Mitigating Deep Space Travel Acceleration

[Mr Shifty]

Probably most are familiar with the idea that travel to extremely distant locations is possible in a single person's lifetime if we can maintain a high constant acceleration using some science-fictional technology like a ram scoop or vacuum energy drive or something. We could, for instance, travel to the Greater Magellanic Cloud galaxy at a distance of 163,000 light years in 23 years (ship relative) if we could maintain a constant 1g of acceleration (towards the destination until the halfway point then decelerating after that.) This is possible due to time dilation and Lorentz length contraction at relativistic speeds.

Or, if we wanted to time travel 1000 years into the future, we could do a trip at 1g out to 500 light years from Earth, turn around and come back. (You'd have to switch acceleration directions twice: once halfway out and again halfway back.) The whole trip would take about 24 years, ship relative. You could do it in less than a decade if you could accelerate at 3g. But living at 3g for a decade would be pretty difficult (if not fatal) for a human body. I was wondering if there's a way around it.

General relativity tells us that acceleration in a spaceship at 1g in deep space is indistinguishable from standing on the surface of the Earth (neglecting tidal, Coriolis, and centrifugal forces.) So, we can conjecture that since we don't have a way to mitigate a constant gravitational pull on Earth, there would also be no way to do it (through, say, a rotating habitat) on the spacecraft. Buoyancy through water immersion seems like a possibility; I wonder if it would be possible to design gear so that humans could live in a fully immersed environment 24/7, and what the ramifications would be for this kind of system in 3g. Also, with the spacecraft, we have the advantage of theoretically being able to change the thrust of the engines at will, so we don't have to have a constant acceleration; you could have everyone get into the tanks for 8 hours out of every 24 to sleep and accelerate at 7g during that period and 1g otherwise, which would average out to 3g.

Any other creative solutions?

[ZetaX]

I am not convinced that buoyancy will help enough. It will lower the strain on your normal muscles because you only need to lift the effective weight, which will be close to zero (ignoring the water's resistance), but it won't help much to mitigate the strain on the body's inside, e.g. organs or the bloodstream/the heart.

[Rakaydos]

I am not convinced that buoyancy will help enough. It will lower the strain on your normal muscles because you only need to lift the effective weight, which will be close to zero (ignoring the water's resistance), but it won't help much to mitigate the strain on the body's inside, e.g. organs or the bloodstream/the heart.

If you're only accelerating at 3G, it shouldnt really be an issue. Boyancy will bring the body's AVERAGE weight to 0, so your bones will want to fall while your fat will want to float- but that will only be a fraction of the actual acceleration subjected to you.

Another idea is to uplift a species with a higher G capability. If we can make, say, an octopus sapient and keep them in a native aquatic enviroment, their lower density differential (lacking high density bones and large lungs full of low densidy air), they should be able to handle 3Gs indefinately, if not higher.

[Tommygun]

With water immersion, humans would have to be in a skintight dry suit all the time, otherwise their skin would start to come apart after too much water absorption.

I think you might be able to reduce some of the effects by having the crew live horizontal relative to the acceleration.

Not an easy thing ether and may cause log term health issues too.

Exoskeletons and implants that help with blood flow may be a way to go and could be combine with horizontal living.

As a last resort bioengineering might be the only way to do it.

I'm not a big fan of doing this, as I fear ending up with unstable genes years or several generations later.

Edited October 9, 2014 by Tommygun

[Nuke]

1g is the perfect acceleration because you dont need a centrifuge to get the benefits of gravity. for military craft where the crew is physically fit and conditioned to high g loads you might have typical accelerations of 1.5-2gs and possibly short duration at higher g loads in emergencies.

you might also choose short stocky builds for higher g voyages. you might selectively breed to produce a beefier bone structure. this would be essential for colonizing high g worlds as well. you can also go the other way and selectively breed for low g loads. you might end up with a wide variety of human strains adapted to life in all kinds of strange conditions.

[ZetaX]

With water immersion, humans would have to be in a skintight dry suit all the time, otherwise their skin would start to come apart after too much water absorption.

I am not convinced this will happen. Do you have a source¿

[magnemoe]

An waterbed would work just as well and be easier to handle, you can get in and out fast, and its dry.

In the mote in the goods eye books it was common, however they did day long burns inside solar systems not year long one.

Main benefit of doing hard burns is that you get up to speed fast, even if you had something able of doing 5g infinite you would probably not bother as the value of the acceleration in the last 25% and less of the distance to turnover is low.

If you talk about 20 years travel you can reach the core of the galaxy with 10+10 years at 1g 30 year to andromeda, yes well before this time you run into other issues like light in front of you is very hard x-rays. interstellar hydrogen behaves like they have visited Cern for a joyride, dust is nukes and other minor issues.

[Tommygun]

I am not convinced this will happen. Do you have a source¿

I can't find any medical sources as I don't know the correct medical terms for it.

Unfortunately the only thing I can find is about an attempt from a magician named David Blaine that spent seven days under water.

The skin on his hands, feet and face started to peel away and were one of the health reasons he cited as stopping.

From BCC news:

http://news.bbc.co.uk/2/hi/entertainment/4983768.stm

His hands after seven days.

Edited October 10, 2014 by Tommygun

[Lukaszenko]

By the time we need to worry about this, and the way medical advancements are going, we will be able to do away with the human body altogether (except maybe for the brain). I imagine we'll be able to pick and choose whatever strength body we want for the journey (and the destination), organic or otherwise.

If we ever get a full understanding of the brain, we might be able to just do away with that as well. Then you can just beam your information to the destination at the speed of light, and then "3D print" yourself there. But that's a bit more far-fetched. At least the first paragraph is medically sound and plausible.

[KerikBalm]

I think this will probably never be a problem.

Slower travel with something like cryogenics seems much more practical.

Reduced life support requirements, and it is much easier to develop a powerplant that accelerates you at a constant 0.1G, than 10g.

From Earth's frame of reference, they won't much are if you go at .9c or .999c

[cantab]

Buoyancy has indeed been considered for this. I believe 3 g wouldn't be a problem.

Higher and you start having problems because the air in the lungs isn't the same density as the water around. A potential solution to this problem is to breathe an oxygenated fluid instead. I believe that so far we haven't found a fluid that can be oxygen-rich enough for humans, though somewhat-successful tests have been done with other animals (as in the animal didn't die straight away). This approach might enable humans to withstand 100 g, though practical issues like eating and excretion would limit the duration.

Then again, maybe you could just put people in water-filled pods, induce sleep, and supply oxygen and nutrition and remove wastes intravenously.

[-Velocity-]

I don't think this will ever be a problem. There is no where in nature to harvest antimatter in sufficient quantities to power a large starship. We will probably never be able to produce it and store it in sufficient quantities either. That leaves us with nuclear fusion, which will have a much lower thrust.

[KerikBalm]

Even if we could produce that much antimatter... see my previous post.... Why bother with an acceleration beyond 1g?

If we have that sort of technology, surely cryostasis or just increased longevity or some form of hibernation would be better.

Its more energy efficient, the dry mass can be lower due to reduced need for life support and no water filled tanks.

Going at .95 or .9995c wont make much of a difference to those not on the ship.

If you can make those on the ship able to handle longer voyages, then why not crank the thrust back and take it a little slower?

[shynung]

Even if we could produce that much antimatter... see my previous post.... Why bother with an acceleration beyond 1g?

If we have that sort of technology, surely cryostasis or just increased longevity or some form of hibernation would be better.

Its more energy efficient, the dry mass can be lower due to reduced need for life support and no water filled tanks.

Going at .95 or .9995c wont make much of a difference to those not on the ship.

If you can make those on the ship able to handle longer voyages, then why not crank the thrust back and take it a little slower?

It matters a lot if it's a military ship. A ship capable of only 1G acceleration is a lot easier to target and attack than one that can pull 6G. In that case, an effective padding/restraint system to keep the occupants able to work is an essential requirement.

Note, the engine doing these kinds of thrust doesn't have to be the main propulsion. The ship could have fusion engines for long voyages, and auxiliary nuclear pulse engines for when someone is throwing relativistic projectiles at it.

Edited October 10, 2014 by shynung

[ZetaX]

If done right, you won't have any reasonable reaction time against relativistic projectiles.

[shynung]

That is where the high-thrust engine shines. If a target is accelerating erratically, the shooter would have plenty of difficulty making the shot count. It's like trying to shoot a rabbit that moves constantly in random directions with a sniper rifle.

Of course, a good enough shooter will still hit it. It's just that shooters of that skill level isn't very common.

Edited October 10, 2014 by shynung

[Mr Shifty]

Even if we could produce that much antimatter... see my previous post.... Why bother with an acceleration beyond 1g?

If we have that sort of technology, surely cryostasis or just increased longevity or some form of hibernation would be better.

Its more energy efficient, the dry mass can be lower due to reduced need for life support and no water filled tanks.

Going at .95 or .9995c wont make much of a difference to those not on the ship.

If you can make those on the ship able to handle longer voyages, then why not crank the thrust back and take it a little slower?

Well it's all hypothetical. We're talking about a future here where we have the technology for constant acceleration, say by harnessing the vacuum energy of spacetime or something, but not to induce human cryostasis. The impetus for faster travel is purely for the sake of those on the vessel, not for the folks left behind, and from their perspective, the difference between 0.95c and 0.9995c is a factor of 10 reduction in the distance they have to travel. A trip to Alpha Centauri at a constant 0.95c would take about 15 months, while a trip at 0.9995c would take about 7 weeks.

[ZetaX]

That is where the high-thrust engine shines. If a target is accelerating erratically, the shooter would have plenty of difficulty making the shot count. It's like trying to shoot a rabbit that moves constantly in random directions with a sniper rifle.

As soon as you are very close to c, any more thrust won't have a significant effect on the ships speed (from the "outside" frame of reference, or the bullet's). Thus your random changes have almost no effect.

[shynung]

Even if the thrust's axis were perpendicular to the direction of travel? How about retrograde? Please enlighten.

Edited October 10, 2014 by shynung

[Mr Shifty]

That is where the high-thrust engine shines. If a target is accelerating erratically, the shooter would have plenty of difficulty making the shot count. It's like trying to shoot a rabbit that moves constantly in random directions with a sniper rifle.

The limitations placed on a vessel's acceleration by having human passengers make this an unwinnable game. Current-tech missiles can undergo accelerations of 100g with no problems. But this discussion is somewhat off-topic.

[shynung]

The limitations placed on a vessel's acceleration by having human passengers make this an unwinnable game. Current-tech missiles can undergo accelerations of 100g with no problems. But this discussion is somewhat off-topic.

Yes, it is rather off-topic. Should I open a new thread?

Anyway, a spaceship has a considerably more delta-v capacity than a missile. The first few km/s of a missile's delta-v budget is already spent to accelerate it towards the target spaceship. If the target ship jinks enough (changes its orbit by thrusting in a random direction), the missile could have expended all its fuel before reaching the ship, making it easier to dodge.

[Mr Shifty]

Even if the thrust's axis were perpendicular to the direction of travel? How about retrograde? Please enlighten.

Consider that a relativistically moving vessel would already have to have robust countermeasures simply to prevent enormous explosions from happening due to collisions with trace interstellar matter. There are additional problems with trying to hit a relativistically moving target: it can only be 'seen' in the rest frame from directly in front of it, and by the time you see it, it will already be here. (If you were able to detect an object travelling toward Earth at 0.99c when it crosses Jupiter's orbit, you'd have about 24 seconds to do something about it by the time the light got to you.)

[shynung]

I see what you mean. It'd be a really difficult task of avoiding that projectile.

Note, however, that trying to shoot a target able to pull 6G in almost any direction from several light-seconds away will still be a difficult matter. The speed of the projectile won't matter at all if the aim was off in the first place.