# Building an interstellar starship during transit

Would it be feasible to have an interstellar vessel be constructed in flight after smaller sections are boosted up to speed? The idea here is about using laser sails which would be easier to use with somewhat smaller designs, because of problems with areal density, the mass carried per unit of area(a consequence of the square/cube law).

The problem is that once you're up to speed, you'd really want artificial gravity. A ship small and light enough to make laser sails practical would be hard to give decent artificial gravity. So one potentially interesting solution occurred to me. Instead of building the ship before launching it, take smaller independent sections of the ship and launch them individually, building the full sized ship once they are all together. Does this seem feasible, once you accept the ideas of laser sails and interstellar vessels in the first place?

Another upside is that there is something of an interesting dramatic hook to the idea that the smaller ships are launched independently and form their own societies before coming together.

EDIT: To clarify, I mean in deep space, at a fairly high speed of around 10-20% of light speed. I'm assuming the big risk during construction would be that of collisions.

• Clarification: wouldn't such a large vessel already be constructed "in flight"? After all, it's moving in orbit as it's being built. Or, do you mean sections are sent off into the depths of space and each succeeding component has to be sent out to catch up? Mar 31, 2020 at 0:51
• Docking with an unfolded laser sail seems like a good way to lose your laser sail. Mar 31, 2020 at 1:01
• Why can't you give your small light ships artificial gravity? Cant you spin them? Mar 31, 2020 at 1:10
• @Wilk You can, but it is vastly easier with a larger design. I'm very loosely basing this on something from GURPS Spaceships, in which the largest possible ship with a solar/laser sail is SM+12(100k tons), and the smallest ship with 1G via spin gravity is SM+14(1M tons). Mar 31, 2020 at 1:30
• I'm not really seeing the benefit of splitting up the ship, particularly when it does not carry its own propellant. Accelerating 10 ships of mass M to speed V will require 5MV^2 J, which is the exact same amount of energy required to accelerate one ship of mass 10M to speed V. A ship will accelerate at the exact same rate as one weighing 10x as much, so long as it has 10x as large laser sail. Mar 31, 2020 at 13:28

## 5 Answers

I don't think this is wise for laser-sail propelled craft.

Okay, you've thrown Spacecraft 1 at 10% of the speed of light, spending several months to years on your laser array running it up to that speed. Time to throw spacecraft 2.

You now have to throw Spacecraft 2 at Spacecraft 1, and throw it significantly faster. Upon reaching Spacecraft 1, Spacecraft 2 must guide itself close enough to spacecraft 1, and match velocities and dock.

How this happens is an interesting question; Perhaps Spacecraft 1 is using its laser sail as a mirror to reflect the laser now propelling it back at Spacecraft 2's sail. But if this process fails, Spacecraft 2 closes on Spacecraft 1 with a relative velocity at closest approach of a small fraction of the speed of light. The best-case scenario in this situation is that the spacecraft miss each other, decelerate at the destination, and rendezvous at saner velocities.

Oh, and Spacecraft 3 is on the way, moving even faster.

Either way, it is significantly less hassle to either build your laser sail colony ship at your spacedock, where you can make sure everything fits together and tests properly while there's still a shipyard within reach, or to throw multiple spacecraft that will function independently for the entire length of the journey.

• Why can't you throw them simultaneously? Mar 31, 2020 at 22:02
• @AsteroidsWithWings If throwing them simultaneously is an option, then the proper thing to do is probably to attach them to each other before throwing anything, allowing you to cut down spacecraft and mission complexity, and reduce total mass thrown by eliminating redundancies. Mar 31, 2020 at 22:13
• I conceptually love the idea of reflecting the laser back on the second craft, it makes a beautiful space ballet. Here's how I imagine it: When the second craft gets close enough to the first, the first deforms it's sail to spread the incoming light out just a bit, and the second deforms it's sails to have a hole in the middle and catch the light being reflected from the first. This solves the breaking problem in a mesmerizing way. I think though what matters is the time frames you would need between craft. I think it does work on some timescale, but what is that timescale, and at what speeds? Apr 1, 2020 at 2:10
• @GarrettMotzner You're right about the difficulty of speeds & timescales. Timescales for the laser-sail boost phase are important too. Back-reflection is one way to match velocities. Nice idea really. By "breaking problem" I think you mean "braking problem." Our fingers disobey our brains with frightening frequency. Apr 1, 2020 at 6:17
• @AsteroidsWithWings See my comment to notovny below. It's relevant to your simultaneous launch suggestion. Apr 1, 2020 at 6:19

Hula hoop ships.

Your sail ships are large, light hoops. The sail is interior. The hoops are spinning. This serves double duty - the hoop will stay oriented on the laser pushing it because of gyroscopic stabilization. Also you will have artificial gravity everywhere in the hoop because of the spin.

If the spin rate is standardized that should not interfere with joining ships via stacking (not edge to edge). You would just need to match speeds.

• An amusing answer, but it doesn't answer the actual question. This is purest hand-waving. If you just do it, it works. Does this make it feasible? Who knows? Sure centrifugal spin solves the artificial gravity problem, but that wasn't the question. Mar 31, 2020 at 22:02
• @a4android I think that this answer would work as a frame challenge for an X-Y problem. Apr 1, 2020 at 5:21
• @nick012000 Not really. It doesn't come close to being a frame challenge answer. I like good frame challenges, they can be the best answers, but this isn't one. Apr 1, 2020 at 6:11

Not if they all embark from a parking orbit at the same time

Arranging for intercepts is essentially not possible.

The whole idea behind a laser sail is that it's a very small force applied for a very long time to achieve a very high velocity along a very specific vector. None of that lines up with what you'd need to achieve an intercept, which would be large forces along a different vector in short times, so that you can change your orbit to match your targets.

That would need to be achieved via conventional thrusters, which means that if you launch at dissimilar times, you're stuck carrying a TON of fuel, because in order to catch up you'd need to go MUCH faster then the very fast thing, and then slow down a LOT.

However, if they are launched in formation, they should have very similar relative speeds. This is identical to 'floating' next to eachother in similar orbit. The energy and fuel necessary to bring the pieces of the craft together after the desired orbit has been achieved should be fairly minimal, because their orbits will all be extremely similar.

This answers the logistical orbital questions, but it completely elides some, serious, larger concerns about this plan:

1. You are accelerating via laser sail, which means you will be accelerating for a very, very long time
2. You are bringing humans along
3. A laser sail cannot be reversed

So, you have at least one manned ship in this fleet that needs to carry supplies and living quarters for the crew, and those supplies must last for at minimum, the entire duration of the acceleration, which is likely measured in years.

You're traveling at .1C relative to where you left, which is almost certainly a relative speed to the destination body that is a significant fraction of C. This also means that any mass that you pass along the way probably also has an enormous relative speed, which means gravitational capture or reverse slingshot is essentially off the table - You're going too fast for that to work.

You could use a laser sail to slow down, and launch a really powerful laser ahead of your crew - But you don't have one large enough for the new giant craft you just built.

Assuming you manage to engineer a solution to the issue of supplies, you still have a crew that is basically riding a space bullet with no brakes. You need an alternate system of propulsion capable of supplying the same order of magnitude of delta V that your construction fleet received from your space laser array - Which brings to mind the question of why you didn't just embark with one of those instead.

The concept is feasible. Launching multiple smaller ships that later on combine into a bigger vessel could be workable. There will be issues about matching velocities. This depends on either launching them all simultaneously or at different times.

However, smaller ships carrying personnel to form their own societies would have to be fairly big in their own right. This goes against most definitions of small. There would be no need for them to combine together to make a bigger ship.

Most ships boosted by laser sail work best if they have low mass. So if a swarm of small ships with small crews are launched and boosted up to speed by laser sails, then when they come together reorganizing multiple tranches of small groups of people into one much larger social organization that could be very tricky. As anyone with people management experience would tell you. Consider how something like organizational hierarchy would need to be established when you have many multiple groups where each small group has equivalent sets of skills as any other small group. So how do you decide who is going to be in charge? Tricky!

• They play limbo, last person standing is captain. Mar 31, 2020 at 1:18
• The problem with velocity is what magnetic sails are for, as they allow the ships to match velocities as they brake against the interstellar medium. The downside is that this extends the trip. As for governance, a sort of parliamentary system might be interesting, in which the captain of each small ship becomes the section's representative to the larger council, electing an overall captain. You could also have the interesting political position of Shadow Captain, from the opposition parties. Mar 31, 2020 at 1:53
• @AdamReynolds Magnetic sails? Your question didn't them. Matching velocity is easy if all the ships are close. The real problem is laser boost time and distance separation.All the small ships boosting together, easy. Boost times could be centuries to get to 20% lightspeed, add deceleration to bring them all to cruising velocity & assemble. This might take as long as the trip itself. Mar 31, 2020 at 21:49
• @AdamReynolds A parliament is a good idea. But privilege rank? The electorate could be drawn from all the crew(s). Much fairer that way. Mar 31, 2020 at 21:51

A related, though slightly different idea that would take advantage of the natural self-replicating capabilities of human beings.

(1) Send a small group of humans to a Kuiper belt object, say 100km in diameter. (2) Build a fusion reactor and use it to power a mass driver at the surface. (3) Use the mass driver to accelerate bulk material from the surface at 30km/s, say. The KBO will gradually gain orbital energy and will spiral out of the sun's gravitational field. (4) Continue to operate the mass driver and adjust the course of the KBO such that it is on course for a close flyby of one of the closer stars, say, Proxima, which is 4.2LY away (39.7tn km). With a final escape velocity of 20km/s, the KBO would reach its destination in 63,000 years. (5) In this transit time, the initially small group of humans would devote their efforts to paraterraforming the KBO, using the fusion reactor as a source of energy. Their numbers would gradually increase and as their destination neared, they would cannibalize the KBO to build colony ships, that would decelerate into orbit about the star.

• This isn't a good idea in some ways, in terms of efficient use of resources, but it would likely not work. If you need to take this long to get to another solar system, there is virtually no chance that you'll arrive before someone else who comes up with a faster way to get there. This problem is known as the wait calculation. This is an argument in favor of something like a laser sail in the first place, because you'll likely get there fast enough that this doesn't matter. Mar 31, 2020 at 5:34
• This is an interesting suggestion, however, it doesn't answer the actual question. laser sails won't be fastest way to the stars, but faster than a KBO colony ship. The OP wants to get his ships up to 0.1 to 0.2 c, & proposes laser-sails as the way to do this. I did like the fact your answer was well-thought through and hope to see more contributions from you here. Keep up the good work. Apr 1, 2020 at 6:31