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Emergency braking in space

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57 pointssklargh88 comments

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.

>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.

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.

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.

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.

> 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.

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

This is akin to the landing impact when you jump from something low enough that you won't die from it. You wouldn't survive it in a sustained manner, but the brief jolt, while maybe causing your innards to ache, is tolerated.

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.

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

> 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.

Indeed. It's also not pulling down with that same weight on each of their internal organs.

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.

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.”

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 %!

Acceleration-added weight is carried better than a backpack. It's better distributed, the same way the rest of your normal weight is distributed, just more of it.

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.

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

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.

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.

The relativistic rocket calculator linked in the comments is quite interesting.

https://gregsspacecalculations.blogspot.com/p/blog-page.html...

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.

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!

In Star Trek they had "inertial dampeners".

From Memory Alpha:

'According to the Star Trek Encyclopedia (2nd ed., p. 205), inertial dampers were "invented" by Star Trek writers to explain how the crew avoided becoming "chunky salsa" when starships accelerated or decelerated. ' [1]

and further list them as a requirement to achieve warp. I would imagine that "merely" instantly decelerating from 0.25c to a dead stop would be trivial in comparison.

[1] https://memory-alpha.fandom.com/wiki/Inertial_damping_system

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.

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.

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).

They might want to consider diamagnetic levitation to counter g-forces:

https://www.ru.nl/hfml/research/levitation-explained/diamagn...

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.

Love that book.

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.

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?

assuming constant acceleration it kinda does, but constant acceleration requires asymptotic infinite amounts of energy.

As measured relative to the destination, yes, but if so then that means the passengers get crushed under infinite gs.

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

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.

That’s just what it shows after rounding up. The site correctly doesn’t let you enter 1c for velocity since it’s not possible.

He mentions Expanse and yet not the smart "skyscraper" design of it's ships? Where the engines push from the "bottom" (decks-wise), not from the back as in a literal seaship. With that arrangement, the issues of "back wall is suddenly the floor" disappear.

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

This reminds me of the device used in Jules Vernes " From the Earth to the Moon". The projectile has "thick lining of leather fastened on springs of best steel", together with some sort of hydraulic system, meant to absorb the shock of shot.

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?

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.

You could consider the start point (roughly) as the reference point. Or the destination point.

The problem of deceleration was already „solved“ in the classic movie Forbidden Planet from 1956: https://youtu.be/78nbTCgkYn4

The phrase "raspberry jam delta-v" comes to mind.

Also, the obligatory clip from Spaceballs...

https://www.youtube.com/watch?v=NAWL8ejf2nM

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

Michael Winslow is one of the greats.

He was on AGT a few years ago.

> 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.

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.

I imagine amelius intended a regular "flip" like a hospitalized patient to avoid bedsores, not constant rotation.

Immersed in liquid would be the way. Internal organs would still have to bear the load, but your skeleton and musculature could be spared.

How do you brake in a vacuum?

There's no air resistance, nothing to create friction.

You use thrust to decelerate. Same thing you use to accelerate.

Flip the ship around and thrust

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.

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.

>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.

So full stop means either go to geosynchronous orbit or fall to the surface?

Since human occupied spaceships are never meant to fall from space, except for the illegal suicide vessels found near Vermiticus 4, I think the practical answer would be geosynchronous orbit.

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.

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.

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