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Emergency braking in space

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Re: Emergency braking in space

#51

Post by ryandrake »

_bz1e wrote: post #13
throwaway09223 wrote: post #7

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.

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?

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.

Re: Emergency braking in space

#52

Post by hallway_monitor »

avhon1 wrote: post #50
verisimi wrote: post #37

How do you brake in a vacuum? There's no air resistance, nothing to create friction.

It's not a perfect vacuum out there, there is an interstellar medium. You could create non-negligible drag with a huge sail or magnetic field, like a Bussard Ramjet. https://en.wikipedia.org/wiki/Bussard_ramjet

I was looking for this as I have always seen the ramjet as the only feasible possibility for non-magical long-term propulsion. Of course that's if we don't find a way to trick the universe into FTL travel.

Re: Emergency braking in space

#53

Post by mateo1 »

dhosek wrote: post #14
throwaway09223 wrote: post #7

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.

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 pla…

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.

Re: Emergency braking in space

#54

Post by paulsutter »

>The two most unscientific words in Star Trek are probably "full stop"

Such a promising article, and then they completely miss the mark

THERE IS NO SUCH THING AS A FULL STOP IN SPACE. There is no frame of refernece to stop against. It's completely meaningless, and has nothing to do with limitations of deceleration

Re: Emergency braking in space

#55

Post by marcosdumay »

tablespoon wrote: post #36
hammock wrote: post #30

Earlier quoted context omitted.

That’s a really interesting point. I wonder if there are “artificial exohearts” or something that we could install on the extremities to keep vital body fluids like blood and lymph flowing when the heart is not strong enough.

> That’s a really interesting point. I wonder if there are “artificial exohearts” or something that we could install on the extremities to keep vital body fluids like blood and lymph flowing when the heart is not strong enough. Isn't that basically a g-suit, like fighter pilots already wear? https://en.wikipedia.org/wiki/G-suit Also, from that page: > The resting g-tolerance of a typical person is anywhere from 3–5 g…

Yes, but that tolerance is for seconds of exposition, not days.

Re: Emergency braking in space

#56

Post by nomel »

paulsutter wrote: post #54

>The two most unscientific words in Star Trek are probably "full stop" Such a promising article, and then they completely miss the mark THERE IS NO SUCH THING AS A FULL STOP IN SPACE. There is no frame of refernece to stop against. It's completely meaningless, and has nothing to do with limitations of deceleration

Oh, that’s silly, for a practical context, which flying a spaceship would be. Practically, it would be relative to the nearest large body/galaxy/pair, and depend on context, which would almost always be understood. For example, if you’re observing a planet, full stop would be relative to that planet.

Re: Emergency braking in space

#57

Post by spuz »

paulsutter wrote: post #54

>The two most unscientific words in Star Trek are probably "full stop" Such a promising article, and then they completely miss the mark THERE IS NO SUCH THING AS A FULL STOP IN SPACE. There is no frame of refernece to stop against. It's completely meaningless, and has nothing to do with limitations of deceleration

Presumably at those speeds, the frame of reference would be the galaxy and the interstellar material in which the ship travels. If your ship were to match the average speed of this material, then you could claim to be at a stop. If the concern was to prevent damage due to collision with this material, then it makes sense that in order to "stop" you would actually have to match the speed of the material that you are moving through.

Re: Emergency braking in space

#58

Post by gridspy »

pavel_lishin wrote: post #4

I wonder what sort of spaceship would be capable of emergency deceleration at 4g for nine days, but would choose a leisurely 0.1g cruise to get up to .1c over the course of over a year instead. edit: actually, what sort of rotational rings would withstand that 4g load? Surely they'd break apart. I guess that might be part of the fun and exciting plot - the rush to move vital equipment into the core of the ship, and c…

That, or when the ship was built with engines capable of delivering 4G they designed the rings to handle it.

You could tether the rings to the outside of the ship (towards the nose) like a suspension bridge to support them during high thrust periods. Perhaps spinning the rings requires the tethers to be removed, or perhaps the rings must not be spinning while the engine thrust above 0.5G (because high trust locks the bearings of the rings).

Re: Emergency braking in space

#59

Post by jrlowe »

KineticLensman wrote: post #32

I think this illustrates how hard it is to write SF plots involving 'mundane' interstellar travel (no warp drives or wormholes) that make approximate sense in terms of the laws of physics but which also reflect the enormous energy required and the sheer complexity of the task. Going to a different star is nothing like going to the moon but with a much larger Apollo. Simple plot devices (there's something in the way t…

Yes, I completely appreciate the handwaving that sci-fi authors regularly do to avoid having to switch to the character's great-greatX1000 grandchild upon arrival to Alpha Centauri....on page 2 of 500!

Re: Emergency braking in space

#60

Post by stickfigure »

jefftk wrote: post #26
throwaway09223 wrote: post #25

Earlier quoted context omitted.

1g isn't all that limiting. Under 1g acceleration you'll reach lightspeed in about a year, at which point you can't go any faster. A year to reach the top speed of the universe isn't so bad. The truly limiting factor is that we don't have engines that can produce 1g for a sustained period. We can't carry and propel enough reaction mass and reactionless drives only exist in science fiction.

> you'll reach lightspeed in about a year, at which point you can't go any faster I don't think that's how relativity works?

It would have been better phrased "at which point you can't go any faster relative to the departure location".
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