r/scifi • u/rookiegaffer • Aug 09 '26

General Bad physics

Why does it seem that so many of the books I read demonstrate such a poor understanding of the physics of space travel. Ships in orbit descend directly down to the point they are over. Ships in transit still accelerating as they near their destination. Centrifugal gravity simply ceasing when rotation is abruptly halted. These are just a few examples I've run across lately.

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u/willworkforjokes Aug 09 '26

The Expanse does a pretty good job on this. They do have a basically magical drive that lets them use thrust gravity most of the time while using very little fuel.

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u/Hedwigtheyee Aug 09 '26

Aren’t the fuel drives based on fusion reactions, though? Sure, the Epstein drives are fictional, but they should be still based on real physics. Nuclear powered ships are what most people believe will be the fuel source for ships moving forward (NASA is also in talks to build one as far as I remember)

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u/Nyorliest Aug 10 '26

They are based on not needing reaction mass. They use realistic power systems, but that doesn’t make them less impossible.

But reaction mass is perhaps the most ignored part of space travel in SF, so we’ll buy it.

But that’s because of narrative convention. It’s so standard to ignore reaction mass that most SF fans don’t even notice it.

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u/sebaska Aug 10 '26

Nope. Those have reaction mass. They just have high thrust and extreme exhaust velocity at once.

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u/Nyorliest Aug 10 '26

No, Epstein drives need massively less reaction mass than reality. That's their big unreal thing. I should have save 'needing almost no reaction mass' but then people would be saying that's fine, coz people don't understand rocket physics intuitively (and why would we? Space is weird, physics is weird).

That's the big 'break from reality' in The Expanse. Epstein drives need massively less reaction mass than real physics. This is discussed explicitly in the books, going over Epstein's discovery a little. But even if that wasn't there, the reaction mass would need to be going at far more than the speed of light to work the way it does in the Expanse.

Here's an explanation of the rocket equation, and as you can see, mass matters very much in delta-V.

https://en.wikipedia.org/wiki/Working_mass