Acceleration Gravity

Terraformer · 2007-10-13 10:53:17

Would it be economical to acclerate at 1-2 g for half the journey, turn the craft round, and deccelerate at that g?

And how much g can humans stand for a long time, anyway?

Austin Stanley · 2007-10-13 15:02:38

Would it be economical? Heck no! If you accelerated at 1G constantly, then flipped around and decelerated at 1G you could get to Mars in 1-2 days, depending on the position of the planets when you left. You could get to Jupiter in about a week. Saturn in slightly more, and Pluto in about 3 weeks. After that assuming you can keep it up you would be able to hit Alpha centari in about 3 years (your time), the center of the galaxy in about 20 years (your time) and Andromeda in about 28. This is all assuming you turn around to stop. If you just fly bye, you'll get there even faster.

Of course you can't do this because the fuel requirements are nuts. You would need something with an ISP up in the tens to hundreds of thousands to make it possible even for intra-solar trips. Such acceleration really isn't realistically possible (even if we assume for total conversion of matter to energy) beyond about a month, at which point you've accelerated past .10C. I'm firmly of the opinion that if you had ISPs in the tens of thousands the best option would be to continue taking the slow Hoffman paths and increase your cargo load.

As for how long the human body could take it, obviously we could sustain 1G acceleration indefinitely as that is the amount of force we are normally exposed to. With proper precautions you could probably take 2Gs for longer then your fuel supply could realistically last (ie weeks).

Terraformer · 2007-10-13 15:30:48

If your fuel supply could last for a week you could get to Mars like that. Then to Jupiter. Then stop. Then to Saturn. And so on and on.

Tom Kalbfus · 2007-10-13 15:37:48

Seems like your talking antimatter, Austin. I think Fusion may have the necessary ISP for 1 to 2 day trips to Mars, the question is can it accelerate at 1g? I think for Mars a case might be made that time is money, and the heck with the fuel requirements. Mars is part of the inner Solar System and 2 days vs 9 months, very few people would want to take an 2 year business trip. I can picture a situation where a high muckity much busnessman might want to make a quick trip to Mars to negotiate a deal face to face. Teleconferencing between Earth and Mars is very slow and inefficient, a corporate CEO may be willing to spend the money for a two day trip to Mars, negotiate with their partners on location and then return to corporate headquarters on Earth. The time of a CEO is valuable so the heck with the cost and energy expenditure. Also the heads of nations and diplomats might have their own "space yachts" that are especially dedicated to flitting VIPs all throughout the Solar System, and these people don't have 2 years to spend just getting there.

Austin Stanley · 2007-10-13 16:28:06

Well I wasn't necessarily talking about anti-matter. Either some sort of fusion drive or possibly even a gas core nuclear reactor could have the ISP necessary for 1G acceleration for short trips like to Mars or Jupiter. Most fusion torch drives have ISP up around a million, but if you crank their thrust up to 1G it generally drops.

In terms of Serious long term application of 1G applications of thrust I was speaking of total conversion of mass to energy. IE some magical method of instantly converting matter into energy with 100% efficiency. Anti-matter comes close to this, but is generally not 100% efficient, as some of its energy is almost always unharnessed (the best designs being maybe 60% efficient). But even with such an engine (the best theoretical possible) you really can't sustain such high levels of thrust for very long.

See for a 100% efficient photon drive (ie total conversion) the mass ratio of craft is equal to e^(a*t). My calculator seems to be broken right now, but just eyeballing it I can tell you that even for short periods of time the mass ratios are still huge.

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As for speed vs payload, there may be payloads where this tradeoff makes sense. However you could carry litteraly millions of times more stuff if you went slower, which seems to indicate to me that for a great deal of time-insensitive cargo going slow seems more logical.

Tom Kalbfus · 2007-10-14 12:52:22

Yeah, that makes sense. humans are somewhat perishable cargo though, so the emphasis will be made to shorten their trips, while Cargo which just has to get their eventually, can take the slow boat.

I think the slow boat for interstellar travel will be from 1-5% of the speed of light, while people carrying ships will tend to be around 10% to 20% of the speed of light. Staging would be required of this, and the stages would be non-recoverable, as they'll tend to travel in approximately straight-lines past the destination as the payload slows to match the velocity of the target. I think the best bet for slowing down starships would be the magnetic plasma sail, so you would just need staging to achieve interstellar cruise velocity. I think an interstellar trip for Humans would be a once in a lifetime event, unless the human lifespan is significantly expanded. The cargo cruisers would be single stage to 1-5% of the speed of light and would also use magnetic plasma sails to slow down, but since they are of one piece, they could potentially be used for a return trip. But as you throw away the starship for passenger travel, that will be expensive.

A more efficient means of interstellar travel would involve beamed propulsion. the best candidate I've heard of involves min-light sails and a giant laser projector.

The giant laser projector accelerates the mini-light sails, which are optimised to reflect a narrow range of wavelengths with maximum efficiency, and the laser projector is within that range. The mini-light sails accelerate swiftly to a high percentage of the speed of light towards the starship. That starship then fires another laser at the incoming stream of mini-light sails, this laser beam is outside the range of wavelengths that the light sails reflect efficiently, and thus each light sail absorbs the energy from the ship-originated laserbeam and then vaporizes, becoming an incoming stream of plasma. The incoming stream of plasma closes fast enough so that it doesn't have enough time to disperse enough to avoid the magnetic plasma sail of the starship, and the momentum of the stream gets transferred to the starship causing the starship to accelerate. Slowing down after that is elementary, in that the same magnetic plasma sail intercepts the stellar wind from the target star and uses the momentum from that to slow down and match velocities with that star, then some minor maneuverings with rockets will put the starship in orbit around the target planet.. The stellar wind is not enough to accelerate a starship to interstellar velocities but it can be used to slow down from them. I don't see much practical limit to the final velocity that is attainable to beamed starship propulsion, except of course the speed of light and considerations of your velocity relative to the interstellar medium and so forth.

Terraformer · 2007-10-14 13:23:32

wrote:

I think an interstellar trip for Humans would be a once in a lifetime event, unless the human lifespan is significantly expanded.

How about magmnetic rail gums? If we could use them to accelerate to 0.9c and use plama sails to slow down it would take less than five years to reach Alpha Proxima. Return journeys would be possible.

JoshNH4H · 2007-10-15 06:47:15

How much energy would it take to accelerate a spacecraft big enough to support however many people for 10 years. An how do they return to earth?

samy · 2007-10-15 10:02:56

Target values:

m = 100 000 kg
a = 0.00981 m/s/s (milligravity, one thousandth of a g)
t = 5 years (after halfway point, you have to start accelerating in the opposite direction so you won't overshoot your destination) = 158E6 seconds
v(peak) = a*t = 1.5E6 m/s = 0.5% of c (0.005c) velocity at the 5 year mark
E(peak) = 0.5mv^2 = 120E15 Joules expended to get to the halfway point
P = E/t = 759E6 W
v(average) = 0.5v(peak) = 774 000 m/s average speed for the whole trip
x(total) = v(average)*2t = 244E12 m ~ 1630 AU ~ 0.03 light years

So assuming 100% efficiencies all around (theoretical maximum) a 760MW power plant could accelerate a 100 ton ship at milligravity (1/1000g) for 10 years, getting 0.03 light years away in that time.

1/100g would require 76GW of power, would reach a peak of 0.05c at the five year mark, and would get 0.3 ly away.

1/10g would require 7.6TW of power, would reach a peak of 0.5c at the five year mark (relativity effects not included), and would get 2.6 ly away in those 10 years.

Unless we discover an antimatter mine in the inner solar system, TW ain't gonna happen. Even 76GW is unlikely.

760MW though, I could see as a space power plant one day.

Austin Stanley · 2007-10-15 16:17:00

Samy's pretty much right. Another issue, even if you have an anti-matter power plant is disposing of waste heat. If the plant is not 100% efficient (and no power plant is) then you have some waste heat to dispose of. In the case of a GW or TW plant this can be considerable amounts. For example consider the 7.6 TW plant he proposes (not enough to get your to 1G even for a 100MT spacecraft, which is pretty light for 10 people for 10 years). Assuming it is 60% efficent (which would be very good) you would have to dissipate ~3 TW of energy a second. Which is a heck of a lot of energy, equivalent to a about 700 tons of explosives every second.

Of course this estimation is actually highly inaccurate as it doesn't take into account the fact that you must accelerate your fuel as well as your ship. So to accelerate a 100 MT payload to .5c in fact requires much more fuel.

Pluging into the rocket equation dV = isp*ln(mf/me)
where dV = .5c (150,000,000 m/s)
isp = 30,000,000 (maximum possible 100% efficiency total conversion)
mf = mass fueled (x+100,000 kg)
me = mass empty (100,000kg)

Which means this sucker takes about 15,000 MT of fuel, giving it a mass ratio of 150 which is virtually impossible even for a multi-staged rocket. Of course it gets worse when you consider relativistic effects which are significant at .5c. The lorenzt factor is something like 1.15 for that speed.

Worse yet that just considers acceleration to .5c, if you want to get going that fast, stop, then come back and stop at the other end things get much worse. The dV for that set of maneuvers is equivalent to 2c, and the mass requirements are much worse. Some ~50 billion MT. The mass ratio for this is of course absurd.

Tom Kalbfus · 2007-10-15 23:14:04

Rockets are fairly impractical. Beamed propulsion is the way to go. You basically need a stream of matter traveling at relativistic velocities to transfer momentum to the spaceship. I think if you were to achieve 0.5 c by this method, you'd still need to come up with some other method to slow down. Stellar wind only works when you approach a star, and it is questionable if you could slow down from 0.5 c in the amount of space you'd have if you approached a normal star, well this leaved interstellar ionized gas clouds, such as the Orion Nebula. If you could get your speed up high enough to enjoy the relativistic effects of time dialation, you could then use a stellar cloud of ionized gas to slow your ship down using a magnetic sail, this is somewhat risky as their may be other particles in that cloud other than ionized gases, and the Ship's magetic sail won't affect those. Perhaps a heavy erosion shield could precede the habitat section to impede any particles should they threaten the crew. The ship would then end up in a stellar nusery, which is not a bad place to be. There is lots of material to mine, you have young stars in the vicinity, and perhaps some protoplanets to terraform.

JoshNH4H · 2007-10-16 14:05:59

nebulae, if I understand correctly, aren't very dense. Also, we could only travel to nebulae, and we could never return.

Tom Kalbfus · 2007-10-17 21:24:57

That's just as well, M42 - The Great Nebula in Orion is thought to be more than 900 light years away, and it is one of the closest, the diameter of the Nebula is about 30 light years across. Other Astromoners put the distance of this Nebula at 1300 light years and 1900 light years respectively. The Time dialation for a starship to get their within a single Human lifetime, would have to be 23.75 to 1, for an 80 year trip or 47.5 to 1 for a 40 year trip, in any case you'd return to Earth about 2,000 years after you left, so you might as well make it a one-way journey. I'd suggest to bring everything you need to make you life in the Orion Nebula a comfortable one. I think about 30 light years of ionized gases ought to be enough stopping distance.

Terraformer · 2007-10-19 15:29:31

Accelerating near the speed of light does do away with the aging problem.

Maybe a load of O'Neill habitats as payloads if you want to live in a nebula. And a shipyard. Plus some way of extracting O2, metal, H, and all the other stuff you need from the nebula.

Austin Stanley · 2007-10-19 23:26:57

Rockets are fairly impractical. Beamed propulsion is the way to go. You basically need a stream of matter traveling at relativistic velocities to transfer momentum to the spaceship.

Your right that this is better in some ways then a simple rocket, but it is not without issues.

Firstly the issue of inefficiency and dispensing with the massive amounts of waste heat generate by GW and TW scale power sources. Dealing with it at the plant may not be an issue, but the space-craft still has to dispense with the waste heat generated by any inefficiencies at its end.

Secondly obviously there is the issue of building such large power plants. Be they lasers, partical beams, or rails guns. To give you an idea of the scale, all of Human civilization currently consumes like 2 TW or so right now. So building these things is a huge effort.

Lastly (and most importantly) focusing these beams of interstellar distances is difficult to impossible. As the spacecraft speeds away it gets harder and harder to keep the beam on it. Worse, diffraction and to a lesser extent diffusion (for particle beams) also limit the distance you can beam power. It's really not possible to beam power out past a few tens of AU. This limits the distance in which you can accelerate, and unless you are willing to accept absurd G forces, you can't get going all that fast (maybe .3C) in that time.

I think if you were to achieve 0.5 c by this method, you'd still need to come up with some other method to slow down. Stellar wind only works when you approach a star, and it is questionable if you could slow down from 0.5 c in the amount of space you'd have if you approached a normal star, well this leaved interstellar ionized gas clouds, such as the Orion Nebula. If you could get your speed up high enough to enjoy the relativistic effects of time dialation, you could then use a stellar cloud of ionized gas to slow your ship down using a magnetic sail, this is somewhat risky as their may be other particles in that cloud other than ionized gases, and the Ship's magetic sail won't affect those. Perhaps a heavy erosion shield could precede the habitat section to impede any particles should they threaten the crew. The ship would then end up in a stellar nusery, which is not a bad place to be. There is lots of material to mine, you have young stars in the vicinity, and perhaps some protoplanets to terraform.

Well unless you are going considerably faster then .5c most Nebula are far out of range. The nearest the helix nebula is some ~500ly away. Of course any trip to the stars is probably going to take at least 50 years or so, but the nearest nebula are some ways away. And you are right that a nebula could be dangerous. Heck anything is dangerous whey you are traveling at a fraction of light speed, even interstellar dust. Which, incidentally, provides another method of slowing down. Magnetic braking against the interstellar dust. The problem with this is the dust is (thankfully) very thin, so it takes a long time to slow down this way. Which will increase your trip time. Alternatively some sort of rocket may have to be employed to stop.

How about magmnetic rail gums? If we could use them to accelerate to 0.9c and use plama sails to slow down it would take less than five years to reach Alpha Proxima. Return journeys would be possible.

Are you talking about trying to use a rail gun to accelerate the ship? This is entirely impractical. The length of a rail gun strong enough to accelerate a vessel to .9c (or even much smaller fractions) is impractically large. And if you are constrained to human survivable G forces, then it just gets worse. Recall that at 1G acceleration you would get out to pluto in about a week but even at that point while you are certainly trucking, you haven't even reached .1c yet (more like .01c).

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I also think interstellar travel will almost certianly be one-way. The distances and time involved are FAR to great for it to be anything else. Not to mention the colossal expenditure of energy/production. I'm also dubious that people, as we understand them today, will ever travel to the stars. Any ship providing for human comforts would be massively larger and the trip time (probably longer than 40 years at least) is prohibative on the human lifespan. This requires either hybernation or generation ships, which are even more stupidly huge.

Most likely to me is the seeding of some-sort of von-neuman probe. And possibly by the time we the capability to do that it will be possible to digitise and transmit the human conconiousness and transmit that. That the only way I think we will ever travel to the stars... BUT it would be at the speed of light

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In any case my favorite method of interstellar travel is remarkably simple. Drop a very large mirror (500km+) very close to the sun (.1AU or closer, as close as you can get without burning it). Counterbalance this mirrors mass and reflected energy with the suns gravity to keep it stable. Then focus its beam on a lightweight sail pulling your spacecraft. If your sail is light enough you could achieve extreme acceleration this way, like greater then 9Gs. Not much good out past Earth, but you are going way fast by that time anyways.

Tom Kalbfus · 2007-10-20 00:45:20

Lastly (and most importantly) focusing these beams of interstellar distances is difficult to impossible. As the spacecraft speeds away it gets harder and harder to keep the beam on it. Worse, diffraction and to a lesser extent diffusion (for particle beams) also limit the distance you can beam power. It's really not possible to beam power out past a few tens of AU. This limits the distance in which you can accelerate, and unless you are willing to accept absurd G forces, you can't get going all that fast (maybe .3C) in that time.

Let me describe the set up once more.

1) The laserbeam is short range, its purpose is to accelerate the "pellets" which come in the form of very tiny laser sails. Each tiny laser sail's reflectivity is optimised to reflect the laser light coming from the Solar System based laser. One after another of these tiny sails are accelerated by the giant laser. If heat disappation is a problem then you use thousands of laser sails and you combine them together with a diffraction lens in one single beam.

2) Since the laser sails are very small they can undergo tremendous acceleration and the acceleration distance to reach say 0.999 c is relatively small. After the laser beam spreads out to the point where its no longer effective the laser sails continue on in the interstellar darkness, all of them heading in the same direction at the same velocity.

The Starship simply has to get in front of them and then laser each mini sail as it comes within range, vaporizing and ionizing each so it can hit the magnetic field of a loop of superconducting wire which is in turn tethered to the starship.

The superconducting wire is pushed forward when the plasma stream made out of former mini-sails hits its magnetic field. The superconducting loop is pushed ahead of the starship and a cable connecting the two tows the starship, perhaps at 1g acceleration.

3) The length of the accelerator laser beam may be less than a light day before it diffuses, but that is enough to get the light sails up to speed. And since a line of minisails are solid objects, although very thin, they don't diffuse as a stream of charged particles would. All each sail would have to do is follow the sail in front of it and maintian a constant distance. Perhaps a feedback mechanism would be employed with a tiny electrical ion drive for maneuvering. The tiny laser sails would simply have to stay in formation and be aimed at the target star or object, a task that is much easier than aiming for the starship.

4) The Starship can maneuver in front of the line of sails, vaporizing each one as it comes close, at which point the resulting plasma begins to diffuse, but not enough to miss the starship's magnetic field and impart momentum to the starship.

As you see, its not a matter of aiming the giant system laserbeam at the starship, its a matter of keeping the laser sails in front of the beam while the beam is aimed at the target star, and then its a matter of keeping the starship in front of the stream of sails. Perhaps some form of radar can detect them as they approach, the starship gets in front, and while the starship might not be exactly on target with regard to the star, it can get close enough to use its stellar wind to slow down. And the Great Orion Nebula is 30 light years wide, so if we can aim a telescope at it to take its picture, aiming a laser sail at it shouldn't be too much of a problem.

After that the Starship uses the same magnetic field coil to slow down. 30 light years of nebula ought to be enough.

Now you are in a "Star factory" Much of the material that would go towards making stars in out in space and accessible. Perhaps with "Von Neuman" machines, the colonists there could build their own planets made to order with precisely the right mass and at precisely the right distance from the still forming star. Making a planet ought to be fairly easy, just create a gravitational mass in the cloud and its gravity will attract more dust.

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#1 2007-10-13 10:53:17

Re: Acceleration Gravity

#2 2007-10-13 15:02:38

Re: Acceleration Gravity

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#4 2007-10-13 15:37:48

Re: Acceleration Gravity

#5 2007-10-13 16:28:06

Re: Acceleration Gravity

#6 2007-10-14 12:52:22

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#7 2007-10-14 13:23:32

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#8 2007-10-15 06:47:15

Re: Acceleration Gravity

#9 2007-10-15 10:02:56

Re: Acceleration Gravity

#10 2007-10-15 16:17:00

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#11 2007-10-15 23:14:04

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#12 2007-10-16 14:05:59

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#14 2007-10-19 15:29:31

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