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Comment by throwaway09223 | Hacker News Reader

If a ship is capable of continuous 1g acceleration then it shouldn't be designed with rotating centrifugal rings for gravity.

Instead, design the ship with the floor towards the engine and travel at a constant 1g to wherever it is that you're going. Decelerate at the same speed.

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I'm really surprised by this oversight as other things (magnetic boots) from The Expanse were explicitly mentioned.

In The Expanse, this is exactly how ships are set up. The "floor" is towards the engine and the continuous 1g acceleration provides "gravity". When a ship starts decelerating there is a "flip and burn" where everyone straps in while the ship literally turns around and starts accelerating at 1g in the opposite direction (meaning it's slowing down relative to its destination).

Most ships in The Expanse travel under much lower (1/6-1/2 G) accelerations because they didn't magic away all fuel consumption just made the fusion drives fantastically efficient. As you get to poorer ships more time is spent "on the float" between target and destination. Even our main character's ship doesn't accelerate all the time while travelling.

From what I remember, not just more efficient but also a LOT more thrust than realistic.

It could just flip much more slowly and maintain ~1g, with minimal course correction.

Presumably the mid-point is where the ship would be travelling at maximum velocity, so the course correction may not be as minimal depending on how slowly we're talking. It might be this would use more fuel/mass.

There's no such thing as maximum velocity. If constant 1g acceleration is available, there's not really a fuel concern. It will certainly use a little more fuel, but even just going to Mars would take a week, so half an hour's extra fuel usage works out to 0.3% extra fuel. For interstellar journeys, you're talking less than 0.001%.

Also, the course correction can easily be part of the manoeuvre, if you're willing to rotate around more than one axis.

Yeah, I could imagine some spiraling maneuver that would rotate 180° while maintaining constant acceleration. However, a flip and burn would certainly be more efficient (no cosine losses).

As well as strapping in to gel couches before high G maneuvers, the crew also connected themselves to IVs for auto injection of anti-clotting drugs so they could minimize the chance of a stroke or aneurysm.

> In The Expanse, this is exactly how ships are set up. The "floor" is towards the engine

This is pretty common in written science fiction but the fact that it is uncommon in movie/tv sci-fi is pretty damning really. Having spacecraft fly around like aircraft or cruise like ships, with decks "horizontal" and engines at the "back end" is just so bloody stupid. If you can't get basic physics right you have no business making science fiction. /rant

That was how the interstellar ship¹ in The Sparrow by Mary Doria Russell worked. For the first half of the trip they accelerated at 1g, then the ship rotated and they decelerated at 1g for the second half of the trip.

⸻ 1. Which was actually a hollowed-out asteroid and used the asteroid itself as propellant to achieve continuous acceleration and deceleration for the trip. One of the better attempts at providing a plausible mechanism for interstellar travel. Good enough that I neither declared it magic nor spent a long time thinking that it wouldn’t work.²

2. Doubtless someone will reply here with a detailed explanation of why it wouldn’t work.

Requires a magic-based energy source; the usual problem with this sort of thing.

Why would an ion thruster engine not be feasible?

The X3 ion engine has a weight of 230kg and a thrust of 5.4N. This is about 500 times less force than is needed to propel the engine forward at 1g (even if we pretend the entire rest of the spaceship doesn't exist - in reality it would need to be thousands of times more powerful)

Ion engines cannot currently produce anywhere close to this level of force. Adding more engines won't help because each engine can't even push its own weight at one gravity.

ELI5: Ion engines are the slowest tortoise you can imagine and to feel like gravity you need the hare.

It's just barely feasible with fusion energy densities. To accelerate a million tons to 0.1c you need as much kinetic energy as is theoretically contained in a million tons of hydrogen you'll fuse. Maybe with antimatter, but traveling so fast causes a ton of other issues, like every single particle you encounter is also traveling at ~0.1c.

This checks out! Kinetic energy = 0.5 mv^2, where v = 0.1c => E = 0.005mc^2 (relativistic effects not too important at this speed). Fusing a kilo of hydrogen converts ~6g to energy; at E = mc^2, that's 0.006mc^2.

So the energy needed to accelerate a given mass of hydrogen to 0.1c is almost exactly equal to the energy produced by fusing that hydrogen.

I believe the math works out such that any slag you create while refining construction or fuel materials needs to be launched out the back of the ship as fast as you can (frequency and velocity) so that you're not accelerating it along with the rest of the rock.

Though probably not exactly straight out the back, since that would create a navigation hazard for the next ship...

Achieving a given acceleration takes a certain amount of force per unit of mass. If one made the (dangerous) assumption that the propulsion system doesn't wear out over time, then it's more plausible to assume that at the beginning of the voyage you would achieve less than 1g, hopefully ramping up fast enough to keep atrophy to a moderate risk. Toward the end, as the asteroid gets lighter, you would run the engines at less than 100% for comfort. In this scenario the acceleration phase continues well past the halfway point.

And even without magic engines, any acceleration to match velocity with the destination doesn't need to be cancelled out, so the first 'half' of the trip is going to be slightly longer than the back half, even if you could do 1g the whole way (which the rocket equation has some problems with).

> If a ship is capable of continuous 1g acceleration then it shouldn't be designed with rotating centrifugal rings for gravity.

OP talks about accelerating at 0.1g on the outbound trip, and this is an emergency situation - maybe this ship isn't capable of continuous 1g acceleration without straining the engines past their operational parameters.

Although I can't imagine a ship with rotation ring segments built to withstand a year of 0.1g acceleration not immediately coming apart when suddenly subjected to a 4g load.

I don't think _my house_ would withstand a 4g load, and it was designed (and has successfully withstood) over a hundred years of a 1g load.

Is this 9.8m/s2? So after 1 year at this is 3.156e7*9.8/1000*3600km/h ?

Assuming it's just moving in space away from any massive objects in a straight line...1 billion km/h.

Isn't it too fast?

Not to mention the power required to continuously accelerate.

Or what happens to anything hitting you when you're traveling at that speed.

Or needing to turn.

Every answer is just a disguise for four more problems.

Sounds like all engineering! On a grander scale than usual though

Even worse, what happens when you get to your destination? Will you find a perfect analog of Earth and with the same kind of star? Assuming the star is very similar to our Sun, then you can ask about the atmosphere of the planet. Pretty much you need 25% oxygen and 75% nitrogen and 14 pound per square inch pressure on the surface. Fine, is the gravity the same? If not, there may be problems getting around. Okay, so what about food? You don't want to die of hunger. Finally, what about viruses and bacteria on the planet? They may attack you instantly and you die a miserable death in two weeks. At that point you may say to your self, Why did I come here.

Viruses and bacteria would not really be my worries. Unlikely they are compatible with our physiology. Maybe bacteria in sense of flesh eating or producing toxins as by-product, but nothing as infection I think.

You have to correct your calculations for relativity. A spaceship accelerating at 1G for 1 year (earth time, not spaceship time) will reach a little less than 0.5c. Accelerating at 1G for 10 years will reach 0.98c and take 4.5 years in the frame of reference of the spaceship. 1G for 100 years reaches 0.9998c and takes almost 9 years for the spaceship. None of these figures account for deceleration.

EDIT:

I echo the comment that it's useful to play around with the relativistic rocket calculators available online. TLDR: If you could actually make an engine that accelerated your spaceship at 1G continuously, that's really all you need. It's good enough for human life scale trips pretty much anywhere. You could travel the diameter of the milky way (approx. 100K light years) in 22.5 years of ship time, including deceleration to stop at the other end. Want to get to the Andromeda galaxy? 28.6 years of ship time. Of course, everything you know back home will be millions of years gone by the time you get to your destination.

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