What are the tradeoffs of docking multiple small launches in orbit vs a single heavy launch?

Question

What are the benefits of having a heavy lift launch vehicle, compared to using several smaller launches and assemble the spacecraft in orbit?

There must've been more than a hundred successful dockings (and berthings) in space, including in Lunar orbit, and especially for building and supplying the ISS. The only docking failure I know of is the Progress M-34 to MIR in 1997 (they seem to have shut down the radar and looked out of the window holding a laser range finder...) and even then they didn't lose the mission. So docking seems mature and doesn't add substantial risk.

How important would the costs of docking systems and fuel for maneuvering in orbit be? Is the logistics of timing multiple launches a problem? Could we use today's small launchers to spend several years to build a Mars spaceship in LEO, like we did with the ISS, and fuel it and send it on its way whenever it is ready? Do economies of scale benefit large launchers for large missions, or is it the other way around because small launchers can be mass manufactured? What are the key issues with multiple small launches versus a single big one?

Answer 1

Many more smaller launchers means smaller modules. That is a limit on absolute volume limits.

Inflatables are a way to handle this. Launch them folded, expand in space, in a later launch deliver water that is stored in the walls as additional radiation shielding. So smaller launch, additional launches for outfitting.

Volume is not always a limit though. You can live with many small rooms, vs one or two big rooms. (Washrooms are a good example. Soyuz has two habitable modules in orbit, only one is landed. The second is most useful since someone can use the 'facilities' while the other two hang out in the other module allowing privacy.)

Docking hardware, wiring, plumbing, and assorted overhead of connecting multiple modules can start to eat into your margin very quickly.

If your structure needs to withstand thrust (Say Mars Transit vehicle) your docking structure must be that much stronger (and concomitantly heavier).

You need to provide a maneuvering system, or a tug, to deliver your modules together, more overhead.

If you do it all in one launch, and follow Zubrin's Shuttle-Z style model, the upper stage engine that put you in orbit can also do your TMI maneuver, so you could save on a set of engines mass. Even if you needed to be refueled.

Refueling, while non-trivial is a good way to launch most of the mass in one launch, one piece and deliver the fuel/oxidizer/propellant in a second similarly sized launch, without only minor overhead.