[HN Gopher] Emergency braking in space
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Emergency braking in space
Author : sklargh
Score : 47 points
Date : 2022-06-07 13:19 UTC (9 hours ago)
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| KineticLensman wrote:
| 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 that we didn't
| detect before launch and didn't otherwise consider) really don't
| stand up very well.
| jrlowe wrote:
| 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!
| pavel_lishin wrote:
| 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
| choosing to abandon the rings, and adapt to a week of high-g,
| followed by a long low-g cruise period.
| Someone wrote:
| If power requirements as a function of
| acceleration/deceleration rate are above linear, it might be
| more economical to accelerate slowly, especially.
|
| Also, the faster acceleration/deceleration might be less
| comfortable, destroy cargo, wear out the engine faster, etc.
| gridspy wrote:
| 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).
| amelius wrote:
| They might want to consider diamagnetic levitation to counter
| g-forces:
|
| https://www.ru.nl/hfml/research/levitation-explained/diamagn...
| nickt wrote:
| The relativistic rocket calculator linked in the comments is
| quite interesting.
|
| https://gregsspacecalculations.blogspot.com/p/blog-page.html...
| franky47 wrote:
| One point is left out from this calculation: what's the reference
| point we're at a "full stop" from?
|
| Aren't you always orbiting something when in space (however large
| or distant)? If you were to kill your orbital (lateral) velocity,
| you'd only be gaining radial velocity by being pulled towards the
| orbiting body, some form of thrust would be needed to compensate
| that.
|
| TL;DR: can you really be "at a full stop" in space?
| 323 wrote:
| You could consider the start point (roughly) as the reference
| point. Or the destination point.
| falcolas wrote:
| A very good point. Even if you're at a "full stop" in the frame
| of reference of the average of the local stellar bodies, those
| dust particles (asteroids, planetoids, etc) the deflector is
| used for aren't necessarily.
| ericbarrett wrote:
| I think it would be impossible for humans to walk around at more
| than ~1.5 g of acceleration for extended periods of time, and
| even that is asking a lot.
|
| Take the g-force and multiply it by your body weight. That would
| be how heavy you feel when standing. If you are 70 kg (154 lbs)
| and under 1.5g of acceleration that is an extra 35 kg (77 lbs) of
| weight, which is about what we ask a modern soldier to carry. But
| the soldier gets to set their pack down when they rest, and the
| weight isn't applied to their internal organs. Perhaps gradual
| introduction of the acceleration over weeks would allow people to
| build conditioning, if all the crew is young and very fit.
|
| Go higher and it gets even less plausible. 2.0g is like carrying
| your twin. Surely this is impossible to sustain for more than an
| hour or two without some kind of acceleration couch--setting
| cardiac health aside entirely--and injury would be very likely if
| you were active.
| ghusbands wrote:
| You could discount a lot of extra weight over time
| (physiological adaptation), especially as you're not carrying
| it in your arms or over your shoulders, but tripping up would
| suddenly become a lot more fatal. Everyone's reactions and
| instincts are tuned for 1g.
| kqr wrote:
| To some extent there's a more direct comparison: a pregnant
| woman gains somewhere around 10--20 % bodyweight. This cannot
| be set down and it applies pressure to internal organs.
|
| I can't imagine making it 50 %!
| ginko wrote:
| >Go higher and it gets even less plausible. 2.0g is like
| carrying your twin.
|
| FWIW there are people who weigh well beyond 150kg so I'd argue
| it would be plausible. Will people be able to perform at peak
| physical level? No. Will they probably manage for a couple of
| days? I'd say so.
| TremendousJudge wrote:
| It's not the same though. When at higher gravity, you're not
| carrying more weight like a backpack or like extra fat --
| your regular tissues weigh more. This includes your blood and
| other fluids, but your heart is still the same strength. I'd
| expect that to make a difference.
| Johnny555 wrote:
| Wall-E found a solution this problem - people don't need to
| walk, they have mobile beds to move around.
|
| https://www.youtube.com/watch?v=s-kdRdzxdZQ
| hammock wrote:
| 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.
| nradov wrote:
| Athletes already use compression boots which apply pulsed
| pressure to the lower legs as a sports recovery modality.
| But those are only used for maybe an hour at a time. I
| doubt whether they would compensate for the physiological
| stress of sustained high acceleration.
| KineticLensman wrote:
| Then you have to toughen up the internal tissues so that
| blood vessels aren't ruptured by high-pressure blood
| (which can cause strokes, permanent vision loss if
| retinal arteries are affected, etc). High blood pressure
| also affects kidney and liver function, amongst other
| things.
| tablespoon wrote:
| > 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 depending on the person.
| marcosdumay wrote:
| Yes, but that tolerance is for seconds of exposition, not
| days.
| sneak wrote:
| > _FWIW there are people who weigh well beyond 150kg so I 'd
| argue it would be plausible._
|
| Those people have been training for months/years to carry
| that weight.
| foxyv wrote:
| You are completely right. I guess 15-30 minutes or so is the
| limit unless there is some new technology that can allow for
| long term gravity exposure.
|
| https://www.newscientist.com/article/dn2076-hypergravity-exp...
|
| > "The experiment will not progress very far, because loads of
| 1.5 to 2G can only be tolerated for about 15 minutes and even
| then it severely impacts on sensory systems, like balance,"
| Elmann-Larsen told New Scientist. "People can withstand forces
| of even 3.5G, but the time length is absolutely crucial."
| jimbokun wrote:
| In this book, a space flight to Alpha Centauri uses an asteroid
| which is constantly accelerated at 1g for half the flight, then
| constantly decelerates at 1g for the second half of the trip:
|
| https://en.wikipedia.org/wiki/The_Sparrow_(novel)
|
| That eliminates need for "artificial gravity" and really drove
| home to me that 1g is really the limiting factor for long range
| human space flight.
| throwaway09223 wrote:
| 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.
| jefftk wrote:
| _> 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?
| scrumbledober wrote:
| assuming constant acceleration it kinda does, but constant
| acceleration requires asymptotic infinite amounts of
| energy.
| lrem wrote:
| Yup, it would take 6 years instead. Ship time ;)
|
| Source: https://gregsspacecalculations.blogspot.com/p/blog-
| page.html... and ignoring all the other fun aspects.
| stickfigure wrote:
| It would have been better phrased "at which point you can't
| go any faster relative to the departure location".
| dhc02 wrote:
| Love that book.
| jimbokun wrote:
| I love it and hate it. Beautiful and brilliant, and combines
| religion with sci-fi without demonizing the religious
| characters.
|
| But events later in the book, while completely consistent
| with the premise, were pretty gut wrenching to read.
| verisimi wrote:
| How do you brake in a vacuum?
|
| There's no air resistance, nothing to create friction.
| dreamcompiler wrote:
| You use thrust to decelerate. Same thing you use to accelerate.
| avhon1 wrote:
| 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
| hallway_monitor wrote:
| 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.
| Rastonbury wrote:
| Flip the ship around and thrust
| EddieDante wrote:
| The phrase "raspberry jam delta-v" comes to mind.
|
| Also, the obligatory clip from _Spaceballs_...
|
| https://www.youtube.com/watch?v=NAWL8ejf2nM
| xeromal wrote:
| This specific moment where he still makes the noise after
| having the microphone smacked from him makes me laugh the same
| as it did when I was a tiny kid.
|
| https://youtu.be/rGvblGCD7qM?t=95
| EddieDante wrote:
| Michael Winslow is one of the greats.
| timbit42 wrote:
| He was on AGT a few years ago.
| paulsutter wrote:
| >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
| jcrawfordor wrote:
| This really all stems from a misunderstanding of what "full
| stop" or "all stop" (all engines, for vessels that commonly
| operate only some engines for cruising) means. "full stop" is a
| position on the engine telegraph that tells engineering to stop
| the engine or take it out of gear, depending on the type of
| propulsion setup. It has nothing to do with the speed of the
| ship relative to anything, after the "full stop" order is given
| the ship will continue to move forwards by momentum. If the
| captain actually wants to stop as soon as possible they will
| order "full astern," which signals engineering to run the
| engine in reverse at normal full speed, effectively braking.
| Because oceangoing ships stop on their own reasonably quickly
| this is usually more of an emergency maneuver.
|
| The fact that motion is relative is already quite true at sea
| in our own world, where in at-sea operations your position
| relative to other vessels can matter much more than your
| position relative to the earth. In other words this issue is
| not at all new or specific to space. More basically, though,
| today and presumably centuries into the future "full stop" is
| not an order to stop the ship, it's an order to stop the
| engine.
|
| The order is "full stop" because large marine engines are
| traditionally directly coupled to the propshaft and cannot
| "idle" per se. On these types of systems, still common on large
| vessels, there is some nuance depending on the engine setup
| between "standby," "stop," and "finished" which are
| traditionally all positions on the engine telegraph that do
| more or less the same thing but give different instructions to
| the engineer operating the engine as far as preparations for
| the near future. On top of this most ships today the "engine
| telegraph" is not really used when underway and the telegraph
| sender on the bridge actually controls the engine directly via
| automation, but usually this only allows for speed changes and
| not stopping or reversing, which still requires that
| engineering take over engine control due to the preparations
| and checks that must be done when stopping and starting the
| engine. Rather than telegraph bells this is more likely to be a
| phone call these days.
| nomel wrote:
| 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.
| paulsutter wrote:
| So full stop means either go to geosynchronous orbit or fall
| to the surface?
| [deleted]
| spuz wrote:
| 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.
| i_like_apis wrote:
| In Star Trek they had "inertial dampeners".
| amelius wrote:
| > Can humans withstand 4 g non-stop for 9 days?
|
| They might if you keep rotating them, so the force is not along
| one axis all the time.
| jermaustin1 wrote:
| Seems like that would cause terrible motion sickness. Rotating
| would cause centripetal force, so another vector of
| acceleration on your body PLUS a constantly changing vector of
| acceleration due to deceleration.
| tantalor wrote:
| I imagine amelius intended a regular "flip" like a
| hospitalized patient to avoid bedsores, not constant
| rotation.
| sydthrowaway wrote:
| WHen are we going to get a new Einstein to sort all this out.
|
| Surely, we can start putting some neural enhancers in baby
| formula.
| throwaway09223 wrote:
| 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.
| Mister_Snuggles wrote:
| 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).
| ghusbands wrote:
| It could just flip much more slowly and maintain ~1g, with
| minimal course correction.
| joncrocks wrote:
| 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.
| ghusbands wrote:
| 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.
| satori99 wrote:
| 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.
| rtkwe wrote:
| 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.
| syndic8_xyz wrote:
| 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?
| ryandrake wrote:
| 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.
| bena wrote:
| 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.
| FredPret wrote:
| Sounds like all engineering! On a grander scale than usual
| though
| dhosek wrote:
| 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.
| mateo1 wrote:
| 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.
| hinkley wrote:
| 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...
| titanomachy wrote:
| 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.
| [deleted]
| hinkley wrote:
| 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).
| rsynnott wrote:
| Requires a magic-based energy source; the usual problem with
| this sort of thing.
| tomrod wrote:
| Why would an ion thruster engine not be feasible?
| throwaway09223 wrote:
| 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.
| hinkley wrote:
| ELI5: Ion engines are the slowest tortoise you can
| imagine and to feel like gravity you need the hare.
| pavel_lishin wrote:
| > _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.
| luis8 wrote:
| What about using a long ship and then using a high tech spring to
| apply the deceleration over the length of it. Relocate the
| passenger at the front and repeat until you reached the desired
| speed.
|
| Just thinking out loud maybe that will be counterproductive in
| the amount of time needed
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