Author Topic: Constant Acceleration at 1G and Beyond  (Read 60008 times)

Offline Bob Shaw

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Re: Constant Acceleration at 1G and Beyond
« Reply #20 on: 06/24/2014 09:40 AM »
Might be going rather off topic, but despite the mind boggling difficulty of interstellar travel when viewed in a single human lifetime, When considering the search for extraterrestrials the troubling problem is how mindboggly easy it appears.

Even at a thousandth of c. an exponentially growing civilization should have been able to colonize every star in the galaxy about a hundred times over. The time to stop and colonize each world become irrelevant due to the rapidly growing number of worlds willing to engage in the next wave.  It strongly suggests we are entirely alone, or worse, the unknown hurdle is still before us and our chances of surviving to achieve 1000th of c. are very slim indeed.

Absolutely correct, sadly. Even assuming a wholly robotic, Von Neuman machine civilisation or post-Singularity ex-biological one (is there a difference?), it takes a reasonably short time to colonise an entire galaxy with rather slow ships. If you can colonise a galaxy, then you almost certainly manipulate the environment, increasing dust, decreasing dust, changing the percentage of stars with high metallicity, altering stellar rotation rates, emitting gravity wave signals and so forth. All these signs and portents can be observed at great distances, but to the best of my knowledge there are no results from astronomical observations of galaxies which suggest any statistical outliers liable to be the relics of distant civilisations. Such signs would be distant in both time, and space, and with a spread right across the development of the universe, too - our 'local' searches are not just local in terms of nearby stars, but are also recent in terms of time and so see only a tiny slice of the history of our own neighbourhood. In short, to seek out civilisations you have to look to the past, and far away for their lights and industrial wastes - and nothing has been seen.

Life, though, is a different matter. Were I a betting man, I'd lay money on life being everywhere. And I can easily extend that to the detection of life in the nearish future. The trouble is, it won't be big critters with eyes, brains and fur (or whatever) but will instead be the perfect life-form, highly adapted to its environment and really quite advanced. Well, advanced for slime, anyway...

So: no signs of relict industry across the observable universe, no signals locally, and lots of slime everywhere. Multi-cellularism may well be a real rarity, and us guys rarer still.

Offline gospacex

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Re: Constant Acceleration at 1G and Beyond
« Reply #21 on: 06/24/2014 09:53 AM »
I think constant acceleration at 1g is a nice well defined problem despite the various reasons we probably wouldnt attempt it.

On topic, it could be worth noting that constant 1g acceleration would be pretty easy for a standard chemical rocket if there where depots strung out at just the right positions and velocities ahead of it.. of course that just pushes the problem onto how to get the depots there, but perhaps you have more options for unmanned depots than for a crewed ship. They could be shot with massive acceleration or take centuries to align.

Isp is WAY too low.
The total mass of chem depots needed even for a small craft to reach any sort of interstellar-class speed is not feasible. Billions of tons.

If you have to have high accel, how about electromagnetic acceleration a-la maglev/railgun.

For a fixed length track and acceleration, final velocity is:
v = sqrt(2*a*l)

At a=10 (one gee), l=1million km, you'd attain 100km/s.
With ten gees and 10 million km track, it's 1000km/s (0.3%c)

Offline john smith 19

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Re: Constant Acceleration at 1G and Beyond
« Reply #22 on: 06/24/2014 03:03 PM »
The problem is not acceleration as such it's sustained acceleration and the energy you need to sustain it.  :( Look at the voyager science package, knock it down by a factor of 10 to allow for 40 years of improvements and work out how much energy it takes to accelerate that to even 1% of C. That would tell you quite a lot about any planets it found, but how would it tell us about them?

If I understand correctly this is the sort of thing some kind of quantum entanglement comms device would be perfect for.  :(

Obviously if you can do higher g you can accelerate faster, possibly using remote power systems and beamed power while still in the solar system. The limits in the 50's seemed to be about 60g for 15 minutes in a water tank. People have also suggested ingesting oxygenated flourcarbons (actually used for breathing assist in patients with badly damaged lungs), possibly also introduced into the body cavity to support internal organs. 1g is 1C in 6 months but 60g?

Another option is a pulse systems would be to string out a series of mass driver rings across the solar system. The ship threads the needle of each one, which then recoils until brought to rest by either thrusters on the ring or by the repulsion of the approaching nearest neighbour.

Obviously the level of recoil depends on the relative masses of the accelerator ring to the payload it's accelerating but any way you slice it this would a massive project.

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Offline Vultur

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Re: Constant Acceleration at 1G and Beyond
« Reply #23 on: 06/24/2014 05:25 PM »
Absolutely correct, sadly. Even assuming a wholly robotic, Von Neuman machine civilisation or post-Singularity ex-biological one (is there a difference?), it takes a reasonably short time to colonise an entire galaxy with rather slow ships. If you can colonise a galaxy, then you almost certainly manipulate the environment, increasing dust, decreasing dust, changing the percentage of stars with high metallicity, altering stellar rotation rates, emitting gravity wave signals and so forth.

What if "mechanical" (as opposed to basically-biology-as-we-know-it-but-with-a-little-bit-different-chemistry) Von Neumann machines simply aren't possible or aren't invented? Living things won't do well in vacuum and I have a feeling that "nanotech" if it happens will be more like biology (viruses/bacteria) than really tiny robots and factories.

Also, that assumes the desire to both colonize the galaxy and change it in observable ways (a quadrillion O'Neill colonies orbiting a billion stars galaxywide would be a colonized galaxy, but not one that the colonization was detectable from a distance). I don't see why that is a natural assumption.

And what if there are lots of altered galaxies? Would we recognize them as artificial rather than just a different type of galaxy? We could have already seen them and they are simply not recognized.

Online Stormbringer

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Re: Constant Acceleration at 1G and Beyond
« Reply #24 on: 06/24/2014 06:00 PM »
one reason this may be a false paradox is we might not even be able to perceive or recognize the signatures of such civilizations. for example; what if such civilizations use slightly larger than plank length wormholes to spy on planets rather than go to all of them? there are cosmic ray signals that would be the only signature of such virtual probes. according to Dr Kramer there are cosmic ray events that would match the signature of such probes. this would not ordinarily bring to mind alien activity. how would we know the difference between natural cosmic ray signals and wormholes spying on us?

What if it's only mankind's arrogance to assume that we can predict what a more advanced civilization's signatures would be? they'd have to do this and this and this and have this or that... how in the heck do we know that?
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Offline john smith 19

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Re: Constant Acceleration at 1G and Beyond
« Reply #25 on: 06/25/2014 08:20 AM »
What if it's only mankind's arrogance to assume that we can predict what a more advanced civilization's signatures would be? they'd have to do this and this and this and have this or that... how in the heck do we know that?
And so we are back to Haldane's conjecture that "The universe is not only stranger than we know it is stranger than we can imagine"

Although obviously as we've learned more what we can imagine has expanded a lot.
BFS. The worlds first Methane fueled FFORSC engined CFRP structured A380 sized aerospaceplane tail sitter capable of flying in Earth and Mars atmospheres. BFR. The worlds biggest Methane fueled FFORSC engined CFRP structured booster for BFS. First flight to Mars by end of 2022. Forward looking statements. T&C apply. Believe no one. Run your own numbers. So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

Offline KelvinZero

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Re: Constant Acceleration at 1G and Beyond
« Reply #26 on: 06/25/2014 10:13 AM »
I think constant acceleration at 1g is a nice well defined problem despite the various reasons we probably wouldnt attempt it.

On topic, it could be worth noting that constant 1g acceleration would be pretty easy for a standard chemical rocket if there where depots strung out at just the right positions and velocities ahead of it.. of course that just pushes the problem onto how to get the depots there, but perhaps you have more options for unmanned depots than for a crewed ship. They could be shot with massive acceleration or take centuries to align.

Isp is WAY too low.
The total mass of chem depots needed even for a small craft to reach any sort of interstellar-class speed is not feasible. Billions of tons.

No ISP is not relevant in this example. The catch is that you have gotten all these depots up to speed ahead of you somehow. Thats where you have to put all your energy in. The chemical rocket becomes just a way of taking this energy from the depot and putting it into the vehicle. You could just bounce off each depot and steal their kinetic energy that way, but that would be a rough trip.

Its not something Im proposing to do, just thought it might spark some ideas. Myself I favor some sort of beamed propellant propulsion, which you can think of as an extreme case of the above idea.


Offline gospacex

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Re: Constant Acceleration at 1G and Beyond
« Reply #27 on: 06/25/2014 10:44 AM »
one reason this may be a false paradox is we might not even be able to perceive or recognize the signatures of such civilizations. for example; what if such civilizations use slightly larger than plank length wormholes to spy on planets rather than go to all of them? there are cosmic ray signals that would be the only signature of such virtual probes. according to Dr Kramer there are cosmic ray events that would match the signature of such probes. this would not ordinarily bring to mind alien activity. how would we know the difference between natural cosmic ray signals and wormholes spying on us?

What if it's only mankind's arrogance to assume that we can predict what a more advanced civilization's signatures would be? they'd have to do this and this and this and have this or that... how in the heck do we know that?

Maybe you are in a vat with neural feed sending you fake picture of the virtual reality. How in the heck can you know that?

See? Not a useful position.

Offline gospacex

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Re: Constant Acceleration at 1G and Beyond
« Reply #28 on: 06/25/2014 10:46 AM »
I think constant acceleration at 1g is a nice well defined problem despite the various reasons we probably wouldnt attempt it.

On topic, it could be worth noting that constant 1g acceleration would be pretty easy for a standard chemical rocket if there where depots strung out at just the right positions and velocities ahead of it.. of course that just pushes the problem onto how to get the depots there, but perhaps you have more options for unmanned depots than for a crewed ship. They could be shot with massive acceleration or take centuries to align.

Isp is WAY too low.
The total mass of chem depots needed even for a small craft to reach any sort of interstellar-class speed is not feasible. Billions of tons.

No ISP is not relevant in this example. The catch is that you have gotten all these depots up to speed ahead of you somehow. Thats where you have to put all your energy in. The chemical rocket becomes just a way of taking this energy from the depot and putting it into the vehicle. You could just bounce off each depot and steal their kinetic energy that way, but that would be a rough trip.

You missed the word "chemical" in the phrase "standard chemical rocket" in the post I replied to.

If it's "chemical", it means "chemical propulsion" for depots and for the rocket.
« Last Edit: 06/25/2014 10:47 AM by gospacex »

Offline KelvinZero

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Re: Constant Acceleration at 1G and Beyond
« Reply #29 on: 06/25/2014 10:59 AM »
Nah, I was clear that the depot was not launched by the same means, or by explained means at all. It was just to give ideas, not a solution.

That magnetic rail idea could have promise, don't use it to launch your ship but your propellant that follows after. This way you can inflict much higher accelerations and build something a million times smaller and have it running for years instead of minutes.

I do think beamed propellant is the most promising. Even if you like antimatter, you could greatly increase your efficiency if you beamed the antimatter. It is easy for some sort of particle projector sitting on a moon to launch particles at a much greater fraction of light than you expect your ship to reach, so with beamed propulsion you escape the exponential problem of the rocket equation. You are pushing directly against the moon for the whole trip.

The big problem I see is what sort of beam you can focus on that range? There is light of course, but then you have to postulate amazing materials for the sail, especially if you are postulating 1g. If you could focus a plasma or gas then from the vehicle's point of view it could be riding on a column of gas no hotter than a chemical rocket the whole way.

If it helps, the beamed material could probably be arbitrarily large. It would all ablate into plasma as it gets near the craft's 'nozzle' I expect.

Offline gospacex

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Re: Constant Acceleration at 1G and Beyond
« Reply #30 on: 06/25/2014 11:03 AM »
Nah, I was clear that the depot was not launched by the same means, or by explained means at all. It was just to give ideas, not a solution.

So what is "chemical" about this scheme?

Offline KelvinZero

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Re: Constant Acceleration at 1G and Beyond
« Reply #31 on: 06/25/2014 11:15 AM »
Nah, I was clear that the depot was not launched by the same means, or by explained means at all. It was just to give ideas, not a solution.

So what is "chemical" about this scheme?
Its absolutely clear which bit is chemical. It is the use of the word scheme that might be misleading because it implies solution. It was a principle to provoke ideas. If it didn't for you, thats fine.

It did for me, because it underlines how beamed propellant can push something at constant acceleration as comfortably as if it were pushed by a chemical rocket, only dealing with chemical-equivalent stresses. No unobtainium, at least on the vehicle end.

Offline frobnicat

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Re: Constant Acceleration at 1G and Beyond
« Reply #32 on: 06/25/2014 02:36 PM »
And what if there are lots of altered galaxies? Would we recognize them as artificial rather than just a different type of galaxy? We could have already seen them and they are simply not recognized.

Not on topic but for those interested the following serious research bubbled up in some forums about detection of large scale ET technologies :
http://www.seti.org/weeky-lecture/wise-search-large-extraterrestrial-civilizations-complementary-approach-traditional
Video quality of the talk is a bit lousy but content I find worth : "The WISE all-sky mid-infrared survey has dramatically improved our ability to detect such civilizations and to distinguish them from "natural" astrophysical sources"

Online Stormbringer

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Re: Constant Acceleration at 1G and Beyond
« Reply #33 on: 06/25/2014 05:33 PM »
one reason this may be a false paradox is we might not even be able to perceive or recognize the signatures of such civilizations. for example; what if such civilizations use slightly larger than plank length wormholes to spy on planets rather than go to all of them? there are cosmic ray signals that would be the only signature of such virtual probes. according to Dr Kramer there are cosmic ray events that would match the signature of such probes. this would not ordinarily bring to mind alien activity. how would we know the difference between natural cosmic ray signals and wormholes spying on us?

What if it's only mankind's arrogance to assume that we can predict what a more advanced civilization's signatures would be? they'd have to do this and this and this and have this or that... how in the heck do we know that?

Maybe you are in a vat with neural feed sending you fake picture of the virtual reality. How in the heck can you know that?

See? Not a useful position.
actually... that depends. in the same way you aren't paranoid if people really are out to get you. if advanced societies develop forms and methods that we cannot extrapolate from our own stuff then we might not be able to recognize the signs of their existence.

it is useful because it reminds us we aren't the be all and end all of technology or biology. it tells us to look at things with a different perspective from time to time. it's not the cave men looking at a rock that advances stuff. it's the caveman that picks up the rocks and bangs them together.
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Offline JasonAW3

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Re: Constant Acceleration at 1G and Beyond
« Reply #34 on: 06/25/2014 06:01 PM »
Since no one else seems to have run the numbers, let me do that.

<SNIP>

Now suppose you want to go exactly 27,000 light-years to the galactic core. I'll only do the 1g figures

Ship time: 19.8 years
Earth time: 27,002 years
Peak velocity about 77 cm/sec less than c.

Assuming a perfect anti-matter drive (exhaust is light-speed and all in the right direction), then the mass fraction for the 1g trip is 39.5. That is, for every kilo of payload delivered to Alpha Centuri, you would need 38.5 kilos of fuel (half matter and half antimatter).

For the 0.1g trip, that falls to 3.7 and it's only 1.5 for the 0.01g trip.

For the trip to the galactic core, it's about 777 million, so funding may be difficult. :-)

Source for formulas: http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html

I assume that you're talking constant acceleration without midpoint turn over and deceleration.  In the case of the 1g acceleration, I would figure that it would take about 4 as long objectively, as you'd never reach peak velocity, but only about half of that.  Unfortunately, I don't have the formula to figure out the time dilation subjective for the ship.  I figure at best, they'd reach about .5c in velocity.
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Online Stormbringer

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Re: Constant Acceleration at 1G and Beyond
« Reply #35 on: 06/25/2014 07:38 PM »
Since no one else seems to have run the numbers, let me do that.

<SNIP>

Now suppose you want to go exactly 27,000 light-years to the galactic core. I'll only do the 1g figures

Ship time: 19.8 years
Earth time: 27,002 years
Peak velocity about 77 cm/sec less than c.

Assuming a perfect anti-matter drive (exhaust is light-speed and all in the right direction), then the mass fraction for the 1g trip is 39.5. That is, for every kilo of payload delivered to Alpha Centuri, you would need 38.5 kilos of fuel (half matter and half antimatter).

For the 0.1g trip, that falls to 3.7 and it's only 1.5 for the 0.01g trip.

For the trip to the galactic core, it's about 777 million, so funding may be difficult. :-)

Source for formulas: http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html

I assume that you're talking constant acceleration without midpoint turn over and deceleration.  In the case of the 1g acceleration, I would figure that it would take about 4 as long objectively, as you'd never reach peak velocity, but only about half of that.  Unfortunately, I don't have the formula to figure out the time dilation subjective for the ship.  I figure at best, they'd reach about .5c in velocity.
time dilation effects are negligible until you get really close to light speed in terms of the crew's perception of trip time. it mucks with electronic timing and stuff long before it get's to be useful to humans. at .5 c the crew would basically spend the full trip time and consume the full trip time's consumables like water air and food and wear and tear on equipment and clothing and stuff like that. if they were going .9 c they would begin to reap the benefits of time dilation.
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Offline aceshigh

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Re: Constant Acceleration at 1G and Beyond
« Reply #36 on: 06/25/2014 08:49 PM »
well, the borehole however was a straight drilled hole downwards. If we mine like an inverted pyramid, we could go much further, without the rock pressure problems (although atmospheric pressure itself will get stronger.

Offline kch

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Re: Constant Acceleration at 1G and Beyond
« Reply #37 on: 06/25/2014 09:07 PM »
well, the borehole however was a straight drilled hole downwards. If we mine like an inverted pyramid, we could go much further, without the rock pressure problems (although atmospheric pressure itself will get stronger.

Wrong thread?  ;)

Offline RanulfC

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Re: Constant Acceleration at 1G and Beyond
« Reply #38 on: 06/26/2014 04:53 PM »
On the notion of "Where is Everyone?" I recall one proposal that we might be part of a cycle of "waves" of intelligence around the galaxy. In a short story the background of the planet being colonized it was mentioned that it showed "layers" of possible alien civilizations followed by a "sterilized" period after which it was "reseeded" back to its original bionome. The general "theory" was that civilizations would colonize the planet and then grow to the point where they decided to leave and "reset" the planets biosphere for the "next" wave.

I think the main "issue" with the idea that someone would have "colonized-the-galaxy" by now is rather self-centerered and really unsupported by the evidence of our own civilization. When most of these thought-problems were introduced the general idea was populations and civilizations that COULD would continue to expand indefinitly both in population and area. That doesn't seem as "true" as it did then, and coupled with the fact that "space" is hard to get to, much harder than it would be to "colonize" still open "areas" of our own planet it doesn't seem the "incentive" to expand at "all-costs" is as logical as it might have been assumed.

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Offline dror

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Re: Constant Acceleration at 1G and Beyond
« Reply #39 on: 06/26/2014 10:02 PM »

... You are pushing directly against the moon for the whole trip...
...The big problem I see is what sort of beam you can focus on that range?
The big problems I see:
Wont it inflict the opposit push on the moon? How big is it considering the vast amount of energy mentioned before?
Can you point it to the same direction of the traveling ship, while the moon rotates around the earth and sun?
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