Emergency braking in space • Hacker News Board

Re: Emergency braking in space

#41

Post by sneak » Tue, Jun 07, 2022, 4:33 PM UTC

ginko wrote: post #19

ericbarrett wrote: post #18

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 s…

>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.

> 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.

Re: Emergency braking in space

#42

Post by nradov » Tue, Jun 07, 2022, 4:35 PM UTC

hammock wrote: post #30

TremendousJudge wrote: post #28

Earlier quoted context omitted.

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.

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.

Re: Emergency braking in space

#43

Post by tomrod » Tue, Jun 07, 2022, 4:36 PM UTC

rsynnott wrote: post #27

dhosek wrote: post #14

Earlier quoted context omitted.

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…

Requires a magic-based energy source; the usual problem with this sort of thing.

Why would an ion thruster engine not be feasible?

Re: Emergency braking in space

#44

Post by scrumbledober » Tue, Jun 07, 2022, 4:56 PM UTC

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?

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

Re: Emergency braking in space

#45

Post by ghusbands » Tue, Jun 07, 2022, 5:13 PM UTC

joncrocks wrote: post #38

ghusbands wrote: post #34

Earlier quoted context omitted.

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.

Re: Emergency braking in space

#46

Post by foxyv » Tue, Jun 07, 2022, 5:40 PM UTC

ericbarrett wrote: post #18

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 s…

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.”

Re: Emergency braking in space

#47

Post by throwaway09223 » Tue, Jun 07, 2022, 5:55 PM UTC

tomrod wrote: post #43

rsynnott wrote: post #27

Earlier quoted context omitted.

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.

Re: Emergency braking in space

#48

Post by Rastonbury » Tue, Jun 07, 2022, 6:00 PM UTC

verisimi wrote: post #37

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

Flip the ship around and thrust

Re: Emergency braking in space

#49

Post by jimbokun » Tue, Jun 07, 2022, 6:42 PM UTC

dhc02 wrote: post #23

jimbokun wrote: post #22

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.

Re: Emergency braking in space

#50

Post by avhon1 » Tue, Jun 07, 2022, 7:06 PM UTC

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