The Expanse: Sustained Gs during space travel

Question

In The Expanse series, characters experience sustained Gs during space travel, not just during acceleration, which doesn't make sense. For instance, here's several passages from Leviathan Wakes, the first book in the series (tried to omit anything that could be a spoiler):

Walking through the Rocinante felt surreal. [...] To walk through the spare, functional corridors, thrust gravity holding him gently to the floor...
[p 359]

And:

Alex had the Rocinante running at three-quarters of a g for 2 hours while the crew prepared and ate dinner. He would run it back up to three when the break was over [...] Once the gravity had dropped from the crush of high acceleration, the whole crew quietly gathered in the galley...
[p 366]

Both passages suggest that there are more Gs during acceleration, but always some amount of g-force just from moving. However real-life physics dictate that you feel g-force only when you accelerate. If you're driving on the highway at a constant speed of 80mph, you aren't being pushed back into your seat. How is the Rocinante generating constant g-force, unless it is constantly accelerating? After hours and hours of travel under any amount of g-force from thrust, wouldn't the speed become unsafe? Or is there something I missed?

Answer 1

The Expanse uses torch ships on Brachistochrone trajectories

Due to the Epstein Drive, fuel is not a (major) consideration so they don't need to worry about it. That means they can use the fastest/most convenient method to get from A to B; that is, accelerate until you're halfway there and then "flip and burn" and decelerate for the other half - a Brachistochrone trajectory. This is the minimum time/maximum delta-V (i.e. fuel) trajectory.

The "fastest" is by accelerating as hard as you can but that is incredibly punishing on the people. The "most convenient" is by accelerating at a comfortable rate - 1g for Earther's 1/3g for Martians, less for Belters and some compromise that upsets everyone for a mixed crew. That means that the ships experience "normal" thrust gravitation for most of their flight.

In combat or emergency circumstances, everyone straps into crash couches and takes dzhush to push their physiology to withstand higher acceleration, up to a point, of course. Of course, the main engine only thrusts in the forward direction; yaw, pitch and roll are done with much less powerful thrusters that give the ship the same maneuverability irrespective of the ship's speed (because vacuum) or whether the main engine is firing (again, within limits; too much acceleration in a direction the couches aren't compensating for and splat).

Real spacecraft do have to worry about fuel because they use real engines not technomagic ones so they use the most efficient rather than the fastest way to get from A to B. This consists of a short burn to get you heading to your objective, a few mid-course corrections and then a capture burn if you want to get into orbit; possibly with aerobraking if your target has an atmosphere.

For going from one (near) circular orbit to another (e.g. Earth to Mars, or Low Earth Orbit to the Moon) this is the Hohmann transfer orbit; the maximum time/minimum delta-V transfer. However, this orbit puts Mars 9 months from Earth and is only available every 26 months; the launch window. Compare this with a 1 g torch ship which would take 2 to 30 days depending on if Mars is on this side or the other side of the Sun (assuming you don't want to get too close to the Sun).

For interplanetary probes, you can do all sorts of cool gravity assists to save even more fuel because probes don't care how long it takes to get there.

Answer 2

I understood "gravity holding him gently to the floor" to imply 'slow burn' steady acceleration, whereas "the crush of high acceleration" to me implies 'high acceleration' like when in combat and on juice, when racing to some destination in a super time-critical fashion, etc. From what I recall this is pretty consistently represented in the books (and I read the entire series, most of it twice); but if you are spoiler tolerant seen my final comment.

So these are both are cases of acceleration (often linear acceleration), and thus both cases of artificial gravity, but low acceleration = low gravity (fractions of Earth normal), whereas high acceleration = high (i.e. crushing) gravity (multiples of Earth normal). Note that the effect of gravity on mass (in non-relativistic mechanics) is measured exactly the same way acceleration of mass is: distance divided by the square of time, as in the approximately 9.81m/s^2 at Earth's surface

At the end of your question you mention the notion of 'unsafe speed': in space velocity is relative, so ideas like 'unsafe speed' that would make sense in the air, on the group, or on or under the water do not really apply.

In later books the proto-molecule, and other technological remnants of the proto-molecule engineers exhibit control of physics beyond human ken… including inertialess motion.

Answer 3

Yes, they're constantly accelerating (sometimes more, sometimes less).

Safe or not, the only way to get across the solar system in days or weeks rather than months or years is to get up to very high speeds, and the only way to get up to very high speeds while limiting acceleration to values that don't turn your crew and passengers into paste is to keep accelerating at around 1g for a very long time.

The fastest way (not the most efficient, but the fastest) to get from point A to point B is to accelerate towards B until you reach the halfway point, then flip 180 degrees and accelerate in the other direction; you will burn off all of your velocity just in time to come to rest at the destination (give or take some tweaks for gravitational effects). This is convenient if you rely on acceleration to let people walk around the ship, because you will be accelerating for 99.9% of the trip.

Answer 4

Yes, in the expanse, ships that are traveling somewhere are constantly accelerating. The ludicrous performance of the Epstein drive allows ships the luxury of continuously accelerating/decelerating non stop throughout the flight, instead of coasting like spaceships have to do IRL. It even allows firing the engines without a destination, just to have gravity if needed, e.g. for treating injuries, since blood pools inside the body in freefall.

Answer 5

The only 'gravity' in space travel in The Expanse is from acceleration or deceleration. In The Expanse more powerful ships are usually either constantly accelerating or constantly decelerating because the engines are so efficient they can do so without fuel worries so they can live with simulated gravity. When they are "on the float" they are neither accelerating nor decelerating and the crew experiences zero-g.

Answer 6

No, you understand the issues correctly. It is the universe that's lacking some concepts, thereby confusing some ideas.

You are correct, even 1g acceleration soon makes speeds rather spectacular and unsafe. For example, it would allow to go from Earth to Mars at their farthest (if they're both in aphelion and on opposite sides of the Sun) in about 10 days (50% accel, 50% decel and detour around Sun).

This distance is 401 million kilometers, maximum speed will be about 1.4% of c.

It doesn't sound much, but taking into account zodiacal dust this will mean that ship, encountering a speck of dust on the bigger end of the scale (which is 0.1 pg to 0.1 mg), will be hit with a force of anything between 15 MJ to 30 GJ (for comparison 155 HE artillery shell produces about 55 MJ, 10kT tactical nuke is about 480 GJ). Since dust density is about 5 particles per cm3, this would be near-constant bombardment of the ship's bow...

I'd say no ship would survive that, double-hull, anti-spalling and all that...

The other problem is sudden loss of gravity. Not so much for humans, but for the spaceships themselves... Space ships must have very robust structure, to deal both with positive and negative g forces, and those are very different creatures in terms of what ship herself, it's equipment and parts can be subject to. Contrary to popular belief, this is a huge problem for engineers.

And, of course, there is the question of breathable atmosphere inside of a ship... This is also tremendous force in space. We can build light on Earth, because atmospheric pressure is equalized both inside and outside your house, car, whatever. But remove atmosphere from one side of that equation and you have either boom or splat. There is an awesome video on the internet showing what happens to a railcar cistern when air is pumped out of it... [Of course, there are vessels built to withstand that (i.e. a submarine easily deals with 1 atm of outside pressure on it's hull), but I think it's still an awesome visualisation of the atmosphere problem] This air pressure is number one structural concern in every vessel we currently have in space (as a humanity). Definitely ISS has that issue in spades (inflatable compartments notwithstanding, no matter how cool they are - and they are very cool!!). And, of course, atmosphere has a mass too - which is 1.3 kg/m3, so if ship is big enough it needs to be accounted for.

Yet another problem is mass distribution. It is not a great example, but in a space shuttle astronauts and their equipment/cargo are carefully laid out, not just piled on top of one another... And there's a reason EVERYTHING there is strapped down - astronauts including (and they're not moving during boost for that reason as well; though it's hard to do anything at 3g). Sudden, even small change in distribution of mass inside ship during accel/decel and results can be catastrophic. Now, space shuttle is not the Donnager, but those rules still apply...

So, bottom-lining it all, I agree with you that there's a lot missing in there to complete the picture...