# I have a question regarding a science fiction story I am writing.

I don't know how often you get questions like this here, if ever, but I was hoping you could help me solve a problem I have come across in writing a story. It is not exactly Hard Sci Fi, but it is definitely not soft science fiction, and I want to get the physics at least mostly right. Here it is:

There is this generation ship, and the way I have it laid out is that every few years, it turns on it's engines for a bit to accelerate it up to, uhh, really fast. (still WAY less than light speed, mind you). The way I have artificial gravity worked out is that the habitation section of the ship is a big sphere that spins around to generate gravity. My question is, would the acceleration pretty much just generate gravity to where the people would just be pushed towards the back, making it more believable to just have a non rotating hab section where the gravity is generated by thrust? Or when the ship is gradually decelerating in the time between engine activation, would there be pretty much 0 G, and the rotating habitation complex would work better?

If anyone had no clue what I just asked feel free to ask me what the heck I am talking about.

## Answers and Replies

Related Writing and World Building News on Phys.org
Nabeshin
Or when the ship is gradually decelerating in the time between engine activation
Why would the ship decelerate in between engine firings? In space, there isn't any friction which is going to slow the ship down, so it will just coast along at whatever speed you last left it (ignoring, of course, gravity wells which I'm assuming you're steering clear of). Regarding artificial acceleration, bearing the above in mind, it sounds like you want to have a (comparatively) brief period of acceleration at the beginning of the voyage, and then just coast? The other option is continuous acceleration for half the trip, then deceleration for the other half.

The problem with the continuous acceleration is that it takes a colossal amount of energy to accelerate for all that time, so it is almost prohibitively expensive to accelerate at, say, 1g for the duration of the trip. Not to mention that if you did accelerate that rapidly, time dilation effects would make it so that a trip which might seem to take tens of thousands of years to an Earth-bound observer would take your crew only a few decades.

The moral of the story is, if your ship will just be coasting after a brief initial acceleration period, you need to have some spinning apparatus to generate your artificial gravity field.

phinds
Gold Member
2019 Award
on-again off-again thrust is a really bad idea. you need to settle on a continuous propulsion method that is at least remotely feasible and then design the crew quarters to rotate but take into account the thrust (as nabeshin said, you are NOT going to have 1G thrust). I don't know if that can even be done in a way that doesn't make everyone nauseous.

By the way, if you plan on being a writer, you might find it helpful to learn things such as the fact that "it's" is not a possessive pronoun, it is a contraction meaning "it is", so that the phrase "it's engines" means "it is engines".

@phinds Excuse me for writing that when I was tired. Sometimes people make mistakes. Though I think I will not have continuous propulsion, because the amount of energy required would be too much, which is why I proposed on and off propulsion in the first place. I think I will just have the engines fire once at the beginning of the voyage, as Nabeshin said, and have rotation generated gravity instead of acceleration generated gravity.

How would such a ship stop upon reaching it's destination?

How would such a ship stop upon reaching it's destination?
The same way as it accelerates. Turn it around (maybe stop rotation first to avoid gyro effects) and switch thrust on.

DaveC426913
Gold Member
The same way as it accelerates. Turn it around (maybe stop rotation first to avoid gyro effects) and switch thrust on.
Note that this doesn't simply double the fuel requirement. When you take the mass of ship, and the amount of energy it requires to get it to its destination, it comes out to X. When you then add on top of X the amount of fuel it would take to stop at the other (which is X), your original X becomes much muuuuuch larger. The amount of fuel spirals up rapidly.

eg. Say it takes 1000kT of fuel (a very conservative estimate) to accelerate your 100kT craft to its nominal speed for the journey - whatever that speed is. You must stop it at the other end, which will take the same 1000kT of fuel.

But now you have to go back to the start of your journey and add in that extra fuel you'll be carrying. You'll see that your craft is not 100kT, it's actually 1,100kT (of craft plus fuel). Your original 1000kT of fuel isn't enough. How much fuel will it take to accelerate your 1100kT craft to nominal speed? Well, 10x, so 11,000kT.

Your 100kT craft is now weighing in at 11,000kT - more than one hundred times its unfueled weight.

There are interstellar journey calculators out there that will help you with this.

Unless, of course, your story contains some sort of exotic propulsion where the mass of the fuel is largely irrelevant (ramscoop? zero point energy?).

It definitely isn't just rocket propulsion, it is some kind of nuclear propulsion. (I don't know how much that would affect those totals.)

It definitely isn't just rocket propulsion, it is some kind of nuclear propulsion.
That doesn't have to be mutually exclusive. Nuclear propulsion usually is rocket propulsion.

DaveC426913
Gold Member
It definitely isn't just rocket propulsion, it is some kind of nuclear propulsion. (I don't know how much that would affect those totals.)
Regardless of the motor, it needs fuel.

And my ballpark 10x fuel per unit payload is actually very generous. Nuclear propulsion would be far less efficient - like 100x. For efficiency, what you want is an antimatter drive, and even with that you're still talking 10x fuel to payload.

D'oh! I wish I could find that website that charted all this!

DaveC426913
Gold Member
Here's A resource, not sure if it's the one you're thinking of Dave,
http://www.desy.de/user/projects/Physics/Relativity/SR/rocket.html
Sortta. That will do. Thanks.

So my estimates were far too generous. See the charts right near the bottom of the page.

Even with a motor that is 100% efficient, converting all its fuel to thrust, a trip to A.Centauri, right in our backyard, requires a ship whose fuel masses 38 times its payload. And no motor is anywhere near 100% efficient....

Would carrying a LOT of compressed helium 3 or hydrogen along on the voyage to use for stopping work reasonably? (This is a generation ship, implying that it it HUGE.) The means of propulsion is a fusion drive of some sort though I'm not going to be too hard SF about that part.

Also, the ship is not going too incredibly fast, at least compared to some science fiction I have read. Definitely very much less than the speed of light. I'm putting 300-400 years as a rough estimate for the trip.

@Nabeshin Isn't that page regarding a ship that is constantly accelerating? (If that is the case would it even apply to the ship I am describing?)

Thought of another question: Could the ship be stopped in a short amount of time? (a few years?)

"Note that this doesn't simply double the fuel requirement. When you take the mass of ship, and the amount of energy it requires to get it to its destination, it comes out to X. When you then add on top of X the amount of fuel it would take to stop at the other (which is X), your original X becomes much muuuuuch larger. The amount of fuel spirals up rapidly."

Why?

(If the ship wasn't accelerating constantly at 1 G, but just coasting, would this still apply?)

Last edited:
DaveC426913
Gold Member
Also, the ship is not going too incredibly fast, at least compared to some science fiction I have read. Definitely very much less than the speed of light. I'm putting 300-400 years as a rough estimate for the trip.

@Nabeshin Isn't that page regarding a ship that is constantly accelerating? (If that is the case would it even apply to the ship I am describing?)
It still applies but to a lesser degree.

Thought of another question: Could the ship be stopped in a short amount of time? (a few years?)
It could decelerate as fast as it could decelerate. (No, that's not a typo.) It's up to you how fast you want it to decelerate.

But I ask you this: if you decided it could decelerate faster than it first accelerated - that means they did not have the engine operating at full power while under initial acceleration. Why not?

Presumably, they would want to accelerate at full power, since that will net them the time dilation advantage sooner. So the outcome is that they can only decelerate at the same rate.

Barring some sort of emergency. If you really needed them to decelerate faster, that would be a plot device.

"Note that this doesn't simply double the fuel requirement. When you take the mass of ship, and the amount of energy it requires to get it to its destination, it comes out to X. When you then add on top of X the amount of fuel it would take to stop at the other (which is X), your original X becomes much muuuuuch larger. The amount of fuel spirals up rapidly."

Why?
Th\e fuel required to stop at the end of your journey is dead-weight. It counts as payload, not as fuel, for the journey out. So, to accelerate on the outward-bound leg, you must account for the mass of the ship plus all the fuel you're carrying to the end of the trip. Essentially, your ship is 10 times (or 40 times) the mass you thought it was.

(If the ship wasn't accelerating constantly at 1 G, but just coasting, would this still apply?)
Yes, that would make a big difference.
If your ship is making a 100 light journey, but only accelerates for, say, five light years, coasts for 90, then decelerates for 5 light years, then you only need enough fuel equivalent to a journey of 10 light years, not 100. Fuel-wise, the coasting leg of the journey is free, yes. (Though you'll still need power to keep life support going.)

Ryan_m_b
Staff Emeritus
Question: Have you thought about the other requirements of this vehicle besides propulsion? Often I see (especially here) budding SF authors spending large amounts of time trying to work out how the propulsion will work without taking any thought to questions like how will the ship contain a stable ecology, how will the ship contain a full industrial complex, how will the ship maintain a specialised workforce of tens of millions, how will the society be set up so as to be stable for long time periods? This are totally non-trivial ideas.

I think a lot of problem with modern SF comes because of the pervasive use of the word "ship" which is a false analogy. Ships travel from hospitable area to hospitable area through a hospitable area with enough supplies to make it. Some sort of colony sent through space is going to have to be the equivalent of an island nation rolled up and given a nudge. If you want to read more along these lines author Charles Stross has given it a lot of attention.
http://www.antipope.org/charlie/blog-static/2007/06/the-high-frontier-redux.html
http://www.antipope.org/charlie/blog-static/2010/07/insufficient-data.html
http://www.antipope.org/charlie/blog-static/2010/07/mediocrity.html
http://www.antipope.org/charlie/blog-static/2011/10/trick-question.html

Last edited:
Note that this doesn't simply double the fuel requirement. When you take the mass of ship, and the amount of energy it requires to get it to its destination, it comes out to X. When you then add on top of X the amount of fuel it would take to stop at the other (which is X), your original X becomes much muuuuuch larger. The amount of fuel spirals up rapidly.

eg. Say it takes 1000kT of fuel (a very conservative estimate) to accelerate your 100kT craft to its nominal speed for the journey - whatever that speed is. You must stop it at the other end, which will take the same 1000kT of fuel.

But now you have to go back to the start of your journey and add in that extra fuel you'll be carrying. You'll see that your craft is not 100kT, it's actually 1,100kT (of craft plus fuel). Your original 1000kT of fuel isn't enough. How much fuel will it take to accelerate your 1100kT craft to nominal speed? Well, 10x, so 11,000kT.

Your 100kT craft is now weighing in at 11,000kT - more than one hundred times its unfueled weight.

There are interstellar journey calculators out there that will help you with this.

Unless, of course, your story contains some sort of exotic propulsion where the mass of the fuel is largely irrelevant (ramscoop? zero point energy?).
eg. Say it takes 1000kT of fuel (a very conservative estimate) to accelerate your 100kT craft to its nominal speed for the journey - whatever that speed is. You must stop it at the other end, which will take the same 1000kT of fuel.

--- But the ship now weighs less. Also, you would more likely go into orbit than come to a dead stop.

DaveC426913
Gold Member
--- But the ship now weighs less.
True. That does affect the numbers.

Also, you would more likely go into orbit than come to a dead stop.
At the velocities we're talking about for the journey, orbit is a dead stop.

10,000+ miles per second down to 5 miles per second is pretty much the same as 10,000+ miles per second down to 0.

Last edited:
Don't forget that you can build the ship in space, if you don't plan to land it you can make it from spheres rotating for example, and the acceleration you use can be built up slowly over a extended period of time, so you don't need to consider the constructible integrity that much.

What you need to consider though is pressure and radiation, the faster you go the more hard radiation will hit your assemble. But you can use very light materials (nano engineered maybe with some metal to shield? Or whatever that suits your imagination, something from the moon?) and build a lot of rotating spheres, interconnected through some framework. You could alternatively use electromagnetic fields to lock them in place, maybe?

It's a really difficult idea, spaceships. We have one, it's called Earth.