4 ms·

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

by 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.
      - garmaine 6y ago
        True, I didn't consider aneutronic fusion reactions. I don't think the sources I'm thinking of do, either.
        DennisP 6y ago
        Here's one source that does: http://toughsf.blogspot.com/2019/10/the-expanses-epstein-drive.html http://toughsf.blogspot.com/2019/10/the-expanses-epstein-dri...
        6y ago

by ivalm 6y ago

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.
    - garmaine 6y ago
      True, I didn't consider aneutronic fusion reactions. I don't think the sources I'm thinking of do, either.
      - DennisP 6y ago
        Here's one source that does: http://toughsf.blogspot.com/2019/10/the-expanses-epstein-drive.html http://toughsf.blogspot.com/2019/10/the-expanses-epstein-dri...
        6y ago