Amazing that mlees is the first one to bring up actual construction difficulties without gravity to hold everything in place. Attaching A to B is much, much harder if A and B float freely in three dimensions.
For this reason, I suggest building your space ship on the moon. Dig a hole, put a plastic roof on it, inflate to 0.25 atmospheres or such pure oxygen, which you can obtain from silicates, then dump a layer of rocks on top to keep out the radiation. Now you have a floor to work on and enough gravity to be helpful but launching into space costs you a fraction of the energy you’d need on earth.
The main downside is that you only have sunlight two weeks out of the month. So build close to the pole and run a wire to the other side where you put your second solar panel array. Or maybe set up shop at the equator for easier launching and use batteries or something to solve the energy issue.
A second downside is that you’re much further from earth, but I’m pretty sure the better environment and availability of many materials makes up for that.
Extraction of raw materials–water, silicates, iron, nickel, et cetera–would require relatively modest advances in technology. At Earth orbit, the energy available in the form of sunlight is enormous (1300 to 1400 W/m[SUP]2[/SUP]) and can be readily thermalized by numerous methods (optical concentration, heating of gas or molten fluids, conversion to electricity by thermoelectric or photovoltaic). The more difficult problem is establishing a sufficient infrastructure to support the manufacture of large or complex components; everything from rolled or extruded metals to microchips requires a massive investment even in a terrestrial environment, and would have to be highly automated in orbital facilities. This argues for making use of space-bourne materials in as close to raw form as possible (e.g. water ice for propellant or substrate, separated hydrogen and oxygen for consumable and energy storage, silicates as reinforcement) and delivering metals and rare earth elements back to Earth for processing until such time as that capability can be established in orbit with sufficient breadth and throughput to be self-sustaining.
However, this limits the construction of complex structures–such as spacecraft–to modular forms that can be carried from Earth to orbit by conventional launch vehicles (hence why the ISS is a series of cans and trusses rather than the elaborate rotating wheel portrayed in early space station concepts). A potential bridge to larger scale structures is to use modular forms but build upon them with minimally processed materials; for instance, silicate-reinforced water ice makes a fairly ideal hull material for a spacecraft or habitat provided you can insulate it from direct solar incidence and don’t subject it to inertial loads that will exceed its structural capability; not only is it readily available in space and fungible, it also makes an excellent radiation shield, provides an easily repaired barrier, and provides thermodynamic moderation. Of course, the material has to be replaced as it liquifies or sublimates, but with good thermoregulatory design the rate can be well controlled. Eventually, as space based industry matures, it would be possible to manufacture more complex structures which are fabricated or “grown” from robust materials.
As for the need for “gigantic spacecraft” this is almost assured; barring some science-fictional power sources and propulsion systems that are efficient beyond believe, any extant or proposed spacecraft will either generate a lot of internal waste heat which will have to be radiated away, or will require months or years to transit between planets. In addition, as our knowledge of space medicine has grown it has indicated the need to maintain a terrestrial-like environment for the health and functionality of crew; astronauts in freefall for many months or years will suffer degradation and chronic health impacts that can only be mitigated by spacecraft large enough to provide simulated gravity via rotation, radiation shielding, hydroponics for fresh food and recycling, and space to avoid the emotional stress of confinement. Interplanetary spacecraft will need to be much, much larger than what can be flown as a single payload or even as a handful of modules. Inflatable habitat modules help to increase interior volume, and when the outer layers are filled with water or waste, provide additional protection against radiation, but large modular structures are going to be subject to the thrust and tidal loads previously mentioned, either requiring distributed propulsion or complex structural design. By contrast, a large but geometrically simple spin-stabilized habitat or spacecraft can provided sufficient space and protection for hundreds to tens of thousands of crew or population within existing material limits.
