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In the Expanse, gravity is created by accelerating / decelerating the ships. Humans from Earth are used to 1G, inhabitants of Mars about 0.4G and Belters even less. We've seen in S01E01 of the Expanse series that Earth gravity can be used to torture Belters.

At what acceleration do ships operate with the inhabitants of the different regions at standard and maximum? This is specially interesting for mixed crews as a normal acceleration for Earthers is a torture to Belters but a normal acceleration for Belters is difficult to operate in for Earthers and takes a lot longer to travel to the destination. Answer from the series or books are accepted.

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    With constant boost, it doesn't take "a lot longer" at lower acceleration -- it's just about linear, i.e. 1/10 G takes 10x as long as 1 G. This will turn days into weeks, but it still isn't years (or many years) as you'd have on Hohmann trajectories to/from or within the outer system.
    – Zeiss Ikon
    Dec 20, 2021 at 12:13
  • @ZeissIkon Yes. However, if you operate a business freighter, taking 10x as long (or even double the time) makes huge differences in operating cost and time to pay the crew. In a military context, you can basically double your fleet at a location if it takes only half the time to arrive at locations.
    – Shade
    Dec 20, 2021 at 13:20
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    If you operate commercially, you post your running acceleration in your hiring advert, anyone who can't take your (0.3 G, 0.6 G, 1 G) ship gravity is welcome not to apply. In the military, they make considerable use of acceleration baths and run at much higher than 1 G when needed.
    – Zeiss Ikon
    Dec 20, 2021 at 14:23
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    @ZeissIkon it's not very linear actually. To go 1 AU at .1 g's takes 6.4 days, at 1 g it's 2 days. Time is squared in the equation. Dec 20, 2021 at 15:58
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    I don't think there's a single answer -- it's going to depend on each crew and the urgency of each trip. Martians and Belters can train themselves to tolerate higher g's so a crew with Martians and/or Belters doesn't necessarily have a lower acceleration capability than an all-Earther crew. Similarly, if the trip is urgent (for whatever reason -- low supplies, trying to escape a pursuer, etc.) the crew will likely accept whatever acceleration is necessary, even if it is difficult for the crew (whether Earther, Martian, or Belter) to tolerate.
    – Null
    Dec 20, 2021 at 17:42

1 Answer 1

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Much of the time, the novels don't specify exact figures - 'heavy burns' are referenced without further detail. That said, there are some clues in odd passages of text.

There are several factors to consider. With the Epstein Drive, continuous thrust is possible - higher thrust leads to shorter travel times (as discussed in the comments), but is both less fuel efficient, and less comfortable. For crews that will plan on returning to a planet, maintaining their ability to cope with the surface gravity is an added incentive.

Further details below, but as an outline:

Belter ships (or with Belters on board), 1/3g for normal civilian burns seems typical.

Martian civilian ships likely run at about 0.4g; military crews routinely train at 1g - unclear if this is a normal burn, or interspersed with 0.4g runs.

Earth civilian ships up to 1g.

In an emergency, higher burns were known - civilian ships would rarely go to 2g, but could manage higher; capital ships appeared to generally top out around 8-10g, without much difference between origin - even Belters (if trained, and on the juice) could survive this acceleration for a while; smaller ships like the Rocinante could hit 15-20g - although using this power was almost unheard of.

Laconia developed technology to go to 30g, but it wasn't much fun and not routinely used.

Specific examples:

Canterbury, with mixed crew, 1/3 g in Leviathan Wakes:

Holden knelt beside her chair. In the one-third g of their current thrust, it was perfectly comfortable. (Leviathan Wakes, Chapter 1)

Two g for a civilian crew was manageable but uncomfortable; they would go higher in a true emergency however:

Ten minutes at two g, and Holden’s head was already starting to ache.

[...]

The thrust suddenly dropped to a tolerable two g and all the ship’s sensors flared into overload. (Leviathan Wakes, Chapter 5)

The Rocinante (still the Tachi at this point) hits 12 g - Alex was military trained, but still a Civilian pilot at this point:

The hole had been punched out by a jagged end of Amos’ broken tibia when the suit had pushed against it at twelve g. (Leviathan Wakes, Chapter 9)

Theoretical maximum was higher - with doubts that the crew could manage it:

The actual limit was one of those trivial numbers, a way to brag about something your ship would never really do. Fifteen g, was it? Twenty? (Leviathan Wakes, Chapter 51)

Martian civilians find a 1g burn uncomfortable, but the military routinely train in it:

The Dae-Jung’s captain was being pressured by the diplomats to get them to Earth as quickly as possible, so the ship was running at a near-constant one g acceleration. While this made most of the Martian civilians uncomfortable, it suited Bobbie just fine. The corps trained at high g all the time and did lengthy endurance drills at one g at least once a month. (Caliban's War, Chapter 8)

The civilian ship sent by RCE to New Terra/Ilus used a sustained 1 g burn:

I mean apart from burning at a full g. (Cibola Burn, Chapter 2)

For Martian capital ships, 8-10 g was described as top end:

“Eight to ten g to start, curving down, which means they’re running at the limit of their drives.” (Nemesis Games, Chapter 28)

Recognising that speed is not the same as acceleration, it is implied that trained Belter ships can go match fairly hard burns by others:

Eight gs, maybe? Maybe more.

[...]

The two ships were bearing down on the Roci, pushing hard to narrow the distance. She didn’t know if that was bold or foolhardy. Probably they didn’t either. Ships full of Belters weren’t known for loving high-g burns, but this was war. (Babylon's Ashes, Chapter 27)

Laconia can manage 30g with a liquid buoyed crash couch, which was entirely outside any other (survivable) burn:

Surviving a sustained thirty-g burn in a conventional crash couch would have been about as likely as living through a free fall drop from orbit onto a pile of knives. (Tiamat's Wrath, Chapter 19)

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