Author Topic: Galaxyship  (Read 2242 times)

Offline qraal

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Galaxyship
« on: 02/13/2017 11:17 AM »
Robert Page Burruss described, in the Sept/Oct 1987 "The Futurist" magazine, his concept for an intergalactic space vehicle to M31. He described a disk, some 1,000 miles in diameter, composed of a billion hexagonal sub-units, propelled by matter-antimatter. Initial mass was 1 trillion tons, but would decrease to 10 billion tons once it had arrived in Andromeda after trekking "2 million lightyears" (now know to be ~2.55 million) at 0.4c. The population would be initially 100 million, rising to perhaps 10 billion over the journey. Essentially an uber-scale Worldship.

The acceleration to 0.4c would take 50,000 years, after an initial maneuver to line up with M31 to avoid the dust-lanes of the Milky Way.

When I first read the description, almost 30 years ago, I thought it was supposed to be spherical. However, after finally getting a copy, it was initially a disk, though it could reconfigure as convenient.

The mass-ratio of 100 as a seemingly arbitrary choice seems odd. A cruise speed of 0.4 c has an energy-fraction minimum when the mass-ratio is about 5. The actual fraction of antimatter to final payload mass is quite markedly different - 0.845 vs 0.3. Burruss imagined the antimatter would be made from the Sun's energy via some means, but I think the whole premise of needing to carry antimatter was misguided. Make the Galaxyship reflective enough and fire all those sun-collection beams straight at it. The middle-man is eliminated.

At destination, a giant magnetic-sail would be sufficient. I can't blame Burruss for not using it, since it didn't get officially invented until c.1988 or so. But Freeman Dyson and Greg Matloff had independently discussed magnetic drag options in the late 1970s.

Well that was 30 years ago. Can we do something better?


Offline qraal

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Re: Galaxyship
« Reply #1 on: 02/13/2017 07:51 PM »
Area of the Galaxyship is 2E+12 m2 and the starting mass is 1E+12 tons, thus 500 kg per m2. 99% is reaction mass. If take the payload mass and the same area, then to reach 0.4 c in 50,000 years needs a flux of 60 kW/m2. Thus a reasonable reflectivity requirement is sufficient to avoid over-heating. Keep the beam focused on the Galaxyship will need a huge optical system, but nothing onerous for a Type-II Civilization that can comtemplate building a Dyson Shell or similar power supply.

Exactly how one builds a ship that masses ~5 kg per square metre is a whole other challenge!

Offline lamontagne

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Re: Galaxyship
« Reply #2 on: 02/13/2017 09:31 PM »
Wonder why they didn't spin it?  Guess mere gravity was a technicality for such a grandiose plan!
And why so fast?  40% of the speed of light is more trouble than it's worth if you're planning on this scale, I believe.
Wonder how this would compare to a large carbon fiber orbital with walls and an atmosphere?

Offline qraal

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Re: Galaxyship
« Reply #3 on: 02/14/2017 06:36 AM »
Structural limitations, I'm guessing. Spin-gravity also ties up a lot of energy in the vehicle, which would make potential impacts quite catastrophic. I'm trying to imagine how it could spread 5 kg of payload per square metre. Air alone has a 1.2 kg/m3 density.

 
Wonder why they didn't spin it?  Guess mere gravity was a technicality for such a grandiose plan!
And why so fast?  40% of the speed of light is more trouble than it's worth if you're planning on this scale, I believe.
Wonder how this would compare to a large carbon fiber orbital with walls and an atmosphere?

Offline qraal

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Re: Galaxyship
« Reply #4 on: 02/15/2017 08:28 PM »
With a mass loading of 5 kg/m2 and a perfectly reflective surface for the laser frequency used, the flux required for an acceleration of 9E-4 m/s2 is 675 kW/m2.

Offline gospacex

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Re: Galaxyship
« Reply #5 on: 02/15/2017 10:30 PM »
Intergalactic flight is hard. For example, you never know when you are ready to launch your ship. Because distance is so huge, even 0.0001% improvement in cruise speed thanks to future technology improvements means you are better to wait a century or two for this to happen :)

Offline lamontagne

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Re: Galaxyship
« Reply #6 on: 02/16/2017 01:23 AM »
Structural limitations, I'm guessing. Spin-gravity also ties up a lot of energy in the vehicle, which would make potential impacts quite catastrophic. I'm trying to imagine how it could spread 5 kg of payload per square metre. Air alone has a 1.2 kg/m3 density.

 
Wonder why they didn't spin it?  Guess mere gravity was a technicality for such a grandiose plan!
And why so fast?  40% of the speed of light is more trouble than it's worth if you're planning on this scale, I believe.
Wonder how this would compare to a large carbon fiber orbital with walls and an atmosphere?
I don't know, as the ship is going at 40% of the speed of light, the added velocity/energy of rotation is negligible, isn't it?  Each segment is independent anyway, as far as impacts go.  And at these velocities a multilayer whipple shield would be pretty effective, as anything that hit the firt layer would turn into a plasma...


Offline qraal

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Re: Galaxyship
« Reply #7 on: 02/16/2017 09:24 AM »
The energy I am considering is in the vehicle's boosted reference frame. A structural failure would quickly become catastrophic if the whole 1,600 km structure spun.

Structural limitations, I'm guessing. Spin-gravity also ties up a lot of energy in the vehicle, which would make potential impacts quite catastrophic. I'm trying to imagine how it could spread 5 kg of payload per square metre. Air alone has a 1.2 kg/m3 density.

 
Wonder why they didn't spin it?  Guess mere gravity was a technicality for such a grandiose plan!
And why so fast?  40% of the speed of light is more trouble than it's worth if you're planning on this scale, I believe.
Wonder how this would compare to a large carbon fiber orbital with walls and an atmosphere?
I don't know, as the ship is going at 40% of the speed of light, the added velocity/energy of rotation is negligible, isn't it?  Each segment is independent anyway, as far as impacts go.  And at these velocities a multilayer whipple shield would be pretty effective, as anything that hit the firt layer would turn into a plasma...

Offline lamontagne

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Re: Galaxyship
« Reply #8 on: 03/06/2017 01:44 PM »
The energy I am considering is in the vehicle's boosted reference frame. A structural failure would quickly become catastrophic if the whole 1,600 km structure spun.

Structural limitations, I'm guessing. Spin-gravity also ties up a lot of energy in the vehicle, which would make potential impacts quite catastrophic. I'm trying to imagine how it could spread 5 kg of payload per square metre. Air alone has a 1.2 kg/m3 density.

 
Wonder why they didn't spin it?  Guess mere gravity was a technicality for such a grandiose plan!
And why so fast?  40% of the speed of light is more trouble than it's worth if you're planning on this scale, I believe.
Wonder how this would compare to a large carbon fiber orbital with walls and an atmosphere?
I don't know, as the ship is going at 40% of the speed of light, the added velocity/energy of rotation is negligible, isn't it?  Each segment is independent anyway, as far as impacts go.  And at these velocities a multilayer whipple shield would be pretty effective, as anything that hit the firt layer would turn into a plasma...
Lot's of redundancy? 
Still, the idea isn't to redesign the ship but to understand it.  Wonder what they felt about the long term effect of 0g?  did they really expect the crew to still be 'humans', or some kind of augmented cyborg, or evolved entity?
And what was the plan once Andromeda was reached?  Zip around and look at the sights, I guess?


Offline stefan r

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Re: Galaxyship
« Reply #9 on: 04/23/2017 02:50 PM »
The energy I am considering is in the vehicle's boosted reference frame. A structural failure would quickly become catastrophic if the whole 1,600 km structure spun.

Structural limitations, I'm guessing. Spin-gravity also ties up a lot of energy in the vehicle, which would make potential impacts quite catastrophic. I'm trying to imagine how it could spread 5 kg of payload per square metre. Air alone has a 1.2 kg/m3 density.

 
Wonder why they didn't spin it?  Guess mere gravity was a technicality for such a grandiose plan!
And why so fast?  40% of the speed of light is more trouble than it's worth if you're planning on this scale, I believe.
Wonder how this would compare to a large carbon fiber orbital with walls and an atmosphere?
I don't know, as the ship is going at 40% of the speed of light, the added velocity/energy of rotation is negligible, isn't it?  Each segment is independent anyway, as far as impacts go.  And at these velocities a multilayer whipple shield would be pretty effective, as anything that hit the firt layer would turn into a plasma...

1600km can fit a 500 meter centrifuge in a lot of places.

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