Jump to content


Getting an "Insecure Connection" warning for Exisle? No worry

Details in this thread

Light sails in the Andromeda universe


  • Please log in to reply
5 replies to this topic

#1 Ilphi

Ilphi
  • Islander
  • 4,071 posts

Posted 23 April 2003 - 12:32 PM

Heya everyone… well I’ve been interested in Light Sails for ages now (and have posted several threads both here and on SlipstreamBBS) so now I’m back again ^_^

Okay, my hypothetical situation is this: A race is advanced and developed good space travel, infrastructure and technological development (EG At the level for nanobots – And yes I know its ridiculous to gauge a cultures development on one piece of technology but what I mean is they have AG fields, Nanobots, basically most of the other stuff the High Guard did) BUT they are not in the known worlds so have encountered no other races (and since only 2 races are known to have developed Slipstream independently, the Vedrans and the Than) they DON’T have slipstream drive.

So their path to the stars will be attempted with solar sails.

My question is, with AG fields that IIRC can “make Andromeda’s mass that of a glass of water” and some fairly ludicrous manufacturing capabilities (somehow making the material that is practically 100% reflective), how quickly could they go? I’ll also assume that this race has advanced reactors capable of generating the hundreds of terawatt beam needed to accelerate these things and the targeting capabilities to track them at many light years away.

(Bear in mind at this point I’m working on assumptions alone because I know little about the actual physics involved)

The sail has a diameter of say 300km and is capable of having the whole sail deal – booms, masts and stays (http://www.ugcs.calt...pes/3-axis.html) what kind of speed could they be getting out of this since their weight will be extremly reduced? So they could have a ship with massive mass (say 30,000,000kg inertial mass, but reduced to such a miniscule amount) and i'll just say that they have the extrodinary industrial abilities to make this gigantic project.

Yes I know the concept is pretty fanciful but what can I say… just interested.
Yea, ere my hot youth pass, I speak to my people and say:
Ye shall be foolish as I; ye shall scatter, not save;
Ye shall venture your all, lest ye lose what is more than all;
Ye shall call for a miracle, taking Christ at His word.
And for this I will answer, O people, answer here and hereafter,
The Fool - Padraic Pearse

#2 Christopher

Christopher
  • Demigod
  • 32,915 posts

Posted 23 April 2003 - 03:28 PM

Well, in space, the only limit on an object's speed is how long you can keep accelerating it.  Of course this would have to be a laser-propelled lightsail rather than a solar sail per se, since sunlight becomes too weak to be practical for propulsion beyond the star's own planetary system.  So it's entirely a question of how powerful a laser beam you can generate to push it and how long you can keep it firing.

Although my past conversations with Paul, Zack & Ash suggest that as a ship goes faster, increasing its inertial mass relativistically, that makes the mass-reduction AG fields less efficient, and you reach a point of diminishing returns.  So that suggests that if the AG fields are comparable, then the practical limit on velocity would be analogous to that of a High Guard vessel, maybe 45-50% of lightspeed.

Of course, one plus of this system is that deceleration is easy -- just increase your mass to normal, and your velocity plummets in order to conserve momentum.  Normally with an interstellar lightsail you'd have to have a decelerating laser already built in your target system, unless you use Robert Forward's idea of concentric rings that split into two, the outer one moving ahead and reflecting the beam from the home system back onto the inner sail from the front, slowing it down.

Now, I'm not sure your scheme would be practical, since AG fields consume power, and it would still take years or decades for the sailship to reach its destination.  It might not be able to carry enough fuel to go the distance.  And the more fuel you put on, the more mass you have to reduce, so the more fuel you need -- it's the same diminishing-returns spiral as any vessel that carries its fuel with it.  So that pretty much negates the fundamental advantage of lightsail craft, the fact that they don't need to carry their own fuel.
"You don't use science to show that you're right, you use science to become right." -- xkcd

"The first man to raise a fist is the man who's run out of ideas." -- "H. G. Wells," Time After Time


Written Worlds -- My homepage and blog
Facebook Author Page

#3 Ilphi

Ilphi
  • Islander
  • 4,071 posts

Posted 23 April 2003 - 04:27 PM

Very true, thanks for the insight.

Do we have any kind of idea how much power a AG field is supposed to guzzle?

When it comes to acceleration, most of the schemes I have seen say that maximum acceleration is about 0.2gees. Am I correct in assuming this limit is imposed because at higher acceleration the particle matter in front of the sail could very well tear the sail, and also because of the apprant "flimsy" nature of the sailers construction, and the idea that the whole thing would just disentegrate?

Well, working on more assumtions that the AG fields can also deflect away the incoming particulate matter and coupled with the impressive construction materials withstand more, could you increase the maximum acceleration?
Yea, ere my hot youth pass, I speak to my people and say:
Ye shall be foolish as I; ye shall scatter, not save;
Ye shall venture your all, lest ye lose what is more than all;
Ye shall call for a miracle, taking Christ at His word.
And for this I will answer, O people, answer here and hereafter,
The Fool - Padraic Pearse

#4 Delvo

Delvo
  • Islander
  • 9,273 posts

Posted 23 April 2003 - 04:47 PM

Ilphi, on Apr 23 2003, 07:11 AM, said:

most of the schemes I have seen say that maximum acceleration is about 0.2 gees. Am I correct in assuming this limit is imposed because at higher acceleration the particle matter in front of the sail could very well tear the sail, and also because of the apprant "flimsy" nature of the sailers construction, and the idea that the whole thing would just disentegrate?
The main limiting factor is really just the fact that light doesn't push matter very efficiently, which means you need to produce lots of light to get even a low-mass object moving at all, and producing lots of light tends to mean using large, powerful equipment, which adds to your ship's mass. 0.2 g isn't very fast and space matter is very thin, so that's certainly not where the limit comes from. Many airplanes, missiles, and space vehicles flying today can do multiple-g acceleration flying in a straight line, and that's against much thicker air; the difference is that their engines have more power relative to the planes' mass, and a light engine is just inherently a low-power way to try to get an object moving.

Low power can be alright if it's constant and long-term. That's the idea behind ion engines for deep-space probes; the push isn't very strong but it's always there, minute after minute, day after day, and the fuel lasts a long time before running out and leaving the thing coasting at whatever speed it had reached by then. But getting that out of a laser-driven light vehicle would mean spending LOTS of energy on that laser.

#5 Ilphi

Ilphi
  • Islander
  • 4,071 posts

Posted 23 April 2003 - 05:04 PM

Well, so the problem isn't with the ship at all, but the fact that the laser pushing it would need to be scarily powerful?

#6 Christopher

Christopher
  • Demigod
  • 32,915 posts

Posted 23 April 2003 - 06:44 PM

Delvo, on Apr 23 2003, 08:31 AM, said:

0.2 g isn't very fast and space matter is very thin, so that's certainly not where the limit comes from. Many airplanes, missiles, and space vehicles flying today can do multiple-g acceleration flying in a straight line, and that's against much thicker air...
But you're forgetting that the lightsail itself is mere molecules thick.  Breathe on it and you tear a hole in it.  Since it's so fragile, it would be vulnerable to a much, much thinner concentration of gas than the thick metal skin of an airplane or missile.

However, I don't know enough to say that the friction of the medium in space would be the reason for that acceleration limit -- just that it can't be ruled out.
"You don't use science to show that you're right, you use science to become right." -- xkcd

"The first man to raise a fist is the man who's run out of ideas." -- "H. G. Wells," Time After Time


Written Worlds -- My homepage and blog
Facebook Author Page


0 user(s) are browsing this forum

0 members, 0 guests, 0 anonymous users