3 ms·

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

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...
        Symmetry 6y ago
        I think I was underestimating how much energy could be lost to x-rays on some of those. D2-He3, the aneutronic fusion everyone likes to talk about is still over 80% efficient but H1-B looks really bad if you're not driving a heat engine with it.

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...
      - Symmetry 6y ago
        I think I was underestimating how much energy could be lost to x-rays on some of those. D2-He3, the aneutronic fusion everyone likes to talk about is still over 80% efficient but H1-B looks really bad if you're not driving a heat engine with it.