Author Topic: Constant Acceleration at 1G and Beyond  (Read 44075 times)

Offline cjackson

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Constant Acceleration at 1G and Beyond
« on: 06/20/2014 04:50 AM »
Exactly what kind of technological leap and how much energy would be needed to accelerate/decelerate at 1g on a trip to another star system? I understand that, barring wormholes, constant 1g acceleration is the only way to achieve relativistic effects and get a spacecraft across significant distances in a human lifetime as measured aboard the spacecraft.

Offline scienceguy

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Re: Constant Acceleration at 1G and Beyond
« Reply #1 on: 06/20/2014 05:05 AM »
OK let’s see. Say the craft has a mass of 10^4 kg. To accelerate at 1 G you need F = ma = (10^4 kg)(10 m/s^2) = 10^5 N. To get a force of 10^5 N over 10^16 m (10 ly), you would need (10^5 N)(10^16 m) = 10^21 J. Antimatter is the most energy dense material we know. To get that from antimatter you would need m = E/c^2 = 10^21 J/10^17 m^2/s^2 = 10^4 kg. Therefore your entire ship would have to made out of antimatter and react with some extra matter to propel itself at 1 G.
e^(pi*i) = -1

Offline gospacex

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Re: Constant Acceleration at 1G and Beyond
« Reply #2 on: 06/20/2014 08:38 AM »
constant 1g acceleration is the only way to achieve relativistic effects and get a spacecraft across significant distances in a human lifetime as measured aboard the spacecraft.

Not really. I'd settle for measly 0.1g ;)

Offline john smith 19

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Re: Constant Acceleration at 1G and Beyond
« Reply #3 on: 06/20/2014 10:29 AM »
In space 1g is actually a very high acceleration.

I cannot say about getting to the next star system but just to travel to say Saturn (about 30 AU) is a serious struggle.

What I have seen was a NASA study on a theoretical fusion ship to Saturn. In there the key metric to get to Saturn in < 1 year was that you needed to exceed the suns gravity at the Earths orbit.

The figure they gave for this was 0.6 milli g. It was stated above that the standard transfer ellipse orbits collapse to straight lines.

Maintaining such an acceleration over time is the tough part, but it's 1/1666x easier than 1g.

I'll leave others to comment if this problem is that much easier IRL.
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Offline Nomadd

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Re: Constant Acceleration at 1G and Beyond
« Reply #4 on: 06/20/2014 11:01 AM »
constant 1g acceleration is the only way to achieve relativistic effects and get a spacecraft across significant distances in a human lifetime as measured aboard the spacecraft.

Not really. I'd settle for measly 0.1g ;)
I'm trying to do math in my head here. That would make for something like 12 years to get to Alpha Centauri, assuming you didn't want to blow through the system at .95C?
 The difficulty of relativistic travel tends to be underestimated. Look at it this way. To get to the speed where time travels half as fast for the ship, you also end up doubling the mass of the spacecraft. That extra mass has to be paid for somehow. It's paid for by the energy you impart to the ship through acceleration. If you accelerated that ship by imparting energy from the outside with a perfect, 100% efficient system, you'd have to turn a kilo  of matter into pure energy with no losses for every kilo you increased the ships mass.
 And it's much worse with a self contained rocket. You also need to accelerate the fuel that you haven't used yet.
 So, it comes down to, to accelerate to a speed where time for the ship is 10% that measured by someone standing still and slow down again once you reach your destination, you'd need the most perfect imaginable system (with today's physics) and spend 99.99% of your initial mass to do it.
 Anything better will need more than technology. It will take different physics than we understand now. 
« Last Edit: 06/20/2014 11:23 AM by Nomadd »

Offline KelvinZero

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Re: Constant Acceleration at 1G and Beyond
« Reply #5 on: 06/20/2014 11:44 AM »
I think three big parts of the solution could be
* Beamed power and reaction mass.
* Exploiting a LOT of solar energy.
* Self replicating machines to build stupendously large infrastructure.
eg, eat mercury to create a massive swarm of solar powered lasers, particle beams, or something like that.

The power/propellant you would need to beam from the sun would have to keep increasing, but this could also mean that you can launch before your array is at full strength.

... but there is plenty to do in this solar system before worrying about the next one. There are about 100,000 rocks > 50km in diameter, and each of those has a volume of about 3000 manhattans.. Also there were 178 episodes of Seinfeld. Thats enough room for 5.3 billion episodes of Seinfeld. :)

Offline gospacex

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Re: Constant Acceleration at 1G and Beyond
« Reply #6 on: 06/20/2014 12:00 PM »
constant 1g acceleration is the only way to achieve relativistic effects and get a spacecraft across significant distances in a human lifetime as measured aboard the spacecraft.

Not really. I'd settle for measly 0.1g ;)
I'm trying to do math in my head here. That would make for something like 12 years to get to Alpha Centauri

About right.
At 1g, you get to relativistic speed in about 1 year.
At 0.1g, you won't even reach relativistic speeds on a a Cen trip (4.37ly). So, it'll take several times longer than 4.37 years to reach it.
Farther destinations would not take much longer, since relativistic time dilation would kick in. I estimate you can reach Milky Way core under 100 years of ship time.

BTW, I think 12 years to a Cen is very good. Fantastically short, in fact.
Most of today's astronauts waited longer than that to get to LEO!

Offline aceshigh

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Re: Constant Acceleration at 1G and Beyond
« Reply #7 on: 06/21/2014 11:08 PM »
what about Bussard Ramjet. Couldnīt it provide enough fuel mass for fusioning your way to another star system at constant 1g acceleration?

Offline Burninate

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Re: Constant Acceleration at 1G and Beyond
« Reply #8 on: 06/21/2014 11:39 PM »
Exactly what kind of technological leap and how much energy would be needed to accelerate/decelerate at 1g on a trip to another star system?
The entire time?  A big one.  A lot of energy.

Ignoring the rocket equation, in kinetic energy alone, to accelerate 1000 tons at 1G for 1 *month*, you would need about the same amount of energy as the human race consumed through all sources in 2013 - or around 400 terawatts of power.  That gets you to about 0.1C.  Somewhere around this point, interstellar gas starts to become dangerous ionizing radiation, and interstellar dust molecules become bullets.

Quote
I understand that, barring wormholes, constant 1g acceleration is the only way to achieve relativistic effects and get a spacecraft across significant distances in a human lifetime as measured aboard the spacecraft.
No, there's nothing *special* about 1G, that's just the default number for some science fiction scenarios because it provides a nice non-rotating habitable acceleration frame for the colony of humans on their magical schoolbus to the end of the universe.  Ignore them, as a practical matter.  Also ignore FTL, at least until a physicist demonstrates some theoretical principle under which it might work.

Interstellar craft are such an enormous engineering challenge that it is entirely possible that human flesh will never be a cargo, because by the time we develop the resources for it, humanity will be living in the cloud as uploads.  Controlled human cryogenic hibernation is a fairly reasonable expectation next to the implausibility of interstellar craft.  If we never get that, generational ships are the only option - propelled by extremely high ISP nuclear engines, which thrust at a small fraction of 1G (likely < 1 milli-G) for the entire mission, turning around at some point to match velocities.  We have some of these ideas down on paper, like the fission fragment rocket and nuclear pulse propulsion - but they're just paper designs for now.  We likely wouldn't launch even an unmanned probe-class mission to another star until we have invested perhaps 10,000 times as many resources in spaceflight, as we have up to this point - and a self sufficient manned mission is going to be still several orders of magnitude harder.  The solar system is a big place, and we have many generations of mission concepts yet to be invented before we start leaving it.

what about Bussard Ramjet. Couldnīt it provide enough fuel mass for fusioning your way to another star system at constant 1g acceleration?

Basically anything you or I can come up with that is definitely physically possible is likely to be 'necessary but not sufficient' - for example it might be the case that we'd never get to 0.001G without a Bussard Ramjet to provide motive mass without increasing initial mass up to intractable amounts.  1G is really a very large number.  But these kinds of questions are so far in the future as to not be worth qualifying.
« Last Edit: 06/21/2014 11:51 PM by Burninate »

Offline Greg Hullender

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Re: Constant Acceleration at 1G and Beyond
« Reply #9 on: 06/22/2014 12:19 AM »
Since no one else seems to have run the numbers, let me do that.

First, assume 1g acceleration from Earth halfway to Alpha Centauri and then 1g deceleration from there to the target. Total distance: 4.3 light-years.

Ship time: 3.6 years
Earth time: 5.9 years
Peak velocity: 95% c

Same thing but at 0.1g

Ship time: 12.7 years
Earth time: 13.6 years
Peak velocity: 57% c

Again, but at 0.01g

Ship time: 40.7 years
Earth time: 41.0 years
Peak velocity: 21% c

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
« Last Edit: 06/22/2014 12:26 AM by Greg Hullender »

Offline KelvinZero

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Re: Constant Acceleration at 1G and Beyond
« Reply #10 on: 06/22/2014 02:05 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.

Offline Vultur

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Re: Constant Acceleration at 1G and Beyond
« Reply #11 on: 06/22/2014 08:09 PM »
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.

That involves quite a few assumptions...

-maybe most intelligent species are in environments where developing spaceflight tech is unlikely (say, the majority of living worlds are Europa/Enceladus type, under ice, and they never even learn that space exists) or lack hands (say, dolphins or ravens or parrots become sapient instead of primates)

-or, maybe developing significant technology is simply rare (most of the human race's history was spent as hunter-gatherers)

-or, maybe most intelligent species don't have a 'drive to explore' and never get beyond the continent they originated on, much less into space

IMO we don't even know enough for the "Fermi Paradox" to be a meaningful problem.

EDIT: also, I don't think having the technology for interstellar travel implies exponential expansion through the galaxy, by a long shot. We aren't really colonizing the oceans or turning them into giant farms (eg Arthur C Clarke's "The Deep Range") though we totally could, or trying to turn the deserts into farmland, etc. --- because agricultural technology has more than kept pace with population growth*, and population densities are becoming higher with urbanization. We don't really need more room. Moon or Mars colonization will be driven "because it's there" if it happens IMO, by people like Elon Musk who are visionaries, rather than by pure economics.

*And most first world nations have below replacement birthrates, the 'demographic transition'. So assuming exponential growth for high tech civilizations doesn't seem supported by the one example we have - ourselves.

EDIT x2: Also, the universe is huge. Even if we really are the only sapient species in the galaxy or the Local Group, there could still be a huge number total (universe-wide). At say 1% of c there might not have been time to colonize from distant galaxy clusters.
« Last Edit: 06/22/2014 08:18 PM by Vultur »

Offline KelvinZero

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Re: Constant Acceleration at 1G and Beyond
« Reply #12 on: 06/23/2014 07:04 AM »
That involves quite a few assumptions...
Not really, you are just pointing out all the ways we might be a special case. Of course we could be, but we are unlikely to be. Postulating we are 'alone' does not make us a special case because that is just postulating the average distance between species is some large value. Anyway Im pulling this off topic. I will continue it in another thread if you want to start it.



Offline gospacex

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Re: Constant Acceleration at 1G and Beyond
« Reply #13 on: 06/23/2014 08:38 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.

The hurdle may be already behind us: maybe the appearance of *sentient* (or even just multicellular) life is very rare.

On Earth, it took less than 1 billion year after planet's creation for life to appear. And the environment was quite a bit harsher than now, maybe we should count from Late Heavy Bombardment? Then first life appeared even faster after that event.

But then it took almost 3 billion years for multicellular organisms to appear. It might have been a fluke!

Offline Stormbringer

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Re: Constant Acceleration at 1G and Beyond
« Reply #14 on: 06/23/2014 03:53 PM »
multi cellular...fluke. Haaaa!  ;D
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Offline Nomadd

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Re: Constant Acceleration at 1G and Beyond
« Reply #15 on: 06/23/2014 04:17 PM »
 Somebody has to be first. Might as well be us.

Offline kevin-rf

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Re: Constant Acceleration at 1G and Beyond
« Reply #16 on: 06/23/2014 04:20 PM »
The Fluke might be as simple as Mitochondria and Chloroplasts.

There is a train of thought that they are a bunch of bacteria that ended up being absorbed by large single cell organisms and instead of being digested wound up in a symbiotic relationship with the larger parent single cell organism.

In the case of of plants, Chloroplasts (Plant photosynthesis engines) may have evolved from the symbiotic relationship between the larger cell and cyanobacteria (Which evolved into Chloroplasts).

The odds of single cell life evolving may not be that low, but the odds of single cell organisms absorbing and then entering into symbiotic helper relationships with other single cell organisms that then allow for large multi-cellular organisms may be very, very, low.

See Endosymbiotic Theory: http://en.wikipedia.org/wiki/Endosymbiotic_theory
« Last Edit: 06/23/2014 04:22 PM by kevin-rf »
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Offline Vultur

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Re: Constant Acceleration at 1G and Beyond
« Reply #17 on: 06/24/2014 01:28 AM »
The odds of single cell life evolving may not be that low, but the odds of single cell organisms absorbing and then entering into symbiotic helper relationships with other single cell organisms that then allow for large multi-cellular organisms may be very, very, low.

Why should it be though?
It happened at least twice on Earth (mitochondria and chloroplasts) and organelles also get traded (the sea slug Elysia chlorotica has stolen chloroplasts from algae).

I think the origin of life (which we know basically nothing about -- there are hypotheses but testing them is incredibly difficult to impossible -- even if we could synthesize life from base chemicals in a lab, it wouldn't mean that's how it happened naturally) or the origin of intelligence is a much more likely point than the jump from single-celled to complex life.

Offline KelvinZero

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Re: Constant Acceleration at 1G and Beyond
« Reply #18 on: 06/24/2014 09:02 AM »
Going waaay off topic guys. Sorry if I started this. 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.


Offline gospacex

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Re: Constant Acceleration at 1G and Beyond
« Reply #19 on: 06/24/2014 09:28 AM »
Somebody has to be first. Might as well be us.

Sure. Somebody has to be first. That's not what puzzles people in this regard.

The apparent contradiction is that Universe is 13 billion years old already and we don't see aliens coming to Solar System.
Our Galaxy has 100s of billions of stars, and there are many more galaxies close by. Why none developed starfaring civilization when evidently Earth needed only 4.5 billion years to get there? (almost. give us ~500 more years!)
There has to be a reason for that.

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