Will building spaceships in space allow for gigantic spacecraft?

In My Humble Opinion
1

Just as a layperson casually looking at the different sizes of land, air, and sea vehicles, I noticed that the seaworthy ones seem to be the biggest. If I had to guess, I’d say that’s because the sea has the convenient property of supporting the weight of heavy vessels as well as low-energy costs to get them moving, compared to the land.

With this in mind, I have to wonder if a space-based manufacturing facility would allow for enormously sized ships to be built, like the largest class of Star Trek enterprise strips. Space as a low-energy cost for moving around as well.

On the other hand, maybe the manufacturing process itself would be the limiting factor. Obviously we have nothing now, and maybe it’s unrealistic to assume we can even build such a facility in the future.

Can any engineers comment on this?

2

Not an engineer but you have to get all that material up there somehow.

3

A spacecraft that only travels in space doesn’t need to worry about Earth’s gravity or withstanding the forces involved in takeoffs & landing, and so yes; such craft could be made far larger than one built on Earth.

Unless you use material that’s already there, like an asteroid.

4

Yes. As you say, without the need to have the ship be capable of supporting itself against gravity, there’s effectively no limit to the size of ship you could build, given infinite building materials. For an existing example, the ISS was assembled in orbit, and is about 4x too big/heavy to be lifted into orbit by even the largest rocket ever built.

However, as running coach pointed out, you have to get the materials there to build things with. Lifting them from the planet’s surface is tremendously expensive, and while it’s conceivable that we might start using resources that are already there, in the form of captured asteroids, that capability is many decades or even centuries away.

5

If this super ship can’t withstand gravity, you need to find some reason to build it, no?
Maybe just a robot cruising around and sending back pics? How large would that need to be?

If you want it to actually carry something, how do you load/unload it?

6

However, we still usually build them on land. I don’t know if that means anything.

7

I was just thinking about his watching the Orion launch & return. It’s the beginning of the Mars era! Really? You’re going to send astronauts to Mars in that? How the hell are they supposed to get back, nevermind people somehow staying alive, sane and healthy for so long, in such a small capsule?

It seems we’re actually thinking about launching a small craft from Earth and having it make it all the way to Mars and back when it seems to make more sense (and more expensive of course) to assemble something more like the ISS, and send that to Mars and back.

8

Non practicing structural engineer checking in.
I assume the OP is referring to thea common sci-fi trope where space ships in the future are often miles long for one reason or another. Warships like the mile long Imperial Star Destroyer (which are dwarfed by their command ship, the Executor). Mining ships like the Red Dwarf. Capital ships from games like EVE Online or Halo. So on and so forth.
It’s a misconception that “weightless” is the same thing as “massless”. Being in space far enough from a planet, a large ship won’t experience gravity. Which basically just means it won’t collapse under it’s own weight like a house of cards. Also, unlike a terrestrial mega-ship like the USS Nimitz class aircraft carrier or Maersk Triple E class cargo ships, a ship in space doesn’t have to worry about air or water resistance as it moves.

However all objects have mass. And the more mass an object has, the more energy that is required to push it around. E=1/2 mv^2. And unlike a seagoing vessel or aircraft, a space ship has to bring with it all the mass it will need to move it long. Newton’s Laws.

So IOW, most of your cargo hold for your mile long space freighter will likely consist of reaction mass needed to move the damn thing.
Theoretically, we probably could build large orbital structures that are mostly thin-skinned hollow habitats. Kind of like the International Space Station at a much larger scale. But barring exotic technologies like “inertial dampeners” or “structural integrity fields” (which are really just “change the laws of physics emitters”), I think at some point such a large object would be affected by tidal forces (as in the parts closer to Earth feel gravity more strongly than those parts further away).

9

Well, gravitational force decreases with the square of distance, but never disappears.

If its motion is unrestrained, it won’t “feel” gravity. And this is true even very close to a planet (e.g. in low earth orbit).

10

There was this 90 million mile gold spaceship once…

Once you get beyond a certain size, you start losing utility to bracing and structural elements. If you could build a monster spacecraft…you might be better off building a fleet of smaller craft instead. You’d get increased mission versatility, if nothing else.

11

NASA does not plan to send astronauts all the way to Mars in just that small capsule. Orion would be used to transfer the astronauts to some kind of Mars Service Module (which has yet to even be designed). This module would be launched separately, possibly in multiple launches and assembled in orbit. Once the module was ready and stocked with all the food, water and air needed for the trip, the astronauts would ride into orbit in the Orion and dock with the Mars ship.

12

Another drawback to building a ship a mile long by a half mile wide by a quarter mile tall. Besides the issues of pure mass pointed out by msmith537 is being able to apply the force to get it moving without tearing it apart. Then, heaven forbid, turning it without having a handy gravitational field in exactly the right location to apply a fairly evenly distributed force to modify your vector. Everything you do to beef up the structural integrity is most likely going to be adding even more of the previously mentioned mass.

13

What about the size of the ship increasing the likelihood of it hitting something or something hitting it?

Is this a factor or are we talking a miniscule increase in risk?

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Will building spaceships in space allow for gigantic spacecraft? - Straight Dope Message Board

Not an engineer, but I do understand the basics.

Building in freefall (there’s actually not any such thing as “zero” gravity, especially not in Earth orbit) could help build bigger stuff, but there are definite limits. Generally, the bigger something is, the more it masses. Without exotic materials to reduce mass while retaining high strength, you quickly run into structural limits unless you’re never planning on subjecting the craft to any maneuvers beyond fractions of a G.

If you build it big enough, you probably don’t have to worry about that because you’ll run into the limits of our drive technology. We’ve currently got 3 main methods of moving mass around in space: 1) Chemical, 2) Electrical, 3) Light sail.

Chemical rockets can move things fast, but the necessary reaction mass is high also. You quickly reach the point of diminishing returns, where it costs you enormous amounts of fuel just to move your fuel around, much less the structure the engine is attached to. Not practical for moving anything big very far very fast, unless you have a way of getting fuel en-route.

Electrical propulsion (ion drives) don’t move anything very fast, but they are really fuel efficient and (eventually) can get you moving at a pretty good clip. Unfortunately, the amount of thrust you get from our best engines is really, really small. As in “insect-like” small. But you can run the engine continuously for tens of thousands of hours. To use a car analogy, an ion drive has a “0-60” time of a snail, but its top “speed” will (eventually) blow the doors off a Ferrari. Hope you don’t mind taking a few years to back it out of the “garage” as it were.

Light sails are theoretically possible, and sorta-kinda proven on a small scale, but no one has made a real-life version bigger than a few kg. The acceleration on these would be pretty abysmal too.

If you’re thinking about carrying humans — which is presumably the reason you’re thinking about building bigger ships — you need to make some parts pretty strong. You can make anything outside the habitat as flimsy as you need to, but the parts people actually need to use and move around inside of need to be strong enough to hold atmosphere at breathable pressures, have enough radiation shielding so we don’t cook, and robust enough that one of us hairless apes doesn’t accidentally tear through the hull.

15

Earth’s orbit is starting to get pretty crowded with space debris. A giant 2 mile long ship would certainly make a pretty large target. Of course, it’s so big that a hit by lug nut from an a old Soviet satellite traveling at orbital velocities would be a lot less catastrophic.
Of course, if you large structure didn’t have to move (like a space station for instance), then the limiting factor once again becomes a function of economics and cargo capacity of the rockets that deliver the building components.

Technically, there is no reason from an engineering perspective why you couldn’t build a mile long aircraft carrier or container ship on Earth. But the economics don’t justify it and from a logistics standpoint, few, if any, ports or canals could support a ship that large.

16

Just ran across this Slate article by Phil Plait, the Bad Astronomy guy. He’s critical of the Orion & SLS plans, and thinks a company like SpaceX could launch more components into space, more cheaply.

Probably a discussion for a different thread though.

17

The government agrees. Orion was originally part of the Constellation Project which was cancelled in order to let private industry build spacecraft and launch vehicles more efficiently. So they went with the Commercial Crew and Cargo program which was then massively underfunded and is why we are paying Russia 75 million a shot to send our astronauts to the ISS. And soon to be forced to send our astronauts to train on a “Russian” naval base in the Crimea which we don’t even officially recognize as theirs.
Project Constellation had several stages which would begin with trips to the ISS developing countermeasures to various health issues pertaining to prolonged spaceflight. By about 2020ish we would be making trips to the Moon and maybe an asteroid. On the moon they would test pressurized rovers and habitats and stuff to develop equipment to use on Mars. It would be around this phase that the Mars transit vehicle would be designed and built. Whatever the end up coming up with, Orion (or a private equivalent like the CST-100 or Dragon would be the spacecraft that gets the astronauts to the larger vehicle. It wouldn’t be taking them all the way to Mars or asteroids.

That would make a great thread.

18

Structurally, if your massive kilometer long ubership can accelerate at 1 G, and the rocket engines are at the bottom, then the structural load is the same as a skyscraper on the ground.

In reality, since the momentum transfer from a rocket is proportional to m*v, and the kinetic energy of the exhaust stream is proportional to velocity squared, every time you double your rocket engine’s fuel efficiency you increase the energy needed by a factor of 4.

There are various methods to produce enormous amounts of energy, such as a nuclear reactor, and then apply that energy to propellant via microwaves and electric fields and things and get very high exhaust velocities for your propellant (and therefore high efficiency). The problem is that all these methods produce enormous amounts of waste heat which must be radiated, reducing the rate at which you can accelerate.

This means that those kilometer long ships, assuming they use efficient engines so that most of the ship doesn’t have to be thrown away as stages, will never have more than teensy rates of acceleration. 0.01 g or so. This means you can make them proportionally longer : structural engineers, does decrease gravity by a factor of 100 mean that the same structure can be 100 times larger in all dimensions, or is the relationship nonlinear?

19

Watching youtubes of Newport News building an aircraft carrier, it struck me how gravityaffects the construction process. Is gravity even used to “hold” or “stabilize” pieces in place while it is being welded?

Construction involving large masses being manipulated in a zero gee and vacuum environment seems to me to involve a whole different set of skills, tools, and challenges than the 1g environment we operate in.

20

These ships most certainly WILL feel the effects of gravity, or gravity equivalent, as they accelerate and decelerate.