Another space question - propulsion this time

I just found the show on iTunes, so I downloaded a season, but... I'll maybe get the books, too! Yay!

And, no, I really DON'T want to get into hard scifi! I just want my soft scifi to have a framework that's satisfying to me. Probably won't even make it into the books.

 

Then I think The Expanse is a good fit as an example. Enough realism to make you feel that realism is in play without finding yourself in a physics lecture hall, boring you with esoteric in-group details that only in-group members would actually care about.

 

The setting sounds perfect - very similar to what I'm planning. Now I just have to worry about being derivative...

 

In that case I have a much better and more entertaining suggestion. The Alex Benedict books, by Jack McDevitt are just plain fun! It's the only series of books, besides Harry Potter, that I've read every one. To be sure, they'll not tax your brain, but they are well done with some nifty mysteries to solve.

 

@BayView


That’s a good question. Unfortunately, a little physics is needed. Your opening point is wrong unfortunately regarding jets. Using jet engines will also confuse... Sorry for the length but hope it helps.

http://www.isaacarthur.net/

this guy is very good. No matter who you ask you need to read these three first and it applies to everything on earth or in space...


Newton's First Law:

A body will remain at rest or in motion in a straight line unless acted upon by a force.

Newton's Second Law:

Change in motion is proportional to the applied force and parallel to it.

Newton's Third Law:

To every action there is an equal and opposite reaction.


By Newton’s first law standing still or coasting along is very cheap fuel cost wise. However, sitting still is a little hard to define - do I mean compared with the Sun, or Mars or the Milky Way or Universe? Most of the time we are in an orbit of some sort, so sitting still doesn't really make sense. What we need to think about is how to change from one kind of orbit to another kind of orbit.

Newton’s Third Law is how propulsion works. If A exerts a force on B, then B exerts an equal and opposite force on A. Or in the case of a spaceship, if a mass(m) of fuel is pushed out of the exhaust exit, then the spaceship will accelerate in the opposite direction to the direction the fuel went.

Important now... when the mass (m) of fuel is exploding out of the exhaust exit it exerts its force opposite onto the spaceship pushing it forward. The ship accelerates in speed. Once the fuel is exhausted then our spaceship will return to Newton’s first law and remain at that increased speed.

Your Jet View...

This is Pushing against something... Explode some fuel in a sealed box. What happens? Force will be exerted in all directions pushing against the walls. Now puncture a whole in one end, say the left side and do the same thing. The box will fly right and you’ll see flames coming from the left side hole. The fuel is pushing against the box not any external force. Does this make sense?

Question:

I’m on the International Space Station just free floating not moving. Then I fire a Hand-Gun at my Russian college. What do you think happens next? Think about it before reading on...



You first off are in an orbit, but it looks stationary a float inside the ISS? Newton’s first Law. The ISS is an Oxygen rich environment so as the hammer of the gun strikes, it ignites the gunpowder inside the bullet’s cartridge. The powders reaction turns it into an expanding gas inside the tight cartridge space pushing against the walls.

The weakest wall point is where the cartridge is mated to the bullet. Mating breaks forcing the out towards my Russian college (side effect, along with the bullet). Force is applied in the opposite direction i.e. towards you...

You under Newton’s first law, now turn to his third law and are accelerated backwards. To then change back to the first law (as all the gas is exhausted from the cartridge) with new constant higher speed. Not slowing down...



Regarding Story Writing...

I’ve just started on this writing thing so know nothing compared to you. However, I noticed even short or simple sci-fi fiction comes with heavy criticism if the physics is slightly wobbly. This due to a wider understanding by the general public. In fact, Stars Wars if released now is laughable and wouldn’t achieve the same standing.

The Expanse, you should look at, The Martian also. Gravity inside of and the shapes of ships... OR invent a magic FTL drive wormhole thing. I started to watch Origin a new Sci-Fi tv show on YouTube...

https://en.wikipedia.org/wiki/Origin_(TV_series)

Only few years back, this would be a smash it. I just couldn’t finish the first series due to bad flaws in the physics. Story, acting and CGI all great but I need/demand better. Amazon and Netflix turned it down, but not sure why. Try the free episode and see if you can spot them...

Anyway, sorry for the length but hope it explains your question...



Norfolk nChance

 

I didn’t get into it until lots of episodes in—Himself was watching it and I was “Another episode of that thing? Well, I can be polite as long as he’s not binge watching it.” Then I was suddenly fascinated, we were out of episodes, and I demanded that we re-watch it from the beginning. So I experienced most of it as a sort of prequel.

 

I'm on the fourth episode, but my sucky internet is making it VERY frustrating...

Still, I'm enjoying it. I want the storylines to combine, though... I want them all in one place, sometime soon.

 

@BayView - You might want to get hold of this book (and there are others in the series as well.) It sounds as if it's just what you need. It's a sci-fi guide (written by one of the best sci-fi writers, Ben Bova) for writers who need to understand how the principles work. I used to own three of the books in the series (and really enjoyed them) but I passed them on, as I have no intention of writing sci-fi myself. I do love to read sci-fi, though, and these books were fun AND informative. Just enough science to keep you right, but not enough to bamboozle you.

https://www.amazon.co.uk/Space-Travel-Science-Fiction-Writers/dp/1542584906

Some people think it's out of date, some people think some of the information is wrong, but others think it's fine ...so take some of it with a grain of salt. However, Ben Bova IS one of the big names in Sci-fi writing, so he must know something....

 

Should I give the series another chance or jump into the books first?

 

Others may have an opinion. I haven't read the books.

 

To answer both in one go...

The way the books present the different characters and their arcs is rather different to the way the show does. Book 1, Leviathan Wakes, is almost solely the story that take place aboard the Canterbury (The Cant) and the characters that come from that ship. In book form, you don't even meet Chrisjen Avasarala or GySgt. Bobby Draper until book 2, Caliban's War.

I love the show and the books equally for different reasons. I love that Bobby and Chrisjen are presented sooner and are part of the story for much longer in the show than in the books. I love the books for other reasons, to include the much ballsier character that Chrisjen is in written form, and I fell in love with Amos Burton the way he was written more than the way you see him in the show. Show Amos is waaaaayyyy too pretty for my taste and as a representation of who he is in written form. Give me big bear book Amos any day.

ETA: Also, a huge swath of Naomi Nagata's story is not present in the show. Not sure why they clipped her back so heavily, but her whole OPA story is missing in the show.

 

There is a bunch of incorrect stuff in this thread.

First, rockets do not push against anything. They do not require air, dark matter, or quantum foam. They will work in a perfect vaccuum.

They also aren’t being pushed by the expanding gas behind them.

They are simply capitalizing on the third law of thermodynamics dynamics. If you are in space (or any frictionless state) and you throw something in one direction, that force of the throw is split in half. Half of the energy goes to moving the ball away from you and the other half goes to moving you away from the ball. This is what rockets do but instead of balls, it’s molecukes of fuel. For every molucule the rocket pushes out, half the energy pushes the rocket forwards.

 

Well...several of us already pointed to the Third Law, so....

 

I know but there are always misconceptions about what it really means.

 

I'm BACK.

I'm trying to figure out what distances/speeds are reasonable.

I've more-or-less settled on 5% the speed of light for my fusion-powered ships. There would be military ships out there that could go faster, but my guys aren't super high-powered.

So... can someone check my math on this?

5% the speed of light would be 15 000 km/second (according to Google's lovely calculator)
but acceleration and deceleration would be important to consider
apparently most humans can handle 5G if wearing the right clothes, properly strapped in, etc... and I think 5G would come from about 50 m/second squared of acceleration.

So... that would mean that to go from a standstill to 15 000 km/second, accelerating at 50 m/second squared... convert 15 000 km to 15 000 000 metres, then divide by 50? so it'd take 300 000 seconds to reach maximum speed? And that's 84 hours?

And it would be reasonable to slow down at the same rate?

But someone probably couldn't withstand 5G for 84 hours straight, right?

Say they could, though... to travel from Earth to Neptune, it's about 4 500 000 000 km... if you were already at max speed, it would take 83 hours to get there, but really you'd have to spend most of your time accelerating and decelerating...

I want to keep my people within the solar system. I wonder if it'd be simpler to have them go slower. Like, have the maximum speed be way less, and not worry as much about acceleration and deceleration...

Is my math on this making ANY sense?

 

Did you ever read “The Forever War”?

They would travel near light speed, but accelerate constantly (I might have stolen this for my sci-fi novel).

Instead of rotating, the floor of the ship lines up with the thrusters. You accelerate half way, flip the ship over, and slowdown half way. Mars is like 4 days away at 1G.

It was called the forever war because time and space contract as you continue to accelerate to the speed of light. The crew would experience like 5 years while the earth experienced 101 during their 100 light year journey.

Military ships would maintain 2G exceleration when flying to emergencies in the solar system, giving the crew constant injuries from tripping and breaking limbs, and panic attacks waking up unable to breath.

Even better, no spinning.

One of the problems with nuclear power is that the escaping high energy neutrinos degrade your ship, but you can conceptualize (sp?) engines that eject even the neutrinos without touching the ship, so the ship remains intact.

 

300k seconds is 83.33333333- and on Hours or 83.4 hours just to make things simpler.

I am sure you could survive 5 G for that long, but you are not going to feel all that great.
While it would take a bit longer to get anywhere try accelerating at 1-2 Gs over a longer
period of time, so as the travelers don't notice the effects during accel/deccel.

Once you get up to speed, you will have to maintain it with micro bursts along the way,
as your ship will slow down over time due to debris impacts, drag, and gravitational
forces pulling/pushing on the ship along the way.

Though you can kinda save on fuel during slowing down, by using Saturn's gravity well
to slow you down, provided it is in the same general area as your travel vector.

However, you could technically move your speed up to 10% C and get there a bit faster.
And if your society has fusion tech, then they might have a way of launching from orbit
by means of a space elevator or an orbital ring, thus saving on fighting gravity and the
atmosphere before you get up into space. And perhaps even a way to scoop up hydrogen
en-route to maintain fuel levels for your fusion reactor. Though you have to consider how
you want to insulate your ship from radiation from both the reactor and from the space
outside. You can do this by using lead or water inside the walls of your ship. Though you
could in theory just get more water from space ice so you don't have to worry about stocking
up on it in immense quantities before you leave, and can use it to both protect your passengers
from radiation, and keep them hydrated.

Consider the formula F=ma (force = mass x acceleration).
So the less you mass you have the easier you can move your ship
up to faster velocities in a shorter amount of time. So a ship half
the size of the space shuttle can go twice the speed of the space
shuttle under the same amount of accel. as the larger shuttle.

So as long as your ship is lighter than it looks, you could go virtually anywhere
in the solar system if it is made of a light and sturdy material to keep the weight
of it down, but strong enough to survive the hazards of space. Perhaps lots of
carbon fiber and titanium would be your best bet on keeping the mass down
to a manageable level to move it around easier.

Neptune is 17 times thr mass of Earth, so I assume you won't be landing on
it since it is a ball of gas.

 

Oh god, you need to scale things way back. G force is basically a multiplier for how much you weigh. A fighter pilot can take 4-5 seconds of 5gs (vertical, so pulling down from the head) without passing out:

A top fuel dragster:



is averaging 4gs, peaking near 6g, over the course of a few seconds. A gentleman never asks nor speculates on a lady's weight, but if someone like Vicky Vale in the 1989 Batman movie, who weighed somewhat over 108 pounds, were to be in a spaceship traveling at 5g, she'd clock in at over 540lbs.



Her heart would explode even if she were lying down in some sort of a waterbed.

This site has what looks like some pretty good answers on 1g travel around the solar system. To Mars, accelerating at 1g continuous, then flipping the spaceship around and "putting on the brakes" at 1g comes up to just under two days at Mars' closest approach to Earth. Neptune clocks in around 15 days.

The thing is, you're going to need to do some serious propulsion handwaving now. I don't think we have anything close to generating 1g of propulsion for anything of size over a sustained period. The closest thing I know of is the old Project Orion, which (vastly oversimplified explanation) involved tossing a nuclear bomb out the back end of the spaceship and closing the door. Once the blast has got you moving, do it again, lather rinse repeat. Obviously there are issues with this approach.

Anytime you start putting too many facts in your SF you either end up scrapping your story or turning it into The Martian, where the facts become the primary antagonist.

 

Hi,

Agree with the G force issue. In essence your body is designed to live and work in one G. Every tiny bit you increase your g force above one, your body starts to suffer, And it's not just falls that you have to worry about. Organs are being squashed including your brain, heart and lungs. Blood isn't flowing so easily which means the heart has to pump harder to keep you going - this is why people black out at three or four Gs. Tissues are being damaged continuously at a higher rate than normal, which means excretion of wastes - kidneys / liver etc, have to work harder, and you get inflammatory responses building up in the body - that includes blood clotting which is why the risk of stroke goes up. You can compensate with pressure suits and tanks of liquid to float in, but only to an extent and for a short while.

As for your time to reach 5% ignoring relativity, the basic calculation is that light travels at 330,000,000 m/sec, and in a one G environment you accelerate at 9.8 m/sec (call it ten) which means to reach the speed of light requires 33,000,000 secs or 388 days. 5% requires one twentieth of that time or about nineteen days.

Hope that helps,

Cheers, Greg.

 

Technically we are all moving at the speed of light, relative the particle horizon.

 

Unless you're writing hard scifi, you can also just hand-wave stuff like this. I give my starships inertial compensators which allow them to accelerate much faster than the human body would be able to survive. Some of the poorer worlds use ones that have crummy efficiency, so they can only change acceleration so quickly. If the compensator fails, everyone turns into goo instantly, but that's just something else for the heroes to be worried about.

Some other hacks to use:

• Genetically engineered humans may be able to survive higher acceleration
• Humans who use special drugs and crash couches may be able to survive higher acceleration. See: The Expanse
• If you accelerate slowly, then you'll be fine. Acceleration is just change in velocity. Once you're already moving, that motion is free. You'll just keep on moving in space, and won't feel a thing.
• Hyperdrives. Who says the ships even have to move that fast? They can just jump between destinations.
• Warpdrives. The ship itself isn't moving, but the warp bubble around it is.

 

Or slingshot around grav-wells. Blood chem for solid suspension. Or something like Bear's Anvil of Stars

 

I've been toying around with a combo hand-wave and galactic speed/rotation, whereby dropping out of a galactic plane results in time manipulation. Like removing yourself from all equations. Then there's nospeed and nospace, and you pop into whatever destination you want instantaneously. "Helmsman, give me full nospeed!" "Entering nospace now, sir."
Just don't screw up the math, or you'll be a billion years early to the party!

 

I read some SF book ages ago that had an Instantaneous Drive, but both ends of the journey had to take place in very gravitationally "flat" space. As a result, a trip from Earth to Alpha Centauri could take a couple years, but in the form of Earth to the Oort Cloud: 1 year, Oort Cloud to Alpha Centauri's equivalent: 1 unit of Planck time, and inwards to the habitable planets using conventional physics, another year (or whatever). I found the concept intriguing.

 

I say thee, yay. The notion is all about turning theory inside out, which is what Einstein, and others I'm surely ignorant of, did to their last day.
Of course, "Ooops, we screwed up" is a fun adventure, too.