Caplan Star Ship-Side

Caplan Star Ship-Front

Caplan Star Ship-Back


I am writing a story with where the main character discovers this ship traveling to Alpha Centauri because our solar system has been devoured by a black hole. Although I know that a sci-fi can't completely adhere to real science, I would like as much of this ship to be based as much in real science as possible. What parts of this ship design are unrealistic? This ship is based on the Caplan Thruster. The center sphere (yellow) is what's left of our sun after the black hole. The pink shape is the Caplan Thruster. The purple rings are based on the Shkadov Thruster but instead of being the main proponent of thrust, are used to angle all the sunlight around the sun back to the point where the Caplan thruster pulls Hydrogen/Helium from the sun (to heat the surface and push Hydrogen/Helium up into the Caplan Thruster). The orange ring is the control ring that rotates and uses powered moving mirrors to:

helps reflect the light reflected from the front of the sun to the mirrors at the back of the sun (where the Caplan Thruster is) uses those same powered mirrors to reflect some of the light back to the mirror rings (both the front and back) at differing intensities based on their location to push the mirror rings towards the direction the ship wants to turn in (since the control ring rotates, the mirrors that is reflecting the most back to the mirror rings are constantly changing, but the location of the mirror that is currently reflecting the most does not (like the lights used on the this drone to indicate the forward direction here https://www.youtube.com/watch?v=taU7mRTnrtY starting at 2:53 in video)) The blue ring is the actual living area for humans and is like a Halo ring. It is fully enclosed but has ring wide windows facing the sun (but with polarized films to not let too much sunlight in) and rotates along with the control ring (orange) so that the living area ring (blue) maintains earth-like gravity. Of course, there are also plants and animals everywhere in there and the whole inside looks like a slice of Earth with woods, forest, suburbs, cities, farms, ect.

This ship also has a self-sustaining water and oxygen cycle. The sun shining through the ceiling windows heats up the lakes and streams, evaporating the water, where the water then goes up to the glass ceiling where it condenses and rains back down (to keep the rainfall and sunlight diffuse, the ceiling windows have transparent cones all over the inside surface, scattering the sunlight and making sure the condensed water doesn't all run down to the edges of the living area and cause flooding at the edges).

Is there any part of this that doesn't line up with science? Or is there anything missing?

  • $\begingroup$ Some better formatting would be a good thing. Headers and sections. Section one premise /setup. Section two physical configuration. you describe functionality of some of the parts, but nothing of the base composition dimensions etc. The physical configuration could perhaps be split further if you have enough details. Details such as the habitat information. $\endgroup$ Jun 1, 2023 at 21:48
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    $\begingroup$ "What's left of our sun after the black hole" Strikes me as problematic, if a black hole got close enough to strip mass from the sun, how was this ship constructed and placed without being destroyed? $\endgroup$ Jun 1, 2023 at 21:50
  • $\begingroup$ @GaultDrakkor aside from that, if there's enough left to make a star...*that's still a freaking star*. This "generation ship" would destroy any system it visits just by being there. What do the builders have against Alpha Centauri? $\endgroup$ Jun 1, 2023 at 22:25
  • $\begingroup$ @GaultDrakkor by “what’s left of a star” I figured that the black hole moved into the general vicinity of the sun and stripped off its outer layers as the sun swirled around it. However the sun’s core and inner layers escaped and , since there was less mass and heat being generated, had shrunk down to the size of Proxima Centauri. $\endgroup$ Jun 1, 2023 at 22:39
  • $\begingroup$ @ChristopherJamesHuff because Alpha Centauri is so far away and the generational ship sucks the mass from the sun to power the ship, by the time they get to Alpha Centauri, the sun will be gone. The ship is only being used to get to Alpha Centauri B as a new home world. $\endgroup$ Jun 1, 2023 at 22:42

1 Answer 1


Not given the scenario

It is hard to assess the design given that without knowing the size of the "sun" built out of "what's left of our sun after the black hole" but there are some obvious issues, as alluded to by Gault Drakkor in comments. The huge one is that taking the maximum acceleration for the Caplan thruster from the Wikipedia article as being 10^-9 m/s^2, so even ignoring how the mini-sun is built and somehow put into (Earth?) orbit around the black hole, it will only increase its velocity by about 3 x 10^-2 m/s each year

Assuming that it starts in Earth orbit (ignoring how the components were accelerated to that orbital speed), it will take over 10,000 years to reach escape velocity from the solar system. During this time its material needs to withstand constant gamma ray bombardment from the-black-hole-formerly-known-as-Sol and be made of material with sufficient tensile strength to be able to withstand the strain of being spun quickly enough to simulate 1G. We cannot calculate the latter without knowing the size of the megastructure, but it is worth noting that Larry Niven had to create a handwavium material call scrith for his famous Ringworld since no known material had the required strength.

Assuming all that worked, then the construct faces a journey of hundreds of thousands of years to Alpha Centurai, accelerating to near halfway and then ponderously reversing its configuration to decelerate the rest of the way. Which makes this close to being an evolution ship rather than a generation ship. One problem is that in the comments you state that the mass of the mini-sun will be used up by the time the structure reaches Alpha Centurai. The consequence of this is that the Halo ring will become progressively colder and colder during the journey as "stellar" output drops (assuming the mini-sun remains stable and does not catastrophically fail, which is a pretty dubious assumption.) This renders the Halo ring uninhabitably cold before the structure reaches the vicinity of Alpha Centurai...

In summary - stellar engines are supposed to be used to get an entire star moving for millions of years. It is impractical to build a scaled-down version in orbit around one system and take it to another system.

  • $\begingroup$ I appreciate the acceptance, but it is generally best to wait 24 hours before accepting an answer. I could be wrong and not everyone is in this time zone. $\endgroup$ Jun 1, 2023 at 23:16
  • $\begingroup$ for the material problem, is that only for the living area and Control ring? Would separating the large structures into smaller asteroid sized (maybe the size of a state) moons that are still orbiting in a ring pattern be possible? If that does work, could I do the same to the Caplan Thruster, separate it into smaller parts that orbit each other but still interact like a Caplan Thruster (just with higher energy necessary to operate)? $\endgroup$ Jun 2, 2023 at 2:14
  • $\begingroup$ Separating the large structures - no for everything except the halo ring, qualified no for the ring. The main "propulsion" structures are like the chamber of a rocket motor - if you build it in pieces that are unconnected then the pressure in the chamber pushes them apart and the force you want to use for propulsion instead disperses the structure. If you connect the pieces with cables and leave gaps between them then you will be losing thrust through the gaps - it's slow enough already. You can connect the habitable ring pieces with cables, but it does not solve any problems since (cont'd) $\endgroup$ Jun 2, 2023 at 2:39
  • $\begingroup$ (cont'd) the cables and the "solid" sections they are attached to still need to have "scrith-level" tensile strength, you are just losing habitable area. $\endgroup$ Jun 2, 2023 at 2:41
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    $\begingroup$ @Vesper the star in the center is the remnants of our sun after a black hole has absorbed most of it, so yes, still need to escape from the (black-holed) Sun's graity well - which may be more massive depending on the initial size of the black hole that consumed it. (If the black hole had gone on its merry way after eating most of the sun then our solar system would not be inherently uninhabitable, we'd just need to move the planets in closer.) $\endgroup$ Jun 2, 2023 at 8:35

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