half the fun of science fiction is in exploring those. If you just call it magic and define it by a set of arbitrary rules, then your fictional universe will look shallow and inconsistent.
  - krapp 6y ago
    >But you have to try and explain the magic. If you didn't, your universe would break. Depending on the postulated mechanisms of action, the principles behind the magical invention will have vastly different impact on societies. Star Trek goes into intricate detail trying to explain its warp drives and comes off looking far less grounded than the vast majority of sci-fi properties with FTL or jumpgates or whatnot that either don't explain anything, or just offer a few details. Whether it breaks your universe depends on the themes you're trying to explore, such as the nth order effects of radical technologies on society. Most of the time, the story isn't about the technology (in the same way that Westerns aren't always about animal husbandry and gun manufacturing), and it's sufficient that you push some buttons, pull a lever and spaceship go brrr.

saberdancer 6y ago

Physics is realistic, engineering not so much? :)

ivalm 6y ago

I am not sure how unreasonable is prolonged 1g. You can’t do it for months (since if it’s all in one direction you will start getting relativistic, one month of 1g gets you to almost 0.1c) but at medium speeds it isn’t too bad. Suppose you have a megaton ship moving at 100km/s with 10 m/s^2 acceleration Power = m v a = 1e9 x 1e5 x 1e1 = 1e15 W While petawatt is a lot, a megaton ship is also huge. Epstein drive is a fusion drive of sort, the easiest aneutronic reaction would be B+p -> C + 16 MeV So 2.5e-12 Joules per fusion Which means you need to burn 1e15 J/s /(2.5e-12 J/atom)*(2e-26 kg/atom) = 8 kg/s Suppose your engine runs at 10% efficiency, it means you are powering a megaton ship on less than a 100kg/s of material. So if your megaton ship starts as 10% fuel you have ~115 days of continuous thrust.

garmaine 6y ago
You're assuming way too high an efficiency. A real fusion rocket would fall far, far short of that. 2% efficiency would be generous. Googling fusion rocket mass efficiencies shows estimates of around 1 ton / MW, so you're not too far from the mark. But that gets to what the Epstein drive basically is, in-universe: a 100% efficient fusion drive. No physical laws being broken, technically, just the nuclear rocket equivalent of a perpetual motion machine. They do in fact go around at 1G (or 1/3 G) thrust for months at a time in the books and show, accelerating full speed for half the trip, then turn around and decelerate for the other half, for trips to/from the outer solar system. In the inner solar system it's only a few days to weeks to get anywhere. This lets them operate in artificial gravity without spinning the spacecraft, which is convenient for filming (although it's in the source novels too).
- ivalm 6y ago
  My point is that even if it is 10% efficient, which is only 5 times more than your estimate， you have 4 months. Even if it is 2% efficient, you get ALMOST a month. Re: accelerating for many months at 1G is definitely bad physics, not only are you relativistic, you start getting into the kind of speeds where a minor meteorites will wipeout your ship. 1 gram meteorite with a relative velocity of 0.5c has 1.4e13 J of kinetic energy = 3.5 kton of TNT, so a small nuke.
  - garmaine 6y ago
    yeah the speed they travel through the inner solar system is pretty ludicrous, especially when you consider how much space junk is generated by all that activity.
- TwoBit 6y ago
  I think comparing future technologies with today's Googke searches is unrealistic.
  - garmaine 6y ago
    Exploratory engineering does put firm limits on future capabilities. There are many possible steam engine designs, and I'm sure those we use today were unimagined in the 1820's. Yet none are more efficient than an ideal Carnot steam engine. The same principle applies here.
    - Symmetry 6y ago
      Yes, and for aneutronic fusion reactions the maximum drive efficiency is really high. Bremsstrahlung x-rays are still a problem in terms of heat dissipation but shouldn't cut into efficiency too much. EDIT: Well, I should say that the propellant mass/delta-v is really high and that most of the fusion energy will can end up as kinetic energy of the propellant and ship. But the vast majority of that kinetic energy will be the propellant rather than the ship. To maximize ship kinetic energy over fuel energy you want the propellant to exit the ship at the same velocity the ship is traveling which is very far from what you see in an Epstein drive. But fusion provides tons of energy so that isn't going to be a figure of merit we care about.

magicalhippo 6y ago

> the Epstein drive might as well be magic For me, good sci-fi does just that, ie limit the magic to one thing or technology, and use that to explore something interesting. Same with Altered Carbon. What would happen to society if we could swap bodies but keep our minds? That one piece of technology ("the stack") is magic, but from it follows lots of interesting exploration of the possible impacts on society etc. What I hate is when they try to explain the magic. Don't do that. It never works. Just assume it's there, someone discovered it, invented it whatever.

TeMPOraL 6y ago
But you have to try and explain the magic. If you didn't, your universe would break. Depending on the postulated mechanisms of action, the principles behind the magical invention will have vastly different impact on societies. For example, it matters how is it the ships can casually sustain a 1G burn for a week at a time. If it's efficient nuclear fusion, then the ships get bulky, energy is no longer a problem in the society at large, and the exhaust of said engine is a weapon of mass destruction. If it's subspace field shenanigans, then aforementioned impacts don't necessarily apply, but there is an open question of how small such subspace field combobulators can get, and what else could you do with them. Technologies have second-order consequences, and societies shape to accommodate them. Half the fun of science fiction is in exploring those. If you just call it magic and define it by a set of arbitrary rules, then your fictional universe will look shallow and inconsistent.
- krapp 6y ago
  >But you have to try and explain the magic. If you didn't, your universe would break. Depending on the postulated mechanisms of action, the principles behind the magical invention will have vastly different impact on societies. Star Trek goes into intricate detail trying to explain its warp drives and comes off looking far less grounded than the vast majority of sci-fi properties with FTL or jumpgates or whatnot that either don't explain anything, or just offer a few details. Whether it breaks your universe depends on the themes you're trying to explore, such as the nth order effects of radical technologies on society. Most of the time, the story isn't about the technology (in the same way that Westerns aren't always about animal husbandry and gun manufacturing), and it's sufficient that you push some buttons, pull a lever and spaceship go brrr.