Artificial Gravity General Thread (Formerly Magnetic Boots)

[Skye]

Magnetic Artificial Gravity. MAG.

Imagine this: you have a spacecraft, there are lots of small magnets under the floor everywhere, and in the soles of the astronauts’ boots, you also have magnets (but these ones can swivel to face up or down). There is a dial somewhere on the astronaut’s boot that they can turn. The more they turn it, the more magnets face down (let’s say there are 20 magnets, and each one is tuned so that it will increase the attraction to the floor by the equivalent of 0.1g, so you can go between 0 & 2g). They would be able to walk around in earth gravity, Mars gravity, moon gravity, etc.

This probably would not work in terms of health effects like needing exercise, because the rest of the person would not be attracted to the floor, but it could be a decent way for easier traversal of a large spacecraft or station?

I just mainly want to know if it would negate health effects or not, how much extra mass it might be, & how easy it could be to implement into a new craft.

It would certainly be good for some things like sleeping without uncomfortable straps to keep you from floating away! And it could also lead to some cool phenomena (e.g: an astronaut’s feet are rooted to the ground but the rest of them feels like it’s in microgravity, or an astronaut can walk around a craft, stuck to the floor, & can see things floating around them, like one of those aquarium tunnels where you walk under a massive fish tank, but instead of fish, it’s science and stuff that’s floating around you)

I’ve thought of this for a while, but never had a good place to ask if it would work or not.

Hope this brings some good discussion! 

[MicrobesInSpace]

What would its applications be on planets/moons be? For instance on the moon, they could have surface bases with this tech as the low gravity could be enough to sort of keep you healthy, but one G could allow easier and more fluid operations and movement?

[Crispy]

"Magboots" are a scifi idea as old as time. You'd only need magnets in the boots for it to work btw, so long as the floors are steel.

The trouble is that this would not be "walking" as we know it. Gravity is required for the gait to work, in terms of pendulum action on the legs and the tipping moment of the body being translated into forward motion. In 0g, sticky feet would feel more like "dragging" yourself forward. Your upper body would want to stay where it is, while you pull forwards with the attached foot. It would induce a rotation of your whole body. You'd be limited to a slow "shuffle" so you can constantly "straighten up" to counteract it.

If you look at astronauts floating round the space station, you see that they don't "stand up" and move forward along the vector pointing out of their belly button. They "fly" on their long axis and keep their propelling hand touches as close to their body as possible, to keep the induced rotation to a minimum.

[Crispy]

Actually there's a good experiment you can do the next time you're at a swimming pool. Float on the surface with both arms outstretched holding the edge. Now move one arm forwards, let go with the other, and pull yourself along with one hand. You'll find it's almost impossible to stay perpendicular to the edge. Your body will "fall" behind. Its only really possible if you shuffle along with both arms working at the same time; the leading arm pulling forward and the trailing arm pushing to keep your body oriented.

[Skye]

Yeah, can’t believe I forgot steel is magnetic 

Shame it doesn’t work, it would be so cool.

In that case, I can just turn this into a general artificial gravity thread? How many alternative methods of AG are there? There’s spinning, but any others?

[StraumliBlight]

In that case, I can just turn this into a general artificial gravity thread? How many alternative methods of AG are there? There’s spinning, but any others?

There's also continuous thrust, as seen with The Expanse tv show's Epstein Drive, enabling 1G 'gravity' on board.

[Skye]

Thought of that after I last posted, got the idea from Proxima. Would be amazing, but (with chemical & NT rockets, at least,) requires a huge amount of fuel for it to work. You need a T:W of 2 (assuming you want to get to the moon) for 1 & 1/2 days, and then you need to flip over and do the same in the opposite direction, and that’s nothing compared to Mars or Jupiter! You could throttle down along the journey to retain the 2:1 T:W, but keeping that much prop for continuous acceleration would be insanely hard, plus having an engine that powerful with such a wide throttle range would also be insanely hard, though you could just have lots of smaller engines, with some shutting off along the journey. I sadly didn’t have time to look at the Epstein drive, but I’ll post a reply about it when I can 

Edit: damn, that’s impressive, shame it requires fusion 

[Twark_Main]

Should be easy to accomplish with switchable magnets.

https://en.wikipedia.org/wiki/Magnetic_switchable_device

The trouble is that this would not be "walking" as we know it. Gravity is required for the gait to work, in terms of pendulum action on the legs and the tipping moment of the body being translated into forward motion. In 0g, sticky feet would feel more like "dragging" yourself forward. Your upper body would want to stay where it is, while you pull forwards with the attached foot. It would induce a rotation of your whole body. You'd be limited to a slow "shuffle" so you can constantly "straighten up" to counteract it.

There's no need to "drag" or to "constantly straighten up."  Once your body is moving, it will continue to move in that direction under inertia until you "lean back" to stop yourself. 

Obviously you can (and will) use arms and handholds to assist when necessary.

Note that the usual procedure on the space station when you want to "stand" (ie hold yourself in one place to do work) is to awkwardly hook your feet under the handholds. This is why astronauts lose their callouses on the bottom of their feet and develop callouses on the tops of their feet. Basically a poor man's magboot. 

[Twark_Main]

In that case, I can just turn this into a general artificial gravity thread? How many alternative methods of AG are there? There’s spinning, but any others?

There's also continuous thrust, as seen with The Expanse tv show's Epstein Drive, enabling 1G 'gravity' on board.

...

Note that canonically, belter ships or ships with mixed crew (like the Rocinante) typically run at 1/3rd G. This is why the belter prisoner is shown being vulnerable to 1G 'gravity torture' in the first episode.

https://scifi.stackexchange.com/questions/258414/what-is-the-acceleration-for-different-crews-in-the-expanse

[Twark_Main]

Actually there's a good experiment you can do the next time you're at a swimming pool. Float on the surface with both arms outstretched holding the edge. Now move one arm forwards, let go with the other, and pull yourself along with one hand. You'll find it's almost impossible to stay perpendicular to the edge. Your body will "fall" behind. Its only really possible if you shuffle along with both arms working at the same time; the leading arm pulling forward and the trailing arm pushing to keep your body oriented.

This effect is mostly because water is 800x as dense as air. It's "almost impossible" because you're fighting immense drag forces.

[Twark_Main]

You need a T:W of 2 (assuming you want to get to the moon) for 1 & 1/2 days, and then you need to flip over and do the same in the opposite direction

You don't need a TWR of 2. If you're going from the Earth's surface you only need a TWR of >1, and if you want to land on the Moon's surface then you need a TWR >1/6, but if you start and end in orbit then there's no lower limit on the acceleration necessary.

Also if you're following a 2G brachistochrone trajectory, the transit is going to be a lot faster than 3 days. This ain't your grandfather's Apollo anymore!    I calculate it's around 3 hours.


As a side-note, in The Expanse there's little reason for them to (dramatically) shut off their engines and coast during the flip-and-burn. You can just keep the engine running and do a 180. This is better for crew safety and operational simplicity (no need to "batten down the hatches" and strap into your seats), and the small angle approximation means that any tiny sideways kick costs essentially zero penalty in terms of lost fuel or time.

[InterestedEngineer]

Thought of that after I last posted, got the idea from Proxima. Would be amazing, but (with chemical & NT rockets, at least,) requires a huge amount of fuel for it to work. You need a T:W of 2 (assuming you want to get to the moon) for 1 & 1/2 days, and then you need to flip over and do the same in the opposite direction, and that’s nothing compared to Mars or Jupiter! You could throttle down along the journey to retain the 2:1 T:W, but keeping that much prop for continuous acceleration would be insanely hard, plus having an engine that powerful with such a wide throttle range would also be insanely hard, though you could just have lots of smaller engines, with some shutting off along the journey. I sadly didn’t have time to look at the Epstein drive, but I’ll post a reply about it when I can 

Edit: damn, that’s impressive, shame it requires fusion 

s = 1/2at², going half way (150e6 m) and accelerating at 10m/sec² means 1.5 hours accelerated 1.5 hours decelerate, 3 hours of constant acceleration at 1G, to get to the moon.

If you want to keep mass ratios at 7 or less, and you want to thrust continuously for an entire transit of 3 to the Moon, for a 200t dry spacecraft that's 1200t of fuel over 3 hours or 400t / hour = 111 kg/sec mass flow rate (mdot)

Now, force = mdot * exhaust velocity.  To accelerate a 1400t spacecraft at 1G requires 13.72MN.

Your exhaust velocity needs to be 124km/sec.  Your kinetic energy rate of that exhaust is 850GW.

I'm not sure fusion gets you there.

Even a 1% inefficiency means heat dissipation capability of 8.5GW.

the rocket equation is terribly inefficient.  And the Expanse is science fiction.

[Twark_Main]

s = 1/2at², going half way (150e6 m) and accelerating at 10m/sec² means 1.5 hours accelerated 1.5 hours decelerate, 3 hours of constant acceleration at 1G, to get to the moon.

Note that the discrepancy in our numbers comes from InterestedEngineer using an Earth-Moon distance of 300,000 km instead of 384,000 km, and thus rounding 1.72 hours down to 1.5 hours for each leg.

At 2G I got 1.23 hours for each leg, and then I fudged a bit upward to account for the ascent phase in Earth's atmosphere (which actually isn't that bad), plus the fact that brachistochrone trajectories aren't straight lines.

If you tried to take off from the surface at a constant 10 m/s² it would take quite a long time to ascend, because you're barely exceeding the pull of gravity. I get 12 minutes to reach the Karman line, vs under 2 minutes at 2G.

So it's roughly 2.5 hours at 2G, versus 3.7 hours at 1G. This shows how the travel time goes as the square root of the acceleration: accelerating 2x harder only decreases travel time by a factor of sqrt(2).

[Hobbes-22]

"Magboots" are a scifi idea as old as time. You'd only need magnets in the boots for it to work btw, so long as the floors are steel.

The trouble is that this would not be "walking" as we know it. Gravity is required for the gait to work, in terms of pendulum action on the legs and the tipping moment of the body being translated into forward motion. In 0g, sticky feet would feel more like "dragging" yourself forward. Your upper body would want to stay where it is, while you pull forwards with the attached foot. It would induce a rotation of your whole body. You'd be limited to a slow "shuffle" so you can constantly "straighten up" to counteract it.

If you look at astronauts floating round the space station, you see that they don't "stand up" and move forward along the vector pointing out of their belly button. They "fly" on their long axis and keep their propelling hand touches as close to their body as possible, to keep the induced rotation to a minimum.

In addition, a magnetic field tends to have a large gradient: moving the boot by a small distance produces a huge increase or decrease in force, so your boot will tend to slam into the floor. That would be uncomfortable.

[InterestedEngineer]

"Magboots" are a scifi idea as old as time. You'd only need magnets in the boots for it to work btw, so long as the floors are steel.

The trouble is that this would not be "walking" as we know it. Gravity is required for the gait to work, in terms of pendulum action on the legs and the tipping moment of the body being translated into forward motion. In 0g, sticky feet would feel more like "dragging" yourself forward. Your upper body would want to stay where it is, while you pull forwards with the attached foot. It would induce a rotation of your whole body. You'd be limited to a slow "shuffle" so you can constantly "straighten up" to counteract it.

If you look at astronauts floating round the space station, you see that they don't "stand up" and move forward along the vector pointing out of their belly button. They "fly" on their long axis and keep their propelling hand touches as close to their body as possible, to keep the induced rotation to a minimum.

In addition, a magnetic field tends to have a large gradient: moving the boot by a small distance produces a huge increase or decrease in force, so your boot will tend to slam into the floor. That would be uncomfortable.

I now wonder in the era of sub-millisecond responds times and power electronics whether that steep gradient can be linearized with real time control of the power on an electromagnet.

What a fun control theory problem!

[Skye]

You need a T:W of 2 (assuming you want to get to the moon) for 1 & 1/2 days, and then you need to flip over and do the same in the opposite direction

You don't need a TWR of 2. If you're going from the Earth's surface you only need a TWR of >1, and if you want to land on the Moon's surface then you need a TWR >1/6, but if you start and end in orbit then there's no lower limit on the acceleration necessary.

Also if you're following a 2G brachistochrone trajectory, the transit is going to be a lot faster than 3 days. This ain't your grandfather's Apollo anymore!    I calculate it's around 3 hours.

Goddamn that’s fast. Even with turning around at halfway for deceleration the whole second half?

Welp, at least T:W isn’t as hard as I thought it was! I’m not great at in-space Gee calculations, I must’ve mixed up w/ Earth ascent? Thanks for the correction  the only problem is that propellant is… an issue…

Your exhaust velocity needs to be 124km/sec.  Your kinetic energy rate of that exhaust is 850GW.

I'm not sure fusion gets you there.

Even a 1% inefficiency means heat dissipation capability of 8.5GW.

the rocket equation is terribly inefficient.  And the Expanse is science fiction.

Oh. My. God.

And this is just to the moon!

So it's roughly 2.5 hours at 2G, versus 3.7 hours at 1G. This shows how the travel time goes as the square root of the acceleration: accelerating 2x harder only decreases travel time by a factor of sqrt(2).

So maybe something like 0.5g is worth? I’ve not done any calculations, but maybe it’s around 5 hours? I’d go for that, way less ΔV. Plus it could be a kind of midpoint/gradient from Earth g to Lunar g?

[InterestedEngineer]

s = 1/2at², going half way (150e6 m) and accelerating at 10m/sec² means 1.5 hours accelerated 1.5 hours decelerate, 3 hours of constant acceleration at 1G, to get to the moon.

Note that the discrepancy in our numbers comes from InterestedEngineer using an Earth-Moon distance of 300,000 km instead of 384,000 km, and thus rounding 1.72 hours down to 1.5 hours.

At 2G I got 1.23 hours, and then I fudged a bit upward to account for the ascent phase in Earth's atmosphere (which actually isn't thay bad), plus the fact that brachistochrone trajectories aren't straight lines.

If you tried to take off from the surface at a constant 10 m/s² it would take quite a long time to ascend, because you're barely exceeding the pull of gravity. I get 9 minutes to reach the Karman line, vs 1.2 minutes at 2G.

So it's roughly 2.5 hours at 2G, versus 3.7 hours at 1G. This shows how the travel time goes as the square root of the acceleration: accelerating 2x harder only decreases travel time by a factor of sqrt(2).

Does that mean that energy is to the fourth power of time decrease?

[Twark_Main]

So it's roughly 2.5 hours at 2G, versus 3.7 hours at 1G. This shows how the travel time goes as the square root of the acceleration: accelerating 2x harder only decreases travel time by a factor of sqrt(2).

So maybe something like 0.5g is worth? I’ve not done any calculations, but maybe it’s around 5 hours? I’d go for that, way less ΔV. Plus it could be a kind of midpoint/gradient from Earth g to Lunar g?

At 0.5G we're talking apples to oranges. The other times I quoted are surface-to-surface, but at 0.5G you can't lift off from the Earth's surface.

You can still land on the Moon though. If you start in Earth orbit, at 0.5G it would take 5 hours (good guess!), and at The Expanse standard acceleration of 0.3G it would take 6.5 hours.

My favorite fact I learned is that elevators have a peak acceleration of 1/8 G. This means that to accelerate upward as gently as an elevator, you would ascend through the entire Earth's atmosphere in only 7 minutes of additional trip time. 

Does that mean that energy is to the fourth power of time decrease?

Yes, if you hold mass ratio constant then the energy scales as the inverse fourth power of travel time.

So if you want to use the same propellant to get you there in half the time, you need 16x as much energy (and of course, 2⁵ = 32x as much power).   

[Skye]

Maybe a future craft (S1 almost to orbit, S2 finishes the job) can have a high T:W S1 and a 0.5 T:W S2 for continuous thrust Lunar Transit?

Thanks, I didn’t even use any number, just a rough guess 

Very interesting! Though if powered by an engine, it’d be very inefficient to have a T:W of 1.125 

[InterestedEngineer]

Does that mean that energy is to the fourth power of time decrease?

Yes, if you hold mass ratio constant then the energy scales as the inverse fourth power of travel time.

So if you want to use the same propellant to get you there in half the time, you need 16x as much energy (and of course, 2⁵ = 32x as much power).   

The 2020s version of TL;DR, here's the prompt you feed chatGTP to derive the 5th power

Given a constant mass ratio of a rocket, show me how the energy rate scales with the 5th power of travel time between two points when using a conventional rocket

Alas it's not possible to paste the result here.  Try it yourself. It appears to be a valid derivation.


I need to make a "let me chatgtp that for you" website.